Post on 28-Mar-2015
FME 706/FML 2007- 08 Air Conditioning 1
AIR CONDITIONING
FME 706/FML 2007- 08 Air Conditioning 2
SCOPE AND USE OF AIR CONDITIONING
Not restricted to cooling only but might include:o Control of temperature at all times by heating or coolingo Control of air humidity by humidification or dehumidificationo Control of air movement at a desirable velocityo Introduction of outdoor air as requiredo Control of air quality by removal of dirt particles and odorous gaseso Control of sound generated by the air conditioning equipment
Environmental control Used for two purposes:
o Comfort (people)o Process control (as required)
FME 706/FML 2007- 08 Air Conditioning 3
PSYCHROMETRICS
Study of air-water vapour (binary) mixtures Content of water vapour can change A/C processes may involve both sensible and latent heat transfer
SOME IMPORTANT PARAMETERS IN PSYCHROMETRICS
Dry Bulb Temperature (TDB) – sensed with a normal thermometer bulb/sensor Wet Bulb Temperature (TWB) – sensed by a thermometer whose bulb is wrapped with water
soaked wick in rapidly moving air Dew Point Temperature (TDP) – Temperature at which water vapour starts to condense at
constant pressure Humidity Ratio/Specific Humidity (W) – Mass of water vapour divided by the mass of dry air
(mv/ma kgv/kga) Relative Humidity ( or rh) – Ratio of actual water vapour pressure in the air to the water vapour
pressure at saturation at the mixture temperature va - volume of a mixture containing one kg of dry air (m3/kga) h – enthalpy contained in a mixture containing 1 kga (kJ/kga) va and h involve (1+W) kg of mixture
FME 706/FML 2007- 08 Air Conditioning 4
PSYCHROMETRICS (Cont’d 1)
PSYCHROMETRIC CHART
FME 706/FML 2007- 08 Air Conditioning 5
SOME IMPORTANT PSYCHROMETRIC PROCESSES
Thermodynamic Wet Bulb Temperature (Adiabatic Saturator)
Thermodynamic Saturator
Adiabatic wall
mw
Water
Air
Make-up water
FME 706/FML 2007- 08 Air Conditioning 6
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 1)
Adiabatic Saturator H2O Process Sling Psychrometer
T
s
Pv2=Pvs2 Pv1
T2=Tw
f2
Handle
Dry bulb thermometer
Wet bulb thermometer
FME 706/FML 2007- 08 Air Conditioning 7
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 2)
22221111 vvaawwvvaa hmhmhmhmhm
aaa mmm21
)WW(mm 12aw
)hh(Whh)hh(W wv2aawv1 2121
hw = hf2, hv1 = hg1, hv2 - hw = hfg2,
21
212
fg
fg2aa1 hh
hW)hh(W
2
2
2
22
622.0
s
s
PP
PW
FME 706/FML 2007- 08 Air Conditioning 8
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 3)
IMPORTANT RELATIONSHIPS
Specific Humidity
Enthalpy
or
a
v
a
v
a
v
a
v
vv
av
a
a
v
v
a
v
P
P
P
P
P
P
M
M
RP
RP
VP
TR
TR
VP
m
mW 622.0
9645.28
01534.18~
vvaa hmhmmiH
va
va
a
hm
mh
m
mi hWhhWi va )1(
gopgopppgop WhtcWhtWccWtchtchvava
)()(
FME 706/FML 2007- 08 Air Conditioning 9
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 4)
