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MarketManager » Modeling HVAC Systems
Q: How to deal with fan CFM and kW when modeling air-cooling units and PTAC systems?


While aircooling is indeed a "zonal system" its equipment can be modeled either as zonal (one aircooling unit attached to each zone under this system) or as central (one, bigger, aircooling unit attached to PLANT, which would be in lieu of all the individual units).

Regarding fans: If the aircooling unit is attached to PLANT (central equipment) then the supply cfm assumed is given by a FAN attached to SYSTEM and its schedule(s). If no fan is attached to SYSTEM then the indoor fan specified in the aircooling unit takes over. If you do have a fan attached to SYSTEM, make sure you zero out the indoor fan KW only (to avoid double-counting), not the fan cfm. The fan cfm is used as a reference for the performance of the unit (see the sensitivities of output capacity and input power with respect to air flow). In other words, this cfm should be set to the value for which the unit performance is specified in spec sheets, which may or may not equal the cfm of the supply fan attached to SYSTEM.

If the aircooling units are modeled as zonal equipment, i.e. there is one separate unit attached to each zone, then the actual supply air flow cfm for each zone is assumed to be that specified in the aircooling unit.

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