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[EnergyPlus_Support] Re: air to air heat exchanger
The equations for sensible and total heat recovery in the
Engineering Document are incorrect (i.e., the temperature difference
is not shown correctly, page 248). In the program, the sensible heat
transfer is being calculated as:
(mdot,supply) * (Cp,supply) * (Tsupairout - Tsupairin)
Similar for total heat transfer: (mdot,supply) * (Hsupairout -
Hsupairin).
In winter the supply air exiting the heat exchanger is warmer than
the cold outdoor air, so the sensible heat exchange is positive. In
summer, you generally get negative sensible heat exchange (when the
outdoor dry-bulb temp is higher than the relief air temperature).
Your point is well taken that the "sign" of the heat transfer should
always be positive. We'll address this issue in the next release of
the program (possibly add separate output variables for when the
outdoor ventilation air is heated and when the air is cooled).
As stated in the documentation, the current air-to-air heat exchanger
object models heat transfer between the outdoor (supply) and relief
(exhaust) air streams whenever it is scheduled to operate
(availability schedule) and when air flow is present. Additional
control options will be available in the next version of EnergyPlus
(e.g., suspend heat exchange based on outdoor air or relief air
temperature, enthalpy, etc. to prevent overheating the supply air
when zone cooling is required ---> economize).
You state that "When the relief air enthalpy is greater than outside
air enthalpy, the heat exchanger should be turned off." This is true
if zone cooling is required (e.g., summer), but is not true when zone
heating is required (e.g., winter).
--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, "zhaicq98" <chaoqinz@a...>
wrote:
> Hi,
>
> I have an air to air heat exchanger, which transfer sensible and
> latent heat between the outside air and building relief air. When I
> output its heat recovery energy, lots of nagative numbers happen in
> summer, while in winter, most are positive numbers.
>
> In my opinion, there should not be any nagative number. When the
> relief air enthalpy is greater than outside air enthalpy, the heat
> exchanger should be turned off. Is it correct?
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