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Re: [EnergyPlus_Support] heat exchanger
Yes it is true that cooling energy may increase in winter when heat
recovery is used. See the description for Supply Air Outlet Temperature
Control in the Heat Exchanger:Air To Air:Generic section of the
Engineering Document. It discusses how to modulate wheel speed or bypass
a plate heat exchanger to avoid overheating the supply air in winter.
Zhen Tian wrote:
>Hi, Richard:
>
>Thanks for your reply. But I have more questions regarding heat recovery.
>
>>From your introduction, in the simulation, the cooling energy will increase if using a
>heat recovery unit, isn't it?
>But in In a real situation, if cooling energy use will increase with heat recovery?
>
>In my simuation, I found that the main cooling energy use happens when the outside
>temperature below the lower limit for economizer. For example, when the economizer
>process for outside air ranges 20C (high limit) and 4C (low limit), the cooling energy
>use will increase when outside temperature ranges bwtween -10C and 4C in winer.
>
>I am wondering if there is method to avoid the overheating of outside air by controlling
>mixed node air temperature instead of heat recovery outlet node temperature? I tried to
>set the "set point manager" for mixed air temperature instead of heat recovery outlet
>temp, but it seems to do not work very effective. If is it because of the mixed node
>temperature contol is next to the heat recovery outlet node and the program does not know
>what ratio of return air will be mixed ?
>
>Thanks a lot.
>
>Regards,
>
>Zhen
>
>--- Richard Raustad <RRaustad@xxxxxxxxxxxx> wrote:
>
>
>
>>
>>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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>>>
>>--
>>Richard A. Raustad
>>Senior Research Engineer
>>Florida Solar Energy Center
>>University of Central Florida
>>1679 Clearlake Road
>>Cocoa, FL 32922-5703
>>Phone: (321) 638-1454
>>Fax: (321) 638-1439 or 1010
>>Visit our web site at: http://www.fsec.ucf.edu
>>
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--
Richard A. Raustad
Senior Research Engineer
Florida Solar Energy Center
University of Central Florida
1679 Clearlake Road
Cocoa, FL 32922-5703
Phone: (321) 638-1454
Fax: (321) 638-1439 or 1010
Visit our web site at: http://www.fsec.ucf.edu
UCF - From Promise to Prominence: Celebrating 40 Years
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