AIR CONDITIONING
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Transcript AIR CONDITIONING
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)
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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
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PSYCHROMETRICS (Cont’d 1)
PSYCHROMETRIC CHART
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SOME IMPORTANT PSYCHROMETRIC
PROCESSES
Thermodynamic Wet Bulb Temperature (Adiabatic Saturator)
Air
Water
Adiabatic wall
mw
Make-up water
Thermodynamic Saturator
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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
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Sling Psychrometer
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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
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h g1 h f 2
0.6222 Ps2
P 2 Ps2
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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
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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
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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
0.6222Ps 2
P 2Ps 2
Heating and Cooling at Constant W (Sensible)
Q
W
(b)
(a)
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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
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(b)
or
or
w m
a (W1 W2 )
m
m
a (h 1 h 2 ) m
a h w (W1 W2 )
Q
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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
A2
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.
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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,
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h 2 h1
hw
W2 W1
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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
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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)
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SOME IMPORTANT PSYCHROMETRIC
PROCESSES (cont’d 11)
EXAMPLE OF A SIMPLE CENTRAL AIR-CONDITIONING SYSTEM
Outdoor
air
Filters
Fan
Cooling &
dehumid
Coil
Heater
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0
Exhaust
2
1
Space
Q=Qs+Ql
5,6,7
3
4
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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)
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m a1
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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Instantaneous
heat gain
Afternoon
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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)
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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
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ROOM AIR DISTRIBUTION (cont’d)
VxCL
Vo
I
II
III
IV
x
Ao
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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
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ROOM AIR DISTRIBUTION (cont’d 2)
High sidewall diffuser – good for cooling
Cooling
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Heating
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ROOM AIR DISTRIBUTION (cont’d 3)
Ceiling Diffuser
Excellent for cooling
Large diffusion surface area
Handles large quantities of air
Beam
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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
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ROOM AIR DISTRIBUTION (cont’d 5)
Room Characteristic Length (L)
L
L
High
Sidewall
Diffuser
Ceiling
Diffuser
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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
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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
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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
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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.
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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
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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
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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
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Return
fan
Room(s)
Return
air
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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
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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
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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
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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)
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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
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s1
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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)
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HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 9)
Multizone Systems
Cooling
Heating
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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
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HVAC SYSTEMS, EQUIPMENT &
CONTROL (cont’d 11)
Heating
Hot duct
Filters
Mixing
box
To zone
Cooling
To zone
Cold duct
Return
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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
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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
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Terminal
unit
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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