4.4 vav systems
TRANSCRIPT
Variable Air Volume Systems
Space Load
• Envelope, such as wall, roof, floor and window
– Design condition (Toa,d, 92F@Miami): maximum cooling load.
– Off-design condition: Toa<Toa,d
– Negative cooling load means heating load.
• People (always positive)
– Design condition (fully occupied): maximum cooling load
– Off-design condition: partially occupied or unoccupied
• Electrical (always positive)
– Design condition: all lighting and office equipment are used.
– Off-design condition: partially used.
• Question
– How to operate HVAC system to match variable space load?
Space Thermal Control
• Space heat balance
• Maintain space comfort by– _________ airflow
– _________ discharge air temperature using reheat
• Increasing supply air temperature at AHU is not an option. Why?
• Terminal box (TB): – Damper with an actuator
– Reheat coil with a valve or an electrical relay
• Question– Which measure is used?
])[(][1.1]/[
)(
FTTCFMQhBtuq
TTcQq
sarmsas
sarmpasas
−⋅=
−⋅= ρ
Example 1
• One VAV terminal box
with a hot water
reheat coil serves four
12 ft by 12 ft interior
rooms
• Design airflow for
each room is 135CFM.
• Size diffusers using
Titus TMS
Required Throw
L=6 ft
T50=3.6-7.2 ft
Diffuser Sizing
Four (4) 12” x12” module 6” diameter
T50=8ft
Example 2
• Analyze the HVAC system performance if each room is occupied by one person w/o electricity consumption.
1. Constant volume (CV) with reheat: adjust supply discharge air temperature at the terminal box by the reheat coil
2. Variable air volume (VAV) w/o reheat: adjust airflow at the terminal box by the damper
• Condition– Sensible: Btu/h
– Latent: Btu/h
• SA temperature (design)=55F
• Airflow (design)=135CFM
• Discharge air temperature
• Reheat from SA to discharge (TB)
• Space humidity
• HVAC cooling load (AHU)
Measure 1: CV with Reheat
FQ
qTT
d
s
rmdis3.73
1351.1
25075
1.1=
×−=−=
%____
/_______1354845
2000083.0
/0083.0%90,55@
=
=×
+=
==
rm
rm
rhFsadis
lbmalbmvW
lbmalbmvWW
φ
hBtuqqqqrhlsHVAC
/170,3720,2450 =+=++=
hBtuq
TTQq
rh
sadisdrh
/______)553.73(1351.1
)(1.1
=−××=
−=
EC364
Terminal Box
RH coil
Discharge Air
T ,Wsa sa
T ,Wdis dis
Terminal Box
Cooling Coil
AHU
qs ql
qrh
Q=Qd
T ,Wrm rm
• SA temperature (design)=55F
• Reheat
• Discharge air=55F w/o reheat
• Actually airflow
• Space humidity
• HVAC cooling load (AHU)
Measure 2: VAV w/o Reheat
CFMTT
sarm
s _______)5575(1.1)(1.1=
−=
−=
%_____
/0119.04.114845
2000083.0
/0083.0%90,55@
=
=×
+=
=
rm
rm
rhFsa
lbmalbmvW
lbmalbmvW
φ
hBtuqqq lsHVAC /450450 ==+=
EC364
Terminal Box
Damper
Discharge Air=Supply Air
T ,Wsa sa
T ,Wdis dis
Terminal Box
Cooling Coil
AHU
qs ql
Q
T ,Wrm rm
0=rhq
Performance under 135CFM
Performance under 11CFM
Comparison (Each Room)
System CV
w/ Reheat
VAV
w/o Reheat
VAV
w/ Reheat
Airflow
(CFM)
135 11.4 (11.4-90)
Ventilation Good Poor
ADPI Good Poor
Cooling
(Btu/h)
3,170
(High energy)
450 (450-3,170)
Reheat
(Btu/h)
2,720
(High energy)
0 (0-2,720)
Humidity
(%)
46% 64% (46-64%)
• SA temperature (design)=55F
• Min airflow (30%)=41CFM
• Actual airflow= CFM
• Discharge air temperature
• Reheat from SA to discharge (TB)
• Space humidity
• HVAC cooling load(AHU)
VAV with Reheat
FQ
qTT s
rmdis5.69
411.1
25075
1.1=
×−=−=
%50
/0093.0414845
2000083.0
/0083.0%90,55@
=
=×
+=
==
rm
rm
rhFsadis
lbmalbmvW
lbmalbmvWW
φ
hBtuqqqqrhlsHVAC
/104,1654450 =+=++=
hBtuq
TTQq
rh
sadisrh
/654)555.69(411.1
)(1.1
=−××=
−=
EC364
Terminal Box
RH coil Damper
Discharge Air
T ,Wsa sa
T ,Wdis dis
Terminal Box
Cooling Coil
AHU
qs ql
qrh
Q=0.3Qd
T ,Wrm rm
Comparison
System CV
w/ Reheat
VAV
w/o Reheat
VAV
w/o Reheat
Airflow
(CFM)
135 11.4 41
Ventilation Good Poor OK
ADPI Good Poor OK
Cooling
(Btu/h)
3,170 450 1,104
Reheat
(Btu/h)
2,720 0 654
Humidity
(%)
46% 64% 50%
Summary for TB Performance
• Required cooling airflow:– Qreq=qs/1.1/(TRM-TSA)
• Minimum airflow =30% of design airflow• Required airflow=[min%, 100%]
– Cooling without reheat– Tdis=TSA
– Qsa=qs/1.1/(TRM-TSA)– qrh=0– qhavc=qs+ql
• Required airflow<min%– Reheating and cooling with a constant minimum airflow– Qsa=min%Qsa,d
– Tdis=TRM+(-qs)/1.1Qsa
– qrh=1.1Qsa(TRM-TSA) +(-qs)=1.1Qsa(Tdis-TSA)– qhavc=qs+ql+qrh
• Reheat coil load @design heat load (qh<0)– qrh=1.1Qsa,min(TRM-TSA) +(-qh)– In example (qh=0 for interior zones): qrh=1.1x41x4x(75-55) =3,600Btu/h=3.6 MBH
Terminal Box Performance
under Partial loads
Critical parameter is minimum airflow setting
• 30% for offices although it should be determined by ADPI.
• 100% for hospitals and museums (heating all the time)
Heating design
Terminal Box Airflow Control
Flow Sensor and Measurement
• Sensor measures airflow by
velocity pressure:
where
A=cross-section area
C=flow coefficient due to uneven air
distribution
][][][
].[4005][
)4005
][()(
2].[
2
22
fpmVftACCFMQ
winchPfpmV
fpmVVwinchP
⋅⋅=
∆⋅=
==∆ρ
Controller Settings
Terminal Box Sizing• Cooling design airflow :
135x4=540CFM
• Design reheat capacity:
3.6MBH
• Size: 7
• One-row reheat coil