ACHRNEWS

Compressor protection starts with a knowledge of refrigerant-oil basics

May 3, 1999
In light of today’s need for more energy-efficient refrigeration, air conditioning, and heat pump systems, we find refrigerant control becoming a more important factor in gaining reliability.

The resultant reduction in compression ratio, calling for larger condensers and larger refrigerant charges, all point to the need to re-emphasize refrigerant control; one of the major causes of compressor failure is damage caused by liquid refrigerant entering the compressor in excessive quantities.

A well-designed, efficient compressor for refrigeration, air conditioning, and heat pump duty is primarily a vapor-moving pump designed to handle a reasonable quantity of liquid refrigerant and oil.

Designing and building a pump to handle more liquid would require a serious compromise on one or more of the following: size, weight, capacity, efficiency, and noise, not to mention the overall important factor of cost.

Regardless of the design, there are limits to the amount of liquid a compressor can handle; these limits depend on factors such as the internal free volume of the compressor, oil charge, type of system and controls, and normal operating conditions.

Proper control of liquid refrigerant is an application problem, and largely beyond the control of the compressor manufacturer.

The potential hazard increases with size of the refrigerant charge and usually the cause of the damage can be traced to one or more of the following:

  • Excessive refrigerant charge;

  • Frosted evaporator;

  • Dirty or plugged evaporator filters;

  • Failure of evaporator fan or fan motor;

  • Incorrect capillary tube selection;

  • Incorrect selection or adjustment of expansion valves; and/or

  • Refrigerant migration.


Refrigerant-Oil Relationship

To correctly analyze system malfunctions, and to determine if a system is properly protected, a clear understanding of the basic refrigerant-oil relationship is essential.

In refrigeration compressors, oil and refrigerant mix continuously. Refrigeration oils are soluble in liquid refrigerant and at normal room temperatures they will mix completely.

Since oil must pass through the compressor cylinders to provide lubrication, a small amount of oil is always in circulation with the refrigerant. Oil and refrigerant vapor do not mix readily, and the oil can be properly circulated through the system only if gas velocities are high enough to sweep the oil along.

If refrigerant velocities are not sufficiently high, oil will tend to lie in the bottom of the evaporator tubing, decreasing heat transfer and possibly causing a shortage of oil in the compressor.

As evaporative temperatures are lowered, this problem becomes more critical, since the viscosity of the oil increases with a decrease in temperature. For these reasons, proper design of tubing is essential for satisfactory oil return in a refrigeration system.

One of the basic characteristics of a refrigerant-oil mixture in a sealed system is the fact that refrigerant is attracted by the oil and will vaporize and migrate through the system to the compressor, even though no measurable pressure difference exists to cause the movement.

On reaching the compressor, the refrigerant will condense into a liquid. This migration will continue until the oil is saturated with liquid refrigerant. The amount of refrigerant the oil will attract is primarily dependent on temperature, increasing rapidly as the temperature increases and approaching a maximum at normal room temperature.

When the pressure on a saturated mixture of refrigerant and oil is suddenly reduced — as happens in a compressor on start-up — the amount of liquid refrigerant required to saturate the oil is drastically reduced and the remainder of the liquid refrigerant flashes into vapor, causing violent boiling of the refrigerant and oil mixture.

This causes the typical foaming often observed in the compressor on start-up, which can move all of the oil out of the compressor in less than a minute. (Not all foaming is the result of refrigerant in the compressor; agitation of the oil can also cause foaming.)

One condition that is somewhat surprising when first encountered by service personnel is the fact that the introduction of excessive liquid refrigerant into the compressor can cause a loss of oil pressure or oil delivery to the bearings, even though the level of the refrigerant and oil mixture may be observed in a sight glass.

The high percentage of liquid refrigerant entering the compressor not only reduces the lubricating quality of the oil, but on entering the oil pump intake may flash into vapor, restricting the entrance of adequate oil to maintain proper lubrication of the compressor bearings.

Should this oil-dilution effect continue, compressor failure occurs.

Typical Defects Of Improper Control

Liquid refrigerant problems can take several forms, each with its own, distinctive characteristics. Following are descriptions of the various problems you may encounter.

Refrigerant migration: Refrigerant migration is a term used to describe the accumulation of liquid refrigerant in the compressor during periods when the compressor is not operating.

It can occur whenever the compressor becomes colder than the evaporator, since a pressure differential then exists to force refrigerant flow to the colder area.

Although this type of migration is most pronounced in colder weather, it can also exist at relatively high ambient temperatures with remote-type condensing units for air conditioning and heat pump applications.

Liquid refrigerant flooding: If an expansion valve should malfunction, or in the event of an evaporator fan failure or clogged air filters, liquid refrigerant may flood through the evaporator and return through the suction line to the compressor as a liquid rather than vapor.

During the running cycle, liquid flooding can cause excessive wear on the moving parts because of the dilution effect on the oil and the loss of oil from the compressor.

During the off-cycle, after running in this condition, migration of refrigerant to the compressor can occur rapidly, resulting in liquid slugging when started.

Liquid refrigerant slugging: Liquid slugging is the term used to describe the passage of liquid refrigerant and oil through the compressor’s suction and discharge valves. It is evidenced by a loud metallic clatter inside the compressor, possibly accompanied by extreme vibration of the unit.

Slugging can result in broken valves, blown head gaskets, broken connecting rods, and other major component damage. Slugging frequently occurs on start-up, when liquid refrigerant has migrated to the compressor.

On some units, because of the tubing configuration and location of the components, liquid refrigerant and oil can collect in the suction line or evaporator during the off-cycle, returning to the compressor as a solid liquid with extreme velocity on start-up.

