Attending to the electrical components of refrigeration systems is good for contractors, but there are other aspects that can be quite lucrative. Phil Kreveld explains.

It’s not much of a surprise that the complete maintenance of refrigeration systems requires much more than electrical trade experience.

Yet much service work is based on the premise that the electrical side needs practically all the attention ¬– essentially compressor motors and their controls.

This article provides basic information to help in understanding recommended service procedures. It should be of interest to non-specialist contractors who may have to deal with refrigerated equipment.

It consists of a refrigerant compressor (piston, vane, screw, scroll or centrifugal type) driven by an electric motor. The hot, high-pressure vapour refrigerant leaving the compressor is condensed back to liquid by dissipating heat to ambient air or by means of a cooling water spray.

Following condensation, a thermostatic expansion valve passes the refrigerant to the evaporator (the business end), where it becomes a gas again through the absorption of heat from the evaporator’s surroundings.

The low-pressure vapour is compressed again to provide a continuous cooling process.

What can go wrong?
There are lots of pitfalls, but in due course the electric motor will probably fail. (As one compressor manufacturer points out, the replacement will fail a bit sooner.)

The compressor is next to come under suspicion and may be replaced with a new one, which also fails sooner than could ‘reasonably’ be assumed.

The cause is likely to be that endemic problems – those typical of refrigeration systems – are not being addressed. Therefore reasonable assumptions have to be replaced by an analytical approach.

One more obvious example is compressor failure due to ‘slugging’, in which the suction port is damaged by liquid refrigerant entering it, as liquid is not compressible.

Slugging may be due to too much refrigerant coming into the evaporator (indicating too great a refrigerant charge), a problem with the thermal expansion valve, or possibly a faulty accumulator – a device that traps excess liquid refrigerant (and lubricating oil for the compressor).

Moisture, and trapped oxygen in combination with refrigerant, can cause the formation of corrosive acids (hydrochloric, fluoric), and sludge in combination with compressor lubricating oil. These can attack motor windings and cause damage to the compressor.

Because of the dissimilar metals used, in particular copper in pipework, some electrolytic plating action can occur. Copper is deposited on compressor bearing surfaces, and seizure is possible.

For the above problems alone, refrigeration service procedures require a clean-up necessitating removal of the refrigerant, water and sludge. This will be followed by purging with dry nitrogen and evacuation using vacuum pumps before the introduction of fresh refrigerant.

Refrigeration system physiology
We won’t go too far with the medical line, but there is a useful analogy.

The compressor is the heart, the refrigerant is the blood, and the pipework is the arterial network.

Compressor lubrication oil is good if there’s not too much in the refrigerant, and bad if there is. It coats the arteries of the evaporator, thus interfering with heat transfer and requiring the compressor to work harder to maintain the required low temperature. And yes, slugging can be seen as a leaky heart valve in the human system.

There are very expensive oil-less compressors, but refrigeration mechanics usually have to deal with these basics.

Oil and refrigerants
Most refrigerants are soluble in mineral oils. For refrigerants such as R-134 (a halocarbon), the oil is likely to be in solution in the evaporator, and will remain in solution in the low-pressure accumulator (see Figure 1).

However, refrigerants reduce the viscosity of the oil and therefore its lubrication effect. A small amount of oil in evaporators using certain halocarbon refrigerants may increase the heat-transfer coefficient slightly, but the presence of oil is generally detrimental to good performance.

For refrigeration systems with halocarbon refrigerants, the vapour pressure of an oil/refrigerant is lower than for the refrigerant by itself, meaning that the cooling system will run warmer for the same suction pressure of the compressor.

Foaming of the lubrication oil can cause serious malfunction of the compressor, and it can occur after a rest period of the refrigeration system.

Low pressure in the compressor crank case or sump, depending on the type, will cause rapid boiling of the refrigerant and foaming, which effectively negates compressor lubrication.

Losing oil
Insufficient lubrication oil will wreck the compressor, and service people should be acutely aware of the potential.

Oil that carried out of the compressor that is not removed by an oil separator ultimately reaches the low-pressure side of the system.

Information published by the American Society of Heating, Refrigerating and Air Conditioning Engineers confirms these observations. Performance is degraded by as much as 30% due to the build-up of lubricants on internal surfaces. Up to 40% degradation has been observed in systems 20 years old and more.

Depending on the velocity of the refrigerant leaving the compressor, oil can be carried with it. At lower speeds, oil is able to coat the riser at the high-pressure end of the compressor and gradually make its way down to the crankcase, sump, etc.

A frequently used oil separator employs the ‘change of direction’ technique. Heavy oil droplets present in the stream of light refrigerant molecules cannot maintain their initial trajectory because of their mass and initial speed, and refrigerant is directed towards the oil separator outlet.

