Medium-temperature refrigeration systems, such as walk-in and reach-in coolers, employ different methods of controlling case temperature. The controls used on these systems are not only designed to maintain proper case temperature, but they are also used to ensure the evaporator defrosts on a regular basis. Because medium-temperature refrigeration systems generally operate with evaporator coil temperatures of 25° F and below, frost will develop naturally on the coils. The frost must be removed on a regular basis or it eventually will block the airflow through the evaporator coil and cause the unit to malfunction. There are methods to prevent frost buildup. One commonly used method, especially with reach-in coolers, is a temperature controller designed to sense the temperature of the evaporator coil. This method works because there is a relationship between the evaporator's coil temperature and its inlet air temperature. When the system is running, most evaporators have a 10° F to 15° F temperature difference between the coil temperature and the inlet air temperature. This being the situation, it is easy to control the case temperature by monitoring the coil temperature. When the coil reaches a specific temperature it will correspond to a specific case temperature. For example, if the desired case temperature is 35° F and the evaporator has a 10° F temperature difference, when the evaporator coil reaches 25° F it will correspond to a 35° F case temperature. The controllers used with this type of system will have an adjustable cut-out range, which will allow the cooler to be cycled off at different coil/case temperatures. By sensing coil temperature rather than just air temperature, a manufacturer can design the system to completely defrost the evaporator each time the unit cycles off/on. When the compressor cycles off the evaporator fan motor will continue to operate and any frost that has developed on the coil of the evaporator will begin to dissipate. The coil temperature then will begin to approach the case's temperature because the compressor is not operating. The cut-in setting on this type of controller is set at a fixed temperature that will cycle the compressor back on at a high enough coil temperature to ensure the evaporator is completely defrosted and the case temperature does not reach an unacceptably high temperature (usually around 38° F). The actual cut-in temperature can vary between different manufacturers and case designs. Always check with the manufacturer of the cooler to find the correct cut-in value of the controller used. When replacing this type of controller it's usually best to obtain the original component from the case manufacturer. This will ensure that the cut-in and cut- out temperature range is appropriate for the design. However, there are universal temperature controllers available that seem to work fine for many applications. These replacement temperature controllers are available with a constant cut-in setting of 38° F to 41° F and varying cut-out ranges. In addition, when replacing this type of controller make sure that the sensing element is properly secured to the evaporator. Most evaporators will have a well into which the sensing element can be inserted. Make sure the sensing element is fully inserted into the well to ensure that it will sense the proper coil temperature. Replacing these types of temperature controllers is not difficult as long as the right controller is used. Using the wrong temperature controller will lead to problems. The use of thermal heat conductive compound in the wells will provide improved sensing and operation.
|
Another popular method of controlling case temperature is the use of a low-pressure control. This method works on the principle that there is a direct relationship between the saturation pressure of a refrigerant and its saturation temperature. For every saturation pressure, there is an associated saturation temperature. This method works very similarly to the previous method discussed. It will not cycle the compressor back on until the evaporator is completely defrosted and it will cycle the compressor off at the lowest desired coil temperature. Again, this reflects the lowest desired case temperature. Instead of measuring the coil temperature directly, this controller is determining coil temperature by the saturation pressure of the refrigerant in the evaporator. A typical low-pressure control will have two set points: a cut-in setting and a differential setting. In order for the control to maintain proper case temperature, both its cut-in and differential set points need to be adjusted properly. Most manufacturers will have stated values for the proper set points for their systems. However, these stated values usually work well on self-contained or close-coupled systems. If the condensing unit is remotely located, the differential set point may need to be adjusted. The cut-in value used on many systems will reflect a coil temperature that will cycle the compressor back on at a high enough coil temperature to ensure the evaporator is completely defrosted and the case temperature does not reach an unacceptably high temperature. For example, if a system uses R-134a and is designed to cycle the compressor on at a coil temperature of 38° F, then the appropriate cutin value would be 33 psig. If the system uses one of the popular refrigerant blends, then the dew point temperature of the refrigerant should be used to set the cut-in value. For example, if R-401A were used, 34 psig would be the correct cut-in value for a 38° F coil temperature. Once the cut-in value has been set, you must set the differential setting. The differential setting is used to cycle the compressor off at the lowest possible coil/case temperature. The differential setting represents the difference between the cut-in pressure and the cut-out pressure. For example, if the cut-in pressure is set at 35 psig and you want the compressor to cycle off at 15 psig, the differential setting would need to be set at 20 psig (35 psig - 15 psig = 20 psig). The appropriate cut-out value must reflect the lowest desired saturation temperature of the evaporator minus any pressure drop through the suction line. For example, if the lowest designed coil temperature is 25° F, then the cutout value should reflect the associated saturation pressure for a saturation temperature of 25° F minus any pressure drop through the suction line. Normally, a 2 psig pressure drop is assumed. Again, if R-134a were the refrigerant used, then the cut-out value should be 20 psig (22 psig – 2 psig pressure drop = 20 psig). For refrigerant blends, the appropriate cut-out pressure must reflect the average saturation temperature within the evaporator and minus any pressure drop through the suction line. For example, if an average coil temperature of 25° F is desired and the refrigerant blend enters the evaporator at a saturation temperature of 21° F and is at 29° F before the refrigerant becomes superheated, you would set the cut-out pressure to correspond to the dew-point temperature of the higher saturation temperature. In this example, it would be 29° F minus any pressure drop through the suction line. If R-401A were the refrigerant used in the example and a pressure drop of 2 psig was assumed across the suction line, then the actual cut-out pressure would be 26 psig minus 2 psig, which would equal a cut-out of 24 psig. Determining the pressure drop across a suction line and the average saturation temperature for refrigerant blends may be difficult at times. Here's an alternate method that can be used to set the cut-in and differential pressure settings of a low-pressure control:
• First, set the cut-in value of the low-pressure control to the appropriate value for the refrigerant to be used.
• Next, initially set the differential of the low-pressure control to an extremely high value (a value the suction pressure should never reach under normal operating conditions).
• Allow the system to run while monitoring the box temperature.
• Once the box reaches its lowest desirable temperature, slowly adjust the differential counter-clockwise until the compressor cycles off.
• Using this procedure will ensure that the differential settings have been properly set and the system will cycle properly.
|
Temperature controls that sense the air temperature also can be used to maintain case temperature. Normally these controllers will have a fixed-temperature differential. When this type of control is used, normally a defrost timer is also used in the system to shut down the compressor at a predetermined time(s) throughout the day to defrost any frost on the evaporator coils. The defrost timer can be adjusted to shut the compressor off from once a day to as many times as needed, depending on the manufacturer's specifications and the ambient conditions surrounding the cooler. When the compressor is shut down it could be off from approximately 45 minutes to 90 minutes depending on the design, the manufacturer's specifications and the ambient condition surrounding the cooler. Always check with the manufacturer of the cooler for their recommendations. Servicing, troubleshooting and replacing any of these control systems usually is routine as long as the design of the control system is understood. Always strive to determine how a system is designed to operate before attempting to resolve any of its problems.
Joe Marchese, CMS, is the owner of Coldtronics and a frequent contributor to RSES Journal. |
