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MarketManager » Fans
Q: How does MarketManager calculate Fan kW when VSDs are used?


To calculate Fan kW for VAV systems, MarketManager does the following:

  1. Determine PLR from dividing the actual fan cfm by the full-load (or rated) fan cfm
  2. Plug this PLR into the Part Load efficiency curve
  3. Multiply the full-load (or rated) fan kW by the result of (2).

The fan laws are applicable only under ideal conditions of, for example, holding static pressure constant while varying cfm and fan horsepower. In this case we deal with fan kW, not HP (Fan kW = Fan HP / motor efficiency) and motor efficiency is not constant over the range of loads. Also, static pressure is not expected to stay constant.

All boils down to an empirical relationship that is different from the ideal fan laws. The VSD drive is the least different, inlet vanes or discharge dampers are most different.

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