AIR-CONDITIONING FUNDAMENTALSHEAT:

Heat is a form of energy, a distinct measurable property of all matter. Heat cannot be

destroyed but can be transferred from one substance to another, always moving from

warmer to the colder substance. The unit used to measure the quantity of heat is the

British thermal unit (Btu). Btu is the amount of heat energy required to raise the

temperature of one pound of water by one degree Fahrenheit. Conversely, if the

temperature is reduced by one degree then the heat is removed.

CONDUCTION:

Heat transfer by conduction occurs when energy is transmitted by direct contact

between the molecules of two bodies in good thermal contact with each other.

CONVECTION:

Convection occurs when heat moves from one place to another by means of currents

which are setup in the fluid medium.

RADIATION:

Radiation occurs in the form of a wave motion similar to light waves wherein the

energy is transmitted from one body to another without the need for intervening matter.

TEMPERATURE:

Temperature is the measure of the intensity or level of heat. The unit of temperature is

Fahrenheit. Other scale is Celsius. The freezing point of water is 32 deg.F. Whereas the

boiling point of water is 212 deg. F.

Temperature conversion:

Deg. F = 9/5 deg. C + 32 Deg. C = 5/9 (deg. F-32)

SPECIFIC HEAT:

The specific heat of a substance is the amount of heat needed to raise the temperature of

one pound of that substance by one degree Fahrenheit. It requires one Btu to raise the

temperature of water by one degree Fahrenheit as such the specific heat of water is 1.0.

Most other substances require lesser heat resulting in specific heat lesser than 1.0. To

calculate the heat required to change the temperature of any substance, multiply the

mass of the substance in pounds with the specific heat and the temperature rise in

Fahrenheit.

HEAT(R) = Mass (m) x Sp.ht. (s) x temp.rise (t)

SENSIBLE HEAT:

Sensible heat is the heat added to a substance or heat removed from a substance which

results in change of temperature.

LATENT HEAT:

Latent heat is the heat added to or removed from a substance which causes change of

state but without change in temperature.

Latent heat of fusion is the amount of heat needed to change a substance from solid to

liquid. Latent heat of vaporization is the amount of heat needed to change from liquid to

vapor. Latent heat of condensation is the amount of heat required to be removed to

change from vapor to liquid state.

The latent heat of steam is 1070 Btu/lb whereas the latent heat of ice is 144 Btu/ lb.

SATURATION TEMPERATURE:

For any given refrigerant there exists a temperature for a given pressure at which the

refrigerant will vaporize or condense which is called as the saturation temperature.

Saturation temperature for any given pressure is defined as that temperature at which

liquid refrigerant and its vapor remain in contact with each other in equilibrium.

SUPERHEAT: 4

Any addition or removal of heat to a liquid refrigerant and its vapor in equilibrium in a

dosed container will only cause the liquid refrigerant to vaporize or the vapor to

condense. However if the vaporized refrigerant is separated from the liquid portion and

heat is added it will raise the temperature above the saturation temperature

corresponding to the pressure and the rise in temperature is the superheat and the vapor

is called as superheated. Superheat is expressed in deg.F.

SUB-COOLING:

If the liquid portion is separated from the vapor portion and is cooled, then any removal

of heat will lower its temperature than the saturation temperature corresponding to the

pressure. The temperature drop is sub cooling and the liquid is called sub-cooled liquid.

Sub-cooling is expressed in deg.F.

ENTHALPY:

Enthalpy is the heat content of a refrigerant measured from a base saturation

temperature of -40 deg.F. At this temperature and corresponding saturation pressure the

heat content of the liquid has been arbitrarily fixed as O. The unit of enthalpy is Btu/lb.

REFRIGERATION:

Refrigeration is the process of producing and maintaining temperatures below that of

the surrounding atmosphere and this means removal of heat from the substance to be

cooled to reduce its temperature below its freezing point.

AIR CONDITIONING:

Air-conditioning is treating of air to change its temperature & moisture content

simultaneously. An air-conditioning system controls temperature, humidity, motion of

air, air distribution, air pressure, dust, noise, bacteria, and odours. It produces an

atmosphere that is conducive to human comfort or required by a product or process

within a space.

ENERGY:

Energy is the ability to perform work. Energy can neither be created nor be destroyed

but may be changed from one form to another.

PRESSURE:

Pressure is the force exerted per unit area. The unit of pressure is pounds per square

inch. (Psi)

ATMOSPHERIC PRESSURE:

The earth is surrounded by an envelope of atmosphere or air. The pressure exerted by

the atmosphere on the surface of the earth is atmospheric pressure. (Atmospheric

pressure is 14.696 psi)

ABSOLUTE PRESSURE:

Absolute pressure is the pressure in pounds per square inch above a complete vacuum.

(Psia)

GAGE PRESSURE:

Gage pressure is the pressure in pounds per square inch above normal atmospheric

pressure of 14.696 psi. (Psig)

INCHES OF VACUUM:

Any pressure below 0 lb gage is referred as so many inches of vacuum. Normal

atmospheric pressure is 14.696 psi and it will support a column of mercury 29.92

inches in height. Lb change vacuum to absolute pressure, the reading in inches of

vacuum is subtracted from 30, and the result is multiplied by 0.48.

CRITICAL PRESSURE TEMPERATURE:

For each gas there exists a temperature above which it cannot be liquefied, regardless

of pressure. This is called its critical temperature. The critical pressure is the pressure

that causes liquefaction at critical temperature.

TON OF REFRIGERATION:

One ton of refrigeration is equivalent to the amount of heat required to be removed

from one ton of water (2000 lb) at 32 deg. F to make ice at 32 deg. F in a period of 24

hours. One ton of refrigeration (TR) is 12000 Btu/hr.

PSYCHROMETRICS

Psychrometrics deals with determination of the thermodynamic properties of moist air

and utilization of these properties in analysis of conditions and processes involving

moist air. Air is a mechanical mixture of gases and water vapor. Dry air is air without

water vapor and is composed of nitrogen (approximately 78% by volume) and oxygen

(21% by volume) the remaining being made up of carbon-di-oxide and minute

quantities of gases such as hydrogen, helium, neon, argon etc. The amount of water

vapor in the air varies greatly with the particular locality and with the weather

conditions. Water vapor in the air results primarily from the evaporation of water from

the surface of various bodies of water.

RELATIVE HUMIDITY DRY BULB & WET BULB TEMPERATURES

Dry bulb temperature of air is the temperature measured by an ordinary thermometer.

When measuring the dry bulb temperature of the air, the bulb should be shaded to

reduce the effects of radiation. The wet bulb temperature of air is the temperature

registered by a thermometer whose bulb is covered by a wetted wick and exposed to a

current of rapidly moving air having a velocity of around 1500 fpm. A sling

psychrometer has dry and wet bulb thermometers. After saturating the wick with clean

water, the instrument is whirled rapidly for approximately one minute, after which the

readings can be taken from wet and dry bulb thermometers. Whereas a dry bulb

thermometer, being unaffected by humidity, measures only the actual temperature of

air, a wet bulb thermometer, because of its wetted wick, it is greatly influenced by the

moisture content of the air; thus a wet bulb temperature is in effect a measure of the

relationship between the dry bulb temperature of the air and the moisture content of the

air. For any given dry bulb temperature, lower the moisture content of the air, lower is

the wet bulb temperature. The difference between the dry and wet bulb temperatures is

called as the wet bulb depression and is a measure of the relative humidity of air.

Higher the difference lower is the relative humidity and vice versa.

DEW POINT TEMPERATURE

Dew point temperature is the temperature at which condensation of moisture begins

when the air is cooled.

Air has both sensible and latent heat, and the total heat content of the air at any

condition is the sum of the sensible and latent heat contained therein. The sensible heat

of the air is a function of the dry bulb temperature. Since all the components of the dry

air are non condensable at normal temperatures and pressures, for all practical purposes

the only latent heat in the air is the latent heat of the water vapor in the air. The amount

of latent heat in any given quantity of air depends upon the weight of water vapor in the

air and upon the latent heat of vaporization of water corresponding to the saturation

temperature of the water vapor. Since the saturation temperature of the water vapor is

the dew point temperature of the air, the dew point temperature determines not only the

weight of water vapor in the air but also the value of the latent heat of vaporization.

