Reciprocating Chiller Compressor Maintenance

Two blue open drive Vilter reciprocating chiller compressors being maintained in a plant room

Reciprocating Chiller Compressor Maintenance

Reciprocating chiller compressor maintenance for two low temperature ammonia chillers. Reciprocating means a forwards and backwards motion in a straight line. This is achieved by converting the circular motion of the crankshaft, into a linear motion using the connecting rods. The pistons are on the end of the con rods, which slide up and down inside the cylinder liners.

Piston Rings

There are two types of piston rings which are used:

Compression Ring

This is the upper ring and is designed to a high tolerance to prevent the refrigerant vapour from bypassing the piston.

Oil Ring

This is the lower ring and is designed to regulate the oil flow around the piston.

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Cylinder Head

The cylinder head is usually cast iron and serves as a pressure plate. It holds the valve gear and has passageways for the suction and discharge of the refrigerant. The discharge pressure varies according to the kind of refrigerant and application. For a 0°C saturation it can be as low as 7 bar on R134a, or as high as 30 bar on R410a. The discharge temperature is usually around 60°C to 80°C which is recorded on our detailed Tick Sheet during the maintenance.

Valve Gear

Helix springs or reeds are popular with this type of compressor. Wave springs are another design that can be seen in operation on our YouTube channel. The video uses a cut away view and the oil is depicted in yellow. These springs control the suction (intake) and the discharge (exhaust) of the refrigerant…

Suction

The suction valves have the least amount of failures because the refrigerant is cool, low pressure and is carrying oil.

Discharge

The discharge valves, however, can have heavy molecules of hydrocarbons collect on them in the form of carbon. This causes them to not seat correctly, resulting in a deterioration of compressor performance. The difference in pressure between suction and discharge, otherwise known as the compression ratio, is a check that we carry out during the maintenance. The pressure and temperature is higher on the discharge valves, so more stress is exerted on to them. Therefore, they have a reduced lifespan by comparison to suction valves.

Top Dead Centre

The piston needs to come as close as possible to the cylinder head to create the largest amount of compressed refrigerant. This is called the clearance space which is usually less than 0.5 mm.

Discharge Header Safety Spring

This spring is fitted into the cylinder head and allows the valve gear to lift when:
• Liquid refrigerant slugs back to the compressor due to poor heat exchange in the evaporator.
• An oil slugging condition occurs.
• Water is drawn around the system from a burst condenser or evaporator.
As a compressor cannot compress a liquid, the valve gear lifting prevents an expensive compressor smash up from occurring.

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Reciprocating Chiller Compressor Maintenance of Shaft Seal

The compressors in the photo are open drive. Each has a shaft seal which has two, mirror finish rubbing surfaces. One seal turns with the crankshaft and the other is stationary. We check the amount of oil that is seeping from the shaft seal on every visit. When we notice that the level in the oil bottle is too high, we arrange a visit to change the shaft seal. This can be done without disruption to your process, as the other compressors can be left running while we carry out the work.

Reciprocating Chiller Compressor Maintenance of Drives

This kind of compressor is usually driven by an in line electric motor, as in the photo. It can also be driven by ‘v’ belts from an electric motor which is located to the side of the compressor. The ‘v’ belts are checked during the visit to see if there are any cracks on the inside working surface. We replace these with the pre ordered spares that are on site at no extra charge. They are then re tensioned according to standard industry guidelines.

Reciprocating Chiller Compressor Maintenance of Crankcase

This is a cast iron housing that all of the above components fit into. It provides the necessary support and strength for the compressor to operate at its high temperatures and pressures. The crankcase heater keeps the oil at operating temperature during the off cycles. It is usually a bore type which pushes into a hole in the casting. We check for the correct operation of the crankcase heaters and replace them where necessary. This is another spare that is kept on site, so that a return visit is not needed.

Service Ports

These bolt on to the compressor crankcase. They can be positioned in various directions, depending on which way the suction and discharge pipes go. The compressor can be valved off when it is being worked on. These ports are used by engineers to attach their gauges during the maintenance. On each visit, we check the calibration of the system pressure transducers by checking them against our gauges.

Reciprocating Chiller Compressor Maintenance of Oil Pump

This is a gear type pump which is fitted to the end of the compressor crankshaft. The pump sucks the oil through a filter from the sump of the compressor. Then, it is discharged from the pump, down the crankshaft passageways to the connecting rods. From here it travels up the con rod passageways and out through the pistons to the cylinder liners. Here, it provides the essential lubrication between the pistons and the liners. According to the maintenance schedule, we periodically change the oil filters to ensure the optimum running conditions of your compressors.

