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Working With It - Operating


Most manufacturers publish performance ratings on their various models. These ratings are usually specified by a model number at a fixed air flow in cubic feet from minute (cfm) with the following variables:

  • Several fluid flow rates in gallons per minute;
  • Entering fluid temperatures from 25 to 100 degrees;
  • Entering air temperatures at 80 degrees dry bulb and 67 degrees wet bulb for cooling; and
  • Entering air at 70 degree dry bulb for heating.

The following rating values are given for cooling for each of the above variables:

  1. Total capacity in Btu per hour;
  2. Sensible heat capacity in Btu per hour;
  3. Heat rejection to the loop in Btu per hour;
  4. Power input in watts; and
  5. Energy efficiency rating (EER), which is the total capacity divided by the power input, or Btu per watt.

Also, for each of these variables, the following rating values are given for heating:

  1. Total capacity in Btu per hour;
  2. Heat absorption from the loop in Btu per hour;
  3. Power input in watts;
  4. Coefficient of Performance (COP), which is the total capacity divided by the power input converted to Btu per hour, and multiplied by 3.412 Btu per hour per watt; and
  5. Performance includes provision for an antifreeze solution used at 35 degrees and below entering water temperatures, but not the pumping penalty for the antifreeze use.

It's important not to extrapolate from the data tables provided by the manufacturers. Extrapolation removes the data from the context and boundaries of the table, and is not a good engineering practice. This is why most manufacturers say that interpolation between ratings within a table is permissible, but extrapolation is not.

Other published data typically includes:

  1. Correction factors given for various other entering air conditions, dry and wet bulb, and other air flow ratings.
  2. The unit water pressure drop is also published for each flow rate, in either feet of water or pounds per square inch.
  3. Blower performance including fan motor brake horsepower and external static pressure capability.
  4. Electrical data on voltages and current draw.
  5. Physical data including operating weight and refrigerant charge.
  6. Hot water generating capacity.

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