Hence the latent heat content of the air is a function of the dew point temperature alone.

As long as the dew point temperature of the air remains unchanged, the latent heat

content of the air also remains unchanged. As such for any combination of dry bulb and

dew point temperatures, the wet bulb temperature of the air can have only one value, it

is evident that the wet bulb temperature is an index of the total heat content of the air. It

should be noted carefully that although there is only one wet bulb temperature that will

satisfy any given combinations of dry bulb and dew point temperatures, there are many

combinations of dry bulb and dew point temperatures which will have the same wet

bulb temperature.

RELATIVE HUMIDITY & ABSOLUTE HUMIDITY

Relative humidity is the ratio of the actual weight of water vapor per cubic foot of air

relative to the weight of water vapor content in a cubic foot of saturated air at the same

temperature.

Absolute humidity of air at any given condition is the actual weight of water vapor

contained in 1 cubic foot of air at that condition.

SPECIFIC VOLUME SPECIFIC HUMIDITY

Specific volume is the volume of the mixture per pound of dry air and it depends on the

temperature and the barometric pressure.

Specific humidity is the actual weight of the water vapor mixed with one pound of dry

air and is usually expressed in grains per pound.

SENSIBLE HEAT FACTOR

Sensible heat factor is the ratio of the sensible to the total heat.

In air-conditioning we treat air with a view to alter its temperature and moisture content

as such it is important to know how exactly air would behave when subjected to various

processes such as cooling, heating, humidifying and dehumidifying. A psychometric

chart is used to study the psychometric properties of air.

The vertical lines represent the dry bulb temperature. The horizontal lines represent the

moisture content. The curved line on the extreme left of the chart a saturation line and

on this line air is saturated i.e., air is having the maximum possible moisture content in

it and it cannot hold further and is having 100% relative humidity. Similar curved lines

running towards right are the relative humidity lines. The slant lines represent wet bulb

temperature and the total heat content. There are slant lines representing the specific

volume. On the extreme right side there is a scale showing the sensible heat factor.

Alignment circle is located at 80 F db and 50% RH and is used in conjunction with

sensible heat factor to plot the various air conditioning process lines.

PSYCHROMETRIC PROCESSES SENSIBLE HEATING & COOLING

By sensible heating, heat is added to the air which results in temperature rise and there

is no change in the moisture content. In other words, during sensible heating process air

retains constant moisture content and accordingly its condition will move on a

horizontal line corresponding to this constant moisture content. Since heat is added, its

enthalpy rises as such wet bulb temperature also rises. By sensible cooling the

temperature will fall without change in moisture content, resulting in removal heat as

such fall in wet bulb temperature.

ADDITION OF MOISTURE

If moisture is added without any sensible heating, the process will follow the vertical

line. The moisture content, enthalpy and wet bulb temperature Increases.

COOLING AND DEHUMIDIFYING

If heat is removed from air it results in fall in dry bulb temperature. If the surface

temperature of the coil is maintained below the dew point temperature of the air then

dehumidification starts with removal of moisture. The dry bulb temperature, the

moisture content and the wet bulb temperature decreases showing the removal of heat.

HEATING AND HUMIDIFYING

If heat and moisture is added to air it will follow the process just opposite to that of

cooling and dehumidifying. The dry bulb temperature, the moisture content and the wet

bulb temperature increases showing addition of heat.

EVAPORATIVE COOLING

During evaporative cooling the process line follows the wet bulb temperature line

showing that there is no change in heat. During evaporative cooling heat from the air

flowing through the air washer spray banks is taken by water for evaporation as such

heat is removed from the air and moisture is added. The amount of heat removed from

the air is equivalent- to the amount of moisture added to air as such the total heat

remains the same. The spray water temperature remains constantly at the wet bulb

temperature. In a spray chamber, the air approaches a state of complete saturation. The

degree of saturation is termed as saturation efficiency.

Saturation efficiency = (temp of enter. air - temp of leav.air) / (temp of enter. air wet

bulb temp).

The saturation efficiency depends on the spray surface available and on the time

available for the air to contact the spray water surface. The available surface is

determined by the particle size in the spray mist and the quantity of water sprayed,

number of banks of nozzles and the number of nozzles in each bank. The time available

for contact depends on the velocity of the air through the air washer chamber, the length

of the effective spray chamber and the direction of the spray relative to the air flow.

BYPASS FACTOR

By pass factor represents that portion of the air passing through the air washer/heat

exchanger which is considered to be leaving the spray chamber completely unaltered

from its entering condition. The physical and operating characteristics affecting the

bypass factor are the as follows:

A lesser heat transfer surface area i.e., less rows of coil, less coil surface area, wider

spacing of coil and fins.

An increase in the velocity of the air through the air-conditioning apparatus results in

increase in bypass factor as there is less time for the air to have contact with the heat

transfer surface. Decreasing or increasing the amount of heat transfer surface has a

greater effect on the bypass factor than varying the velocity of air through the

apparatus.

CHEMICAL DEHUMIDIFIER

Chemical dehumidifier contains silica gel which is located in the path of the air stream.

As moist air comes into contact with this silica gel, moisture is removed from the air by

the difference in vapor pressure between the air stream and the silica gel. As the

moisture condenses, latent heat of condensation is liberated, causing a rise in

temperature of the air stream and the silica gel. This process occurs at a constant wet

bulb temperature.

HEAT LOAD CALCULATION 5

The space cooling load is the rate at which heat must be removed from the space to

maintain a constant air temperature. To calculate a space cooling load, detailed design

parameters are required. Generally the following steps should be followed.

1. BUILDING CHARACTERISTICS.

a. Building Construction material

b. Physical dimensions of space

c. Ceiling height, columns, beams

d. Windows, stairways, escalators.

2. CONFIGURATIONS.

a. Orientation

b. Shape

c. Shading from Adjacent building

d. Space used for

3. OUTDOOR DESIGN CONDI TIONS.

a. Weather data

b. Latitude/Longitude c. Elevation

d. Period & time

4. INDOOR DESIGN CONDITIONS.

a. Dry bulb temperature

b. Relative humidity

c. Ventilation requirement Permissible Variation/Control limits.

5. INTERNAL LOAD

a. Light

b. Occupancy 6

c. Equipment, Appliances

6. ADDITIONAL CONSIDERATIONS 7

a. Duct heat loss

b. Duct leakage

c. Fan energy

d. Return air system

E. Outside air

f. Type of system.

HEAT LOAD CONCEPTS

A good designer has to calculate the cooling load at optimum design conditions. The

load so calculated should not be too high or too less.

Although there are several methods of load estimating available today, but cooling load

estimate form "Annexure1" is the most acceptable form in the industry for many years.

The space heat gain is a resultant effect of sensible and latent heat.

The sensible heat is the phenomenon of temperature, where the latent heat is the stored

heat in the form of moisture or metabolism rate.

The other heat load components can be classified into-

a) Loads originated from heat sources outside or external to the conditioned space.

b) Loads within the conditioned space. c). Load occurring from heat gains or losses

with moving cool fluids to and from the conditioned space.

OUTDOOR DESIGN CONDITIONS.

While calculating the heat load the outside conditions playa vital role in estimating the

heat load.

In America ASHRAE data are regarded as the industry standard. In India ISHRAE has

started working on the project on establishing and compiling authentic weather data for

various places in India. Presently the outside conditions as per Annexure -2 are

generally acceptable for different places in India.

The ambient air properties and solar intensities changes with different elevation,

latitude and longitude. While selecting the refrigeration capacity of the plant for year

round air conditioning the cooling load for Summer and Monsoon weather which ever

is higher is selected.

In general for Indian climatic conditions 4PM is average time for solar heat gain and

average daily range of temperature (Maximum DB - Minimum DB in a day) vary from

15 to 20 degree F (Local conditions are to be referred)

The table-1 shows the equivalent temperature difference for 15 degree F temperature

difference (Outside temperature - Inside temperature) and 20 degree F daily range. For

other conditions the correction factor as per table -2 is to be considered.

INSIDE DESIGN CONDITIONS.

