In virtually all coolers and freezers, small or large, air is cooled by forced-circulation evaporators that contain propeller fans powered by fractional-horsepower motors. Typically, these fans run continuously, even though, on average, full airflow is only required about half the time.

An inexpensive controller is currently available that slows these fans when full-speed operation is unnecessary. The controller does this simply and inexpensively by taking advantage of a basic principle of motor operation: the lower the voltage applied to a motor, the less rotational force it produces.

In field tests, documented savings varied from 10 percent to 60 percent of overall refrigeration energy, and some users report paybacks as low as one year. Savings vary widely, however, as they are dependent primarily on duty cycle, evaporator motor power, and local utility rates.

What Are the Options?

The controller is available in three models. The "bare bones" unit (see Figure 1) senses when refrigerant has ceased flowing through the evaporator coil and cuts the voltage to the motor by almost two-thirds (usually from 110-115 volts to 20 volts). The net effect is that the controller, by cutting the voltage going to the motor, also cuts the motor's speed—typically from about 1,600 to 400 revolutions per minute (rpm). The lower speed is considered the bare minimum required to provide defrosting and prevent air in the cooler from stratifying into layers of higher and lower temperature.

The next model also monitors and stores temperature, energy consumption, and savings data on a daily basis (see Figure 2). Data for as many as 120 days can be stored and the information can be downloaded via the serial port included in the unit. It includes three warning lights: one to indicate that the temperature has dropped below a set point (set at the factory according to health department codes), another to signal that the evaporator is icing, and the third to indicate that the compressor is running longer than necessary. The controller also monitors and records when the warning lights are activated.

The current top-of-the-line model adds a modem to allow for remote monitoring. The temperature, consumption and savings data, and warning alarms can be sent to or accessed remotely by whomever the owner desires. A full-time monitoring service is also available. The controller sends an alert about any problems with the refrigeration system, allowing for immediate repair before either the food or the refrigeration system itself is affected.

Installation costs are typically about $100 per unit, but they can vary depending on the region and the installer.

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How to Make the Best Choice

Controllers don't work for every application, and so before you decide to install one on a given cooler, you should give some consideration to several issues (Table 1).

The cost-effectiveness of the controller must be evaluated on a cooler-by-cooler basis. But because this control is relatively inexpensive and usually highly cost-effective, these evaluations need not be especially detailed or complicated. To help, we developed a simple calculation tool, available below. Before you begin, you must collect four pieces of information on site to input into the tool:

(1) Evaporator fan power in kilowatts, which may be determined by any of these techniques, listed in order of accuracy:

• Measure the power drawn by the evaporator fan motors using a wattmeter.

• Measure the current flowing through the evaporator fan motors using an ammeter, and measure the voltage applied to the motors using a voltmeter. Multiply these values times each other and times power factor, which for shaded-pole motors is about 0.6 and for permanent-split-capacitor motors is about 0.9.

• Read the rated amperage and voltage off the motor nameplates. Multiply these values times each other and then times the power factor (see previous item).

(2) Compressor duty cycle in percent, which may be calculated by first wiring a clock in parallel with the compressor motor, or by attaching a run time logger to the compressor motor. The amount of time the compressor motor runs divided by the total time of the test equals the duty cycle. If possible, conduct the measurements during a two- to four-week period when conditions are average. For walk-in coolers exposed to outdoor conditions or with outdoor condensing units, conduct a regression analysis that correlates run hours to outdoor temperature.

(3) Electric rate in U.S. dollars per kilowatt-hour, which may obtained from a recent electric bill.

(4) Purchase and installation cost in U.S. dollars, an estimate of which may be obtained from a refrigeration technician.

After you enter this information, click the calculation button. The tool produces outputs including annual savings, both in U.S. dollars and kilowatt-hours, and simple payback period in months.

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What's on the Horizon?

A report from the Centers for Disease Control (CDC), "Surveillance for Foodborne-Disease Outbreaks—United States, 1988-1992," identified bacterial agents as the leading cause of laboratory-confirmed disease outbreaks caused by food. Improper holding temperature was identified as the major risk factor, accounting for 35 percent of the outbreaks. Studies like these have led to an increasing number of voluntary food safety programs.

Recently, however, there has been talk of shifting these programs from voluntary to regulatory. A ordinance proposed by the U.S. Food and Drug Administration would require that temperatures in coolers be recorded twice a day, seven days a week. For those individuals considering a monitoring system, the evaporator fan controller with data logger becomes an attractive system. It meets the monitoring requirements and, as a bonus, provides energy savings. The controller is comparable in price to some of the automated temperature monitors. Less expensive solutions exist (such as temperature probes costing roughly $25-$35), but they require an individual to physically measure the temperature.

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Who are the Manufacturers?