objectives discuss project topics learn to design vav and doas system
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Objectives
• Discuss Project Topics
• Learn to design VAV and DOAS System
3D model – Biomedical Engineering (BME) Building
3D model – Biomedical Engineering (BME) Building
3D model – Biomedical Engineering (BME) Building
3D model – Biomedical Engineering (BME) Building
VAV Example
http://www.trane.com/commercial/library/vol242/v24b6.asp
-One system
-Multiple Spaces/Zones
- VAV box for each space/zone
VAV System Terms & Equations(you can find them in ASHRAE Standard 62)
• Fraction of minimum outdoor air in space/zone supply airXi = VOAi / VSAi
• Critical space
Space with maximum Xi , Z=max(Xi)
• Fraction of outdoor air in system supply air
X = sum(VOA ) / sum(VSA)
• Required Fraction of outdoor air:
Y=X/(1+X-Z)
Example Solution
Space1 Space 2 Space 3 Space 4
VSA
VOA
Xi
X
Z
Y
Other Issues Related to VAV
• Fan control
• Minimum supply air
• Occupancy sensors
Enthalpy wheel
Desiccant wheel
Figure 3 – A desiccant-based cooling system combined with regenerative heat exchanger, vapor compression cooling, and evaporative humidifier (hybrid system).
HVAC ControlExample 1:
Economizer (fresh air volume flow rate control)
mixing
damper
fresh air
T & RH sensors
recirc. air
Controlled device is damper
- Damper for the air - Valve for the liquids
Economizer Fresh air volume flow rate control
mixing
damper
Fresh(outdoor) air
T & RH sensors
Recirc. air
% fresh air
Minimum for ventilation
100%
TOA (hOA)
enthalpy
Economizer – cooling regime
How to control the fresh air volume flow rate?
% fresh air
Minimum for ventilation
100%
If TOA < Tset-point → Supply more fresh air than the minimum required
The question is how much?
Open the damper for the fresh air
and compare the Troom with the Tset-point .
Open till you get the Troom = Tset-point
If you have 100% fresh air and your still need cooling use cooling coil.
What are the priorities: - Control the dampers and then the cooling coils or - Control the valves of cooling coil and then the dampers ?
Defend by SEQUENCE OF OERATION the set of operation which HVAC designer provides to the automatic control engineer
Economizer – cooling regime
Example of SEQUENCE OF OERATIONS:
If TOA < Tset-point open the fresh air damper the maximum position
Then, if Tindoor air < Tset-point start closing the cooling coil valve
If cooling coil valve is closed and T indoor air < Tset-point start closing the damper till you get T indoor air = T set-point Other variations are possible
HVAC ControlExample 2:
Dew point control (Relative Humidity control)filter
fancooling coil
heating coil
filter
mixing
damper fresh air
T & RH sensors
We either measure Dew Point directly or T & RH sensors substitute dew point sensor
Humidity generationHeat gains
We should supply air with lower humidity ratio (w) and lower temperature
Relative humidity control by cooling coil
TDP
Mixture
Cooling Coil
RoomSupply
Heating coil
Relative humidity control by cooling coil (CC)• Cooling coil is controlled by TDP set-point
if TDP measured > TDP set-point → send the signal to open more the CC valve
if TDP measured < TDP set-point → send the signal to close more the CC valve
cooling coil
heating coil
mixing
Fresh air
Tair & TDP sensors
Control valves
• Heating coil is controlled by Tair set-point
if Tair < Tair set-point → send the signal to open more the heating coil valve
if Tair > Tair set-point → send the signal to close more the heating coil valve
Sequence of operation(ECJ research facility)
Control logic:
Mixture in zone 1: IF (( TM<TSP) & (DPTM<DPTSP) ) heating and humidifying Heater control: IF (TSP>TSA) increase heating or IF (TSP<TSA) decrease heatingHumidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA)
decrease humid.
Mixture in zone 2: IF ((TM>TSP) & (DPTM<DPTSP) ) cooling and humidifying Cool. coil cont.: IF (TSP<TSA) increase cooling or IF (TSP>TSA) decrease
coolingHumidifier: IF (DPTSP>DPTSA) increase humidifying or IF (DPTSP<DPTSA)
decrease hum.
Mixture in zone 3: IF ((DPTM>DPTSP) ) cooling/dehumidifying and reheatinCool. coil cont.: IF (DPTSP>DPTSA) increase cooling or IF (DPTSP<DPTSA)
decrease cooling Heater control: IF (TSP>TSA) increase heating or IF (TSP<TSA) decrease heating
Set Point (SP)
Mixture 2
Mixture 3
Mixture 1
DBTSP
DPTSP