AIR CONDITIONING
FME 706/FML 2007- 08
Air Conditioning
1
SCOPE AND USE OF AIR
CONDITIONING



Not restricted to cooling only but might include:
o Control of temperature at all times by heating or cooling
o Control of air humidity by humidification or dehumidification
o Control of air movement at a desirable velocity
o Introduction of outdoor air as required
o Control of air quality by removal of dirt particles and odorous gases
o 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
2
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
3
PSYCHROMETRICS (Cont’d 1)
PSYCHROMETRIC CHART
FME 706/FML 2007- 08
Air Conditioning
4
SOME IMPORTANT PSYCHROMETRIC
PROCESSES
Thermodynamic Wet Bulb Temperature (Adiabatic Saturator)




Air



Water
Adiabatic wall


mw
Make-up water
Thermodynamic Saturator
FME 706/FML 2007- 08
Air Conditioning
5
SOME IMPORTANT PSYCHROMETRIC
PROCESSES (cont’d 1)
T
Pv1
Pv2=Pvs2
Handle
f2


T2=Tw
Dry bulb thermometer
Wet bulb thermometer
s
Adiabatic Saturator H2O Process
FME 706/FML 2007- 08
Sling Psychrometer
Air Conditioning
6
SOME IMPORTANT PSYCHROMETRIC
PROCESSES (cont’d 2)
 a ha  m
 v hv  m
 whw  m
 a ha  m
 v hv
m
1
1
1
1
2
2
2
2
 a m
 a m
a
m
1
2
 w m
 a (W2  W1 )
m
W1 (h v1  h w )  h a 2  h a1  W2 (h v2  h w )
hw = hf2, hv1 = hg1, hv2 - hw = hfg2,
W1 
(h a 2  h a1 )  W2 h fg 2
W2 
FME 706/FML 2007- 08
h g1  h f 2
0.6222 Ps2
P  2 Ps2
Air Conditioning
7
SOME IMPORTANT PSYCHROMETRIC
PROCESSES (cont’d 3)
IMPORTANT RELATIONSHIPS
Specific Humidity
~
mv PvV RaT Pv Ra M v Pv 18.01534 Pv
P
W 



 
  0.622 v
ma RvT PaV Pv Rv M a Pa 28.9645 Pa
Pa
Enthalpy
H  mi  m a h a  m v h v
i
m
m
 ha  v hv
ma
ma
i(1  W )  ha  Whv  h
or
h  c pa t  (hgo  c pv t )W  (c pa  Wc pv )t  Wh go  c p t  Wh go
FME 706/FML 2007- 08
Air Conditioning
8
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
=
Pv
Ps
;

W  0.622
Pv
Pa
; P = Pa + Pv ; and hence
WPa
W ( P  Pv ) W ( P   Ps )


0.622 Ps
0.622 Ps
0.622 Ps
Solution for W1 From Adiabatic Saturator
W1 
c pa (t 2  t1 )  W2 h fg2
hg1  h f 2
FME 706/FML 2007- 08
Air Conditioning
9
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
W2 
0.6222Ps 2
P  2Ps 2
Heating and Cooling at Constant W (Sensible)
Q





W
(b)
(a)
FME 706/FML 2007- 08
Air Conditioning
10
SOME IMPORTANT PSYCHROMETRIC
PROCESSES (cont’d 6)
 a (h2  h1 )  m
 a [(ha2  ha1 )  W (hv2  hv1 )]  m
 a (c pa  Wc pv )(t2  t1 )  m
 ac p (t2  t1 )
Q  m
Cooling and Dehumidification
Q







A
 m
(a)
w
 v m
 w m
v
m
1
2

 a h1  m
 ah2  m
 whw Q
m
FME 706/FML 2007- 08
(b)
or
or
 w m
 a (W1  W2 )
m
 m
 a (h 1  h 2 )  m
 a h w (W1  W2 )
Q
Air Conditioning
11
SOME IMPORTANT PSYCHROMETRIC
PROCESSES (cont’d 7)
 a h w ( W1 - W 2) represents enthalpy carried away by the condensate ( 10ºC) which is
m
negligible compared to the first term and hence
 Q
 Q

Q
s
l
where
 Q

Q
s
A2
 a c p ( t1  t 2 )  m
 a (h A  h 2 )
m
and
 Q

 a ( W1  W2 )h fg
Q
l
1 A  m
1
 a (h 1  h A )
m
Sensible Heat Factor (SHF) (Related to bypass factor)
SHF 

