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Relation of System Air Heat Transfer Rate and Sensible Cooling Rate of Coil

asked 2018-11-29 03:04:07 -0600

zahro gravatar image

updated 2018-11-30 02:31:43 -0600

Hello, I am learning to understand how the heat gain calculation happened in Energy Plus, and I am reading in the ASHRAE Handbook of Fundamental that the instantaneous space sensible load should be same with the heat removal rate of conditioning equipment.

I am looking at the zone air heat balance to look at each components contribution to the zone sensible load. Based on the observation in my result, the Zone Air Heat Balance System Air Transfer Rate number match with the sum of

  • Zone Air Heat Balance Internal Convective Heat Gain Rate
  • Zone Air Heat Balance Surface Convection Rate
  • Zone Air Heat Balance Interzone Air Transfer Rate
  • Zone Air Heat Balance Outdoor Air Transfer Rate

minus the Zone Air Heat Balance Air Energy Storage Rate.

Based on the sentences, what I understand is that the zone air heat balance system air transfer rate should be matched with the zone cooling coil sensible cooling rate, is that correct?

However, when I check the number, there is always a constant difference between those number in each day. Am I using the right parameter to compare? Or there is any parameter that I need to check in order to have the same number with system air transfer rate?

Thank you

(EDIT) image description

For this model, I tried to define all the equipment that would be used in the building, and not using the autosize number for the equipment characteristics.

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I take it the cooling coil rate is greater than the Zone Air Heat Balance System Air Transfer Rate? Can you provide an approximate percent difference?

My first instinct was that it's not capturing solar gains (from the list you included above); however, I think that is either wrapped in with internal gains or winds up being a contributor to surface convection rate (I'm getting my software mixed up).

I'd also confirm that 'Outdoor Air Transfer Rate' includes both ventilation and infiltration air.

jmbattis gravatar imagejmbattis ( 2018-11-29 10:04:52 -0600 )edit

I edit my question with the calculation data of the heat air balance of my model. The cooling coil rate is lower compared to zone air heat balance system air transfer rate as you could see in the image. I have more than one thermal zone in my model, and for each thermal zone, the percentage is different. I am trying to understand where does the difference come from.

zahro gravatar imagezahro ( 2018-11-29 19:48:29 -0600 )edit

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answered 2018-11-29 19:55:01 -0600

The Cooling Coil Sensible Cooling Rate will include fan heat and OA load if present. Otherwise this report should match the energy transfer to the zone. The air transfer rate is calculated as Q = Mdot * Cp * Tsup - Tzone while coil Q = Mdot * Cp * Tin - Tout. So as long as Tcoil, in = Tzone, and there is no fan heat then these reports should match.

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Then if I understand it correctly, the Cooling Coil Sensible Cooling Rate should be higher compared to the Zone Air Heat Balance System Air Transfer Rate because it contains fan heat and OA load. However, as you could see in the excel calculation, the number is always lower. It seems that there is something wrong in my model. Would you have any suggestion which input parameter do I need to check?

zahro gravatar imagezahro ( 2018-11-29 20:02:40 -0600 )edit

I am sorry for not explaining earlier, for this model, I used a defined number based on the equipment datasheet not using the auto-sized number. However, now I am still learning to define good input of the system inside energy plus, and trying to figure it out how this could happen. I am checking with the auto-size number, and the Zone Air Heat Balance System Air Transfer Rate matched with Zone Predicted Sensible Load to Cooling Setpoint Heat Transfer Rate and have quite constant difference with Cooling Coil Sensible Cooling Rate which I suspect comes from fan heat gain.

zahro gravatar imagezahro ( 2018-11-30 02:35:25 -0600 )edit

answered 2018-12-10 11:14:47 -0600

Haohan Sha gravatar image

The unclear results always a problem for the user of energyplus. The problem you described may have different reasons, so you need to check and show more details in your model. For example: Can you show how you define the cooling coil? checking the sizing:parameter, did you change it to smaller one? or you have other air-conditioning devices in the zone?

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Asked: 2018-11-29 03:04:07 -0600

Seen: 298 times

Last updated: Dec 10