Air Conditioning (1)

7/25/2019 Air Conditioning (1)

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Air Conditioning

Definition, advantages and disadvantages,

brief introduction to psychrometric process,

air-cycle and refrigeration cycle.

Summer and winter air-conditioning,

calculation of air-contitioning loads,

Zoning: purpose and advantages.

Air-distribution systems: Ducts and duct systems.

Air-outlets

Air-conditioning methods and equipment: window units, split

units and central Air-conditioning systems.

Location of air-conditioning equipment in buildings.

Architectural requirement of various equipment.


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Air conditioning is the process of altering the properties of

air (primarily temperature and humidity) to more comfortableconditions, typically with the aim of distributing the conditioned airto an occupied space to improve thermal comfort and indoor airquality.

Or

Air-conditioning is a process that simultaneously conditions air;

distributes it combined with the outdoor air to the conditionedspace; and at the same time controls and maintains the requiredspaces temperature, humidity, air movement, air cleanliness, sound

level, and pressure differential within predetermined limits for thehealth and comfort of the occupants, for product processing, orboth.


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An air-conditioning process describes the change in

thermodynamic properties of moist air between the

initial and final stages of conditioning as well as thecorresponding energy and mass transfers between themoist air and a medium, such as water, refrigerant,

absorbent or adsorbent, or moist air itself. The energybalance and conservation of mass are the twoprinciples used for the analysis and the calculationof the thermodynamic properties of the moist air.


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Psychrometrics is the science of studying the thermodynamic

properties of moist air. It is widely usedto illustrate and analyze the

change in properties and the thermal characteristics of the airconditioning process and cycles.

The psychrometric chart is a graphic representation of theproperties of moist airvarious air-conditioning processes and air-

conditioning cycles.

The approximate composition of dry air by volume is nitrogen,79.08%; oxygen, 20.95%; argon, 0.93%; carbon dioxide, 0.03%;other gases (e.g., neon, sulfur dioxide), 0.01%.

The amount of water vapor contained in the moist air within thetemperature range 0 to 100 F changes from 0.05 to 3% by mass.The variation of water vapor has a critical influence on thecharacteristics of moist air.

Psychrometrics


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Important psychrometric properties:

Dry bulb temperature (DBT) is the temperature of the moist air as measured

by a standard thermometer or other temperature measuring instruments.The Dry Bulb Temperature refers basically to the ambient air temperature.

Dry bulb temperature

These lines are drawnstraight, not always parallel to each other, andslightly inclined from the vertical position.This is thetaxis, the abscissa (horizontal)axis. Each line represents a constanttemperature.


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The water vapor in the air will be saturated when air is at a temperatureequal to the saturation temperature corresponding to the partial pressureexerted by the water vapor.

This temperature is called dew-point temperature.

Dew-point temperature

Dew point temperature

From the state point followthe horizontal line of constant humidity ratio to theintercept of 100% RH, also known as thesaturationcurve. The dew point temperature is equal to the

fully saturated dry bulb or wet bulb temperatures.


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Wet bulb temperature

These lines are obliquelines that differ slightly from the enthalpylines. They are identically straight but are not

exactly parallel to each other. These intersectthe saturation curve at DBT point.


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Humidity ratio: These are the horizontal lines on the

chart. Humidity ratio is usually expressed as mass of

moisture per mass of dry air (pounds or kilograms of

moisture per pound or kilogram of dry air, respectively).

The range is from 0 for dry air up to 0.03 (lbmw/lbma)on the right hand -axis, the ordinate or vertical axis of

the chart.


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Specific enthalpy

These are oblique lines drawn diagonallydownward from left to right across the chart that are parallelto each other. These are not parallel to wet bulb temperaturelines.


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Relative humidity

These hyperbolic lines areshown in intervals of 10%. The saturationcurve is at 100% RH, while dry air is at 0% RH.


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A psychrometric chart is a graph of the thermodynamic parameters of moist air at

a constant pressure, often equated to an elevation relative to sea level.

Dew point temperature: From the state point follow the horizontal line of constant

humidity ratio to the intercept of 100% RH, also known as thesaturation curve. The dew

point temperature is equal to the fully saturated dry bulb or wet bulb temperatures.

