In almost all coolers and freezers—small or large—air is cooled by forced-circulation evaporators that contain propeller fans powered by fractional-horsepower motors. These fans typically run continuously, even though, on average, full airflow is only required about half of the time. In the most common applications (those that use single-phase power), motors for the fans are usually shaded-pole or permanent-split-capacitor types, both of which are very inefficient.
Controllers are currently available that slow these fans when full-speed operation is unnecessary. They do this simply and inexpensively by taking advantage of a basic principle of motor operation: The lower the voltage that’s applied to a motor, the less rotational force it produces and the slower the motor speed. Reducing the operating speed also reduces the energy consumption of the fan. In addition, the motor produces less heat at slower speeds, which means that the compressor has less heat to remove from the refrigerated compartment. Anecdotal evidence also suggests that product quality in walk-in coolers can be improved. Because less air is circulated when the fan speed is reduced, items such as flowers, produce, or meat do not dehydrate as much.
In field tests for controllers from one manufacturer, documented savings varied from 10 percent to 60 percent of overall refrigeration energy, and some users reported payback periods as short as one year. Savings vary widely, however, as they are dependent primarily on duty cycle, evaporator motor power, and local utility rates.
Evaporator-fan controllers operate by cutting the voltage to the motor by almost 80 percent (from between 110 and 115 volts to 20 volts in typical single-phase applications). This reduces the motor’s speed—usually from about 1,600 to 400 rpm. The lower speed is considered to be the bare minimum required to provide defrosting and prevent air in the cooler from stratifying into layers of higher and lower temperature.
There are two manufacturers that produce evaporator-fan controllers. One of them, Energy Control Equipment Inc., produces a controller called the Frigitek that can be used in either coolers or walk-in freezers (Figure 1). The Frigitek reduces fan speed in response to a signal from the thermostat to stop the flow of refrigerant. For single-phase power applications, the Frigitek is available in configurations that can handle from 3.5 to 25.0 amps and from 115 to 480 volts (V). These units also have a field-adjustable low-speed setting to accommodate unique application requirements. For three-phase power applications, one available configuration uses a master control unit that can handle 480 V and motors of up to 20 horsepower. Additional power units are added for multiple evaporators.
For single-phase controllers, Frigitek’s prices start at about $500. Three-phase applications are considerably more expensive, particularly if fans from multiple evaporators are to be controlled. Installation costs for single-phase units are typically about $100 per unit, but they will vary depending on the region, the number of fans controlled, and the installer.
The second manufacturer, Functional Devices Inc., produces the ECT4045T, an enclosed, two-speed electronically commutated motor (ECM)/shaded-pole-motor phase controller with temperature differential input of 120 or 240 volts alternating current (VAC) for use in walk-in coolers. Check with manufacturers for current pricing.
Controllers don’t work for every application, so you should give some consideration to several issues (Table 1) before you decide to install one on your cooler.
The cost-effectiveness of any controller must be evaluated on a cooler-by-cooler basis. For coolers with three-phase fans, which are typically found in warehouses or distribution centers, cost-effectiveness calculations can be complicated. They need to factor in the cost of installing conduit and wiring to reach multiple evaporator coils potentially spread across a large area, and the number of master control and power units needed will vary widely by application. But because single-phase applications don’t have these issues, and the controllers for single-phase fans and their installation are relatively inexpensive and usually highly cost-effective, evaluations for their cost-effectiveness need not be especially detailed or complicated. Manufacturers offer tools to help evaluate the cost-effectiveness of the application of their products.