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Damage Analysis in Refrigeration Compressors - Part II

FAILURES CAUSED BY LIQUID SHOTS

Let us first examine the mechanical failures caused by liquid shots. Disassembling a compressor, for example, showed that the intake valve was damaged following an attempt to compress coolant or oil, or both. The introduction of virtually incompressible liquid into the compression chamber causes a characteristic hammering, and the breaking of the intake valve. In this example, the broken valve pieces were wedged into the exhaust valve and the side of the compressed gas passage. In general, the suction valves remain intact, but have radial cracks or are chipped if subjected to liquid strikes. Damage to the piston may be caused by contact with the broken valve pieces; in this case also the cylinders are usually damaged so much that they have to be repaired. Other compressors, in the event of strong liquid shots, may also present the rupture of the intake valve and the spring. Generally the piston head is marked, having come into contact with the valve or spring fragments. Whenever a valve or spring breaks, the motor must be removed and the windings and the inside of the compressor casing must be carefully inspected; valve or spring fragments can be inserted into the casing or in the windings and then cause short circuits. This problem is dealt with in detail under the heading "Electrical faults".

CAUSES OF LIQUID SHOTS

The oversizing of the thermostatic valve is one of the main causes of liquid shots. Even if an oversized thermostatic valve can work well at full load, in the event of partial load it could lose control of the refrigerant flow rate introduced into the evaporator. In fact, with a partial load, the valve tends to maintain a certain overheating, but its oversized seat allows more liquid to pass than is necessary; this feeds the evaporator excessively, causing a rapid reduction of overheating at the evaporator outlet. The valve then closes until normal overheating re-establishes, and then opens again and allows too much liquid to pass. This "all or nothing" operation can allow a part of the refrigerant to pass through the evaporator and be sucked by the compressor, thus causing a shot of liquid. Certain monoblock appliances are sometimes intentionally designed with slightly oversized expansion valves. In these cases, an accurate selection of the valves was made and tests were carried out to ensure proper operation with specific operating conditions. It is therefore necessary to distinguish between this type of choice, made during construction, and a choice made on site with a certain approximation. Very often the valves installed on site are chosen by personnel who do not have enough experience; a maintenance technician should not trust too much in these cases.

THERMAL LOAD REDUCTION

Liquid shots can also be caused by reducing the air flow through the battery direct expansion. This reduction in airflow can be caused by dirty filters, by an air-breaking network that brakes the air, or by fan-level problems. The reduction of the air flow decreases the heat load on the battery and causes the formation of frost on it; the frost reduces the heat exchange surfaces and further decreases the load actually perceived by the battery. With these reduced heat load conditions, the valve becomes relatively oversized and operates "all or nothing" as described above.

IRREGULAR AIR DISTRIBUTION ON THE EVAPORATOR

It is a problem similar to the one illustrated above and it will happen if the air distribution on the surface of a direct expansion battery is not uniform. This will cause a different distribution of the load between the refrigerant circuits and, consequently, an unstable suction temperature, which will be perceived by the expansion valve, causing suction of liquid into the compressor. Uneven air distribution on the evaporator can easily be detected by the appearance of frost or condensation on the battery.

REFRIGERANT MIGRATION

The condensation of the refrigerant fluid in the coldest areas of the refrigeration system causes it to migrate. The liquid migrates to vapor state towards the colder areas (at lower pressure), accumulates in these points and condenses. Certain types of plant are more susceptible to refrigerant migration problems, because they are connected to other exchangers. So the migration problems are particularly serious with dual coolers, ie two compressors. The water circulates in the two evaporator circuits, therefore, when the group works with only one compressor, the difference in temperature is considerable between the condenser and the evaporator of the inactive circuit. Since a compressor could remain inactive for several days, it is easy for a considerable amount of refrigerant to migrate, and to condense in the active circuit. The migration occurs through the solenoid valve or the compressor valves, since these are never completely sealed. This means that, over time, a good percentage of refrigerant will end up in the lower part of the system. When the second compressor is started, after a long period of stoppage in such conditions, the excess refrigerant liquid in the evaporator will be sucked by the compressor and will therefore cause liquid strikes and a dilution of the oil contained in the crankcase. A system to prevent this problem consists in the installation of a timer for restarting in parallel with the "pump down" auxiliary circuit. This device is used to ensure starting at regular intervals (usually every 30 minutes). This system avoids the accumulation of refrigerant in the low pressure side of the refrigerant circuit and prevents liquid blows when the compressor starts.

OIL SUCTION

The oil sucked by the compressor is as harmful as the suction of the refrigerant. A well designed refrigerating circuit must allow a regular return of the oil to the compressor and prevent the formation of oil deposits. The refrigerant circuit must be particularly studied in the case of systems that have to work for long periods with minimum load, given that, since the speed of the gas in the pipes is reduced, the oil return to the compressor is irregular and a shot may occur when the compressor resumes to work at maximum. To avoid problems related to oil returns, with partial load operation, follow the instructions and recommendations of the refrigeration manuals when running a refrigeration circuit. In a system where the oil accumulates excessively when the compressor is at reduced load, it is possible to temporarily solve the problem by preventing the compressor to fall below a minimum capacity that guarantees a sufficient oil return. However, this solution risks causing the compressor to run frequently and to overheat the motor; it will then be necessary to provide, during this transitional period, the addition of a protection against frequent cycles.

Damage Analysis in Refrigeration Compressors - Part II

Author, Riccardo Tigani  Linkedin Page
General Manager at Linea3C Srl

23rd April 2019

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