The human body considers itself comfortable when it can maintain an average body

temperature between 97 degree F and 100 degree F. It becomes the task of air

conditioning to maintain the environment around the body within this comfort zone of

conditions.

In general 75 degree F DB and 50% RH is considered the design conditions for human

comfort. However these conditions may vary depending upon the environmental

requirement and applications.

Some typical inside design conditions for various industries are stated in Table-3

SOLAR HEAT GAIN

The primary weather related variable influencing the sensible cooling load for a

building is solar radiation. The effect of solar radiation is more pronounced on exposed

surfaces.

Room sensible heat is calculated as under. The heat transfer rate q is given by equation

q=UA (T1-T2)

Where q = Heat transfer rate in Btu per hour.

U = Coefficient of overall heat transfer between the adjacent and the conditioned

space in Btu /h.sqft-deg. F.

A = Area of the separating section in sqft. T1= Average air temperature in adjacent

space degree F

T2= Air temperature in conditioned space deg.F

U= 1/R where R = Addition of thermal resistance of all the surfaces coming in

between the conditioned space and adjacent space. (Refer Table-4 for Thermal

Resistance R of various building and insulating materials.)

VARIOUS TYPES OF SPACE HEAT IS CALCULATED AS UNDER.

1. SOLAR GAIN - GLASS

Solar heat gain from glass is the absorbed solar energy transmitted to the conditioned

space by convection and radiation.

The table no. 5 is referred to calculate the solar gain through glass. The glass factor as

per Table-6 is also to be considered based on number of hours the air conditioning is

required.

2. SOLAR AND TRANSMISSION GAIN (WALLS & ROOF)

The heat transfer rate q is given by equation as stated herein above. The factor (T1-T2)

is replaced with Te which is equivalent temperature difference (Refer Table -1 A&B).

The correction factor from Table -2 is applied on table 1 A&B for desired conditions.

3. TRANSMISSION GAIN (EXCEPT WALLS & ROOFS)

The heat gain transmitted through conduction is a direct function of temperature. The

value of Tl may vary widely from the conditioned space. If the adjacent space is kitchen

or boiler room the temperature may vary 15 to 50 degree F above the outside

temperature.

In general where no specific particulars are indicated for adjacent space the (TlT2) is

considered as temperature difference between ambient temperature and inside

temperature for transmission through glass. For partition walls or unexposed floor or

ceiling, the temperature difference is taken less 5 degree F.

For floors directly in the contact with the ground or over an under ground basement that

is neither ventilated nor conditioned, heat transfer may be neglected for cooling load

estimation.

4. INFILTRATION AND VENTILATION HEAT GAIN.

The outside air of high temperature than air conditioned space temperature produces

the heat gain in the space to be air conditioned by leakage through windows or doors

or unintentional openings. This being uncontrolled air quantity, generally heat gain

through ventilated air quantity due to bypass from the coil is considered for heat gain

due to ventilation.

5. HEAT SOURCES IN CONDITIONED SPACE (INTERNAL LOADS)

(i) People - Table-7 gives representative rates at which heat and moisture are given off

by human being in different states of activity.

(ii) Light- The primary source of heat from lighting comes from the light emitting

element or lamps. Lighting is either fluorescent or incandescent.

Incandescent light

lKW = 3410 Btu/h. Fluorescent light lKW = 1.25 x 3410 Btu/h

(iii) Equipment - Power driven equipments such as motors, calculators and other

appliances electrical gas or steam driven produce localized heat load, Table-8

indicates Heat gain from typical appliances.

ADDITIONAL HEAT SOURCES

(i) Supply duct heat gain

If ducts pass through a space whose temperature is higher than that of air, heat gain is

experienced. It is generally expressed in percentage of sensible load of the room and

generally considered 4 to 5%.

(ii) Supply duct leakage

The supply air is transmitted under pressure to the room. The losses from air leakage

out of duct work or equipment is a direct loss of cooling. The losses from leakage is

also considered in percentage and it is generally 4 to 5%.

(iii) Supply air fan heat

Fans that circulate air add energy to the system from fan inefficiency, air static and

velocity pressure and heat generated by motor and drive inefficiencies. Generally 3%

of room sensible heat is considered as fan heat gain for normal application.

Adding the heat gain in the room from the above steps 1 to 5 gives the total room

sensible heat and total room latent heat. Adding the additional heat gain as enumerated

in step 6 (i) to (iii), referred to effective room sensible heat and effective room latent

heat.

These loads are referred to as effective since both the coil leaving temperature and

humidity level must effectively be lower than conditions required at the room in order

to (1) absorb the losses along the way and then (2) absorb the room loads. These two

loads establish the air quantity to be cooled and dehumidified.

(iv) OUTSIDE AIR

The outdoor air, which in required to be introduced to ventilate conditioned space,

added considerable heat load on the system. Though this load is not considered as room

load, but this outside air is required to be treated to the air temperature supplying to the

room. Thus this load is considered the load on the coil to arrive the total capacity of the

plant. Table-9 shows the recommended ventilation standard for the most common

application.

For general application, a basis of estimating the cfm per person is

People not smoking - Recommended 15 Minimum 5

People smoking recommended 40 Minimum 25

The amount of outdoor air required primarily depends on the number of occupants and

on materials and apparatus that may give off odors within the space. To provide for

physiological needs, the outdoor air quantity should never be less than 4 cfm per

person.

(v) Return air system

If return air ducts pass through apace whose temperature is higher than transmitted air

or free air is collected through a space having higher temperature than transmitted air a

sensible heat gain is experienced.

Since the air in the return duct can be at a relatively negative pressure compared to the

air surrounding the duct, an inward leakage of warm air can take place.

This load is also generally expressed in percentage varying from 2 to 5%.

(vi) TYPE OF SYSTEM

If chilled water system is used to produce the cooling in the air conditioned space, then

heat loss in the chilled water piping, water circulating pumps etc are also considered.

Generally 5 to 10% of the total load is added to arrive the refrigeration load.

Thus following the calculations detailed in step 1 to 6 we arrive the total capacity of the

plant required to maintain inside design conditions.

DESIGN GUIDE SECTION

CONTENTS OF SECTION

Air Handling Units

Duct Design

Graphic Illustrations of Design Considerations:

Duct Design - Fan Connections, Duct Turns, Etc.

Duct Design - Diffuser Layouts, Balancing

Damper Locations

Mixing Boxes - Duct Connections, Mixing Baffle

Location, Etc.

Duct Sizing

Design Guide

Duct Design - Diffuser Layouts, Balancing

Damper Locations

Duct Design - Balancing Damper, Extractor Locations

Gauge and Thermometer Locations

Terminal Devices

DESIGN GUIDE MANUAL

AIR HANDLING UNITS

1. Either a variable pitch drive or provisions for the cost of one fixed sheave

change should be specified as it is unlikely that the system static pressure will be

exactly as calculated.

2. The air handling unit specified should have the ability to deliver (including

available HP) sufficient excess air volume and pressure over that required by the

outlets to allow for leaks from ductwork, housing and connections. An excess of

10% air volume and pressure is realistic on the average installation.

3. In addition to the standard automatic outside and return air dampers, provide

manual type opposed blade dampers on the minimum outside air intake louvre

and on the return air ducts at the entrance to the mixed air plenum. These

manual dampers permit proper proportioning and setting of outside air and

return air quantities.

4. Provision should be made for a complete change of air filters in all systems at

the time of balancing (in event filters have become clogged due to fan operation

during construction). Artificial filter resistances can be added to the clean filter

bank by partial blanking off at filter face areas.

5. Manually operated opposed blade or quadrant type dampers must be provided in

each main zone duct of a multizone unit installation to permit the balancing

technician to set each zone to the required air delivery.

6. Care should be taken to assure that the discharge air from cooling towers,

condensing units, relief exhausts, roof exhaust fans, etc., cannot short circuit or

be carried back by wind induced currents to any fan system outside air intake.

7. The inclusion of a return air fan is recommended with system design

incorporating an economy cycle (100% Outside Air to Minimum Outside Air).

If return air fans are not included in the design, either exhaust fans or a power

operated relief system should be incorporated to limit the building pressurization

to approximately .05" WG.

8. The efficiency and operating characteristics of a fan system can be adversely

altered by connecting return or outside air to only one side of the fan inlet

plenum.

