One of the great inventions of our time — in terms of its social impact and influence on the health care and food preservation industries — refrigeration equipment is everywhere, from hotels, restaurants, grocery stores, and hospitals to homes, vehicles, and wherever large amounts of electronic equipment are housed (such as telecom hubs and Internet servers). As with all mechanical/electrical systems, there is always the possibility of malfunction. In many cases, it's the electrician who is tasked with making an initial diagnosis and/or repair. Armed with a basic knowledge of refrigeration systems, an electrician often can service this type of machinery, saving the facility the expense of outsourcing.
However, that doesn't mean you should even think about breaking open the refrigerant circuit, unless you are licensed to do so. The license to handle refrigerant is issued by the Environmental Protection Agency on a federal level. The focus is not on a person's ability to troubleshoot and repair refrigeration equipment, but rather on competency in handling refrigerant so it doesn't escape into the atmosphere and subsequently damage the ozone layer and/or contribute to global warming. As you can imagine, fines for working without a license in this area are very high.
Keeping your cool
Generally, the initial complaint with a refrigeration system is that the temperature is too low or too high. If it's too low, you need to adjust the thermostat. The idea is to tweak it slightly and then check back in about an hour. If the temperature is too high, check to see if the compressor is running. If it is, then the problem is probably ice buildup on the evaporator. You can remedy this situation by shutting the compressor down until the ice melts or — if time is of the essence — removing the ice with a heat gun or handheld propane torch, making sure you don't overheat the aluminum fins or any nearby wiring/components. Several factors can cause the evaporator to ice up, including leaving the door open, a worn door gasket, excessive ambient humidity, or failure of the defrost cycle timer. If the compressor is not working — and fails to come on when you adjust the thermostat — then a little more advanced knowledge and troubleshooting skills are required.
Let's take a look at a typical walk-in refrigeration unit (Photo 1) to gain a better understanding of how these systems work. This particular unit consists of a well-insulated room with a heavy gasketed door with single-acting push hardware on the inside. High up near the opposite wall (hanging from the ceiling), one or more evaporators (Photo 2) are mounted. The cold refrigerant passes through this unit and goes back to the compressor located outside the walk-in via a low-pressure return line. The evaporator consists of pipe arranged in a flat coil configuration with cooling fins attached — quite similar to an automotive radiator. Other features include a fairly powerful electrical motor with a fan blade attached to the shaft, a sturdy guard to prevent injury, and a sheet metal shroud for further protection.
Located just before the evaporator is the diffuser valve. This device, which is the key to the whole mechanism, is equipped with a small orifice (size depends on type of refrigerant). This constriction of refrigerant causes an abrupt drop in pressure. Beyond the diffuser valve, a given quantity of refrigerant occupies a much greater volume so that the temperature drops suddenly.
On the wall behind the evaporator is a thermostat (calibrated in degrees), which is adjustable by means of a hand knob or screwdriver. A high cut-in and low cut-out (opposite of a heating system) exist so the machine won't cycle excessively. Next to the thermostat is a wall switch, which controls power to the thermostat and then to an in-line solenoid valve just before the diffuser valve. The action of the switch or the thermostat deprives the solenoid valve of power, causing it to stop the flow of refrigerant in the high-pressure line.
The thermostat could control the compressor by means of a low-voltage line back to the motor; however, a more elegant solution is employed instead. When the solenoid valve shuts off, an increase in pressure is detected at the compressor, and a high-pressure sensor shuts it down immediately, thus controlling the function without the need of any wiring run back to the compressor.
The compressor and condenser are located some distance away, frequently in a basement directly below or sometimes in a compressor room (Photo 3), along with other similar units connected to other walk-in or reach-in coolers.
Making the call
Although under certain circumstances it's possible for you, as the electrician, to troubleshoot refrigeration equipment, you must decide whether or not a refrigeration technician should be called in for backup. Let's take a look at some practical tips, along with a little background, that will help you make that decision.
Years ago, compressors were belt driven. Refrigerant leakage around the input shaft was a constant problem, which was solved by putting the pump and motor inside a hermetically sealed housing with no external shaft. Most units made today use this design — in which they are immersed in oil and are not repairable. Many hermetically sealed units have no provision for checking, adding, or changing the oil. Because there is no combustion, the oil does not require changing. Because there is no external shaft, leakage is not an ordinary occurrence either. Typically, they run for quite a few years before needing replacement. If and when they do, remember this changeout is a job for a licensed refrigeration technician, because it involves opening the refrigerant circuit and capturing the old refrigerant for recycling purposes or proper disposal.
It's normal for a compressor/motor assembly to run hot to the touch — a derivative of Boyle's Law at work, not the motor overheating. Similarly, the compressor makes a distinctive knocking sound in normal operation, a consequence of the compression process. If this sound becomes abnormally loud, it means the end is near.