Q
s

Q
Important in A/C calculations.
FME 706/FML 2007- 08
Air Conditioning
12
SOME IMPORTANT PSYCHROMETRIC
PROCESSES (cont’d 8)
Heating With Humidification

Q

m
w






(a)
(b)
 w m
 a (W2  W1 )
m
 m
 a (h 2  h 1 )  Q
 a (W2  W1 )h w
m
or

h 2  h1
Q

 hw
 a ( W2  W1 )
W2  W1 m
Equation of a straight line.
For Q = 0,
FME 706/FML 2007- 08
h 2  h1
 hw
W2  W1
Air Conditioning
13
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
2
2
2
1
FME 706/FML 2007- 08
Air Conditioning
14
SOME IMPORTANT PSYCHROMETRIC
PROCESSES (cont’d 10)
Adiabatic Mixing

h2



h3

W2
h1


(b)
(a)
 a h1  m
 a h2  m
 a h3 ;
m
1
2
3
a
m
1
a
m
2


W1


h 3  h 2 W3  W2

h1  h 3
W1  W3
W3
a
m
1
a
m
3
 a W1  m
 a W2  m
 a W3 ;
m
1
2
3
h  h 2 W3  W2
 3

h 1  h 2 W1  W2
a
m
2
a
m
3
 a m
 a m
a
m
1
2
3

h 3  h 1 W3  W1

h 2  h 1 W2  W1
Equation of a straight line (final state lies along this line)
FME 706/FML 2007- 08
Air Conditioning
15
SOME IMPORTANT PSYCHROMETRIC
PROCESSES (cont’d 11)
EXAMPLE OF A SIMPLE CENTRAL AIR-CONDITIONING SYSTEM
Outdoor
air 
 Filters
Fan

Cooling &
dehumid
Coil

Heater
FME 706/FML 2007- 08
0
Exhaust



2
1
Space
Q=Qs+Ql
5,6,7
3
4

Air Conditioning
16
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)
m a4 

Q
h5  h4
Join 0 and 5 locate 1 – adiabatic mixing, i.e.
h2  h1 
W0  W1 m a6

W0  W6 m a1
Q fan

Hence


For known SHFcoil draw line 2-3, and hence 3-4 at constant W
Qcoil = ma1(h2 – h3), Qheater = ma3 (h4 – h3)
FME 706/FML 2007- 08
m a1
Air Conditioning
17
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
18
COMFORT AND HEALTH (cont’d 1)
Examples of Cooling Load Due to Occupancy
Activity
Example
Male Adult Total
Watts
Total Adjusted
Watts
Sensible
Watts
Latent
Watts
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
19
COMFORT AND HEALTH (Cont’d 2)
ASHRAE Comfort Standard 55-81 (1981) (Sedentary)
W
gv/kga
15
Winter
20
10
0
5
Summer
25
30
ºC
FME 706/FML 2007- 08
Air Conditioning
20
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
21
COMFORT AND HEALTH (Cont’d 4)
OUTDOOR DESIGN CONDITIONS
Winter
Station
(Elevation)
Mean
Annual
Extrem
es
99%
C
Nairobi
(1820 m)
7
9
Addis (2363
m)
2
Lagos (3 m)
Dar es
Salaam (14
m)
Summer
97.5%
C
Design Dry Bulb C
1%
2.5%
5%
10
27
27
26
4
5
29
28
19
21
22
33
17
18
18
32
FME 706/FML 2007- 08
Design Wet Bulb C
Outdoor
Daily
Range C
1%
2.5%
5%
13
19
18
18
27
16
19
18
18
33
32
7
28
28
29
32
31
7
28
27
27
Air Conditioning
22
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
23
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
24
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.)
Radiation
Heat
Gain
Heat storage in
furnishings and
structure
Convection
(delayed in
time)
Cooling
load
Convection, infiltration
FME 706/FML 2007- 08
Air Conditioning
25
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)
Heat being
stored
Heat Gain
and Cooling
Load
Removal of
stored heat
Cooling
load
Removal of
stored heat
Morning
FME 706/FML 2007- 08
Instantaneous
heat gain
Afternoon
Air Conditioning
Evening
26
HEAT TRANSMISSION IN BUILDINGS
AND COOLING LOAD (Cont’d 2)
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)
FME 706/FML 2007- 08
Air Conditioning
27
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
28
ROOM AIR DISTRIBUTION (cont’d)
VxCL
Vo
I
II
III
IV
x
Ao
FME 706/FML 2007- 08
Air Conditioning
29
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
Vx
K
Vo
Ao
x
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
FME 706/FML 2007- 08
Air Conditioning
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ROOM AIR DISTRIBUTION (cont’d 2)

