Nick Alty and John Jackson from J & E Hall devise Halltherm training courses on compressor maintenance. Here they’ve pooled their experience and expertise to deliver advice on how to avoid mechanical breakdowns.
WITH over 100 years experience in manufacturing, repairing and servicing refrigeration compressors, J & E Hall’s training courses at the company’s Derby remanufacturing centre continue to attract engineers from leading UK contractors. Here tips on maintaining compressors and simple ways to avoid breakdowns continue to prove popular.
It’s easy to take the humble compressor for granted and our courses remind people that this is a very important piece of equipment in a refrigeration system. A compressor, with a little tlc will run happily for a very long time. Some, manufactured by J & E Hall, have, with the right maintenance, been doing a great job for as long as 40 years.”
In the event that a compressor does have a problem, the reasons behind this are in the main down to a lack of care and attention.
Problems fall into two camps: electrical and mechanical. In this article we will spotlight the mechanical causes and provide a straightforward guide to troubleshooting. The mechanical causes of a breakdown are more than likely as follows: liquid slugging; liquid dilution; liquid washout; high discharge temperature; lack of oil; and contamination.
Liquid slugging is the result of trying to compress liquid in the cylinders. Liquid may be refrigerant or oil or a combination of both. The symptoms can be broken reeds, rods, or a crankshaft; loose or broken discharge bolts or blown gaskets. Pressures generated can be sufficient to puncture the piston or the valve plate.
Maintaining proper compressor superheat, preventing uncontrolled liquid return with accumulators and checking the pump-down cycle could be the way forward here.
The most positive and dependable means of keeping refrigerant out of the compressor crankcase is the use of a pump-down cycle. By closing a liquid line solenoid valve, the refrigerant can be pumped into the condenser and receiver, and the compressor operation controlled by low pressure. The refrigerant can then be isolated during periods when the compressor is not in operation and the migration of refrigerant to the compressor crankcase is prevented.
Liquid dilution is a result of liquid refrigerant returning to the compressor during the running cycle. Oil becomes diluted causing loss of lubrication to the rods and main bearings.
The symptoms can include: worn bearings;
scored and/or broken rods; scored crankshaft; worn oil pump; and rotor drag/shorted stator.
Possible cause and correction can be preventing uncontrolled liquid return with the accumulator, maintaining proper compressor superheat, checking the defrost cycle and checking for oversized TEV.
Liquid washout is a result of refrigerant washing oil off bearing surfaces and the off cycle migration of saturated refrigerant into the crankcase. The symptoms may be:
• worn rods and bearings
• worn pistons and cylinders on lower end
• worn crankshaft and oil pump
• and scored cover bearing and crankshaft.
Locating the compressor in warm, ambient conditions or installing a pump-down cycle could solve the problem. It is also worth checking the crankcase heater operation too.
High discharge temperature is a result of temperatures in the compressor head and cylinders becoming so hot that the oil loses its ability to lubricate. The symptoms may include:
• discoloured valve plate
• overheated or burned valve reeds
• worn pistons, rings and cylinders
• scored rod, bearings and crankshaft.
Check suction superheat, look for low-load and evaporator problems, check for dirty condenser and inoperative condenser fan and also run the rule over the ambient temperature. Blown head or valve plate joints may be a problem, so too could be a leaking valve gear or relief valve. There could be defective start and unload mechanisms. Look out too for air in the system.
Lack of oil is a result of not enough oil in the crankcase to properly lubricate the running gear. Symptoms may include:
• scored bearings
• broken rods
• scored crankshaft
• and low oil in the crankcase.
To sort this one out it is worth checking the oil pressure switch operation, having a look at the unit in low load conditions, checking for inadequate defrost and eliminating any short cycling. Lack of oil pressure could be down to dirt in the system or the oil, a blocked strainer or oil filter and defective oil pumps. It may also be worth having a quick peek at the oil separator if there is one fitted.
Contaminants include moisture, air, non-condensables, along with chips of copper, steel, or aluminium, copper oxide, iron chloride, and many other types of dirt that might enter a system accidentally during installation or servicing. This could lead to: copperisation; overheating; valve leakage; electrical failure; and acid. Ways to handle this could be to evacuate the air, dehydrate the moisture and use strainers and filters to remove chips and dirt. To deal with the acid replace the oil and/or the filter drier.
And finally, in this short guide to tackling compressor failure, if a compressor does develop problems, and the cause has not been found and dealt with, remember that a replacement compressor will fail too.
J&E Hall
01322 394420