* Wet bulb temperature: These lines are oblique lines that differ slightly from theenthalpy lines. They are identically straight but are not exactly parallel to each other. These

intersect the saturation curve at DBT point.

* Relative humidity: These hyperbolic lines are shown in intervals of 10%. The saturation

curve is at 100% RH, while dry air is at 0% RH.

* Humidity ratio: These are the horizontal lines on the chart. Humidity ratio is usually

expressed as mass of moisture per mass of dry air (pounds or kilograms of moisture per

pound or kilogram of dry air, respectively). The range is from 0 for dry air up to 0.03

(lbmw/lbma) on the right hand -axis, the ordinate or vertical axis of the chart.

* Specific enthalpy: These are oblique lines drawn diagonally downward from left to right

across the chart that are parallel to each other. These are not parallel to wet bulb

temperature lines.


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Assume that the outside air temperature is 32C with a relative humidity =60%. Use the psychrometric chart to determine the specific humidity [18

gm-moisture/kg-air], the enthalpy h [78 kJ/kg-air], the wet-bulbtemperature Twb [25.5C], the dew-point temperature Tdp [23C], and the

specific volume of the dry air v [0.89m3/kg]. Indicate all the valuesdetermined on the chart.


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Assume that the outside air temperature is 8C. If the air in a room is at 25Cwith a relative humidity = 40%, use the psychrometric chart to determine if thewindows of that room which are in contact with the outside will become foggy.

The air in contact with thewindows will become colderuntil the dew point isreached. Notice that underthe conditions of 25C and40% relative humidity thedew point temperature isslightly higher than 10C, Atthat point the water vaporcondenses as thetemperature approaches 8C

along the saturation line,and the windows willbecome foggy.


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One of the major applications of the Psychrometric Chart is in air conditioning, and wefind that most humans feel comfortable when the temperature is between 22C and 27C,and the relative humidity between 40% and 60%. This defines the "comfort zone" whichis portrayed on the Psychrometric Chart as shown below. Thus with the aid of the chart

we either heat or cool, add moisture or dehumidify as required in order to bring the airinto the comfort zone.


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In the refrigeration cycle, heat is transported from a colder

location to a hotter area. As heat would naturally flow inthe opposite direction, work is required to achieve this. A

refrigerator an example of such a system, as it transports

the heat out of the interior and into its environment (i.e.

the room). The refrigent is used as the medium which

absorbs and removes heat from the space to be cooled and

subsequently rejects that heat elsewhere.

REFRIGERATION CYCLE

A simple stylized diagram of the

refrigeration cycle: 1) condensing

coil, 2) expansion valve,

3) evaporator coil, 4) compressor
http://en.wikipedia.org/wiki/Condensing_coilhttp://en.wikipedia.org/wiki/Condensing_coilhttp://en.wikipedia.org/wiki/Thermal_expansion_valvehttp://en.wikipedia.org/wiki/Evaporator_coilhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Evaporator_coilhttp://en.wikipedia.org/wiki/Thermal_expansion_valvehttp://en.wikipedia.org/wiki/Condensing_coilhttp://en.wikipedia.org/wiki/Condensing_coil


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Liquids absorb heat when changed from liquid to gas

Gases give off heat when changed from gas to liquid.

For an air conditioning system to operate with economy, the refrigerant mustbe used repeatedly. For this reason, all air conditioners use the same cycle of

compression, condensation, expansion, and evaporation in a closed circuit.

The same refrigerant is used to move the heat from one area, to cool this area,

and to expel this heat in another area.

The refrigerant comes into the compressor as a low-pressure gas, it is

compressed and then moves out of the compressor as a high-pressure gas.The gas then flows to the condenser. Here the gas condenses to a liquid, and

gives off its heat to the outside air.

The liquid then moves to the expansion valve under high pressure. This valve

restricts the flow of the fluid, and lowers its pressure as it leaves the

expansion valve.

The low-pressure liquid then moves to the evaporator, where heat from the

inside air is absorbed and changes it from a liquid to a gas.

As a hot low-pressure gas, the refrigerant moves to the compressor where the

entire cycle is repeated.


