Air to air rotary exchanger rpm

answered 2017-04-18 07:25:36 -0500

For air-to-air heat exchangers, rotational speed is not modeled explicitly. The performance of a rotary heat exchanger is modeled through the supply outlet air temperature and frost controls. A plate HX can be bypassed while a rotary HX can change speed to adjust effectiveness. For rotary HXs, the model assumes the HX can achieve the speed needed to provide the control.

From the IDD:

HeatExchanger:AirToAir:SensibleAndLatent,
  A7, \field Supply Air Outlet Temperature Control
    \type choice
    \key No
    \key Yes
    \default No
  A8, \field Heat Exchanger Type
    \type choice
    \key Plate
    \key Rotary
    \default Plate
  A9, \field Frost Control Type
    \type choice
    \key None
    \key ExhaustAirRecirculation
    \key ExhaustOnly
    \key MinimumExhaustTemperature
    \default None

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Comments

thanks for your answer, while viewing the source code of the program, I understood that in case of rotary air to air heat exchanger, effectiveness is only calculated using temperatures, expecting the wheel to modulate its speed to achieve any setpoint temperature within boundaries where the control fraction will always be 1 in this case. This means that there is no point from entering the rated effectiveness (values at 100% and 75% of rated flow in case of heating and cooling) when using rotary heat exchanger. Do I understand correctly? Please help, thanks again.

Aly ElHefny ( 2017-09-04 02:38:20 -0500 )edit

You still need to enter the effectiveness values so the model knows what the outlet air temperature is before it modulates wheel speed. At wheel speed = 0 the outlet air temp = inlet air temp. At full speed the outlet air temp is calculated based on effectiveness. Modulating wheel speed can adjust outlet air temp between these two values.

rraustad ( 2017-09-05 08:01:33 -0500 )edit

Thanks for your reply, so from your reply and from reading code I understood that for rotary effectiveness is always found using (X1-X2sp / X1-X3) and so it needs the effectiveness values at 75% and at 100% to draw a straight line defining the maximum effectiveness value for each flow rate. You wrote in the code that rotary is never bypassed and so if my flow is always constant then sensible effectiveness for example will be (T1-T2sp / T1-T3) where the value can only be between 0 and effectiveness at 100% of flow.

Aly ElHefny ( 2017-09-05 08:24:52 -0500 )edit

That's correct, except that although the HX is at constant flow, the air flow through the HX doesn't have to be the flow specified in the HX, that's why you give 2 effectiveness values. 100% of flow is when air flow through the HX equals the value specified in the HX.

HeatExchanger:AirToAir:SensibleAndLatent,
  autosize,                !- Nominal Supply Air Flow Rate {m3/s}

rraustad ( 2017-09-06 08:58:29 -0500 )edit

Would it be possible to give more inputs to the effectiveness than the eff at 100 and 75 % airflow rates?For our experience the effectiveness is not linear but rather parabolic so we would like to introduce some extra points.