objectives propose residential system related final project –compare vav systems with doas systems
DESCRIPTION
Commercial Project Zone orientation Commercial building Base size is ft x ft with perimeter zone depth of ft. Floor to floor is 12 ft and floor to suspended ceiling is 9 ft.TRANSCRIPT
Objectives
• Propose residential system related final project
– Compare VAV systems with DOAS systems
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
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.
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.
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)
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
DOAS fresh air configurations
DOAS fresh air configurations
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)
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
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