MAXIMUS ADVANTAGE™

If a compressor smash up occurs and the compressor is found to be obsolete- don’t worry. We can adapt the compressor mountings and the pipework for a different compressor. This is part of what we call The MAXIMUS ADVANTAGE™ Any Chiller- Any Problem- Any Part- Any Refrigerant- Anywhere.

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Grey centrifugal chiller compressor being maintained

Centrifugal Chiller Compressor Maintenance

Centrifugal chiller compressor maintenance keeps your critical plant up and running and your customers happy. The work can be scheduled to be carried out during factory shutdown, so as not to disrupt your production. We can also carry out this work to your compressors while the factory is in production. This is achieved by isolating the compressor that needs to be worked on when it is in an off cycle. The compressors on remaining machines can carry on running.

Tasks during Centrifugal Chiller Compressor Maintenance

Here are some of the tasks that we carry out…

Oil Changes

The oil becomes dirty over time by picking up contaminants that have formed in the system. Two of these contaminants are…

Acid

Compressor discharge is the hottest part of the refrigerant cycle. Acid can be formed from the refrigerant in this part of the system.

Refrigerant Types

A popular refrigerant for centrifs is R134a. Because it consists only of one kind of refrigerant, it does not fractionalise into different component refrigerants. This would be no good for a flooded system because one or more of the refrigerants would end up in the bottom of the evaporator and condenser. The remaining refrigerant would circulate and the whole plant would not function as it should. Refrigerants popular for other kinds of system are zeotropic HFC refrigerants. This means that the different component refrigerants have different boiling points- R407c is a good example.

Metal

White metal from the compressor and copper from the heat exchangers end up in the oil. They will eventually be caught by the system oil filters…

Oil Filter Change

After completing the above, now is a good time to change the filter as the compressor is valved off and has been broken into. It is also when an oil sample is taken depending on the schedule…

Oil Samples during Centrifugal Chiller Compressor Maintenance

Our oil samples are transported using our specialist kits which include the bottles and labels. This saves mix ups in our laboratory during the analysis.

Leak Rectification

The drawback of carrying out the above are leaks because the pressure has been pumped out of the compressor into another part of the system. The ‘o’ rings and shaft seal can now leak, this kind of failure can be rectified whilst still on site by the knowledge of our engineers.

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Oil Pre Lubrication

When the compressor starts, the oil pressure is built up first using an oil pump. This is so the internal components such as the high and low speed shaft are properly oiled before they start to rotate. They can run at 10,000 RPM and are very expensive to replace. Therefore, we check the oil pressure gauges and the system controls to ensure optimum ‘pre lubing’ of your compressors.

Volumetric Efficiency

The ratio between the volume actually compressed and the theoretical volume derived from compressor design calculations. This kind of compressor has a lower volumetric efficiency than positive displacement compressors. It is because the refrigerant is compressed off the tip of the rotating impellor or impellors. The refrigerant moves outwards in a circular path due to centrifugal force. A centrif more than makes up for this lower volumetric efficiency by the high mass and volume of the refrigerant that it circulates around the system.

Design 

The refrigerant is sucked from the evaporator into the centre of the impellor which is a disc of radial blades positioned to direct the refrigerant outwards. Due to the low differential of pressure, multiple stages of impellors are often arranged in series with the discharge being directed onto the suction of the next impellor. This is simple in design with less moving parts than some other compressor types. Modern centrifs can have magnetic, levitating bearings and so remove the need for oil in the system altogether. The faster the speed, the better the efficiency- so high speed rotation of the impellor is achieved at full load. The compressed refrigerant is discharged into the condenser.

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Electric Drive Centrifugal Chiller Compressor Maintenance

Most modern centrifs are driven by variable speed drives. This is for efficiency as the load can be exactly matched. Another reason for this is to soft start the compressor. 415v and 3.3kv are popular volt inputs, although other voltages can be made available by the onsite transformer.

Open

An open drive electric motor is the most popular design. It has the drawback, however, of needing a shaft seal which needs to be replaced at periodic intervals. This shaft seal is also prone to leaking refrigerant and oil.

Semi Hermetic

The photo is of a semi hermetic design of this compressor type. The motor is contained inside the suction housing and so has the benefit of being cooled by the refrigerant. No shaft seal is needed and therefore it has none of the associated maintenance drawbacks.

Steam Drive Centrifugal Chiller Compressor Maintenance

On oil rigs there can be an abundant supply of steam that can be used to drive the compressor. It is often used on multi stage compressors which are used for the liquefaction of natural gas. The steam goes through a turbine which is connected to a shaft- this drives the impellors. The steam flow and pressure can be tested and adjusted during the visit.