Datum: Dry saturated vapour at 0ºC, t in ºC For A/C purposes, cpa 1.005, cpv 1.87 kJ/(kg.K), W 0.01 kgv/kga, hgo = 2500.8 kJ/kg,
cp 1.024 kJ/(kga.K), and hence h 1.024t + 2500.8W kJ/kga
and W
; ; P = Pa + Pv ; and hence
Solution for W1 From Adiabatic Saturator
P
P=s
va
v
P
P622.0W
( ) ( )
0.622 0.622 0.622a v s
s s s
WP W P P W P P
P P P
21
2212
1
)(
fg
fgp
hh
hWttcW a
FME 706/FML 2007- 08 Air Conditioning 10
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 5)
Cpa, t1, t2, hfg2, hg1, and hf2 from tables
Since at 2 air is saturated, 2 = 1 get W2 from
where Ps2 from tables at t2
Heating and Cooling at Constant W (Sensible)
2
2
s2
s22 PP
P622.0W
Q
(a) (b)
W
FME 706/FML 2007- 08 Air Conditioning 11
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 6)
Cooling and Dehumidification
or
or
)())(()]()[()( 121212 1212ttcmttWccmhhWhhmhhmQ pappavvaaaa va
Q
(a) (b)
mw
A
21 vwv mmm )WW(mm 21aw
Qhmhmhm ww2a1a )WW(hm)hh(mQ 21wa21a
FME 706/FML 2007- 08 Air Conditioning 12
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 7)
represents enthalpy carried away by the condensate ( 10ºC) which is negligible compared to the first term and hence
where and
Sensible Heat Factor (SHF) (Related to bypass factor)
Important in A/C calculations.
)W-W(hm 21wa
ls QQQ
)hh(m)tt(cm
2Aa21pa
2As
)hh(m
h)WW(mQQ
A1a
fg21aA1l 1
Q
QSHF s
FME 706/FML 2007- 08 Air Conditioning 13
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 8)
Heating With Humidification
or
Equation of a straight line.
For Q = 0,
Q
(a) (b)
mw
)WW(mm 12aw
w12a12a h)WW(mQ)hh(m w12a12
12 h)WW(m
Q
WW
hh
w12
12 hWW
hh
FME 706/FML 2007- 08 Air Conditioning 14
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 9)
hw = hgT1 – humidification at constant T1 (2’)
hw > hgT1 – heating with humidification (2’’)
hw < hgT1 - cooling with humidification (2)
Spray with liquid water at air wet bulb temperature – Twb remains constant. Basis of evaporative cooling
1
22
2
FME 706/FML 2007- 08 Air Conditioning 15
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 10)
Adiabatic Mixing
; ;
Equation of a straight line (final state lies along this line)
(a) (b)
h1
h2
h3
W1
W2
W3
3a2a1a hmhmhm321
321 aaa mmm 3a2a1a WmWmWm
321
31
23
31
23
a
a
WW
WW
hh
hh
m
m
2
1
21
23
21
23
a
a
WW
WW
hh
hh
m
m
3
1
12
13
12
13
a
a
WW
WW
hh
hh
m
m
3
2
FME 706/FML 2007- 08 Air Conditioning 16
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 11)
EXAMPLE OF A SIMPLE CENTRAL AIR-CONDITIONING SYSTEM
Space Q=Qs+Ql
Exhaust Outdoor air
Filters
Fan
Cooling & dehumid Coil
Heater
5,6,7
0
1 2
3
4
FME 706/FML 2007- 08 Air Conditioning 17
SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 1)
T4, Q, mao, 5, 6, 7, SHFroom and Qfan known.
Draw line from 5, 6, 7 to cross T4 (T5 – T4 10ºC)
Join 0 and 5 locate 1 – adiabatic mixing, i.e.
