Objectives

1
Objectives

• Propose residential system related final project
• Compare VAV systems with DOAS systems

2
One more project topic

• Residential system with mechanical ventilation
• Analyze operation of the heat recovery system

Outdoor Air
OA
72ºF
72ºF
Heating Cooling
Exhaust
Fresh Air
3
Commercial Project
Commercial building
Zone orientation
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.
4
Project
Cooling 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.
5
VAV System Terms Equations(you can find them
in ASHRAE Standard 62)

• Fraction of minimum outdoor air in space/zone
  supply air
• Xi VOAi / VSAi
• Critical space
• Space with maximum Xi , Zmax(Xi)
• Fraction of outdoor air in system supply air
• X sum(VOA ) / sum(VSA)
• Required Fraction of outdoor air
• YX/(1X-Z)

6
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/AR
  E389H/Handouts/VAV.pdf

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www.doas.psu.eduDOAS with multi-split systems
Fresh air?
8
DOAS fresh air configurations
9
DOAS fresh air configurations
10
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)

11
Sequence of operation for the control system
design
Adiabatic humidifier
CC
HC
SA
OA
mixing
RA
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
12
Economizer
fresh air

• Fresh air volume flow rate control

100
enthalpy
Fresh (outdoor) air
TOA (hOA)
Minimum for ventilation
damper
mixing
Recirc. air
T RH sensors
13
Economizer cooling regime

• How to control the fresh air volume flow rate?

If TOA lt 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
fresh air
100
Minimum for ventilation
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Economizer cooling regime
Example of SEQUENCE OF OERATIONS If TOA lt
Tset-point open the fresh air damper the maximum
position Then, if Tindoor air lt Tset-point
start closing the cooling coil valve If cooling
coil valve is closed and T indoor air lt
Tset-point start closing the damper till you
get T indoor air T set-point Other
variations are possible