9. Many multizone units discharge their air at an angle between horizontal and

vertical. Turning vanes should be provided in the initial discharge duct fitting to

direct air flow smoothly to the elbow or transition that follows.

10. Avoid the application of internal duct linings at duct connections to multi zone

units or conventional air handling units with high outlet or discharge velocities.

Start the duct lining after the first transition has been reached where normal duct

velocities exist.

11. To assure proper internal air flow and uniform distribution across coil surfaces,

specify perforated static pressure plates at fan outlets of blow-through air

handling units.

12. Difficulties in air distribution generally arise over static imbalance of zones on a

multi zone installation. To minimize these problems, the ducts to the longer runs

should be oversized and any zone requiring 1.25 modules of the multi zone unit

outlet should be given two modules with the resulting shortage of outlet modules

being absorbed by the larger zones.

13. Single blade dampers with locking quadrant hardware are preferred for use at

each multi zone unit module. Care must be taken in the fabrication of this

damper assure that it closes properly with minimum leakage, as its use many be

required to correct a zone static imbalance.

14. Specify that access holes for tachometer readings be provided in all belt guards,

the newer split causing guards often have the half connections of the same line

as the fan and motor shaft centers, thus making tachometer readings impossible

without removal of the guard. This greatly hampers fan adjustments as well as

verification readings and presents on obvious equipment hazard.

15. Fan capacities must be selected for the elevation above sea level at which they

will be required to function. While delivered calculations are required to

determine exact correction factor, the following guide values may be used to

determine an approximate correction factor.

4% for each 1000 feet elevation above sea level.

2% for each 10 deg F above standard 70 deg F temperature air.

The following formulas apply to fan selections at elevated levels:

a. Fan selection CFM = Design Standard Density Air CFM x correction factor.

b. Fan selection CFM = Actual System SP x correction factor

c. Fan selection RPM = RPM listed in fan chart at corrected CFM (a) and SP (b).

17. Avoid selection or approval of fans that different CFM capacities and HP

requirements at the same static pressure and RPM as specified by the fan chart

curve. Also avoid the selection of fans that indicate an increase in CFM when

the fan RPM is reduced or vice versa.

DUCT DESIGN

1. Splitter type dampers offer little or no control of air volume in ducts. They

should be regarded as air diverters only, with maximum effectiveness when

present on duct systems exhibiting low resistance to air flow. Valid application

of splitter dampers generally occurs at the ends of the branch ducts where need

to reduce or restrict air flow is not required.

2. The application of single blade quadrant volume dampers immediately behind

diffusers and grilles may tend to throw air to one side of the outlet, preventing

uniform air flow across the outlet face or cones.

3. Manually operated opposed blade or quardant type volume dampers should be

installed in each branch duct takeoff after leaving the main duct to control the

amount of air into these branch ducts.

4. Turning vane leading and trailing edges should be always be parallel with the

entering and leaving air stream to minimize air flow turbulence.

5. Manual volume dampers should be provided in the duct drop or takeoffs to

diffusers and registers to limit the total air to the face damper of the register or

neck damper of the diffuser. Sidewall and diffuser dampers cannot be used for.

6 Outside air louvers can create objectionable air noise on large systems. Louver

blades should be widely spaced with all edges rounded (or double folded) to

prevent the generation of high pitched air noises. Intake screens should have

openings of at least 1/2" squares to prevent clogging while offering ample

protection against large entering objects.

7. Manual dampers should be installed downstream of hot and cold zone dampers

on each zone of a double duct system.

8. Double thickness or extended edge turning vanes should be utilized in all

elbows, return as well as supply.

9. Furnish extractors where the main air stream in the main duct is a distance away

from the branch duct takeoff (when located on the inside radius duct wall

following an elbow, etc.).

10. Vertical duct chases or main return ducts require manual balancing dampers to

be installed at each branch duct inlet.

11. Provide extension - ceiling mounted damper hardware wherever possible.

12. Require adequate size access doors to be installed within working distance of

volume dampers, fire dampers, pressure reducing valves, reheat coils, mixing

boxes, blenders, constant volume regulators, etc., to permit required

adjustments.

13. Avoid placing a return air opening directly in or adjacent to the return air

plenum. Sound lining of the duct opening and plenum will not reduce the

transmitted noise to accepted levels.

14. A slight space or opening between blades of an opposed blade volume damper

will generate a relatively high noise level as the air passes through the openings

under system pressure. Damper blades should be sealed with foam rubber or felt

to form an effective seal with the blades in the closed position.

15. Duct leakages may vary from 15 to 45% depending upon workman ship, type of

duct construction and fittings, system design, etc. To minimize this variable, all

duct seams, casing and plenum connections, etc., should be taped, thus generally

assuring a maximum of 5% duct seam leakage factor.

16. Avoid the use of masonry or composition wall vertical air shafts supply or

exhaust systems on multi-storey building.

Where the use of such shafts is unavoidable, extreme care must be taken to seal

not only the connections into the shaft, but the entire masonry or concrete

surface itself. The sealing of this type of shaft after it has been closed is

extremely expensive.

17. Indicate volume damper locations at accessible points and wherever possible, a

distance from a duct transition or fitting. Care should be taken during

installation to make certain that sheet metal fasteners (screws) do not protrude

into the duct and interfere with damper operation.

18. Do not use extractors at branch or main duct takeoffs to provide volume control.

Extractors are principally effective in diverting air to ducts experiencing air

shortages, provided these shortages are not due to a considerably higher branch

duct resistance to air flow than the other branch ducts on the system. With the

condition of higher branch static, the extractor when positioned in the main duct

air stream cannot produce

19. sufficient velocity pressure diverted air to overcome the branch duct resistance.

When severe dampering is required to build pressure, etc., the high bypass and

leakage factors of the extractor nullify its effectiveness.

20. Proportion the sizes ofthe duct split fittings or branches based on the CFM

requirements of each resulting duct. If higher or lower duct pressure

requirements in the branches and outlets are present, adjustment of the

proportion of the split should be made.

21. Splitter dampers should be provided at all duct split fittings to permit balancing

without raising noise levels. Said dampers do not eliminate the necessity for

volume dampers in the resulting branch ducts.

TERMINAL DEVICES

1. Avoid installing diffusers or grilles directly into the bottom or sides of a main air

duct. No amount of adjustment will decrease the noise level generated.

2. Do not design light troffers on the same duct run or zone with standard diffusers

or registers due to the greater pressure requirements of truffers which will

necessitate excessive throttling at the standard outlets and generate objectionable

noise levels.

3. Restrict use of high induction type diffusers to those applications requiring high

air motion (CFM per square foot) when required.

4. Avoid long duct runs with large volume diffusers off the main and branch ducts

terminating in small diffusers or registers.

5. Avoid mixing supply registers and diffusers on the same duct section. The

greater pressure requirement of the diffusers will necessitate extensive throttling

at the registers and generate air noise.

6. Avoid placing diffusers or registers so that air patterns will be discharged into

ceiling mounted light fixtures, or in having pattern follow the ceiling too closely.

7. If possible, provide adjustable extractors at each duct takeoff to a register.

8. Return air registers should be located on or near exterior walls, preferably at or

near floor level.

9. Do not use an outlet with a low induction characteristic where the air volume

being distributed is high and the distance of throw is short.

10. Select air outlets which have damper mechanisms readily accessible for

adjustments. Diffusers with removable cores which expose the dampering

devices are satisfactory; the principal difficulty lies with registers whose

dampering mechanism (OBD) are recessed too far behind the register face or do

not have alignment of the grille face openings with the damper mechanisms

operating key, preventing damper adjustments.

11. Return air grille should be selected for operation at low face velocities (100 to

600 FPM) to minimize noise levels.

12. When designing duct drops to diffusers, provide a minimum length of 2 times

the duct diameter (or square dimension) in length to assure even distribution

from the outlet.

13. Return air grilles and duct connections which open into common return plenums

without return air fans should be oversized when possible.

14. Avoid the passage of return air from one space or zone through that of another to

reach a return air grille.

15. Avoid the use of built-in door louvers for passage of return air when the supply

air system operates at low pressure (ceiling plenum supply, etc.).