Hence
For known SHFcoil draw line 2-3, and hence 3-4 at constant W
Qcoil = ma1(h2 – h3), Qheater = ma3 (h4 – h3)
454 hh
Qma
1
6
60
10
a
a
m
m
WW
WW
1
12a
fan
m
Qhh
FME 706/FML 2007- 08 Air Conditioning 18
COMFORT AND HEALTH
Deep body temperature 36.9ºC If body can easily maintain an energy balance, then feeling of comfort results Body regulatory mechanisms:
Metabolism rate Increase of the rate of cutaneous blood circulation (capillary dilation) Sweating
Metabolism – depends on the level of activity 1 MET (metabolic rate) = 58.2 W/m2
Energy generated by an average sedentary MAN Area (man) 1.8 m2
1 MET 105 W Women 30% lower than men Latent and sensible
Comfort Conditions Depends on activity and clothing
1 clo 0.155 m2.K/W – heavy two piece suit with accessories 0.05 clo pair of shorts
FME 706/FML 2007- 08 Air Conditioning 19
COMFORT AND HEALTH (cont’d 1)
Examples of Cooling Load Due to Occupancy
Activity Example Male Adult Total Watts
Total Adjusted Watts
Sensible Watts
LatentWatts
Seated at rest Theatre, movie 115 100 60 40
Seated, very light work, writing
Offices, hotels, apartments
140 120 65 55
Standing, light work or walking slowly
Retail store, bank
235 185 90 95
Light Bench work Factory 255 230 100 130
Heavy work, heavy machine work, lifting
Factory 470 470 165 300
Heavy work, athletics
Gymnasium 585 525 185 340
FME 706/FML 2007- 08 Air Conditioning 20
COMFORT AND HEALTH (Cont’d 2)
ASHRAE Comfort Standard 55-81 (1981) (Sedentary)
W gv/kga
ºC 20 25 30
5
100
15
Summer Winter
FME 706/FML 2007- 08 Air Conditioning 21
COMFORT AND HEALTH (Cont’d 3)
Cooling T 24ºC Heating T 22ºC Humidity 40 – 50 % Velocity in occupied zone V 0.15 m/s For high activity – special charts (Fanger comfort Charts – ASHRAE HF)
FME 706/FML 2007- 08 Air Conditioning 22
COMFORT AND HEALTH (Cont’d 4)
OUTDOOR DESIGN CONDITIONS
Winter Summer
Station (Elevation)
Mean Annual Extrem
es
99%C
97.5%C
Design Dry Bulb C Outdoor Daily
Range C
Design Wet Bulb C
1% 2.5% 5% 1% 2.5% 5%
Nairobi (1820 m)
7 9 10 27 27 26 13 19 18 18
Addis (2363 m)
2 4 5 29 28 27 16 19 18 18
Lagos (3 m) 19 21 22 33 33 32 7 28 28 29
Dar es Salaam (14 m)
17 18 18 32 32 31 7 28 27 27
FME 706/FML 2007- 08 Air Conditioning 23
COMFORT AND HEALTH (Cont’d 5)
Mean of annual extremes: Average of the lowest temp. recorded each year over 25-30 years
99%: Temp. which has been equaled or exceeded 99% of the time during the three cold months (Ditto for 97.5%)
1%: Temp. equaled or exceeded or equaled 1% of the time during the time during the cooling months
Daily range: Difference between average maximum and minimum temp. for the warmest month – has an effect on the energy storage of structures.
Ventilation Mainly to control odour – recommended standards for different spaces (minimum 2.5 l/s) Filtration, washing, scrubbing, adsorption, odour masking and counteraction The smaller the particle, the more difficult to remove Fibrous media (viscous impingement and straining), electronic air cleaners
FME 706/FML 2007- 08 Air Conditioning 24
COMFORT AND HEALTH (Cont’d 6)
Cooling T 24ºC Heating T 22ºC Humidity 40 – 50 % Velocity in occupied zone V 0.15 m/s For high activity – special charts
Ventilation Mainly to control odour – recommended standards for different spaces (minimum 2.5 l/s) Filtration, washing, scrubbing, adsorption, odour masking and counteraction The smaller the particle, the more difficult to remove Fibrous media (viscous impingement and straining), electronic air cleaners
FME 706/FML 2007- 08 Air Conditioning 25
HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD
Cooling Load Temp. and humidity to be maintained at a comfortable level Heat must be extracted – cooling load Basis of equipment selection (cooling and dehumidification coil, heater, ducts, fans, piping, fans, pumps,
etc.)
