This article is written by Christopher Stephens of JVS Refrigeration in California with just a few additions by me (Bryan) in italics. Thanks, Chris!
Reach in refrigerators are an interesting side of our industry, often looked at as frustrating and troublesome. Often working in kitchens or convenience stores the refrigerators are never located in a convenient place to work on them, and that tends to lead to frustration on the technician’s part. Please understand my article pertains to medium temperature refrigerators. I also advise you to use manufacturers OEM parts when possible as the unit was designed to work with them. One of the more misunderstood and misdiagnosed parts is the temperature controller.
Keep in mind that some refrigeration temperature controls sense the evaporator coil temperature (not the desired box temperature) some use intake air sensors and some use supply air sensors. The medium being sensed (Coil, return air (intake) or supply air (discharge) will greatly impact how the controls function and what impacts them.
Personally, I break temperature controllers down into five different types, please understand that these are generic descriptions and you should always lean on the manufacturer if possible to understand their control strategies.
- Standard Pressure Control – these work on the principle that at any given pressure saturated refrigerant is a constant temperature. This style of control is not used very much anymore as a means of temperature control because it is not very precise and to an untrained technician, it can be hard to set the temperature correctly.To use this control strategy, you need to understand what evaporator T.D. (Temperature differential) your reach-in was designed with, you will need a temperature pressure chart, you will need an accurate set of refrigeration gauges, and an accurate thermometer. With all these tools you can take your desired box temperature and find it on your pressure chart read across the pressure chart and find the corresponding pressure for your desired box temperature and that will be your cut-in pressure to set your control at. Now we need to find the cut-out setting typically we want the system to have about a 5-8 degree differential between the cut out and cut in to reduce system short cycling this will likely be about 8 degrees colder than the cut-in temperature, so take your desired box temperature subtract your differential of 5-8 then subtract your designed evaporator T.D. (specific to the equipment but likely 20 -30 degrees for reach ins) and find that number on the temperature pressure chart than read across the pressure chart and find the corresponding pressure and that will be your cut out setting. Understand that pressure controls are never exact, so you will need to adjust accordingly in the field.
- Constant Cut in Control (electromechanical) – These are one of the most common temperature control’s that you will find in reach in refrigerators because they are the most economical for the manufacturers as they have an off cycle defrost built into them. They work by inserting the sensing bulb into the evaporator coil and they have a set temperature that they turn back on (cut in) at no matter how cold you turn the dial. They work very similar to the pressure control as they are designed with the evaporator T.D. (Temperature Differential) in mind, but instead of using pressure they sense the evaporator temp on the surface of the coil, they do have a knob to adjust the cut-out temperature, but you have no control over the cut in temperature that is why they are called constant cut in. By design they also have a built-in defrost as the cut in temperature is usually 37 to 41 degrees (for a cooler/refrigerator) depending on the manufacturer. They rely heavily on proper superheat and proper refrigerant charge. If the charge is incorrect or the superheat is not correct the coil could get too cold and the control could prematurely shut off. This could lead a technician to diagnose a bad control if they did not understand how they work. If you come upon a reach in that is short cycling and shutting off too soon, make sure to check the charge and measure the evaporator superheat before you diagnose a bad control.
- Constant cut in (Digital) – These work the same as the electromechanical control, but they typically have two probes one to be located in the coil and one to be located in the return air stream. They tend to have more features that are available, such as an added defrost cycle based off time (every four hours, every six hours, etc.…..) while still using box temperature as a fail-safe. For example, say the control has a defrost every four hours if the coil temperature comes above a pre-determined temp say 40 degrees the control will terminate the defrost. The controls can also shut off the evaporator fan motors during the off cycle to save energy and reduce warm is intrusion into the unit. These types of controls are on many HC (Hydrocarbon) units being built today.
- Universal electromechanical – these typically have one sensing bulb that you mount in the return air stream and they turn on and off via the temp setting.
- Universal digital – These are usually aftermarket controls and can have several different control strategies and can usually be customized to do anything, from heating to cooling to defrost depending on the manufacturer.
Something to understand is that reach in refrigerators is usually designed to perform in a certain environment and if something changes such as the ambient temp in that environment, or if doors are left open. The box will not perform correctly, I suggest you take a step back before you start throwing parts at a reach in and evaluate the environment you may find your problem there!
As always I suggest “Commercial Refrigeration for Air Conditioning Technicians” by Dick Wirz as the bible for refrigeration training