Re: [EnergyPlus_Support] purchase air with outside air

[Rahul Chillar <chillarrahul@xxxxxxxxx>]

here is an earlier reply to a some what related query for puchased air.

The purchased air model is very simple but it can produce confusing results when the outside air option is used. The purchased air model takes return air from the zone and mixes it with the specified outside air volume. If the purchased air flow rate is less than the outside air flow rate, then the purchased air flow rate is used and is 100% outside air. It then takes this mixed air condition (dry bulb temperature and humidity ratio) and calculates the enthalpy change required to condition this air to the specified supply air temperature and humidity ratio. This is the total cooling reported. It also determines the sensible change required, and this is the sensible cooling reported. If the mixed air is drier than the specified supply air humidity ratio, then the total cooling will be less
 than the sensible cooling. Purchased air with the outside air option is useful for determining peak loads. If you are using purchased air for an annual load, it is better to introduce the ou
 tside
 air directly to the space using ventilation or infiltration.

Thanks

Rahul Chillar


Ian Doebber <idoebber@xxxxxx> wrote:
I have specified a flow rate for say 0.027 m3/s of outside air to be mixed
with some of the return air from my conditioned zone.  What I am wondering
is how the purchased air option will controll this.

1.  Will the purchased air component continually bring in 0.027 m3/s of
outside air irregardless if the purchased air component is needed to
condition the zone.  Therefor, the annual amount of energy to condition the
inflow of outside air to the setpoint temperature condition is essentially
just added onto the annual amount of energy necessary to condition the zone
when no outside air was specified.

or

2.  Based on the heating and cooling inlet temperatures into the zone
specified in the purchased air feature, for a given load in the zone, the
purchased air component will determine the "exact needed flow rate" of air
to condition that zone load.    Then based upon the flow rate, the purchase
air component will determine the energy to condition the 0.027 m3/s of
outside air and the return flow rate ("exact needed flow rate" - 0.027 m3/s)
to the inlet zone temperatures. This way the outside air is only brought in
when there is a zone load and the purchased air component needs to condition
it.

Which process happens and do I need to specify an "exhaust node" in my
CONTROLLED ZONE EQUIP CONFIGURATION so that air in the zone can be released
to make room for the incoming outside air.

Thanks in advance
Ian Doebber



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Thanks
Rahul Chillar
Grad Student
Building Systems lab
Univ Of Illinois Urbana Champaign


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