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Unitary Air Conditioner Cost Estimator

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Establishing Loads: Sensible Load Lines

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  • Establish high point of the total sensible load line
    • Total sensible load = Total sensible capacity @ design conditions
      • Use rated sensible capacity corrected to design conditions
  • Establish two points for the non-ventilation line
    • Subtract off Sensible Ventilation Load at design conditions
      • SVL = f (ventilation mass flow, tout - tin)
    • Solar and internal gains point (the intercept)
      • Value is calculated based on user's selection of building type and location. Calculation is based on EnergyPlus load models for ASHRAE building types.

Discussion

The RTU Comparison Calculator uses weather data to conduct a binned energy analysis for the rooftop unit in cities across the United States. Weather tape data (outdoor dry-bulb and coincident web-bulb) was binned in 5 degree increments and filtered by the selected occupancy schedule. The result is a database of hours (in each bin) and coincident wet-bulb temperatures for each city and occupancy schedule combination.

The first step in the binned analysis is to determine the air-conditioning load for each weather bin. Since building characteristics are not explicitly entered for the RTU Comparison Calculator, building loads must be inferred by assuming the unit would be installed in a building suitable for its specified capacity. Load behaviors for this hypothetical building are established via a "Total" sensible load line and the "Non-ventilation" sensible load line. These load lines are defined by the yellow line markers in the figure to the right.

Fundamental to the cost estimator is the assumption that total sensible loads and sensible capacity are balanced at design conditions. This balance is achieved by scaling a linear building-load model until the load equals the unit's capacity at design conditions.

Another principal concept is that internal gains and solar loads contribute to the total sensible load. This essentially produces an intercept in the load model such that there is a cooling load even when the outdoor temperature matches the setpoint. This two-point nature to the load model allows for quantitative adjustments to better reflect various building types. For example, the building-type control allows the user to select a building type which is internally dominated and insensitive to weather (high internal gains as with a multi-story office building) or one more coupled to the weather (low internal gains as with a warehouse). The EnergyPlus load models for each building type can be manually edited to produce custom versions of each building-load model. The analysis behind the load models can be reviewed in this document. There is also related discussion in the help for the building-type control.

The Comparison Calculator uses a sensible load analysis formulation. Humidity impacts on performance are accounted for as they affect the unit's sensible capacity to meet the total sensible load (and also the effects on system power draw). Interior humidity conditions can be set to automatically track the outside conditions (by assuming inside humidity ratio is equal to outside humidity ratio), or they can be set to a constant (relative humidity) value. Note that energy usage associated with other humidity controlling devices is not included in the estimates by the Comparison Calculator.