Woops Something Went Wrong

When you suffer a centrifugal compressor failure- don’t worry. You are in safe hands with the team here at Maximus Chillers. We have a team who can get the compressor out and lift it to the workshop on site, or transport it to our Head Office. We have another team who are experts in the remanufacturing of this kind of compressor. A fast supply chain is in place for delivery of the internal moving parts, gaskets and bearings. We are so confident that you will be happy with us- we offer a 12 month warranty on all of our compressor rebuilds.

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Chiller compressor failure of blue Grasso in enclosure

Chiller Compressors

System Testing for Chiller Compressors

Chiller compressors fail often as a result of ineffective servicing and system testing. At Maximus Chillers, we carry out extensive tests during our visits to ensure that small problems are resolved before they become big problems. If we notice a reading starting to become abnormal, we can carry out the diagnosis and then remedy the problem. Some of the compressor readings we monitor are:

Temperature of Chiller Compressors

The suction, discharge, motor windings and bearing temperatures are recorded for comparison to previous visits. These are often available in the PLC for the chiller, or our engineer can take the readings with his test equipment. Problems with the oil cooler can be the cause of higher compressor temperatures, the system running outside of its nominal operating conditions is another reason. Magnetic drive systems have an advantage as they do not use oil.

Accelerometer

Portable vibration sensors are carried in of each of our company vehicles. This is an accelerometer to measure vibration. Along with other system readings, we keep an on going record of the vibration levels around the compressor. When internal components are coming out of alignment due to wear, this causes an out of balance condition in the compressor. This, in turn, causes a knock on effect- causing other components to go out of balance. Catching this condition early will prevent a compressor smash up resulting in the replacement of expensive internal components.

Oil Analysis for Chiller Compressors

Another way of preventing big problems from occurring is periodic compressor oil testing. Samples are taken, usually on alternate visits, which are sent off to a laboratory for analysis. The acid level is tested to provide pre warning of a potential compressor motor windings burn out. This is because acid in the compressor oil rots through the electrical insulation on the motor windings. The presence and quantity of white metal and yellow metal is analysed too. This is a window through to a component starting to wear inside the compressor.

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Screw

The compressor in the photo is a screw compressor. It operates with ammonia refrigerant. This refrigerant is usually used for low temperature applications, mainly associated with food production. This compressor, however, has a 1°C refrigerant saturation and is used to cool computer rooms. Common causes of compressor failure on this kind of compressor are:

Leaking Castings on Chiller Compressors

The various compressor components are sealed together using ‘o’ rings or paper gaskets. ‘O’ rings are especially prone to leaks due to work hardening and flattening of the sealing face. The system can be pumped down and the compressor valved off. Then, our lift and shift team can remove the compressor to our remanufacturing facility for strip down.

Leaking Shaft Seal

The mating surface of a shaft seal has a mirror smooth finish. This is to reduce friction and aid with a better seal. Over time, this starts to wear, causing an ineffective seal with a leak of refrigerant and oil. A service visit can be arranged to change the shaft seal on site. The shaft couplings can be split, the shaft seal can then be removed and replaced. A quick job, then the machine is up and running again.

Slide Valve Potentiometer

This is an electronic device with a slide attached to the moving compressor slide valve. The device has a start and an end position programmed into it during commissioning. The potentiometer converts the slide valve position, usually into a 4-20mA signal which is fed back to the chiller PLC. They are prone to reading out, or the reading being jammed in one position. This results in a trip out from the controls, as the controller is not able to determine the true position of the compressor slide valve. We have an off the shelf stock of slide valve potentiometers for the various compressor range. A service visit can be arranged to replace the part after diagnosis has been carried out. The controls operate the loading and unloading solenoid valves to change the position of the slide valve.

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Centrifugal

Centrifugal compressors are a very reliable kind of compressor but when they go wrong, they can go wrong in a big way. Compressor overhaul is expensive, this can be carried out onsite, or a better option is a lift and shift to our remanufacturing facility. Proximity sensors are usually fitted to monitor the distance between the impeller and the casting. This is an added protection along with the other sensors and transducers around the compressor.

MCS

We are on account with Micro Control Systems- an American company who specialise in building panels to order for specific chiller compressors. We easily fit this control system to any compressor to control the loading of the vanes in accordance to the available load from the process. The control panel has previously been fitted to other machines of the same model number, so any teething issues have already been ironed out. Maximus Chillers can achieve seamless operation of your plant.

