Maintaining the correct temperature has always been critical in the food processing industry. In the past, bulb-and-capillary mechanical thermostats could be found in every warehouse and cold storage unit. As technology has advanced, many of these thermostats have been replaced by their digital counterparts. Digital temperature switches provide the same functions as the mechanical devices while also acting as a timer, an external relay and a thermometer.
The most popular digital temperature switches have a single input and a single output. These controls are used to maintain the process temperature by cycling a compressor, which compresses the refrigerant gas in the system. The hot compressed refrigerant gas flows from the compressor to the condenser, which acts as a heat exchanger and uses ambient air to cool the refrigerant. As the refrigerant gas cools, it becomes a cold liquid-gas mixture, which is cycled to the evaporator.
Acting as another heat exchanger, the evaporator typically transfers heat from ambient air, water or a water-glycol mixture to turn the refrigerant back into a hot, low-pressure gas to feed the compressor. At the evaporator stage, the refrigerant temperatures can be cold enough to drop the temperature of the water flowing through the evaporator close to its freezing point. The evaporator fan pulls air across the cold evaporator coils to refrigerate the space.
Like their digital counterparts, mechanical devices are able to turn on the compressor based on the temperature inside the refrigerated space, but they require a defrost timer to allow the evaporator coils to defrost. If the evaporator coils are not allowed to defrost, the water flowing through evaporator lines can become obstructed to the point that the water freezes, which could damage the evaporator. The single input/single output digital controls have a built-in timing function that eliminates the need for the defrost timer. This function allows the user to define the defrost cycle time and how frequently the systems should defrost.
Because most mechanical temperature switches are nonindicating, refrigeration systems that rely on these devices require a bimetal or gas-filled thermometer to display the temperatures. Digital controllers have a bright LED display that can be read easily in dark areas. This display reduces labor and material cost by eliminating the bimetal thermometer, the extra connection to the pipe, and most importantly, the labor of installing the thermometer.
Because external temperature conditions can affect how fast the coils defrost, a second temperature input probe is available on higher-end systems to turn off the defrost cycle when the temperature reaches a desired level. Having the defrost cycle stop based on temperature allows the temperature to remain constant, which results in less strain and greater efficiency. To perform this function, the defrost temperature probe normally is mounted close to the evaporator coils.
Better Control Through Relay Outputs
Most digital controls can be supplied with up to four relay outputs to allow better control of the conditioned space. For example, in a system with two relay outputs, the primary relay would continue to control the compressor while the second relay would control the evaporator fan. In a refrigeration system with a mechanical temperature switch, the fan would run either all the time or only when the compressor is on, and it would have to be hard-wired to accomplish either of these configurations. A digital control with two or more outputs allows the fan configuration to be programmed in the control. Besides operating all the time or with the compressor, the fan also can be turned on when the compressor is turned off. The fan typically is turned off during the defrost cycle to avoid transferring the additional moisture into the refrigerated space. The fan runs without the compressor to minimize the temperature differential between the evaporator and the inside of the refrigerated space.
A third relay normally is used for applications that require either a gas or electric defrost. These applications tend to require shorter defrost periods due to the sensitivity of the products that are being stored, or because they have ambient air conditions that are not ideal for defrosting the evaporator. The fourth relay is an auxiliary output that can be used to control an audible alarm to notify operators of out-of-spec conditions or to turn off the fan relay when necessary.
Because digital controls are microprocessor-based, advanced software functions can be implemented to provide additional benefits. Minimum compressor on and off time is the most commonly used software feature. These safeguards protect the compressor against chatter and premature wear. Digital controls also have ambient temperature probe adjustments that allow for corrections of the temperature readings. These adjustments can compensate for temperature errors due to the resistance of the extension wire on applications in which the temperature sensors are mounted long distances from the control.
As the technology used in the digital control market continues to advance, mechanical switches increasingly will be replaced by digital temperature controls. These sophisticated devices provide a cost savings by eliminating other components, allowing systems to work more efficiently and offering more user friendly options that did not exist previously.