Heat Gain
Radiation Heat storage in furnishings and structure
Convection, infiltration
Convection (delayed in time)
Cooling load
FME 706/FML 2007- 08 Air Conditioning 26
HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD (Cont’d 1)
Heat gain: Rate at which heat is being received in the space at any time (solar radiation, lighting, conduction, convection, people, equipment, infiltration, etc.)
Storage effect: Heat does not immediately go into heating the room air. Radiant component first absorbed by room materials before being absorbed by room air.
Cooling load: Rate at which heat must be removed to maintain room design conditions (temperature and humidity)
Cooling load
Morning Afternoon Evening
Heat Gain and Cooling Load
Instantaneous heat gain
Removal of stored heat
Heat being stored
Removal of stored heat
FME 706/FML 2007- 08 Air Conditioning 27
Heat Gain/Cooling Load Components Conduction through exterior walls, roof and fenestration (glazing/any light transmitting element) Conduction through interior partitions, ceiling and floor Solar radiation (short wave) through fenestration Lighting and equipment Occupancy Infiltration (Fans, duct heat gain, duct leakage)
HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD (Cont’d 2)
FME 706/FML 2007- 08 Air Conditioning 28
ROOM AIR DISTRIBUTION
Good air distribution is necessary for comfort Effective draft temp. difference from design condition between -1.7ºC and 1.1ºC within occupied zone
(approx. < 1.75 m) Air velocities 0.13 – 0.25 m/s (below or above cause discomfort)
AIR FLOW PATTERNS
The Horizontal Isothermal Jet
FME 706/FML 2007- 08 Air Conditioning 29
ROOM AIR DISTRIBUTION (cont’d)
I II III IV o
x
V
VCL
oAx
FME 706/FML 2007- 08 Air Conditioning 30
ROOM AIR DISTRIBUTION (cont’d 1)
Zone I – Constant centerline velocity Zone II – Transition zone Zone III – Most important and the longest fully developed flow) - Zone IV – Fast velocity decay – regarded as still air – very short
Throw – Distance to a specified velocity, e.g. 0.25 m/s
Important Characteristics Surface effects increase the throw and decrease the drop (c.f. free jet)
Jet parallel to a wall or ceiling tends to hug the surface (reduced entrainment –”ceiling effect” Obstructions e.g. beams, columns etc.
Cold jet – drop Warm jet - rise
x
AK
V
V o
o
x
FME 706/FML 2007- 08 Air Conditioning 31
ROOM AIR DISTRIBUTION (cont’d 2)
High sidewall diffuser – good for cooling
Cooling Heating
FME 706/FML 2007- 08 Air Conditioning 32
ROOM AIR DISTRIBUTION (cont’d 3)
Ceiling Diffuser Excellent for cooling Large diffusion surface area Handles large quantities of air
Beam
FME 706/FML 2007- 08 Air Conditioning 33
ROOM AIR DISTRIBUTION (cont’d 4)
Slot Diffusers Long strip-shaped with one or more narrow openings
Plenum Ceilings Hung ceilings with slots or perforations for air supply (specialized suppliers/installation)
SELECTION CRITERIA FOR DIFFUSERS Capacity – Volumetric flow rate Throw – Axial distance (isothermal) jet travels till the maximum velocity is reduced to a specified
level, e.g. 0.75, 0.5, 0.25 m/s Noise Criterion (NC)
Tabulated Standards for different spaces, ducts, applications, fittings
Pressure - Ps and Pv or Po
FME 706/FML 2007- 08 Air Conditioning 34
ROOM AIR DISTRIBUTION (cont’d 5)
Room Characteristic Length (L)
Ceiling Diffuser
L
High Sidewall Diffuser
L
FME 706/FML 2007- 08 Air Conditioning 35
ROOM AIR DISTRIBUTION (cont’d 6)
Air Diffusion Performance Index (ADPI) Effective Draft Temperature (EDT)
q = (tx – tc) – a(vx – b)
tx - local temp., ºC
Tc – room average temp., ºC
vx – local velocity, m/s
a = 8, b = 0.15 Comfort Conditions: - 1.7 1.1˚C; vx < 0.35 m/s
ADPI – percentage of locations in occupied space of room which meet this criterion
FME 706/FML 2007- 08 Air Conditioning 36
ROOM AIR DISTRIBUTION (cont’d 7)
Example
Terminal Device
Room Load (W/m2)
T0.25/L for max ADPI
Max ADPI For ADPI greater than
Range of T0.25/L
Circular 250 1.8 76 70 0.7 – 1.3
Ceiling 190 1.8 83 80 0.7 – 1.2
Diffuser 120 1.6 88 80 0.5 -1.5
65 1.5 93 90 0.7 – 1.3
FME 706/FML 2007- 08 Air Conditioning 37
BUILDINGS AIR DISTRIBUTION
