There are actually 2 limits on DX coil flow/capacity. One is for rated capacity/sizing, and the other is for operating performance.
The limits are as follows:
1) min/max rated flow/capacity
2) max flow/capacity in cooling or heating mode
3) min flow/capacity in either mode
// Airflow per total capacity range (Regular DX coils)
Sizing Constraints:
Real64 const MaxRatedVolFlowPerRatedTotCap1( 0.00006041 ); // m3/s per watt = 450 cfm/ton
Real64 const MinRatedVolFlowPerRatedTotCap1( 0.00004027 ); // m3/s per watt = 300 cfm/ton
Operating Constraints:
Real64 const MaxHeatVolFlowPerRatedTotCap1( 0.00008056 ); // m3/s per watt = 600 cfm/ton
Real64 const MaxCoolVolFlowPerRatedTotCap1( 0.00006713 ); // m3/s per watt = 500 cfm/ton
Minimum limit:
Real64 const MinOperVolFlowPerRatedTotCap1( 0.00002684 ); // m3/s per watt = 200 cfm/ton
What this means is that the DX coil can be sized to meet the coil sizing requirements, and then simulated to meet the operational requirements (which may exceed the rated requirements).
If you autosize a DX coil you will get a capacity and air flow rate that are constrained to the min/max "rated" flow/capacity limits.
Coil:Cooling:DX:SingleSpeed, ( child object that is used in parent "wrapper")<br>
Furnace ACDXCoil 1, !- Name<br>
FanAndCoilAvailSched, !- Availability Schedule Name<br>
autosize, !- Gross Rated Total Cooling Capacity {W}<br>
autosize, !- Gross Rated Sensible Heat Ratio<br>
3.0, !- Gross Rated Cooling COP {W/W}<br>
autosize, !- Rated Air Flow Rate {m3/s}<br>
If you also autosize the parent object (one that also has a flow rate input), then you will get the same capacity and flow values as with the child component. The parent objects do not have a capacity input since the child component specifies that performance aspect.
However, if you do not autosize the parent object, and that parent has a flow rate input field, then you can operate the coil "outside" the rated flow/capacity limits.
If the parent object does not provide a flow rate input, then the only constraint is with the child component (i.e., the DX coil) and you cannot exceed the rated flow/capacity limit. If the parent does have these inputs, then you can operate the DX coil outside the rated limits.
As an example, the DX coil shown above was either autosized, or hard sized. That coil was then configured in a unitary system parent object. The parent then selects the air flow rate to operate during the simulation. During sizing, the child (the DX coil) uses the "rated" flow/capacity limits. During the simulation, the parent uses the operating flow rate specified.
This way one can select a component with certain design specifications, and operate that component at "off design" operation. For example, I pick a coil with 400 cfm/ton, and then operate that coil at 450 cfm/ton.
Assume here that the DX coil had a 1.5 m3/s design flow rate. In this parent a flow rate of 1.6 m3/s is specified.
AirLoopHVAC:Unitary:Furnace:HeatCool, ( parent object that ...
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