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The refrigeration cycle uses four essential elements to cool.

The system refrigerant starts its cycle in a gaseous state. The compressorpumps the

refrigerant gas up to a high pressure and temperature.

From there it enters a heat exchanger (sometimes called a condensing coil or

condenser) where it loses energy (heat) to the outside, cools, and condenses into its

liquid phase.

An expansion valve (also called metering device) regulates the refrigerant liquid to

flow at the proper rate.

The liquid refrigerant is returned to another heat exchanger where it is allowed to

evaporate, hence the heat exchanger is often called an evaporating coil or evaporator.

As the liquid refrigerant evaporates it absorbs energy (heat) from the inside air, returns

to the compressor, and repeats the cycle. In the process, heat is absorbed from indoors

and transferred outdoors, resulting in cooling of the building.

A simple stylized diagram of therefrigeration cycle: 1) condensing

coil, 2) expansion valve,

3) evaporator coil, 4) compressor

Ci l ti f i t t th d i d t
http://en.wikipedia.org/wiki/Refrigeration_cyclehttp://en.wikipedia.org/wiki/Thermal_expansion_valvehttp://en.wikipedia.org/wiki/Condensing_coilhttp://en.wikipedia.org/wiki/Condensing_coilhttp://en.wikipedia.org/wiki/Thermal_expansion_valvehttp://en.wikipedia.org/wiki/Evaporator_coilhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Evaporator_coilhttp://en.wikipedia.org/wiki/Thermal_expansion_valvehttp://en.wikipedia.org/wiki/Condensing_coilhttp://en.wikipedia.org/wiki/Condensing_coilhttp://en.wikipedia.org/wiki/Thermal_expansion_valvehttp://en.wikipedia.org/wiki/Refrigeration_cyclehttp://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Gas_compressor


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Circulating refrigerant vapor enters the compressorand is compressed to a

higher pressure, resulting in a higher temperature as well. The hot,

compressed refrigerant vapor is now at a temperature and pressure at which

it can be condensed and is routed through a condenser. Here it is cooled by

air flowing across the condenser coils and condensed into a liquid. Thus, the

circulating refrigerant rejects heat from the system and the heat is carriedaway by the air.

The condensed and pressurized liquid refrigerant is next routed through

an expansion valve where it undergoes an abrupt reduction in pressure. That

pressure reduction results in flash evaporation of a part of the liquid

refrigerant, lowering its temperature. The cold refrigerant is then routed

through the evaporator. A fan blows the warm air (which is to be cooled)across the evaporator, causing the liquid part of the cold refrigerant mixture

to evaporate as well, further lowering the temperature. The warm air is

therefore cooled.

To complete the refrigeration cycle, the refrigerant vapor is routed back into

the compressor.

By placing the condenser inside a compartment, and the evaporator in theambient environment (such as outside), or by merely running an air

conditioner's refrigerant in the opposite direction, the overall effect is the

opposite, and the compartment is heated instead of cooled. See also heat

pump.

The engineering of physical and thermodynamic properties of gasvapor

mixtures is calledpsychrometrics.
http://en.wikipedia.org/wiki/Gas_compressorhttp://en.wikipedia.org/wiki/Condensationhttp://en.wikipedia.org/wiki/Condenser_(heat_transfer)http://en.wikipedia.org/wiki/Thermal_expansion_valvehttp://en.wikipedia.org/wiki/Flash_evaporationhttp://en.wikipedia.org/wiki/Evaporator_coilhttp://en.wikipedia.org/wiki/Refrigeration_cyclehttp://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Psychrometricshttp://en.wikipedia.org/wiki/Psychrometricshttp://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Heat_pumphttp://en.wikipedia.org/wiki/Refrigeration_cyclehttp://en.wikipedia.org/wiki/Evaporator_coilhttp://en.wikipedia.org/wiki/Flash_evaporationhttp://en.wikipedia.org/wiki/Thermal_expansion_valvehttp://en.wikipedia.org/wiki/Condenser_(heat_transfer)http://en.wikipedia.org/wiki/Condensationhttp://en.wikipedia.org/wiki/Gas_compressor


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Pressurization


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In multi-storey buildings, stairways and lobbies may be air

pressurised to clear smoke and provide an unimpeded escape

route. The air pressurization is usually between 25 and 50 Pa

depending on the building height and degree of exposure.

