Building HVAC System

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Exam is on November 6, 2012
in class exam
Example exams are posted on the
course website
Lecture Objectives:
• Address HW3 questions
– Use of eQUEST
• Continue with HVAC Systems
– modeling of HVAC Systems
Air-conditioning in
Air Handling Unit (AHU)
AHU
Roof top AHU
fresh
air
mixing
filter
AHU schematic
Exhaust
From room
Return fan
flow control
dampers
Fan
air from
building
to building
Supply fan
Evaporator
Fresh
Outdoor
airair
To room
cool
water
hot
water
Gas/Electric
Heater
Compressor
and Condenser
Processes in AHU presented in
Psychrometric in psychrometric
Case for
Summer in Austin
OA
MA
IA
SA
Refrigeration Cycle
Released energy
(condenser)
T outdoor air
T cooled water
- What is COP?
- How the outdoor air temperature
affects chiller performance?
Cooling energy (evaporator)
Building-System-Plant
HVAC System
(AHU and distribution systems)
Plant
(boiler
and/or
Chiller)
Building
Building HVAC Systems
(Primary and Secondary Building Systems)
AHU – Air Handling Unit
Fresh air
For ventilation
AHU
Primary
systems
Distribution
systems
Air transport
Electricity
Secondary
systems
Cooling
(chiller)
Heating
(boilers)
(or Gas)
Gas
Building
envelope
HVAC systems affect the energy efficiency of
the building as much as the building envelope
Integration of HVAC and building
physics models
Load System Plant model
Qbuiolding
Building
Heating/Cooling
System
Q
including
Ventilation
and
Dehumidification
Plant
Integrated models
Building
Heating/Cooling
System
Plant
Example of System Models:
Schematic of simple air handling unit (AHU)
Tf,inTf,ou t
Mixing box
(1-r)mS
mS
TO wO
TM wM
rmS
TR wR
QC
QH
mS
TS wS
fans
cooler
heater
room
TR wR
Qroom_sensibel
Qroom_latent
m - mass flow rate [kg/s], T – temperature [C], w [kgmoist/kgdry air],
r - recirculation rate [-], Q energy/time [W]
Energy and mass balance equations for
Air handling unit model – steady state case
The energy balance for the room is given as:
Qroom_ sensible  mS c p TR  TS 
mS is the supply air mass flow rate
cp - specific capacity for air,
TR is the room temperature,
TS is the supply air temperature.
The air-humidity balance for room is given as:
Qroom_ latent  mS wR  wS   i phase_ change
wR and wS are room and supply humidity ratio
i phase_ change
- energy for phase change of water into vapor
The energy balance for the mixing box is:
TM  (1  r )  TO  r  TR
‘r’ is the re-circulated air portion,
TO is the outdoor air temperature,
TM is the temperature of the air after the mixing box.
The air-humidity balance for the mixing box is:
wM  (1  r )  wO  r  wR
wO is the outdoor air humidity ratio and
wM is the humidity ratio after the mixing box
The energy balance for the heating coil is given as:
QHeating  mS c p (TS  TM )
The energy balance for the cooling coil is given as:
QCooling  mS c p (TS  TM )  mS wS  wM  i phase _ change
Non-air system
Radiant panel heat transfer model
radiant panel layer (water tube)
Tw_out
Radiant Panel
ti on
c
e
v
Tsurface
c on
Tzone_air
Q rad_pan
rad
iat
ion
Ro om (zone 1)
Tsurounding
m s ,Ts = const.
air supply
system
Qzone
Tw_in
Non-air system
Radiant panel heat transfer model
The total cooling/heating load in the room
Qzone  Qrad _ pan  Qair
Qair  (mcp )sup ply _ air (Tsup ply _ air  Troom _ air )
Qrad _ pan
The energy extracted/added by air system
The energy extracted/added by the radiant panel:
The energy extracted/added by the radiant panel is the sum of the radiative
and convective parts:
Qrad _ pan  Qradiation  Qconv  hradiation,i Apanel (Tpanel  Tsurface ,i ) hconv Apanel (Tpanel  Tair )
The radiant panel energy is:
Qrad _ pan  mc pw (Tw _ out  Tw _ in )
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