16. Avoid the use of combination supply-return outlets. Air quantities handled by

supply and return section of outlet can be accurately measured and adjusted by

means of supply air to return cannot be determined.

17. Avoid designing supply registers and diffusers on the same duct section. The

greater pressure requirements of the diffusers will necessitate excessive

dampering at the registers with possibility of generating air noise.

MIXING BOXES

1. Require that mixing box manufactures set mechanical volume controllers to plus

or minus 5% of design CFM by actual air flow test methods, rather than by

measured spring adjustments. etc., which results in less than 40% of the boxes

being delivered and installed within 10% of the design air requirements.

2. Require that direct access be provided to each mixing box with sufficient

clearance for adjustment and if necessary, removal of the volume controller

element, etc.

3. Pressure testing of high pressure duct runs is an absolute necessity, Spiral duct

can be sprung in shipment or during installation, with the resulting leaks along

the casing walls rather than at the duct joints. Test should be made in the

presence of the design engineer or performed by an independent testing agency.

4. Avoid short discharge duct connections from the mixing box unit to the supply

register or grille due to the extremely large variation in the discharge air

velocities across the box outlet opening. It is possible to simultaneously

experience induced air flow and excessive discharge velocities accompanied by

air noise at the face of the register under the above arrangement.

5. To prevent stratification of warm and cool air in supply duct work,

supplementary mixing baffles (perforated plate, etc.,) should be installed at the

outlet of all mixing boxes. Temperature differences of 20 deg to 25 deg F can be

experienced in branch supply ducts due to this stratification.

6. Require installation of internal duct lining of the discharge duct after leaving

mixing box, said installation to follow mixing box manufacture's

recommendations.

7. Avoid short, abrupt connections from unit outlet to duct split fittings or

branches.

8. Specify mixing box dampers shall not leak more than 3% of the design CFM

when functioning at the design static pressure conditions.

9. On low velocity, double duct systems, install round volume dampers in each hot

and cold duct take off to all mixing boxes. Care should be taken that sufficient

takeoff duct is provided to house the damper mechanism so that the damper

blade does not protrude into the main branch duct, or strike the flexible duct

connection when open.

10. Differential pressure across an orifice offers the best method of assuring constant

volume. The reliance upon point static pressure (Duct or Box) should be avoided

as it is not reliable.

11. Pressure will vary from one side of a duct to the other when a unit is on heating

or cooling, and can give a false pressure signal to the controller. Pilot type static

pressure tips located in the center of the duct are recommended for consistent

readings.

DUCT SIZING

An air duct transmits air from the air handling unit to the space to be conditioned. As

such the ducts must be designed properly taking into consideration the available space,

friction loss, velocity, sound level, heat and leakage loss and gain and first and

operating cost. Generally ducts are designed with a velocity of 1500 fpm and it varies

for different applications.

Air distribution systems are divided into three pressure categories:

Low pressure - up to 33/4" of WG -class I fan

Medium pressure - 3314" to 6314" of WG class II fan

High pressure - 6314" to 121/4" of WG class III fan

The air handling unit in an air conditioning application has to over come the resistance

posed by return air grilles, return air ducting, return air filters, dampers, coils, fan

transformation, outlet dampers, special filters, supply air ducting and grilles.

Air passing through any duct meets a resistance to the flow on account of the friction

between the duct surface and the air stream rubbing against this duct surface.

Ducts are fabricated out of GS sheets, Aluminium sheets and MS sheets and may be

circular or rectangular. Flexible round ducts are also used for terminal connections.

ASPECT RATIO is the ratio of the long side to the short side of a duct. This ratio is an

important factor in duct design. Higher aspect ratio means higher first and operating

costs. Further large duct aspect ratios have more heat gain. A large aspect ratio duct

will call for more sheets of heavier gauge for fabrication and higher reinforcement.

Duct transformations are used to change the shape of a duct or to increase or decrease

the duct area. When the shape of the duct is changed without affecting the cross

sectional area a slope of 1 in 7 in is recommended. It this slope cannot be maintained a

maximum slope of 1 in 4 in should not be exceeded. Often ducts must be reduced in

cross sectional area to clear obstructions. It is a good practice not to reduce the area

more than 20% of the original area.

A variety of elbows are used for both rectangular and round ducts. A minimum throat

radius of 6 in. has to be maintained for the elbows. At a take off collar a splitter damper

or vanes are provided.

Duct may sweat when the surface temperature of the duct is below the dew point

temperature of the surrounding air. If the ducts are not passing around the conditioned

air it has to be insulated. If the length of the duct exceeds more than 50 feet it is a

general practice to insulate the tail end to prevent the heat gain at the tail end. Ducts are

acoustically treated for a length of approximately 15 feet from the fan discharge

internally for cordoning the noise of the fan.

The duct accessories are volume control dampers and fire dampers. A parallel acting

blade damper is used for isolating whereas an opposed blade damper is used for

throttling. Afire damper is a damper with a fusible link which will close in case of fire

by a spring once the fusible link melts.

DUCT DESIGN

In any duct section through which air is flowing, there is a continuous loss of pressure.

This loss is called duct friction loss and it depends on the following:

Air velocity Duct size

Interior surface roughness

Duct length

The friction rate of a duct is expressed in terms of inches of water per 100 ft of

equivalent length of duct work. To determine the loss in any section of duct work, the

total equivalent length in that section is multiplied by the friction rate. A duct designed

with a higher velocity results in smaller ducts and lower duct material cost but it

requires higher operating costs with a larger fan and vice versa for a duct designed with

a lower velocity. A duct is designed by using a duct friction chart or using a ductolator.

The ducts are designed either by equal friction method or by static regain method.

EQUAL FRICTION METHOD

In the equal friction method for a given air quantity an initial velocity is selected to

determine the friction rate. This friction rate is then maintained throughout the system

and the equivalent round duct diameter is selected from the duct friction chart. This

procedure of duct sizing automatically reduces the air velocity in the direction of air

flow. The loss in the duct having the highest resistance will be the basis for the fan

selection. The equivalent length of the duct including the fittings and elbows multiplied

with the friction rate will give the loss in the duct. The equal friction method does not

maintain uniform static pressure at all branches and terminals. As such to obtain the

proper air quantity at the beginning of each branch it is necessary to include a splitter

damper to regulate the flow to the branch. It is also required to have volume control

dampers to regulate the flow at each terminal.

STATIC REGAIN METHOD

In a duct carrying a certain quantity of air at a certain velocity, if the duct area is

suddenly increased with the cfm still remaining the same, the velocity is reduced. This

reduction in velocity is converted into static pressure. When the velocity changes from

V1 to V2 the static regain is (V2 /4005)2 - (V1/4005)2. The basic principle of the static

regain method is to size a duct run so that the increase in static pressure (regain to

reduction in velocity) at each branch or air terminal just offsets the friction loss in the

succeeding section of duct. The static pressure is then the same before each terminal at

each branch. For a given air quantity with the initial velocity initial duct section is

designed. The remaining sections are sized with the L/Q chart and the velocity charts.

With the L/Q chart knowing the air quantity and the length between the ducts L/Q ratio

is determined. With the velocity before take off and the L/Q ratio the velocity after take

off ie., the velocity in the duct section to be designed is determined. With the airflow

and the velocity the duct area is arrived which is helpful in sizing the duct.

The branch ducts designed by static regain method are bigger in size than the equal

friction method. However the increase in first cost is offset by reduced balancing time

and operating costs.

DUCT CONSTRUCTION

The sheet metal gauge used in the construction of ducts and the reinforcing required

depends on the pressure conditions. There is also a wide variety of joints and seams for

the manufacture od ducts. The enclosed table shows the recommended gauges of GS

and aluminium sheets required for duct construction having different dimensions.

AIR TERMINALS AND DISTRIBUTION

Air has to be distributed uniformly to various rooms or zones in proportion to the actual

load. Even though there is a single hall the distribution is important as there may be

more concentration of sensible heat in the peripheral regions than the core of the space.

The air admitted into the room should not create any draft. It is desirable to restrict to a

temperature difference of 15 deg. F between the supply air and the room temperature to

prevent cold drafts. In applications where the apparatus dew point selected is low it is

desirable to take more return air and bypass the same around the cooling coils.