For a single-phase 240V unit, there is a start winding and a run winding, both submersed in refrigerant and electrically independent of each other — except that single legs of each winding are joined inside the hermetic enclosure to form a common wire. Besides economy, the purpose for this configuration is so there will be only three sealed holes instead of four; therefore, there's less chance of leakage. (Note: Larger units are 3-phase, and start windings are not required.)
There are two refrigerant lines connected to the compressor — the larger of which is the low-pressure return line. Under normal operating conditions, it will remain cool (but not cold) to the touch. The smaller pipe (both of these are usually copper tubing) will be hot where it emerges from the compressor. It goes to the condenser, which can be air-cooled or water-cooled. Air-cooled condensers work like an automotive radiator. There is a pipe matrix with cooling fins and a well-guarded fan that directs air through the assembly to get rid of the excess heat. Where the high-pressure pipe emerges from the condenser, it will be close to room temperature. Water-cooled condensers are even better. The high-pressure line emerging from this type of heat exchanger may actually feel cold to the touch, depending on the temperature of the water.
Surrounding these components is a plethora of electrical controls with many variations. The following comments are applicable for a typical single-phase, 240V setup. The whole thing is fairly easy to troubleshoot, especially if you have a schematic. If you don't, you must look over the equipment and try to understand the purpose of the system's various components.
Step by step
First, of course, you must look at the electrical supply, which travels from the entrance panel to a disconnect close by. If there are several compressors for separate walk-in and reach-in coolers, each should have its own disconnect.
If the compressor is not running, verify that both legs are energized at the disconnect output. Next will be a timer, the purpose of which is to shut down the compressor so that the evaporator can defrost. These timers can malfunction, causing the compressor not to run or keeping it from entering a defrost cycle.
If the timer has output so that both legs are energized but the compressor is still not running, then there are typically three additional controls capable of shutting down power to the compressor: high-pressure cutout, low-pressure cutout, and overtemperature cutout. The quick and dirty way to ascertain which, if any, of these is the culprit is to shunt them out individually. Because these devices have a protective function, it's more prudent to take voltage, current, and resistance measurements at appropriate points.
The overtemperature sensor is usually a round button sitting on the compressor adjacent to where the wires come out inside the terminal housing. The high- and low-pressure cutouts are contained within a single pressure switch that has two flexible copper tubes running to it — one from the compressor and one from the refrigerant piping. Because the points often stick, tapping lightly on the outside of the pressure switch may cause the compressor to take off, indicating a new pressure switch may be needed. A sight gauge in the refrigerant line also indicates if a recharge of the refrigerant is necessary.
If both legs are energized at the compressor terminal box, take ohm readings with wires unhooked. Neither start nor run should be open with respect to common or to each other, nor should they be shorted. The reading should be low ohms, depending on the horsepower of the motor. At megohm ranges, there should be good isolation between all three wires and the metal enclosure or any accessible equipment ground. If any of these tests yield unacceptable results, you must replace the pump/motor assembly.
Another cause for “failure to start” is bad capacitors. If the start or run capacitor is visually bad, (i.e., swollen or leaking oil), replacing it will likely solve the problem. Applying an ohmmeter, it should be neither open nor shorted. In a megohm range, the reading should gradually progress to a certain point as the ohmmeter battery charges the capacitor, then retreat to the starting point when leads are reversed. This “counting” action is necessary for a good capacitor but not a guarantee it will perform under full power. If in doubt, replace both capacitors with known good ones of the same values. If the relay will not pull in — even though all controls are positive — the coil may be weak or the points corroded, in which case replacement is appropriate.
Herres is a licensed master electrician in Stewartstown, N.H. He can be reached at firstname.lastname@example.org.
Sidebar: NEC Compliance
Air-conditioning and refrigeration equipment is covered in Art. 440, which follows Art. 430 (Motors, Motor Circuits and Controllers). The requirements of Art. 440 presuppose knowledge of Art. 430, in addition to Chapters 1-3.
For a hermetic refrigerant motor compressor, sizing the disconnecting means, branch circuit conductors, controller, branch circuit, short circuit and ground fault protection, and separate motor overload protection is fairly straightforward. Use the rated-load current marked on the nameplate of the equipment. Where this value is not shown, go to the rated load current on the compressor nameplate. Where present, use the branch circuit selection current instead to size out disconnecting means, branch circuit conductors, controller and branch circuit short circuit and ground-fault protection. Use Chapter 3 tables to size conductors based on the above current values.
Proper grounding and bonding is essential, because unintended fault current could be carried from the compressor and condenser unit back to the evaporator via refrigerant piping, where a poorly grounded metal enclosure could become energized and be a hazard to individuals standing on a damp concrete floor.