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

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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.