FAN Supply the required air to all conditioned space Must provide the required pressure drop to cater for ducts, diffusers, filters, etc. Types:
Axial : a) Vane axial - centerline of duct
- guide vanes before and after wheel (rotor) to control rotation of stream
- high speed (noisy)
b) Tube axial - no guide vanes
c) Propeller - low pressure applications
- high mass flow rates Centrifugal:
a) Forward curved (blades)
b) Radial
c) Backward curved (airfoil)
Most used in A/C – can move large or small quantities of air over wide ranges of pressure
FME 706/FML 2007- 08 Air Conditioning 38
BUILDINGS AIR DISTRIBUTION (Cont’d 1)
Fan Selection Fan characteristics
Capacity and total pressure Efficiency Reliability Size Weight Speed Noise Cost
Duct Design Layout (supply and return) – related to supply diffusers and return grilles, location of machine
room, and other structural and architectural considerations. Selection of size is a compromise between capital and running costs.
FME 706/FML 2007- 08 Air Conditioning 39
HVAC SYSTEMS, EQUIPMENT & CONTROL
HVAC systems may conveniently be divided into two broad categories: Equipment and systems which provide heating and cooling Systems which provide ventilation (air distribution and diffusion)
It is important to understand the (initial) design of the installation, modifications, operation/performance, utilization hours of operation and even maintenance record (for energy management purposes)
HVAC SYSTEMS Related to system organization Energy consumed depends on source of heating/cooling, air distribution, and whether working
fluid is simultaneously cooled or heated.
ALL AIR SYSTEMS Most common Moderate room air by providing conditioned air from a central source via ducts Control by altering the amount of air supplied or its temperature Provide best control of fresh outdoor air (quality) and humidity control
FME 706/FML 2007- 08 Air Conditioning 40
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 1)
Can be used to provide outside air for cooling interior spaces while providing heating for perimeter zones
Drawback – energy consumed in distribution
Components of All Air Systems Air Handling Unit (AHU) – fan, (heating and cooling) coils, filters, humidifier (Supply and return) ducts circulate conditioned air. Sometimes plenum above suspended ceiling
used as part of return path Included in duct system is supplier of outdoor air and another for exhausting some of the return
air
FME 706/FML 2007- 08 Air Conditioning 41
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 2)
Single Zone Air Conditioning System Layout
Outdoor air
Filters
Preheat coil (opt)
Cooling & dehumid coil
Heating or reheat coil (opt)
Supply fan
Supply air
Room(s)
Return air
Return fan
Exhaust air
FME 706/FML 2007- 08 Air Conditioning 42
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 3)
Can be used for all year round control Can use 100% outdoor air – during intermediate cooling seasons – refrigeration
equipment not used
Control of proportion of outdoor air
Exhaust
Min
Max Outdoor air
Return
Mixed supply to AHU
FME 706/FML 2007- 08 Air Conditioning 43
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 4)
Pre-heat coil – in cold climates to prevent cooling coils from freezing Face bypass – provides another method of controlling humidity – but not as good control
as reheat coil
Single zone systems suitable for large open spaces with uniform load, e.g. stores, factories, arenas, auditoriums, exhibition halls, etc
Bypass damper
Face damper
Cooling and dehumid coil
FME 706/FML 2007- 08 Air Conditioning 44
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 5)
Variable Air Volume (VAV) Systems Same as single zone but individual thermostats control the amount of air supplied to
room
VAV
FME 706/FML 2007- 08 Air Conditioning 45
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 6)
High degree of local temperature control Moderate additional capital cost AHU pressure increases (additional P for VAV) AHU needs regulation to balance varying duct P requirements (fan inlet and outlet dampers) Fan would operate off the optimum position – need variable speed drive Supplementary heating may be necessary (minimum air to space must be supplied) Single duct VAV systems most versatile and most widely used for large buildings (except where
