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Equipment Response To Loads: An Example

                 Wichita, KS: Equipment Performance

  • Sensible capacity corrections:
    Steady-state correction due to environmental conditions (ODB, EDB, EWB). Entering mixed air conditions are calculated based on ventilation rate and economizer usage.
  • Load Fraction = Sensible load / Sensible Capacity
  • Part load performance:
    • Linear relationship established by the user specified degradation factor:
      100% capacity at 100% load fraction
      100% - DF% capacity at 0% load fraction
    • Non-linear model (spreadsheet interface only):
      Function of load fraction and outside drybulb.
  • Runtime = Load Fraction / Degradation Factor
  • Staging
    • Units capacity is (optionally) split equally into two stages
    • Load spills to second stage if runtime of first stage is greater than 1 hour.
  • Condenser power draw corrections
    • Steady-state correction due to environmental conditions (ODB, EWB)
  • Evaporator fan power
    • Default evaporator fan: Power_kWatts = (0.0132 * TotalCapacity_kBtuh) - 0.2283
    • User specified on controls page
    • For multi-speed fans, the fan affinity laws are used to estimate fan-power draw at part-load conditions
      • Power_PartLoad = Power_FullLoad * ((CFM_PartLoad)/(CFM_FullLoad))^3
  • Evaporator fan and compressor control strategies:
    1-Speed Fan: Always ON
    1-Speed Fan: OFF When Unoccupied (i.e., always ON when occupied)
    1-Speed Fan: Cycles With Compressor
    2-Speed Fan: Always ON
    2-Speed Fan: OFF When Unoccupied (i.e., always ON when occupied)
    2-Speed Fan: Cycles With Compressor
    Variable-Speed Compressor and Fan: Always ON
    Variable-Speed Compressor and Fan: OFF When Unoccupied


The unit's response to the load starts with total and sensible capacity corrections. Corrections due to environmental conditions (outside [drybulb] and entering mixed air [drybulb and wetbulb]) are applied to the ARI rated gross total capacity. Sensible load fractions are then modified by part load degradation factors to calculate run times.

The unit's capacity is split equally into two stages. Load not handled by the first stage spills into the second stage. Runtimes are converted into condenser energy usage through corrected condenser power values.

Evaporator-fan energy is calculated and added to the units total energy usage estimate. Fan energy calculations are dependent on occupancy schedules, fan and compressor controls strategies, and compressor staging. The following are details associated with the fan/compressor control strategies:

  • A single-speed fan system runs at one speed during both single-stage and two-stage compressor operation. This system, if there are two stages, is assumed to have a face-split evaporator design (i.e., essentially two side-by-side evaporator coils subject to parallel air flow from one blower fan). The face-split design yields first-stage dehumidification performance equivalent to that when both stages are active.
  • The two-speed choice is a valid option only when the unit has two stages. A two-speed fan system has a lower fan speed when running only the first stage. This system is assumed to have either a row-split or interlaced evaporator design (serial air flow from one two-speed fan). Based on fan affinity laws, power consumption at the lower fan speed (1/2 the air flow of the high-speed mode) is assumed to be one eight ((1/2)^3=1/8) of the high-speed power specified in BFn field on the controls page. Note: this control is only visible when advanced controls are turned on. When only ventilating, the two-speed system runs at its lower fan speed.
  • The variable-speed choice assumes the system runs both fan and compressor at capacities proportional to the load. At reduced load, capacity and fan air flow are reduced together, in such a way that dehumidification performance is preserved and approximately equal to that at full load operation. This system is assumed to have either a row-split or interlaced evaporator design (serial air flow from one multi-speed fan). Capacity unloading can be achieved by multiple stages or other variable capacity methods. As with the two-speed choice, fan affinity laws are used to model the reduction in fan power at reduced flow rates (e.g. at 1/3 load, the blower fan is assumed to draw (1/3)^3 = 1/27th of the full-load blower-fan power). A minimum fan speed is set based on the ventilation control. If the ventilation control is set for 10%, then the fan will run at 10% of capacity when there is no call for cooling. The cycling degradation factor is set to zero when either of these variable speed modes is selected.
  • "Always ON" runs the fan at all times (all day, all week), regardless of the occupant schedule. "Cycles With Compressor" runs the fan only when the compressor is running. "OFF When Unoccupied" runs the fan at all times when the building is occupied.

The chart of equipment performance (above right) is generated when the "Show bin calculations" option is selected. This a chart of some of the columns in the bin-calculations tables.

  • The red (TCF) line is the correction to the total capacity.
  • The green (PCF) line is the correction to the system power draw.
  • The brown line is the S/T ratio.
  • The yellow line (OCF) represents the overall effect of all the environmental corrections (to system power draw, to system capacity, and to the sensible to total ratio). Higher values of OCF correspond to higher values of consumption.

    OCF = PCF / (TCF * S/T_CF),
    S/T_CF = S/Tentering / S/TARI
  • The two blue lines are the runtimes for the two stages.
  • The two black lines with square markers indicate system energy usage (and are normalized to allow plotting with the correction factors). The line with green markers is the consumption from the condenser. The line with red markers is the consumption by the blower fan.