Oil System for Chiller Compressors 

Newer centrifugal compressors are oil free so as to eliminate any of the service issues relating to oil. There are a substantial amount of compressors, however, that use oil to lubricate the bearings. This kind of compressor, if properly serviced, can last for 50 years. The oil system picks up impurities which are caught by various filters. These filters can be changed or cleaned according to the prescribed service schedule. Our engineers make sure that spares are ordered and kept onsite prior to a visit.

Cost Effective

Our visits and ongoing upkeep of your plant saves money. Money spent as a preventative measure saves so much more money in the long run. With competitive prices on specialist internal centrifugal compressor parts- Maximus Chillers completes the picture. When compressor failure occurs, you are in safe hands with years of industry experience invested in each of our engineers.

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Scroll

These are some issues affecting a scroll compressor:

High Discharge Pressure

With high discharge pressure, there is a corresponding increase in discharge temperature. This means that the compressor is operating beyond its recommended values. The cause of this is often a poorly maintained condenser. Especially on industrial chillers, there can have been gaps in the schedule where the condenser was not correctly serviced. This condition is often rectified easily by an onsite engineer by giving it a brush down. Where the fins are bent over- we carry a specialist tool to straighten them back out- how they came out of the factory. We also use different formulas of chemicals to rinse the various kinds of dirt from deep within the fins.

High Suction Pressure on Chiller Compressors 

Some chillers are used where very high water temperature can come back from the process if the chiller were to be off line for a short period. Usually, this happens in factories where certain industrial processes are being carried out. When the onsite engineers start the plant back up, the chiller experiences a high heat load to deal with.

MOP Expansion Valves

Maximum operating pressure expansion valves limit the pressure in the evaporator to a given level, regardless of the available heat load from the process. They do this by having a limited amount of liquid refrigerant in the bulb. When this runs out, the power element cannot push the orifice open any further- thus limiting the suction pressure. This is important to prevent scroll compressor failure as it prevents putting added strain on the compressor motor windings due to high suction pressure.

Oil and Refrigerant Shortage

Where there is a shortage in refrigerant, there follows a low oil level condition. The refrigerant mass flow rate carries the oil around the system and back to the compressor. This is greatly impaired when the chiller is short of gas. The oil cools the compressor and lubricates the shaft bearings. These bearings and other internal components wear down and seize causing failure. Maximus Chillers can put together a package to minimise chiller compressor failure.

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Ammonia refrigerant cylinder being winched up a cat ladder using a block and tackle

Industrial Refrigeration Ammonia

The industrial refrigeration ammonia visit was at a site in Aberystwyth. We look after 2 ammonia chillers for a blue chip customer. They are 10 year old chillers that have been designed to need ongoing attention. Quite easy job as the faults they go into are reoccurring round in a circle. They have previously had the same problems.

Ammonia Shaft Seal for Industrial Refrigeration

The shaft seal had started to leak ammonia. It was replaced by removing the shaft seal cover, splitting the coupling and sliding the shaft seal over the shaft. After removing the non condensables, we reintroduced the ammonia back into the compressor- now the 'o' rings leaked.

Leaking Industrial Refrigeration Ammonia 'O' Rings

Leaks on ‘o’ rings occur because they become flattened and plastic like. This is caused by heat around the high side of the screw compressor. When there is pressure in the system- the oil if forced against the gap- making a seal. After the ammonia refrigerant had been handled- the oil ran away- causing a leak when the ammonia was reintroduced.

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Lift and Shift Visit

The pipework and ancillaries had been removed from the compressor prior to the visit. Our lift and shift guys arrived and set up their lifting equipment. They raised the compressor and manoeuvred it on across the roof. They then manoeuvred it up then down some steps. They used lifting equipment to get the compressor, end on- down a cat ladder.

Compressor Strip Down

It is always better to strip down this kind of compressor in a workshop- on a bench with compressed air and all the tools nearby. We unbolted the castings and slid the screw out. The various castings being laid out in a line. We replaced the ‘o’ rings for new the rebuilt the compressor.

Industrial refrigeration ammonia being charged by an engineer wearing breathing apparatus and gloves
Ammonia refrigerant being sucked into the low side of a flooded evaporator

Ammonia Refrigerant Charge

Part of the ammonia refrigerant had been handled during the visit. This left the compressor pipework, compressor and the oil separator open to atmosphere. The non condensables were evacuated. In the photo we are lifting the cylinder up a cat ladder into the plant room. From there it needed to be man handled up some steps, down some steps and out onto the roof. We charged the ammonia plant with liquid ammonia until the superheat and subcooling values were within industry known limits. The sight glass levels and condition were monitored too.

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