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Re: [EnergyPlus_Support] heat exchanger
zhen tian wrote:
>Hi, All:
>
>I have a question about using the heat exchanger object. I found that if I use "Heat Exchanger: air to air: flat plate" , I do not need a set point manager, but when I use "Heat Exchanger: air to air: Generic", it requires a set point manager for heat exchanger. Could somebody tell me why?
>
>
The Heat Exchanger:Air To Air:Generic can modulate to control the supply
air temperature to a set point which requires a set point manager but
only when Supply Air Outlet Temperature Control is set to "Yes". If
Supply Air Outlet Temperature Control is set to "No" a set point manager
is not required. The flat plate heat exchanger can not control to a
temperature set point and does not need a set point manager (modulation
of heat recovery is not supported). It does, however, allow an
economizer mode where free cooling is available when the outside air
controller INCREASES the outdoor air flow rate above the flat plate heat
exchangers design air flow rate. See the IO reference manual for further
details of air to air heat exchangers.
>
>By addding a heat exchanger, I found that the heating energy use decreased a lot but the cooling energy use increased much in winter. I do use economizer within 20C to 4C outside temperature. I check the node temperature, when the heat recovery outlet node temperature can meet the setpoint (14C), but the mixed is about 17C to 20C and higher than supply air temperature 15.6C. This is the result of the mixture of some portion of return air.
>
>Is there any solution to control the mixed temperaure? I use a set point manager for the mixed air temperature control. But sounds like if there was some portion of return air mixture, the mixed air temperature will be higher than supply air temp, when the outside temperature varies between -10C to 4C (winter).
>
>Could sombody tell me how to avoid this situation?
>
>
There is a trade off when using a heat exchanger for energy or heat
recovery. As you describe the winter heating energy can be significantly
reduced when energy is recovered. The cooling energy, however, can
increase in winter when the economizer set points are set to low. The
heat exchanger is active until the upper economizer limit is reached and
pre-heats the incoming outdoor air which may require additional cooling
to meet the desired set point. Increase the upper economizer limit (up
to the return air temperature) and watch the cooling energy in winter
decrease as the set point is moved upward. To optimize the simulation,
select the economizer set points such that ANNUAL energy use is
minimized and compare this annual energy use with the same simulation
without an air to air heat exchanger. The economizer set points will
vary based on geographic location, cooling and heating set points,
internal loads, and building construction.
>
>Thanks a lot for your help.
>
>Regards,
>
>Zhen
>
>
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