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Why is DX coil's rated airflow per rated cooling/heating capacity limited?

asked 2022-10-16 01:46:33 -0600

Keigo's avatar

updated 2022-10-16 09:30:06 -0600

For DX cooling/heating coils, the rated air volume flow rate per watt of rated total cooling/heating capacity is limited from 0.00004027m3/s/W to 0.00006041 m3/s/W (300 to 450 cfm/ton) (I/O Reference, Engineering Reference).

Most VRF indoor units can switch airflow like H/L or H/M/L, but normally, only one power consumption is on catalogues or technical data sheets, so I regard the power consumption the maximum fan power with the highest airflow rate i.e., I input the highest airflow rate to Cooling Supply Air Flow Rate and Heating Supply Air Flow Rate in ZoneHVAC:TerminalUnit:VariableRefrigerantFlow. The drain pump power may be included in the power consumption, but I ignore it unless I can identify it. And I input the lowest airflow rate to No Cooling Supply Air Flow Rate and No Heating Supply Air Flow Rate in ZoneHVAC:TerminalUnit:VariableRefrigerantFlow.

The problem is that the highest airflow rate in product catalogues often exceeds the upper limit of 0.00006041 m3/s/W. The airflow constraints are not in line with the actual products. Below is an extract from DAIKIN's catalogue as an exapmle.

image description

I have a few questions:

  1. Why does EnergyPlus have lower and upper limits for DX coil's rated airflow rate per cooling/heaitng capacity?
  2. Will the limits be relaxed or eliminated in the future?
  3. For now, should we use the airflow that meets the limits rather than the maximum airflow from the multiple airflow listed in product catalogues?
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Some discussion as to 'why' here: https://github.com/NREL/EnergyPlus/is...

ericringold's avatar ericringold  ( 2022-10-16 11:04:40 -0600 )edit

@Eric Ringold Thank you for sharing the thread. @rraustad says "the limits are there to protect the user from operating a DX system out of range on flow rate and is based on review of manufacturers data." If that's the case, the data is old and should be updated. I know that too low airflow rate per capacity may cause coil freezing, so it is reasonable to have a lower limit for the rated airflow per capacity, but we don't need to have the upper limit of 0.00006041m3/s/W (450cfm/ton).

Keigo's avatar Keigo  ( 2022-10-16 12:48:03 -0600 )edit

and.... I don't think E+ crashes when you exceed the ranges. Instead, it gives MANY warnings. If you have reviewed the situation and are confident it's acceptable, then I think you can ignore the warnings

Jim Dirkes's avatar Jim Dirkes  ( 2022-10-17 05:53:00 -0600 )edit

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answered 2022-10-17 06:26:07 -0600

The catalog data you show has a range of possible air flow rates. However, this is the same coil. When a DX coil is "rated" it is tested at a flow rate very near 400 cfm/ton. This coil can then be used in a wide range of applications with a lower or higher operating flow rate. So the coil is rated at a reasonable air flow per capacity and can operate at a wider range of air flow per capacity. This raises the question of autosizing a DX coil. The model looks to see what the air flow per capacity is and adjusts the DX coil capacity to provide a value within the required range. You could also hard-size the DX coil performance while operating that coil outside the rating air flow per capcity specification.

The rated flow per capacity range is as you show:

Real64 constexpr MaxRatedVolFlowPerRatedTotCap1(0.00006041); // m3/s per watt = 450 cfm/ton
Real64 constexpr MinRatedVolFlowPerRatedTotCap1(0.00004027); // m3/s per watt = 300 cfm/ton

The min/max operating flow per capacity range for cooling and heating are:

Real64 constexpr MaxHeatVolFlowPerRatedTotCap1(0.00008056);  // m3/s per watt = 600 cfm/ton
Real64 constexpr MaxCoolVolFlowPerRatedTotCap1(0.00006713);  // m3/s per watt = 500 cfm/ton
Real64 constexpr MinOperVolFlowPerRatedTotCap1(0.00002684);  // m3/s per watt = 200 cfm/ton

These limits do cause a problem when autosizing DX coil as previously described. For example, when a DX coil is in the outdoor air stream the coil entering enthalpy can be quite high which causes a calculation of high capacity while the air flow could be quite low. For this reason the 100% OA DX coils have a different rated and operating air flow per capacity range.

Rated air flow per capacity range:

Real64 constexpr MaxRatedVolFlowPerRatedTotCap2(0.00003355); // m3/s per watt = 250 cfm/ton
Real64 constexpr MinRatedVolFlowPerRatedTotCap2(0.00001677); // m3/s per watt = 125 cfm/ton

Operating air flow per capacity range:

Real64 constexpr MaxHeatVolFlowPerRatedTotCap2(0.00004026);  // m3/s per watt = 300 cfm/ton
Real64 constexpr MaxCoolVolFlowPerRatedTotCap2(0.00004026);  // m3/s per watt = 300 cfm/ton
Real64 constexpr MinOperVolFlowPerRatedTotCap2(0.00001342);  // m3/s per watt = 100 cfm/ton

There has been no discussion of changing these ranges although the issue with autosizing may require some action. You can always apply a DX coil performance "rating" and then operate outside that range.

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Thank you for your detailed explanation. I mistakenly thought that if the air flow per capacity exceeded the upper/lower limits, it would be forcibly corrected to the upper/lower limits. So, the rated air flow per capacity range is not critical. The operating air flow per capacity range is critical, but it is wide enough. We don't have to worry so much about the ranges.

I have one more question. How does EnergyPlus know if a DX coil is for 100% OA or not? Do we need to set 100% Outdoor Air in Cooling and 100% Outdoor Air in Heating "Yes" in Sizing:System?

Keigo's avatar Keigo  ( 2022-10-20 21:53:22 -0600 )edit

Look in the parent object for the 100% OA coil choice. I only see this input in UnitarySystem If air flow per capacity exceeds RATED limits an adjustment is made to the coil size. If air flow per capacity exceeds OPERATING limits a warning is issued.

AirLoopHVAC:UnitarySystem, A16, \field Use DOAS DX Cooling Coil \type choice \key Yes \key No \default No \note If Yes, the DX cooling coil runs as 100% DOAS DX coil. \note If No, the DX cooling coil runs as a regular DX coil. \note If left blank the default is regular dx coil

rraustad's avatar rraustad  ( 2022-11-15 13:44:55 -0600 )edit

Q: Do we need to set 100% Outdoor Air in Cooling and 100% Outdoor Air in Heating "Yes" in Sizing:System?

These are used for the zone heat balance during zone sizing. These inputs only affect the zone loads, which in turn will affect the size of the coil.

rraustad's avatar rraustad  ( 2022-11-15 13:46:11 -0600 )edit

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Asked: 2022-10-16 01:46:33 -0600

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Last updated: Oct 17 '22