Objectives Propose residential system related final project –Compare VAV systems with DOAS...
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![Page 1: Objectives Propose residential system related final project –Compare VAV systems with DOAS systems.](https://reader035.fdocuments.us/reader035/viewer/2022070616/5a4d1c147f8b9ab0599f873e/html5/thumbnails/1.jpg)
Objectives
• Propose residential system related final project
– Compare VAV systems with DOAS systems
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One more project topic
• Residential system with mechanical ventilation– Analyze operation of the heat recovery system
HeatingCooling
72ºF
OA 72ºF
Outdoor Air
Exhaust Fresh Air
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Commercial Project
Zone orientation
Commercial building
Base size is 111.80 ft x 111.80 ft with perimeter zone depth of 15.00 ft.
Floor to floor is 12 ft and floor to suspended ceiling is 9 ft.
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ProjectCooling and heating load for each zone and whole building:
Fresh air requirement based on number of occupants:
To provide acceptable IAQ in the building, ventilation system supply minimal amount of fresh air is in all spaces (except plenums) that produce 0.2 ACH.
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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)
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Other Issues Related to VAV
• Reheaters (sizing and control)• Fan control• Minimum supply air• Occupancy sensors
Some references at:http://www.ce.utexas.edu/prof/Novoselac/classes/ARE389H/Handouts/VAV.pdf
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DOAS fresh air configurations
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DOAS fresh air configurations
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Issues Related to DOAS
• Split of sensible and latent load• Selection of hydronic system • Winter vs. summer operation
– Winter operation with DX systems (heat pump)
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Sequence of operation for the control system design
mixing
CCOA
RA
SAHC
Adiabatichumidifier
Define the sequence of operation for:WINTER operation and:
- case when humidity is not controlled- case when humidity is precisely controlled
Solution on the whiteboard
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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
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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
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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