This pressure is insignificant for movement of personnel.

Pressurization is a technique for protecting escape routes

against the ingressof deadly smoke by maintaining the pressure within the

escape route at a

higher pressure than that in adjacent spaces.

Pressurization is provided to maintain safe conditions within

an area which could be used in the event of a fire for means

of escape/life safety and fire fighter access.

Pressurization


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In large open spaced buildings, CAR PARKS, SHOPPING CENTRES,


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EXHIBI- TION HALLS etc, the usual method of smoke control is by

ventilation - extracting the smoke from the area.

In high rise, multi room buildings, where the Staircases, Lift Lobbies

and Corridors provide the escape route, SMOKE EXTRACTION may

only serve to worsen the situation.


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However, it is possible to hold back smoke from a fire by simplysupplying clean air into the escape routes, thereby developing

excess, or POSITIVE pressure in the spaces requiring protection.

In a PRESSURISATION SYSTEM the air- flow must always be

away from the escape routes to ensure that they will be at a

higher pressure than the surrounding area.


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A pressurization system

consists of two main

components:

Supply air (air injected

into the area that is to beprotected).

Air release (air and

smoke is released fromthe adjoining fire area).


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SUPPLY AIR SYSTEM


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SUPPLY AIR SYSTEM

A pressurisation system has TWO and occasionally

THREE, modes of operation

Mode 1 - DECTECTION PHASE - To raise a

pressure differential in the protection space -

staircase, corridor etc, by the required amount

(50Pa in the UK) when ALL DOORS ARE

CLOSED.

Mode 2 - ESCAPE PHASE - To maintain aspecified AIR VELOCITY (0.75m/sec) through

the OPEN DOOR(S) onto the fire floor with

various other doors open, OR a PRESSURE

DIFFERENCE OF 10+ Pa with the fire floor

door(s) closed and various other doors open

(fig 7-9)Mode 3 - FIRE FIGHTING PHASE - To

maintain a specified AIR VELOCITY (2.0

m/sec) through the OPEN DOOR(S) onto the

fire floor with various other doors open.


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A k l bb i d i t ll b ildi h k t k t l


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A sky lobby is used in supertall buildings, such as skyscrapers, to make travelbetween elevators convenient for all passengers.

These are intermediate floors used to section off the building. Most such

buildings have one elevator, called an express elevator, which only travelsbetween the different sky lobby floors.

Between each sky lobby, there are local elevators that lead to all the floors inthat section of the building.

The number of local elevators in each section typically ranges up to 10, withup to five in a row to make walking between elevators easier for visitors andemployees.In very large buildings, there are numerous floors with a constant flow of

visitors and workers going between the floors.If each elevator served the entire building, then most people wouldencounter a number of stops for others to get off before they arrived at theirdesired floor. This would make elevator travel inconvenient in largebuildings and would lead to massive delays for most elevator travelers.

ll i hi bl k l bb i d hi l bb i i


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To alleviate this problem, a sky lobby is used. This lobby acts as a sectioning

device t hat separates the entire building into several zones. In each lobby, there

are separate elevators that only take travelers to a floor in that zone. For

example, if there is a 20-floor building and each lobby serves five floors, then

the first lobby would have elevators for floors one through five, and the secondlobby would serve floors six through 10.

Getting to a sky lobby also is streamlined. Aside from the local elevators thatserve each lobbys zone, there also is an express elevator. This elevator or set of

elevators only travels between the lobby zones. In this case, even if the travelerneeds the top lobby zone and the elevator stops at every other lobby, the

traveler still will only experience a few stops. Most buildings only contain afew lobbies, so express elevator travel would be easy on all visitors andworkers.Depending on the architecture, there may be 10 local elevators for each skylobby zone. These local elevators are usually placed on opposite ends of thelobby, so the area does not become congested. Each local elevator will only

serve several floors in the lobby zone, so those who need one of the bottomfloors will not impede people who need a top floor.

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