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DX coil sizing and part-load flow component

asked 2015-11-04 00:33:51 -0500

ngkhanh's avatar

updated 2017-03-09 02:12:35 -0500

1/ I continuously get error for autosize a 2 Stage DX cooling coil with dehumidification control if I enable DX DOAS option in CoilSystem: DX cooling. The error describes following statements:

COILPERFORMANCE:DX:COOLING "DOAS DEHUMID PERF 1" -- coil bypass factor calculation did not converge after max iterations.The RatedSHR of [0.556], entered by the user or autosized (see *.eio file), may be causing this. The line defined by the coil rated inlet air conditions (26.7C drybulb and 19.4C wetbulb) and the RatedSHR (i.e., slope of the line) must intersect the saturation curve of the psychrometric chart. If the RatedSHR is too low, then this intersection may not occur and the coil bypass factor calculation will not converge.

I use all autosize option, it's a DOAS airloop system with ventilation requirement sizing and Cooling load control - VAV I read that it's an energyplus error so how to pass this ? May I hard-size the DX coil and SHR or change the performance curve ?

2/ When I try to reduce the airflow of fan at night when DX coil still work for dehumidification, there are a lot warning for frost coil condition while the dehumidifying load at night is very low. How to solve this problem for DX coil with variable flow fan ?

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answered 2015-11-04 11:49:58 -0500

1) If you draw a straight line between a DX coil's entering air conditions and leaving air conditions, then let that line continue to the saturation curve, the intersecting temperature is called the Apparatus Dew Point. This represents the temperature at which air would be cooled to IF the DX coil was 100% effective (reference HERE). The coil bypass factor describes how close the actual leaving conditions are to the coil's apparatus dew point. However, if the straight line between the DX coil's entering air conditions and leaving air conditions doesn't intersect the saturation curve, there is no apparatus dew point and the coil bypass factor can't be calculated.

From the Engineering Reference for Rated SHR during DX Coil Sizing in EnergyPlus, the rated inlet air condition used to size the coil is ALWAYS T_DB = 26.7C and T_WB = 19.4C. Since that can't change, and since you are auto-sizing rated SHR, then EnergyPlus is calculating SHR based upon your supply air conditions. Supply air conditions are set inSizing:Zone and Sizing:System objects depending upon if your DX coil is part of zone HVAC equipment (PTAC, Window AC, etc.) or a central air system. Try adjusting your design outlet air state in these objects to be colder and/or more humid.

2) Frost coil conditions often happen when a DX coil is operating with cold inlet air or low air flows (can read more about freezing coil causes HERE). Since you're operating at night and forcing reduced air flow, you have both impacts together. There are a few things I would check:

  • I'm guessing that you're trying to use only the first stage of your 2-stage DX coil for night dehumidification. Either your first stage capacity is too low, and the second stage is also operating during night dehumidification, or your first stage capacity is too high and causing coil freezing.
  • There is an input field "Outdoor Air DX Cooling Coil Leaving Minimum Air Temperature" at the end of the CoilSystem:DXCooling object. Make sure this value isn't too low (E+ default value is 2.0 C).
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Thank for your repies, I succeed pass-through DOAS coil sizing by lowering main cooling supply temperature but it seems not be a good ideal when the energy consumption increased due to higher cooling coil load and sometimes it make the zone get overcooled. i checked conditons, the rated flow, which is bigger than normal DOAS DX setting, maybe make me confuse. 2/ How to solve problems with staging of DX coil ? will change bypass fraction in coil performance will work or we need change to new DX performance curve? Thanks

ngkhanh's avatar ngkhanh  ( 2015-11-05 13:43:19 -0500 )edit

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Asked: 2015-11-04 00:33:51 -0500

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Last updated: Mar 09 '17