General Overview
A compressor is one of the main components of a chiller. A constant-speed compressor operates at a constant speed to raise the vapor pressure of the refrigerant and power the refrigeration cycle of the chiller.
| Plant | System | Component |
|---|---|---|
| Air-cooled Chilled Water Plant | Air-cooled Chiller | Chiller Compressor |
| Water-cooled Chilled Water Plant | Water-cooled Chiller | Chilled Compressor |
Evaluation of Energy Transfer
The primary energy source for a constant-speed compressor is the electricity used to run the compressor motor. Table 2 provides a summary of measurements needed to quantify the annual power consumption and operating characteristics of the constant-speed compressor and motor.
| Component Quantification | Values to be Quantified | Measurement |
|---|---|---|
| Electricity usage of a constant-speed compressor motor |
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Measurement Strategy
The measurement strategy for a constant-speed compressor and motor involves a one-time measurement of power draw and long-term monitoring of the motor’s operational schedule. The approach assumes that the power draw stays constant throughout the measurement period. Because the motor runs at constant speed and is under a constant load, it is either operating at full power (when on) or it is drawing no power (when off).
A motor on/off data logger is used to record the operating schedule. The true RMS power is measured at main feed to the constant-speed motor. Measurement locations are generically represented in Figure 1.
Measurement Equipment
| Equipment | Description | Measurement (Units) |
|---|---|---|
DENT 16” RoCoil Flexible Rope Current Transformers (CT-R16-A4-U) |
Provides a measurement of true RMS power from voltage and current inputs and records long-term power (kW) and energy (kWh) measurements. Requires ELOG19 software and a USB connection cable for programming and downloading data files. | True RMS Power (kW) |
|
Records outdoor air temperature and relative humidity using internal sensors. Requires HOBOware software and a USB connection cable for programming and downloading data files. |
Calculation Methodology
The general methodology for quantifying the electricity consumption of a constant-speed compressor and motor involves measuring their electrical energy use. Depending on operational variability, daily or weekly models incorporating cooling load or a proxy (such as outdoor air temperature) may be developed to better characterize consumption. The typical energy use of a constant-speed compressor is estimated using its simulated yearly schedule. A more detailed explanation of the calculation methodology is provided below.
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Further Reading
- ASHRAE. (2020). HVAC Systems and Equipment. Atlanta, GA: ASHRAE.
