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Corrections to Tested Performance: Sensible Capacity

Figure 1a. Graphical illustration of the definition of the bypass factor: ratio of segment yellow-blue
to segment red-blue.

 

Figure 1b. Graphical illustration of the iterative technique used to determine
the sensible to total ratio from a given bypass factor and total capacity. This
method answers the question: at what S/T ratio does the cooling process preserve
the bypass factor that is determined at ARI rating conditions.

Figure 2. Total and sensible capacity data normalized by corresponding values at ARI
rating conditions. This graph shows the stronger dependence of the sensible capacity on
the humidity of the entering (mixed) air.

 

 

  • Sensible / Total
    • The sensible/total capacity ratio is determined by an iterative technique based on the apparatus dewpoint and bypass factor method (ADP/BPF). This method is dependent on: the S/T ratio (at rating conditions), air flow rate, total capacity, ODB, EWB, and EDB.
    • This iterative technique but can represented functionally as:
      S/T = S/TADP(ODB, EWB, EDB)
  • Sensible capacity is the product of this ratio and the total capacity.
    • SC = S/TADP(ODB, EWB, EDB) * TC(ODB, EWB)

Discussion

In making predictions of sensible capacity at conditions differing from ARI test conditions, the calculation engine makes use of the iterative apparatus dewpoint and bypass factor method. This method acts to first characterize the cooling coil with a bypass factor based on its sensible and total capacity at ARI test conditions (see Figure 1a). This bypass factor can then be used to predict the sensible to total ratio of the coil at entering conditions, flow rates, and total capacity levels, other than those at ARI test conditions (see Figure 1b).

There is a separate system performance calculation page that serves to dynamically illustrate the ADP/BPF method described here. The results shown in the "S/T" columns are the result of the iterative calculation using the ADP/BPF method. Of the three variations of this S/T result, the S/T (BP-EP) is the one actually used in calculation engine. This version has algorithmic similarities with the form of the method that is used in the EnergyPlus system simulator.

The letters "S" stands for supply, "E" for entering, and "O" for outside, "HR" for humidity ratio, "DB" for drybulb, and "WB" for wetbulb. The "BPF" factor, shown at the top row of the second table, is the calculated bypass factor at the user entered ARI conditions. All columns to the right of and including the "BPF" column are calculated with the ADP/BPF method.

To run the calculation, enter system performance data (at ARI test conditions) in the text boxes in the top row and then press the submit button. This produces a table of predicted coil performance for a variety of entering conditions. The second row of inputs allows you to change the capacity level and air flow rate that the projection table is based on (by default, these values are left equal to the ARI conditions specified in the first row of inputs).