high degree of humidity control is required or high air exchange)
FME 706/FML 2007- 08 Air Conditioning 46
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 7)
Reheat Systems
o
Zone 1
Zone 2
Zone 3
AHU
Reheat coils
Return
m c s1
o
m
z1
s1
c
FME 706/FML 2007- 08 Air Conditioning 47
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 8)
Provides individual zone control of temp. and humidity Wasteful – all air has to be cooled and then heated – double use (waste) of energy (cooling and
then reheating) Constant Air Volume (CAV) and VAV Reheat systems inefficient – highest level for all systems
(CAV reheat systems most inefficient. VAV reheat inactive except when air modulation cannot meet minimum temp. requirements)
CAV and VAV systems with reheat can provide extremely tight control conditions (with humidity control) e.g. museums, printing plants, textile mills and other industrial process settings)
FME 706/FML 2007- 08 Air Conditioning 48
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 9)
Multizone Systems
Cooling
Heating
FME 706/FML 2007- 08 Air Conditioning 49
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 10)
A variation of the single duct CAV reheat system (NOT any system with thermostatically controlled zones – misconception)
Most common systems produce two streams at ~ 38C and ~ 13C Streams blended with dampers to adjust room supply air temp.
Dual Duct Systems Air not blended in the fan room Usually uses high velocity ducts (reduces size and cost of ducts but increased fan energy) with
mixing boxes Limited to buildings with strict temp. and humidity control requirements Dual duct with VAV has efficient control (c.f. CAV) but requires a lot more distribution energy
FME 706/FML 2007- 08 Air Conditioning 50
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 11)
Heating
Cooling
Mixing box To zone
Hot duct
Cold duct
Return
Filters
To zone
FME 706/FML 2007- 08 Air Conditioning 51
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 12)
ALL WATER (HYDRONIC) SYSTEMS Distribute hot or cold water from central plant Terminal units heat or cool room air Ventilation brought in through external wall directly to room or via terminal unit Lower capital cost and requires less space than all air system – H2O has higher density and
specific heat Useful when space is limited e.g. existing building not originally conditioned Disadvantages
Many units – maintenance Control of ventilation air quantities not precise Humidity control limited
Popular for low cost central systems in multi-room high-rise applications Water heated to 60 - 120C or chilled to 4 - 10C and piped to devices – finned heaters or coolers Steam also used
Latent heat 50 times more effective as water (T ~ 20C) But higher volume (~ 1600 times) Per m3 water requires less piping space
FME 706/FML 2007- 08 Air Conditioning 52
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 13)
PIPING CONFIGURATIONS
Single Pipe Series System Least piping Maintenance of any unit necessitates shutdown of entire system Individual unit control not possible T diminishes with distance
Chiller/Heater
Terminal unit
Pump
FME 706/FML 2007- 08 Air Conditioning 53
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 14)
One Pipe Main Offers individual control Special diverting tee – directs some of the water to the tee
Chiller/ Heater
Terminal unit
Pump
Diverting tee
FME 706/FML 2007- 08 Air Conditioning 54
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 15)
Two Pipe - Direct Return Facilitates individual control
Central unit
Terminal units
Pump
FME 706/FML 2007- 08 Air Conditioning 55
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 16)
Two Pipe – Reverse Return Balanced – provides nearly equal flow path
Central
unit
Terminal
units
Pump
FME 706/FML 2007- 08 Air Conditioning 56
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 17)
Three Pipe System Separate heating and cooling supply pipes but common return with appropriate 3 way valves Possible to heat some rooms while cooling others Return can be direct or reverse