The air distribution system must be designed to hold the temperature within tolerable

limits. The variation in temperature shall be limited to a maximum of 2 deg. F.

Temperature fluctuations are more noticeable than variations. The ideal room velocity

for comfort application is around 25 to 30 fpm. The desirable direction of air motion is

towards a person's face. It is tolerable if it is from the top or from the side but not

definitely from the back of the person.

Normally it is not necessary to blow air to the entire length or width of the room. It is

enough if the blow is 3/4th the length of the room. Exceptions may that when there are

local sources of heat at the end of the room opposite to the air outlet.

PRINCIPLES OF AIR DISTRIBUTION THROW

Throw is the horizontal distance that an air stream travels on leaving an outlet. This

distance is measured from the outlet to a point at which the terminal velocity of the air

stream has reached to 50 fpm and at a height of 6.5 ft above the floor.

DROP

Drop is the vertical distance the air moves between the time it leaves the outlet and the

time it reaches the end of the throw.

INDUCTION

Induction is the entrainment of the room air by the air ejected from the outlet and is a

result of the velocity of the outlet air. For two outlets having the same area, the outlet

with larger perimeter has the greatest induction but lesser blow. AB such greatest room

induction and shortest blow will occur with an outlet in the form of a narrow slot. The

air coming directly from the outlet is .called primary air. The room air which is picked

up and carried along by the primary air is called secondary air. Induction ratio is

defined as the ratio of the total air (primary + secondary air) to primary air.

SPREAD

Spread is the divergence of the air stream as it leaves the outlet. Spread occurs both

horizontally and vertically. The vanes in the grilles produce spread.

The outlets may be a rectangular wall grille or a square or round ceiling diffuser or a

linear continuous grill or diffuser. The outlets are fabricated out of MS sheets,

aluminum or plastic.

The wall grille should be located atleast twice its height below the ceiling to avoid

appreciable streaking of the ceiling with dirt. The principle thing to keep in mind in the

question of supply grille location is the fact that the conditioned air must be delivered to

the locations desired and no dependence placed upon the return grilles for proper

distribution. In other words, the location of the return grilles has little effect upon the

air circulation within the room. Some large areas may be successfully and evenly

cooled with only one large return air grille. Return air grilles need not be of any special

design but can be made to suit any desired architectural effect. The one point- to keep

in mind in locating the return air grilles is the fact that the velocity of the air returning

to this point should not exceed 50 fpm in the occupied zone.

The object of room air distribution is to provide satisfactory room air motion within the

occupied zone, and is accomplished by relating the outlet characteristics and

performance to the room air motion as follows:

Total air in circulation = outlet cfm x induction ratio

Average room velocity = 1.4 x total cfm in circulation / Area of wall opposite to outlet

Room circulation factor (K) = outlet cfm/ clear area of wall opp. outlet

= average room velocity / 1.4 x induction ratio

PIPING DESIGN

The materials used for piping systems in air conditioning are:

Mild steel - black and galvanized Copper - soft and hard

Mild steel pipes come in three classes light or Class A (yellow band), medium or Class

B (blue band), heavy or Class C (red band).

Copper pipes are generally used for packaged unit and semi hermetic compressor

refrigerant piping. Black steel heavy class pipes are used for other refrigerant piping.

For condenser and chilled water application either black steel medium or heavy class

pipes are used. Some customers specify heavy/ medium class Galvanized pipes for

condenser water application. Drain pipes are generally with Galvanized medium class

or with pvc pipes.

For pipes running for longer lengths suitable expansion joints/expansion loops and

offsets are to be provided to take care of the expansion and contraction. All piping

should be supported with hangers that can withstand the combined weight of the pipe,

pipe fittings, valves, fluid in the pipe and the insulation. There are recommendations for

support spacing. Pipes are to be isolated not to transmit the vibration from the

equipment to the building structures. Piping will involve fittings such as elbows, tees,

flanges, couplings, unions etc. The pipes are joined either by threading or welding!

brazing.

The valves used in air conditioning applications are back seating globe valves or ball

valves and angle valves for refrigerants and gate/globe/butterfly/ ball valves and

balancing valves for water. Gate valves are used for isolation whereas the globe valves

are used for throttling. Butterfly valves can be used for isolation as well as throttling.

Further butterfly valves cause lesser pressure drop in comparison to a globe valve for

the same size and water flow. Balancing valves are used for throttling and measuring

the water flow.

A non return valve either swing type or lift type is used along with water pump sets.

Lift type check valves are to be used only for vertical lines. Y strainers or Pot strainers

are used near cooling tower and pump sets.

WATER PIPING

The water piping systems may be once through or re-circulating type.

In a once through system water passes through the equipment only once and is

discharged. In a re-circulating system water is not discharged but flows in a repeating

circuit. Both types are further classified as open or closed systems. An open system is

one in which the water flows into a reservoir open to atmosphere. A closed system is

one in which the flow of water is not exposed to the atmosphere at any point. This

system usually incorporates an expansion tank to allow expansion of water.

The water re-circulating systems are further classified into reverse return piping, direct

return piping and reverse return header with direct return rises. If the units have the

same or nearly the same pressure drop one of the reverse return methods are used. If the

units have different pressure drops then it is economical to use direct return. Ina reverse

return piping system balancing the system is easier. In a direct return system balancing

valves are to be provided for system balancing. Reverse return system can be used only

for closed piping applications.

There is a pressure drop in any pipe through which water is flowing. This is due to the

friction in the pipe. Pressure drop is expressed as feet of water per 100ft length of pipe

for various rates of flow. The friction loss depends on the water velocity, pipe diameter,

interior surface roughness and pipe length. System pressure has no effect on the head

loss of the equipment in the system but they will dictate the use of a heavier pipe. To

properly design water piping the friction loss in the pipe, valves, fittings and other

equipment are to be considered. For working out the frictional pressure drop through

the pipe fittings and valves, these items are expressed in terms of an equivalent length

of pipe of the same size. Tables are enclosed for the equivalent length of the fittings and

valves. Pipes are generally sized so as to limit the maximum velocity within 10 fps. The

velocity ranges between 4 and 7 fps for pump suction and drain lines, 4 to 15 fps for

header lines, 8 to 12 fps for pump discharge, and 3 to 10 fps for risers. These

restrictions in velocity is to reduce the erosion of pipes. For sizing of pipes friction

charts are used. There are separate charts for open and closed piping systems.

EXPANSION TANK is used to allow expansion of water in the system due to

temperature rise. If this is not provided then the system will develop dangerously high

hydraulic pressure when water expands due to temperature rise. The expansion tank is

located at the highest point. The expansion tank should be sized so that the difference

between the lowest and highest water levels in the tank will account for not less than

3% of the total volume of water in the entire system. A permanent water supply

connection from an external source has to be connected to the tank with a float valve

and quick fill valve. It will have an over flow and drain pipe. There should not be any

valve between the expansion tank and the piping system as there is chance of

accidentally closing this valve thus making the expansion tank isolated. It may be of

interest to note that for every deg. F rise in temperature of water it will exert an

pressure of 18 psi in a closed water system. It is advisable to fill the system with water

from the lowest point as this will purge out all the air from the system.

Thermometers and pressure gauges are provided at convenient points to measure the

temperature and pressure. Separate thermowells are to be provided. Pressure gauges are

selected so that the normal reading of the gauge is near the mid point of the pressure

scale.

Air vents should be installed in the high points of any water system so that at the time

of initial filling air can be vented out by opening these air vent cocks. Automatic air

vents are also available. Drain plugs/caps are to be provided at the lowest points to

drain out the water and dirt. U trap has to be provided in the drain pipe for the air

handling units to prevent air being sucked through this point this preventing the water

from draining.

PUMPS

Pumps are classified into positive displacement and centrifugal pumps. A reciprocating

pump is a positive displacement pump. A centrifugal pump is simple in construction. It

has the following advantages over other types of pumps as such they are generally used

for air conditioning applications.

Simplicity in construction

Absence of valves

Fewer moving parts

Minimum power transmission losses

Steady non surging flow

Operation at shut off condition without excessive pressure build up

Absence of closer clearances

Compactness and lighter in weight

Longer life

Reasonable cost

Absence of contact between the liquid pumped and lubricant

Easy maintenance

The disadvantages are it is not self priming and it is inefficient for smaller capacities

with higher heads.