Compressors
The main compressor types used in refrigeration systems are piston, centrifugal, scroll and helical-rotary.

Scroll and helical-rotary (or screw) compressors have gradually become more frequently used. Along with the piston compressor, they are positive displacement machines. Centrifugal compressors impart increased velocity to refrigerant molecules, which is the same as increasing their pressure.

Many reciprocating piston-type compressors are integral with the electric motor. The refrigerant from the suction side of the evaporator passes over the motor windings, cooling the motor, before being drawn into the compression cylinders.

This form of package minimises refrigerant leakage, which can occur at the compressor bearings. Reciprocating compressors are gradually giving way to screw compressors, which are widely adopted in air-conditioning applications.

Scroll compressors are very efficient. The compressor has a stationary scroll with a discharge port. As the rotary scroll undergoes angular displacement it compresses refrigerant gas trapped between the two scrolls through the gradual reduction of volume.

The screw compressor works on a similar principle, compressing refrigerant between two mating helical surfaces.

Controlling the compression process is the single most important feature of a refrigeration system. For piston-type multi-cylinder compressors, individual cylinders are unloaded (taken out of the compression cycle) as the heat load decreases, and put back as the heat load increases.

In response to a decreasing load, an electronic controller opens a solenoid valve. This solenoid valve diverts pressurised refrigerant vapour from the compressor discharge to the top of the unloader valve, causing it to close and shut off the flow of refrigerant vapour into the cylinder.

Even though the piston continues to oscillate in its cylinder, it is no longer performing compression, as it cannot take in any refrigerant vapour.

Screw compressors use a slide valve to tap off the refrigerant at the desired outlet pressure. However, variable-speed drives are a simple and very effective solution, providing greater efficiency than mechanical methods of compression control.

On the lookout for trouble
Prevention is better than cure, yet good monitoring practice for refrigeration systems is not always simple.

We can usually assume that the correct refrigerant is in place, but there are many other aspects to watch out for.

As mentioned, trapped air and lubricating oil may necessitate a purging of the pipework, evaporator and condenser.

Defrosting can cause excessive energy consumption as well as interfere with the refrigeration process.

Like car radiators, blocked or partly blocked condensers will effectively negate the process they are designed for.

Refrigerant leakage
The opportunity for refrigerant leakage is very much dictated by the physical layout of the plant.

In typical supermarket installations, the sources are pressure controls and lines to and from them, the typical causes being vibration and mechanical damage. Other points of leakage can occur at brazed, flanged and threaded fittings – most obviously on the high-pressure side.

Other suspects include manifold fittings as found in evaporators, leakage from accumulators through degraded gaskets, and faults in the Schrader (tyre-like) valves that introduce the refrigerant charge.

Condensers can be another source of leaks, in particular remote units where trouble often develops in the finned area. Thermal expansion can cause brass ferrules used in the condenser tubing to dislodge, with consequent contact between the tubing and the frame.

Fan problems can also bring about leaks. Ageing riveted fan blades in older systems can damage condenser tubing.

Walk-in systems also present leakage potential, as there are generally more solenoid valves and check valves to contend with.

Piping for various refrigeration systems in a supermarket is prone to leakage because of the many brazed connections and long runs. In addition, the different thermal expansion of copper piping and steel supports can result in repeated movement of the soft tubing against hard supports, thereby causing leaks.

Corrosion caused by various foods can mean pinholes and condensation in compressor racks, and inappropriate cleaning materials can also cause leakage.

Depending on regulatory requirements and the refrigerants in use, the acceptable leakage rate is about 15 grams a year for commercial refrigeration.

There are many leakage testing methods, including underwater bubble, bubble soap paint, pressure and vacuum decay, and tracer gas detection (halogen, helium and hydrogen). There is a surprising amount of science behind leak testing, meriting an article on its own.

The bubble test is not sensitive, even when enhanced with soap. Other tests involve evacuation of the system, thus adding enormously to the workload. Sniffer tests often rely on the detection of halogenated gases and are thus well suited to the sensitive detection of most refrigerants.

A halogen detector is based on the measurement of positive ion emission due to halide presence in an electronic cell. This ion current is related to the halogenated gas concentration and therefore to the size of the leak.

Preventive maintenance
Studies in 2005 by the UK-based Institute of Refrigeration concluded that continuous monitoring of compressor electrical parameters provides early indication of incipient problems.

Equally important, such monitoring allows changes to be made in condenser running conditions, defrosting, interior lighting of displays, etc, to improve energy efficiency.

For the typical supermarket, more than 50% of energy use is for cool rooms, refrigerated showcases and freezers for ice cream, frozen vegetables and other perishable foods.

So there are opportunities for contractors to gain additional expertise and obtain business additional to basic maintenance in the larger food retailing and preparation sector.