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Unitary Air Conditioner Cost Estimator

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Establishing Loads: An example

                Wichita, KS: Loads and Hours

  • Economizer
    • Sensible Load Reduction = c(Vcap-Vvent)(Tin - Tout)
  • Hours of operation from weather tapes
    • Schedule acts as filter
    • Binned by dry bulb (with coincident web bulb)

Discussion:

Fan capacity, not used in ventilation, can optionally be used in economizing the load. The economizer reduces the sensible load, at outdoor temperatures below the set point, by bringing in additional ventilation air. In the example chart (produced by the RTU Comparison Calculator) for Wichita, the "After Economizer" trace shows the final load data that will drive the calculation.

The final step in presenting the load data to the unit is to determine the number of hours in which the unit experiences that load. In the example chart for Wichita, with a schedule of Monday - Friday, 7am - 7pm, the hour distribution, as binned by outside temperature, is shown normalized, with a fine red line (max value = 254 hours). This line is labeled "ActCond Hrs", for ActiveCondenser Hours. The potential total hours, as would be the case if no schedule were filtering the hours, is shown by the fine dark-brown line, labeled "Total Hours." This Total Hours trace is displayed for use as a reference to give the user a sense of the impact of the filtering by the schedule. When an "All Day, All Week" schedule is selected, these two traces (filtered and unfiltered) are identical and lay on top of each other.

When considering how unit performance varies from city to city it is useful to keep in mind that the load on the unit is scaled to match the capacity at design conditions. As a result of this explicit balance, design temperature is not by itself a primary driver of energy usage, but rather two associated weather characteristics have more impact:

  1. Hour count in the cooling bins (longer cooling season means more energy use).
  2. The skew of the hour distribution (the more skew toward high temperatures the more time at high load and low capacity; the result is longer run times per bin hour).

The RTU Comparison Calculator reports calculation results for any bin where there is a positive cooling load (before economizer reductions).