The impellers are of cast iron or bronze. They may have either gland packing or

mechanical seal

The various types of centrifugal pumps are mono bloc, split casing and back pull out

pump sets.

In any piping system, the pumping head is the algebraic sum of the static head on the

discharge side minus the static head on the pump suction plus the friction losses

through the entire system of fluid flow. With an increase in flow the friction losses

increase approximately as the square of the flow.

Static suction lift is the vertical distance from the surface of the liquid to the centre line

of the pump. The maximum theoretical suction lift of a pump depends on the

atmospheric pressure which at sea level is about 14.7 psi. But atmospheric pressure

decreases as we ascend above the sea level so does the maximum theoretical static

suction lift.

Pumps are selected from the manufacturer's performance curves. The horse power of

the motor selected to drive a given pump must be at least 15% hi2"her than the BHP to

allow voltage fluctuations as per NEMA standards.

BHP = USGPM X PUMP HEAD IN IT X SPECIFIC GRAVITY

3960 X PERCENT PUMP EFFICIENCY

GUIDELINES FOR PUMP INSTALLATION:

The suction line approach to the pump should be as straight as possible.

With an oversized suction pipe an eccentric elbow to be used.

A check valve and gate valve should be installed at the pump discharges of a multi

pump system to service one pump without draining the discharge line.

The suction line for the pump operating with a negative static head should have no

valves other than a foot valve.

The chilled water pump is generally insulated.

The pump has to be mounted on a base frame which has to be located on a concrete

pedestal which is at least two times heavier than the machinery. The base has to be

isolated from the ground by cork or high density thermocole. Unless the pump is self

priming it must be primed before starting. When starting the pump the discharge valve

is usually closed, then gradually opened so as not to run the risk of overloading the

drive motor. A reciprocating pump never has to be run with the discharge valve closed.

REFRIGERANT PIPING

A refrigerant piping has to be designed with optimum pressure drop with respect to

economics, friction loss and oil return. It is economical if the line size is as small as

possible, However this should not result in excessive suction and discharge line

pressure drop as this will result in loss of compressor capacity and excessive Bhp/ TR.

Too small a line size will also cause excessive pressure drop in the liquid line which

will result in flashing of the liquid refrigerant. A refrigerant line has to be designed to

accomplish the following:

To insure proper feed to evaporators. Provide practical line sizes without excessive

pressure drop.

To ensure oil return and orevent excessive lubricating oil fro~ being trapped in the

system.

Prevent liquid refrigerant from entering the compressor during operation and shut

down.

The pressure drop in refrigerant piping depends on the velocity of flow of refrigerant. It

is proportional to the square of the fluid velocity in the pipes. Refrigerant pressure drop

is usually expressed as Deg. F and not in terms of PSI. When the pressure drop is 2 deg.

F, it is the drop in saturation temperature corresponding to the pressure before and after

the drop.

For refrigeration and air conditioning applications, it has been established that the

optimum acceptable pressure drops in each interconnecting piping should be as follows:

Suction line (CHW) - 1 deg. F

Suction line (DX) - 2 deg. F

Hot gas line - 2 deg. F

liquid line - 1 deg. F

Piping design charts are available for design of copper and steel pipes for different

refrigerants.

Following is the procedure for sizing of the refrigerant pipes:

Measure the length of the straight pipe. Add 50% to obtain a trial total equivalent

length.

If necessary, correct for suction and condensing temperatures.

Read the piping chart the size of the pipe line for the tonnage and the equivalent length.

With this pipe size add the equivalent length for fittings and valves to the straight

length of the pipe and cross check this with the trial equivalent length.

Correct the equivalent length in case of difference address elect the pipe.

OIL RETURN

Oil is continuously carried from the compressor through the discharge line into the

condenser and then into the evaporator. This oil is generally in the form of very minute

droplets and they depend on gas velocity to flow through the suction line back to the

compressor. The flow of this gas does not pose much problems as far as horizontal

lines are concerned. It becomes a problem only in vertical lines/rises where gas travels

from one level to higher level in the suction line. For ensuring oil return through this

suction riser, we have to maintain a minimum velocity of gas flow in the suction line.

If the velocity becomes less than this then oil will not rise along with the suction gas

and will therefore accumulate in the evaporator thereby starving the compressor of oil.

It is therefore quite obvious that suction line should never be oversized. It is also

important that oil return goes uninterruptedly at all operation loads of any particular

system. In case of compressors with automatic capacity control, it is necessary to figure

out the likely minimum load at which the system is expected to operate. This minimum

tonnage has to be verified with the chart showing the minimum tonnage for oil

entrainment in suction risers. If ever this minimum partial load is likely to fall below

the minimum recommended load for oil return then alternate arrangement for ensuring

oil return has to be implemented. Generally double suction risers are used. Whenever

the load happens to fall below the minimum tonnage as required for oil return then oil

will not rise up through the suction pipe and tend to collect at the bottom level thereby

filling the trap underneath. This oil trap will render the larger suction pipe ineffective

and so the entire suction gas during such partial load operating conditions will go only

through the smaller pipe in which the velocity will be high enough for carrying the oil.

Refrigerant piping layout is shown for various arrangements including the system with

compressors running in parallel. All these layouts are to be followed strictly for proper

functioning of the system.

There are various accessories in a refrigerant piping:

Liquid suction heat exchangers This will subcool the liquid refrigerant and superheat

the suction gas. This prevents liquid slop-over to the compressor. Generally liquid

suction heat exchangers are used only for a chiller package. Excessive superheat of

suction gas has to be avoided.

Liquid indicator

Every refrigerant system should include a means of checking for sufficient refrigerant

charge. Liquid indicator or sight glass is installed in the liquid line and it shows

bubbling when there is insufficient refrigerant charge and a solid clear glass when there

is sufficient charge. Sight glass should be installed in full size of the main liquid line

and not in the bypass line that parallels the main.

Refrigerant driers

A refrigerant drier is used for low temperature systems to remove moisture entrained in

the system. It is essential for all systems using hermetic compressors since the motor

winding is exposed to the refrigerant gas. A filter drier is used in the bypass line along

with a moisture and liquid indicator. In case of moisture in the system, the refrigerant is

allowed to pass through the drier.

Solenoid valves

Solenoid valves are used in the following places:

In the liquid line for pumping down the system.

Along with the compressor for capacity control.

They are electrically operated and are of two way or three way type.

Charging connections

Refrigerant in liquid form is charged into the liquid line between the condenser shut off

valve and the expansion valve. Refrigerant in gas form is charged for small systems in

the suction line but this is generally not advisable as there is danger of dumping raw

liquid into the compressor. Suitable angle valves should be provided for this purpose.

Further angle valves/tappings are to be provided for connecting gauges and safety

cutouts.

Mufflers

It is used in the hot gas line and installed closer to the compressor. The hot gas

pulsations from the compressor can set up a condition of resonance with certain lengths

of refrigerant piping in the hot gas line. A muffler aids in eliminating such a condition.

Strainers

A strainer is installed ahead of an expansion valve. A shut off valve has to be installed

on both sides of the strainer to facilitate cleaning of the strainer.

Expansion valves

An automatic thermostatic expansion valve meters the refrigerant flow and it has to be

sized properly to avoid both the penalties of being undersized at full load and of being

excessively oversized at part load. Refrigerant pressure drop through the system has to

be evaluated for selection of the expansion valve. Tables are available for the selection.

A minimum superheat of 10 deg. F has to be considered in the selection. The expansion

valve bulb should be located immediately after the coil outlet on the suction line and at

45 degrees above the bottom of the pipe. The valve should be set such that overfeeding

does not occur at times of partial load.

ELECTRICAL ENGINEERING & CONTROLS

MOTORS

The air conditioning and refrigeration systems include fans, pumps and compressors.

The prime mover to put these equipments into motion is the electric motor. Motors may

be three phase or single phase. The various types ofthree phase motors are induction

and synchronous. Single phase motors are either capacitor start, induction run; capacitor

start, capacitor run; split phase; permanent split capacitor etc. Induction motors are

popular. They are fairly cheaper to produce and operate when compared to DC motor or

synchronous motors. Further they offer a very wide choice of operating characteristics

and therefore the flexibilities in selection to various drives.

