General Overview
A constant-speed, constant-volume (CSCV) fan uses a power-driven rotating impeller to circulate air at a single speed. Fans can be either axial or centrifugal.
Table 1 shows the plant and system configurations that may contain a CSCV fan and motor and the most common respective controlling variables.
| Plant | System | Component | Controlling Variable |
|---|---|---|---|
| Air-cooled Chilled Water Plant | Air-cooled Chiller | Condenser Fan | Outdoor air temperature (F) |
| Water-cooled Chilled Water Plant | Cooling Tower | Cooling Tower Fan | Wet-bulb temperature (F) |
| Air Handling Unit | AHUs | AHU Supply Fan | Motor schedule and/or Outdoor air temperature (F) |
| Air Handling Unit | AHUs | AHU Return Fan | Motor schedule and/or Outdoor air temperature (F) |
|
Boiler | Burner fan | Motor schedule and/or Outdoor air temperature (F) |
Evaluation of Energy Consumption
The primary energy source for a CSCV fan is the electricity used to run the fan motor. Table 2 provides a summary of measurements needed to quantify the annual energy consumption and operating characteristics of the CSCV fan and motor.
| Component Quantification | Values to be Quantified | Measurement |
|---|---|---|
| Electricity usage of CSCV fan motor (kWh) | Average hourly true RMS power of the motor | True RMS power of motor that drives the fan (kW) |
| Controlling variable, non-weather dependent | Yearly operating schedule | Fan motor runtime (hours) |
| Controlling variable, weather dependent | Average hourly outdoor air temperature (OAT) | Outdoor air temperature (F) |
Measurement Strategy
The measurement strategy for a CSCV fan and motor involves a one-time measurement of true RMS power and long-term monitoring of the motor’s operational schedule. This approach assumes that true RMS power remains constant throughout the measurement period. Since the motor runs at a constant speed and is assumed to be under a constant load, it either operates at full power when on or draws no power when off. A motor on/off data logger records the operating schedule. True RMS power is measured at the main feed to the constant-speed motor. Measurement locations are generically represented in Figure 1.
If the fans are in a modular configuration (assuming they all run at the same speed), then only one fan needs to be measured, provided substantiating documentation from the building automation system (BAS) shows that all cells are operating equally at the same time. If fans are further staged, all fans should be measured.
In some cases, the motor’s operational schedule is related to the facility’s heating or cooling load. OAT can serve as a proxy variable for this load and can be measured onsite or obtained from a nearby weather station.
Measurement Equipment
Table 3 provides the equipment required to carry out the measurements of this component.
| Equipment | Description | Measurement (Units) |
|---|---|---|
|
Records outdoor air temperature and relative humidity using internal sensors. Requires HOBOware software and a USB connection cable for programming and downloading data files. | |
|
Records when a motor is on and off, as well as runtime. Requires HOBOware software and a USB connection cable for programming and downloading data files. | Motor Runtime (minutes) |
|
Multi-purpose electrical measurement tool used to take true RMS power readings over a short period of time. | True RMS Power (kW) |
|
Used in conjunction with the CTV series of sensors. Records measurements from up to four CTV sensors. Requires HOBOware software and a USB connection cable for programming and downloading data files. | Electrical Current (Amps) |
Calculation Methodology
The general methodology for quantifying the energy consumption of a CSCV fan motor involves measuring the true RMS power of the motor. The estimated annual energy consumption of a CSCV fan is estimated using the simulated yearly schedule of the fan. Many CSCV fans run on a set daily or weekly schedule.
However, the yearly schedule may depend on outdoor air temperature (OAT). If so, operating hours can be regressed against OAT to develop a regression model. Depending on operational variability, daily or weekly models may be created to better characterize the component. This model is then applied to climate normal year data to estimate the typical annual operating schedule, which is used alongside true RMS power to calculate the estimated annual electricity consumption.
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Further Reading
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Boyd, BK.; McMordie Stoughton, KL.; Lewis, T. (2017). “Cooling Tower (Evaporative Cooling System) Measurement and Verification Protocol.” Golden, CO: National Renewable Energy Laboratory. https://www.nrel.gov/docs/fy18osti/70219.pdf.
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Crowther, H.; Furlong, J. (2004). “Optimizing Chillers and Towers.” ASHRAE Journal, Vol. 46, No. 7, July 2004; pp. 34-40.
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Morrison, F. (2014). “Saving Energy with Cooling Towers.” ASHRAE Journal, Vol. 56, No. 2, February 2014; pp. 34-40.
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Tom, S. (July 2017). Cat. No. 11-808-616-01. “CHILLED WATER SYSTEM OPTIMIZER.” Farmington, Connecticut: Carrier Corporation.
