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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 (S&I) point
      • Value is calculated based on users selection of building type and location. Calculation is based on EnergyPlus load models for ASHRAE building types.
      • Daily average S&I load is assumed constant throughout the cooling season.

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 schedule combination.

The first step in the binned analysis is to determine the 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. Assumptions behind these points are in the outline above.

Fundamental to the cost estimator is the assumption that total sensible loads and sensible capacity are balanced at design conditions. This balance at design conditions, is achieved by adjusting the conduction component (see blue line in chart). If the ventilation parameter is increased by the user, the conduction component decreases (as if the building were better insulated). Note that the conduction line and the non-ventilation line only differ by an additive constant, the S&I level (see paragraph below). Ventilation levels (user selections) are not allowed to exceed the point that would cause the conduction component to be negative (better than a perfect insulator).

The Comparison Calculator uses a sensible load analysis formulation. Humidity impacts on performance are accounted for as they affect the units 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.

Another principle concept is that solar and internal gains contribute to the total sensible load. This daily average S&I load, is assumed to be constant throughout the cooling season (independent of outside conditions). This feature of the RTUCC allows for qualitative adjustment of the loads to better reflect building types. An S&I control factor allows the user to adjust loading to be internally dominated and insensitive to weather (high S&I; multi-story offices building) or more coupled to the weather (low S&I; warehouse). By default, both the S&I factor and ventilation flow level are automatically calculated base on EnergyPlus load models, but can be manually adjusted via the S&I control or a custom building-load model. The analysis behind the automatic calculation of these values can be reviewed in this document. There is also related discussion in the help for the building-type control.