ATE 582- Final Project |Instructor- Dr. T. Agami Reddy | Syed Hussaini | Brad Taris

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.

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.

Background

Background

The primary equation used to calculate the Volume flow or cfm of air is given by the following equation :

HVAC Diagram

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 37oF at which point the Total flow is low and Ventilation air is at 60%, and cooling is no longer needed.

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) 56oF,

• 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 55oF to:• lighten the load for the reheat coils• protect the cooling coil from dangerously cold air.

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 2oF out.

• A “benefit” of CAV is no preheat is required

CAV issues

• Worse YET! Since the chiller was sized for the most extreme condition it takes supply air all the way down to 55oF 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 97oF

• For 8,754 hours of the year in this CAV system both the chiller and boiler work on the same air.

HRU selection

ε = (TBin – Tvent out )/ (Tbin – Treturn )

ε =0.8 Tvent out = TBin - (ε * (Tbin – Treturn ))

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 77oF 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.

HRU with CAV and VAV

• However, If the cooling coil is on, and outside temperature is below return (T6) 77oF, 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 77oF 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.

Economizer

To, min = (T1mixed ,2 - mr (Treturn 6’ ) ) / mo,min To, min = (55oF - (( 1 - (2400cfm / 14469cfm )) * 77 oF)) / 2400cfm To, min = -55.6oF

Economizer curve

Manually finding Vo flow so that T1 = 55. Then take

Then take Vo / total flow

and use corresponding Bin temp

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.

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.

Overall comparison

Overall comparison

VAV vs. CAV

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.