High sidewall diffuser – good for cooling
Cooling
FME 706/FML 2007- 08
Heating
Air Conditioning
31
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
32
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
33
ROOM AIR DISTRIBUTION (cont’d 5)
Room Characteristic Length (L)
L
L
High
Sidewall
Diffuser
Ceiling
Diffuser
FME 706/FML 2007- 08
Air Conditioning
34
ROOM AIR DISTRIBUTION (cont’d 6)
Air Diffusion Performance Index (ADPI)
 Effective Draft Temperature (EDT)
 = (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
35
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
36
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
37
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
38
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
39
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
40
HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 2)
Single Zone Air Conditioning System Layout
Preheat
coil (opt)
Cooling &
dehumid
coil
Supply
fan
Outdoor
air
Supply
air
Filters
Heating
or reheat
coil (opt)
Exhaust
air
FME 706/FML 2007- 08
Return
fan
Room(s)
Return
air
Air Conditioning
41
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
Max
Outdoor
air
Min
Mixed
supply to
AHU
Return
Exhaust
FME 706/FML 2007- 08
Air Conditioning
42
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
Bypass
damper
Face
damper

Cooling
and
dehumid
coil
Single zone systems suitable for large open spaces with uniform load, e.g. stores,
factories, arenas, auditoriums, exhibition halls, etc
FME 706/FML 2007- 08
Air Conditioning
43
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
44
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
45
HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 7)
Reheat Systems
Reheat coils
m
o
c
AHU
Zone 1
s1
Zone 2
Zone 3
Return
o
m
z1
c
FME 706/FML 2007- 08
s1
Air Conditioning
46
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
47
HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 9)
Multizone Systems
Cooling
Heating
FME 706/FML 2007- 08
Air Conditioning
48
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 ~ 38C and ~ 13C
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
49
HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 11)
Heating
Hot duct
Filters
Mixing
box
To zone
Cooling
To zone
Cold duct
Return
FME 706/FML 2007- 08
Air Conditioning
50
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 - 120C or chilled to 4 - 10C and piped to devices – finned heaters or coolers
 Steam also used
 Latent heat 50 times more effective as water (T ~ 20C)
 But higher volume (~ 1600 times)
 Per m3 water requires less piping space
FME 706/FML 2007- 08
Air Conditioning
51
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
Pump
Chiller/
Heater
FME 706/FML 2007- 08
Terminal
unit
Air Conditioning
52
HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 14)
One Pipe Main
 Offers individual control
 Special diverting tee – directs some of the water to the tee
Pump
Terminal
unit
Chiller/
Heater
Diverting tee
FME 706/FML 2007- 08
Air Conditioning
53
HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 15)
Two Pipe - Direct Return
 Facilitates individual control
Pump
Central
unit
FME 706/FML 2007- 08
Terminal
units
Air Conditioning
54
HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 16)
Two Pipe – Reverse Return
 Balanced – provides nearly equal flow path
Pump
Central
Terminal
unit
FME 706/FML 2007- 08
units
Air Conditioning
55
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
FME 706/FML 2007- 08
Cold
Terminal
units
Air Conditioning
56
HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 18)
Four Pipe System
 Two separate pipe systems – one for cooling and one for heating
Hot
FME 706/FML 2007- 08
Cold
Terminal
units
Air Conditioning
57
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
58
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
59
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
60
HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 22)
Lint screen
filter
Secondary
air
Mixed
air
Coil
High
velocity
air jets
Primary
air
Induction Unit
FME 706/FML 2007- 08
Air Conditioning
61
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
62
HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 24)



Maintenance for large number of units
Energy wasteful
Up to (approx) 3 tons (~ 10 kW)
Cond. Discharge air
Outdoor air
Condenser
Cond. fan
Compressor
Motor
Evap. fan
Evap. coil
Filter
Room air
Cooled air
FME 706/FML 2007- 08
Air Conditioning
63
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
64
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)
FME 706/FML 2007- 08
Air Conditioning
65