Hot Cold
Terminal units
FME 706/FML 2007- 08 Air Conditioning 57
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 18)
Four Pipe System Two separate pipe systems – one for cooling and one for heating
Hot Cold Terminal units
FME 706/FML 2007- 08 Air Conditioning 58
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 19)
HYDRONIC TERMINAL DEVICES
Radiators Hollow cast iron sections through which hot water flows – free convection
Convectors Heaters – free convection
Unit Heaters
etc
FME 706/FML 2007- 08 Air Conditioning 59
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 20)
Fan Coil Units Small air handling unit No outside air provision (usually) Hot or cold water supply Can be placed anywhere – cooling near ceiling, heating near floor If with outdoor air, known as unit ventilators
Coil
Filter
FME 706/FML 2007- 08 Air Conditioning 60
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 21)
AIR-WATER SYSTEMS Water and conditioned air from central system to individual terminal units Utilize best features of all air and all water systems Water carries most of the energy Usually distributed air only enough for ventilation – usually by high velocity ducts Supplied air distributed via fan coil units, or directly to rooms Most systems use induction units Central air – known as primary air. As it flows through unit at high velocity it inducts room air
(secondary air) – no fan required – minimizes maintenance Induction units popular with high rises Initial cost relatively high Primary air as low as 25% of all air system – not adequate for outside air cooling even for mild
climates – hence chilled water supplied to unit coils
FME 706/FML 2007- 08 Air Conditioning 61
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 22)
High velocity air jets
Mixed air
Lint screen filter
Coil
Induction Unit
Primary air
Secondary air
FME 706/FML 2007- 08 Air Conditioning 62
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 23)
UNITARY SYSTEMS Refrigeration and air conditioning packaged together, i.e. refrig. equipment, fan, fan coils, filters,
dampers and control Usually in or close to air conditioned space Can be all air, all water or air – water. Generally all air and largely inclined to the more simple
such as single zone with or without reheat, or multizone. Categorized as: Room units Unitary conditioners Roof units
Room Units Dampers adjustable to allow outdoor air through cooling coil Low cost and simplicity Ideal for existing building – electrical power upgrading may be necessary No flexibility to handle high latent heat or changing sensible heat ratio – no good humidity control High sound levels Air cleaning quality marginal (only large particles)
FME 706/FML 2007- 08 Air Conditioning 63
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 24)
Maintenance for large number of units Energy wasteful Up to (approx) 3 tons (~ 10 kW)
Room air Filter
Evap. coil
Evap. fan
Cooled air
Motor
Cond. fan
Condenser
Compressor
Outdoor air Cond. Discharge air
FME 706/FML 2007- 08 Air Conditioning 64
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 25)
UNITARY A/C UNITS
In or near space Heating sometimes included Available in vertical or horizontal package Limited ductwork can be connected if air distribution is desired Popular in small commercial application s Normally only condenser not packaged Split system
Condenser and compressor one package and cooling coil (with fan) inside (popular for residential heat pump)
Same advantages and disadvantages as room units Large units have multiple compressors Available up to ~ 50 tons (175 kW)
FME 706/FML 2007- 08 Air Conditioning 65
HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 26)
ROOFTOP UNITS (DIRECT EXPANSION – DX)
Outdoor installation All components packaged together or compressor and condenser may be remote Heating may be incorporated May be used with ductwork Do not use building space Relatively low cost Available with multizone arrangement Humidity control limited Popular in low cost one floor buildings (e.g. supermarkets and suburban commercial buildings)