Basically there are two types of induction motors; cage and slip ring. In cage motors,

uninsulated rotor bars are short circuited at both the ends of the core by heavy short

circuited rings. Hence for a given cage design, the rotor resistance is fixed and the

starting torque, starting current is an inherent charcteristic of a cage motor. In slipring

motor, the rotor is wound with insulated windings and the terminals are brought

through slip rings. By adding external resistance in the rotor circuit, it is possible to

control starting torque, current characteristics and also vary the speed. Slipring motors

are expensive when compared to cage motors. The induction motors may be screen

protected drip proof type (SPDP) or totally enclosed fan cooled (TEFC). TEFC motors

are costlier.

Synchronous motors are inherently and strictly constant speed motors. Their application

is characterised by the high efficiency of conversion of electrical energy into

mechanical energy. Unlike other motors, their speed is unaffected by changes in voltage

or load. OEFINITIONS:

FULL LOAD CURRENT

The current drawn by the motor when producing the rated output at rated speed with

rated input.

PULL OUT TORQUE

The highest torque that the motor can develop while running at rated voltage and

frequency.

SLIP

Slip is the difference between the synchronous speed and the actual speed at full load

expressed in terms of synchronous speed.

POLES

The number of poles of the motor is the frequency in cycles per second divided by half

the speed of the motor in revolutions per second.

To obtain uniformity in applications NEMA has defined specific designs of integral

horspower squirrel cage induction motors. Each design conforms to specific starting

and break down torque, starting current and slip.

The. motor may be said to have been properly selected and successfully applied, if it

srarts the load and accelerates to full speed within reasonable period of time and then

runs as required at rated load without exceeding the winding temperature limit for the

particular class of insulation used.

The first step in selecting a motor is to analyse the following and then match the motor:

1. The load to be driven and its torque speed characteristics

2. The motor operating conditions which include ambient, environment, altitude,

mounting, driving arrangement, speed variations.

3. lnertia of the load

4. Any limitations which may be imposed on acceleration on method of

starting.

5. Maximum torque required under worst conditions.

6. Supply system, voltage, frequency variation, limitations of starting current, if

any, current pulsations tolerated, effect of power factor and efficiency and

temperature rise of the motor etc.

Efficiency and power factor are two characteristics which will suffer to some extent

when special starting current and speed torque characteristics are required.

Induction motors with various types of enclosures are available depending on the

application and location and environmental conditions in which they are required to

operate. Proper selection of motor enclosure cuts down the maintenance cost of the

motor.

A few important types of enclosures are open drip proof enclosure (SPDP) IP 22/ 23

adopted for indoor applications. The ambient air circulates through the overhangs of the

motor, thereby dissipating more amount of heat efficiently. The initial investment for

the motor with this type of enclosure is significantly low.

Totally enclosed fan cooled enclosures (TEFC) IP 44/54/55 are suitable for either

indoor or outdoor clean or dirty location. The ambient air does not circulate through

the motor and thus windings stay clean. Though the initial investment for these motors

is high, the savings in maintenance expenses will offset the difference.

Special types of enclosures such as forced ventilated, flame proof etc. are available in

some ratings and the use of these enclosures depends mainly on the environmental

conditions.

The temperature rise which is associated with the class of insulation used in the motor,

indicates the thermal capability of the motor. Determination of the temperature rise of

the motor for various types of duty is therefore essential to select a proper motor. Final

temperature rise is directly proportional to the losses generated in the motor and is

inversely proportional to the heat dissipation capacity. In addition to motor surface and

ventilation, which decide the heat dissipation capacity of the motor, the ambient

temperature is also an important parameter to limit the final temperature rise and hence

the selection of frame size. The motor is to be derated for higher than the normal

ambient temperature i.e., either the ouput is to be reduced for the same frame size, or

the frame size is to be increased for the same output. The permissible temperature rise

over an ambient temperature of 40 deg. C for the insulation scheme in use are class E

120 deg. C; class B 130 deg. C; class F 155 deg. C and class H 180 deg.C.

As per NEMA standards the motors are oversized by 10 to 15% over the required HP to

compensate for the fluctuations in voltage and drive losses.

PROTECTION

Motors must be employed with protective devices to prevent cafastrophic failures when

the characteristic parameters of the drive motors exceed the rated values. The following

are the few important items to be provided:

RUNNING OVERLOAD PROTECTION

Motors must be protected against overload, the use of devices responsible to either

motor currents or motor temperature or both. Temperature detectors may be embedded

in the motor windings (hermetic compressor motors) which interrupt over current to the

motor.

UNDER VOLTAGE PROTECTION

All equipment are to be provided with under voltage protection in order to minimize

the damage to the equipment in the event of sudden drop in voltage or power failure.

SINGLE PHASING PROTECTION

This form of protection is to be provided to protect the polyphase induction motors in

the event of single phasing.

SPACE HEATERS

Corrosion of mechanical parts and damage to the insulation due of moisture is

eliminated by providing space heaters.

BEARING TEMPERATURE DETECTORS

These detect the increase in bearing temperature which is general due to the loss of oil

film caused by either contamination of lubricating oil or change in alignment. These

devices are to be set at a nominal value above the normal operating temperature in

order to cut off power supply to motor when the bearing temperature crosses the

preset value (This is popular in centrifugal machines which run at higher speeds).

At times there is a need for two or more fixed steps in speed change for the operation

of driven equipment particularly fans. The multispeed operation is obtained by either

multiplying or rearranging the stator windings.

Small integral motors are generally polyphase type; however in many areas of extensive

use, power is available only in single phase. In single phase induction motors there is

only one winding in the stator. The alternating current produces a magnetic field with

alternating polarity, but it does not revolve. Therefore an auxiliary means must be

provided to produce torque to start and accelerate the motors to full speed. At full speed

the single phase motor operates like a polyphase motor in respect to slip, efficiency and

power factor. The single phase motors are popular for fan coil units and window/ split

air conditioners.

There are three sources of energy losses appearing as heat that raise the motor

temperature; a) windings-heat produced by the flow of current against resistance and

equal to the product of the current squared and resistance, b)Iron core-heat produced by

hysteresis and eddy current losses set up by the magnetic field in the stator and rotor

and c) the mechanical losses in the bearings, brushes, fans etc.

STARTERS

Starter selection is integral with motor selection and should be so considered in relation

to the following factors: horsepower rating, permissible current, desirable torque,

necessary protection and combined economics.

There are two fundamental classes of starting equipment serving squirrel cage induction

motors: 1) full voltage, across the line and 2) reduced voltage, reduced current in rush

starters. The factors that influence a. choice between full or reduced voltage starters are:

1) cost, 2) size of the motor 3) current inrush and starting torque behaviour of the motor

with reduced voltage and 4) electricity board restrictions on the use of electric energy.

SWITCHES

A majority of air conditioning and refrigeration installations are low voltage

applications which at times may use manually operated circuit switches as permitted by

local electricity boards. There are several varieties:

1) Disconnect switches for isolating purposes. They have no interrupt rating and

should not be operated with a load on the line.

2) Enclosed safety switches, fused or unfused, available for light duty AC service.

FUSES AND CIRCUIT BREAKERS

The design performance of a motor is delivered under a normal supply of electric

energy. Any disturbance to normal flow leads either to overheating and eventual

destruction of the motor or to non delivery of the required mechanical power. The

greatest hazard to any electrical services is a short circuit, a flow of an enormously

excessive current caused by some fault either in the power line or at the motor.

Sufficient protection must be designed into the system to safeguard the feeder and the

branch circuits as well as to isolate the feeder from a fault in the individual branch.

A fuse is a low cost short circuit protection device. It may be a plug or cartridge type.

A circuit breaker functions both as circuit protector and as a branch circuit disconnect

switch. Its advantage is that upon being opened by short circuit it may be reset without

the necessity of replacement, as in the case of a fused disconnect switch. With the fused

circuit, there exists a danger, namely single phasing in case only one fuse blows. A

circuit breaker disconnects all the three phases.