The velocity and the weight of the liquid slug may be of sufficient magnitude to override any internal anti-slug protection devices designed within the compressor, again causing major damage.

Corrective Action

The potential hazard to refrigeration or air conditioning systems is almost directly proportional to the size of the refrigerant charge in use. It is difficult to determine the maximum safe refrigerant charge of any system without actually testing the system with its compressor and other major components.

The compressor manufacturer can determine the maximum amount of liquid the compressor will tolerate without endangering the working parts, but the manufacturer has no way of knowing how much of the total system charge will actually be in the compressor under the most extreme conditions.

The maximum amount of liquid a compressor can tolerate depends on its design, internal volume, and oil charge. Where liquid migration, flooding, or slugging can occur, corrective action must be taken. The type of action is normally dictated by the system’s design and type of refrigerant control problem incurred.

Table 1 (page 18) lists refrigerant charge limitations for Bristol compressors that may be used without some means of liquid refrigerant control. It is possible on some system designs, particularly systems where large quantities of liquid can get to the compressor on start-up, that even though the system contains refrigerant charges smaller than the limits listed in this table, some protective action may be required.

When liquid refrigerant control becomes a problem, one or more of the following types of corrective action may be necessary.

Minimum refrigerant charge: The best compressor protection against all forms of liquid refrigerant problems is to keep the charge within the compressor’s limits. Even if this is not possible, the charge should be kept as low as reasonably possible.

Use the smallest practical tube size in condensers, evaporators, and connecting lines. Receivers should also be as small as possible.

Charge with the minimum amount of refrigerant required for proper operation. Beware of bubbles showing in the sight glass, which are caused by small liquid lines and low head pressure. This can lead to serious overcharging.

Pumpdown cycle: The most positive and dependable means of properly controlling the liquid refrigerant, particularly if the charge is large, is by means of a pumpdown cycle.

By closing a liquid line solenoid, the refrigerant can be pumped into the condenser and receiver, and the compressor operation is controlled by means of a low-pressure control. The refrigerant can thus be isolated during periods when the compressor is not in operation, and migration to the compressor and crankcase is prevented.

A recycling type of pumpdown control is recommended to provide protection against possible refrigerant leakage through control devices during the off-cycle. With the so-called one-time pumpdown, or non-recycling type of control, sufficient leakage may occur during long off-periods to endanger the compressor.

Although the pumpdown cycle is the best possible protection against migration, it will not protect against liquid flooding during operation. If a pumpdown cycle is used, an approved start capacitor and relay must be used on all compressors having single-phase, permanent-split-capacitor (PSC) motors.

Crankcase heaters: On some systems, operating requirements, cost, or customer preference may make the use of a pumpdown cycle undesirable; crankcase heaters are frequently used in such cases to retard migration.

The function of a crankcase heater is to hold the oil in the compressor at a temperature higher than the coldest part of the system. Refrigerant entering the compressor will then be vaporized and driven back into the suction line.

However, in order to avoid overheating and carbonization of the oil, the wattage input of the crankcase heater must be limited. Also, in ambient temperatures approaching 0ÞF or when exposed suction lines and cold winds impose an added load, the crankcase heater may be overpowered and migration can still occur.

On such systems, in no event should a compressor be started unless the crankcase heater has been energized for a period of at least 12 hrs immediately prior to start-up.

Crankcase heaters are effective in combating migration if conditions are not too severe, but they will not remedy all slugging where liquid floodback has negated the heat input of the crankcase heater.

Suction line accumulators: On systems where liquid flooding is apt to occur, a suction accumulator should be installed in the suction line. Basically, the accumulator is a vessel that serves as a temporary storage container for liquid refrigerant that has flooded through the system, with a provision for metering the return of the liquid to the compressor at a rate to which the compressor can safely tolerate.

This type of flooding typically can occur on heat pumps at the time the cycle is switched from cooling to heating, or from heating to cooling. A suction line accumulator is mandatory on all heat pump systems unless otherwise approved by the manufacturer’s engineering department.

Systems using hot gas defrost are also subject to liquid flooding either at the start or the termination of the hot gas cycle. Compressors on low-superheat applications, such as liquid chillers and low-temperature display cases, are susceptible to occasional flooding from improper refrigerant control.

Since each system will vary with respect to the total refrigerant charge and the method of refrigerant control, the actual need for an accumulator and the size required is, to a large extent, dictated by individual system requirements.

If flooding can occur, an accumulator must be provided with sufficient capacity to hold the maximum amount of refrigerant flooding which can occur at any one time; in some cases this can be well over 50% of the total system charge.

If accurate test data as to the amount of liquid floodback is not available, then 50% of the system charge normally can be used as a conservative design guide or starting point.

Oil separators: Oil separators cannot cure oil-return problems caused by system design, nor can they remedy liquid refrigerant control problems. However, in the event that system control problems can be remedied by other means, oil separators may be helpful in reducing the amount of oil circulated through the system.

They can often make safe operation possible through critical periods until such time as system control can be returned to normal conditions. For example, on low-temperature applications or on flooded evaporators, oil return may be dependent on defrost periods; the oil separator can help maintain the oil level in the compressor during the period between defrosts.

In summary, close attention must be paid to the capacities and capabilities of system components such as the compressor and condensing unit.

On unique systems, it is strongly recommended that the equipment be field tested prior to being placed into the marketplace, to ensure its reliability.

Article provided by the Application Engineering Department of Bristol Compressors Inc., 15185 Industrial Park Rd., Bristol, Va. 24202; 540-466-4121.