1. ATE 582- Final Project |Instructor- Dr. T. Agami Reddy | Syed Hussaini | Brad Taris
2. Background
• This report analyzes a 2 zone VAV and CAV system, at first using
a crude evaluation of the latent space loads based on Example
11.9 of Krieder et al. and in the second approach we use a more
accurate method of obtaining the latent cooling loads.
• Assumptions of Example 11.9 are:
• Ducts assumed adiabatic
• System is located at sea level
• The building is conditioned 24/7 a day
• We have analyzed the efficacy of both CAV and VAV systems
using the Bin Method. The Bin method uses a set of year round
outside air temperatures relative to the summer or winter
condition for a location for the different times of the year.
3. Background
• Constant Air Volume (CAV) and Variable Air Volume (VAV)
systems are an all air type systems, used for air conditioning of
space.
• CAV systems use a constant air volume throughout the space
based on the peak cooling load requirement. CAV systems use a
reheat coil to heat the air when load requirements of the space
have decreased, such that the same volume of air is supplied at a
different temperature.
• VAV systems vary the total flow based on the heating or cooling
load requirements. VAV varies the volume flow rate to meet the
present load requirements rather than reheating the air.VAV
system operates by use of a damper that opens depending on
the space load requirements.
7. VAV solutions
• With VAV the fans can provide less cfm when loads are lower.
This change provides tremendous benefits.
• As sensible loads are reduced in colder bins the flow required
drops until it reaches a minimum (Zone2=1300, Zone1=1100)
which only happens for Zone1.
• it should be the absolute value of flow for max expression…?
• Since the mixed air temperature (T1) is above Zone2 required
temperature (T5a) to meets its loads the Chiller is still on until
outside Bin temperature (Tin) is 37o
F at which point the Total flow
is low and Ventilation air is at 60%, and cooling is no longer
needed.
8. VAV solutions
• There are only two bins in which the chiller and boiler are on. This
is because room supply is between mixed air (T1) and the
temperature after the fan (T4) 56o
F,
• lower than T1 requiring the cooling coil and,
• higher than T4 requiring reheating.
• Once the Chiller turns off, with to the mixed air (T1) being too cold
for the both space’s requirements, the preheat coil is then turned
on.
• The preheat coil brings the mixed air up to 55o
F to:
• lighten the load for the reheat coils
• protect the cooling coil from dangerously cold air.
9. CAV issues
• Ventilation air is at a constant 17% year round, which results in the
mixed air condition consisting primarily of return air.
• During extremes this can be useful as it reduces obtaining extreme
exterior temperature. However, it becomes obviously wasteful when
exterior temperatures could benefit the system.
• Due to high mixed air temperatures (T1) are consistently above
Zone2’s required air supply (T5). As a result the Chiller for CAV is
on year round, even when its 2o
F out.
• A “benefit” of CAV is no preheat is required
10. CAV issues
• Worse YET! Since the chiller was sized for the most extreme
condition it takes supply air all the way down to 55o
F regardless of
needed load.
• So, other than the highest Bin after air is cooled, it needs to be
reheated, before reaching the proper temperature for the space to
combat and balance heat loads.
• As a consequence the boiler is also needed for all bins and hours,
except for the 6 hours at the peak bin of 97o
F
• For 8,754 hours of the year in this CAV system both the chiller and
boiler work on the same air.
11. HRU selection
ε = (TBin
– Tvent out
)/ (Tbin
– Treturn
)
ε =0.8
Tvent out
= TBin
- (ε * (Tbin
– Treturn
))
12. HRU
• The HRU has tremendous ability to exchange the heat between
exhaust and ventilation air.
• Unfortunately, this ability is often not beneficial to the system energy
as a whole.
• In Bins over return air temperature (T6) of 77o
F the HRU brings
outside temperatures down reducing the cooling coil delta T, and
thus saving energy.
• For these cases the HRU is on in four Bins, and 749 hours.
13. HRU with CAV and VAV
• However, If the cooling coil is on, and outside temperature is below
return (T6) 77o
F, then the HRU will heat up the ventilation air and
actually increase the cooling coil delta T, and thus the cooling load.
• This overheating in CAV is harmful since the cooling coil is always on, and
thus you want to bring mixed air (T1) down from 77o
F using ventilation
air as much as possible (until 55).
• In VAV turning the HRU on causes similar problem in mild months,
but in Winter bins it actually causes the cooling coil to turn on. This
means your heating with the HRU to cool with the C/C and then
reheating at the space.
14. Economizer
To, min
= (T1mixed ,2
- mr
(Treturn 6’
) ) / mo,min
To, min
= (55o
F - (( 1 - (2400cfm / 14469cfm )) * 77 o
F)) / 2400cfm
To, min
= -55.6o
F
16. Econ with CAV and VAV
• When Bin/HRU inlet ventilation air temperature is above 77F the
economizer passes the minimum air through, at 77 to 55 the
economizer passes only outside air through. Then when the
Bin/HRU temperature falls below 55 through To,min it uses the
regression calculation above to determine optimum cfm of
ventilation air that allows mixed air condition to be just above 55Fm,
and if Bin/HRU falls below To,min, minimum air flow is again passed
through.
• When the regression calculation is being used a small percentage is
reduced from the flow to ensure that T1 is at least 55.
• With CAV the To, min is constant at -55.6F since outdoor ventilation
is at a constant 17%. However with VAC that percentage goes up
dramatically and thus increases To, min.
17. Econ with CAV and VAV
• If the HRU was to be turned on during colder Bins, even passing
100% outdoor air would be warmer than passing 17% at Bin
temperature.
• This is true for CAV since it uses cold air as a device to reduce
mixed air condition (T1).
• VAV would also have a warmer mixed air temperature (T1), and
thus require winter cooling.
• With VAV The economizer returns to minimum ventilation air when
the cooling coil turns off, as it is coincident with To, min becoming
less than Bin.
• In CAV the economizer is on for all winter Bins since as mentioned
To,min is -55.6F. This contributes to the economizer having a higher
percentage of savings from base as compared to VAV.
21. Future work & Demo
1. The Dedicated Outdoor Air Systems can be analyzed using both
Economizer and Heat Recovery Unit.
2. Explore different duct configurations, such as two, three, and four
duck setups.
3. An enthalpy switch point for the economizer.
4. A bypass damper to allow some mixed air to circumvent the cooling
coil. Giving a more accurate T3 temperature, and avoiding some
reheating.
