Air Cooled Chiller Condenser Testing

Two large air cooled chillers during condenser testing

Air Cooled Chiller Condenser Testing

We at Maximus Chillers have recently carried out air cooled chiller condenser testing at a new 3 year maintenance contract in the North West. The last of 4 systems had gone down on the morning that we first attended site. 1.2 MW of cooling had been lost which resulted in the water system temperature rising to 31°C. These chillers are used to cool an office block, so we wanted to get some of the systems up and running as quickly as possible…

News Article No.16

Air Cooled Chiller Condenser 1 Testing

One of the systems was disabled due to a condenser fan failure. 1 of 5 fans was found to be seized and disconnected. Fan speeds have sufficient redundancy built in to allow for a high ambient and a partially blocked condenser. There was a high ambient condition, but the condenser was found to be clean. Therefore, our engineer decided to run test the system and monitor the condenser pressure…

Fan Speed Transformer

3 fan speeds available on this chiller- low, medium and high. The fan speeds are achieved using a transformer which makes a different voltage for each fan speed. This is quite a good way of controlling the condenser pressure, as all of the fans run smoothly together. The program looks at the condenser pressure using a transducer, then selects the required fan speed.

Medium Fan Speed

As expected, the chiller program selected the medium fan speed. This provided the required saturation pressure in the condenser and so adequately subcooling the refrigerant.

Compressor Protection

All of the system pressures and temperatures were nominal, so the compressor was protected. There was a good oil return and sufficient cooling to the suction housing from the refrigerant. This cooling ensured that the internal motor windings did not overheat.

Monitoring

Having got this system away- our engineer monitored the water system temperature which started to come down. More cooling was required, however, to get the water system temperature down to set point…

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Air Cooled Chiller Condenser 2 Testing

Another system had been the only system running for some time.

Blocked Condenser

Grass pollination had caused a covering of organic material to accumulate on the condenser. The increased head pressure had caused the high pressure switch to trip. This type of material was easy for our apprentice to remove, as it clumped together when brushed.

HP Reset

The high pressure switch was reset and the system was run tested. It did not trip again, but a high pressure condition still persisted in the condenser.

Chemical Clean

Therefore, our engineer decided to carry out a chemical clean. After the whole of the ‘v’ type condenser was cleaned- the pressure came down a little.

Non Condensables

This system stayed running, but in the high fan speed. On the upcoming maintenance visit, a refrigerant diagnosis will be carried out to assess whether there are non condensables in the system. If this is the case, a false reading of subcooling will be recorded, as the non condensables throw out the calculation.

Cool Building

With 2 systems now running, the water system got down to set point. Our customer was really happy as the situation had gone from: office workers walking out of the building- to a cool building before 10 o’ clock on the first day of the contract.

Air Cooled Chiller Condenser 3 Testing

The third system had been on pressure test for the past 2 months with the condenser valved off. There was still pressure in the system, but our engineer decided to confirm the pressure test for himself. He did this by leaving it on pressure while having a drive round the suppliers. Several hours later, the pressure had remained constant, so he was able to start the evacuation process. After this was completed, the system was recharged and run tested. Now there were 3 systems up and running.

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Air Cooled Chiller Condenser PRV Testing

The pressure relief valves on the condensers have recently been changed. Industry guidelines state that they should be tested or replaced every 5 years. There is no guarantee that the PRVs will re seal satisfactorily after they have been tested. Therefore, in practice they are usually replaced. We inspected the date of the replacement, the burst pressure, the associated certification and paperwork- all was found to be satisfactory.

Air Cooled Chiller Condenser 4 Testing

The fourth system has a leak on the condenser with some refrigerant remaining in the system. This refrigerant will need to be decanted on a subsequent visit using a pump out unit and a recovery cylinder.

F-gas Leak Testing

The entire system will be pressure leak tested to identify the location of all leaks. Then, the leaks will be repaired using oxy-acetylene. After this, the system will be pressure tested to ensure its integrity.

Dehydration

Then, the dehydration process will be carried out. The achievable pressure of the vacuum pump will be tested and recorded. Evacuation will be carried out until this recorded pressure is achieved. Our engineers are issued with a powerful 10 cfm vacuum pump to speed up this process. This system will then be recharged and its operation tested.

Air Cooled Chiller Condenser Testing and Calibration

One of many tests that we will carry out on the upcoming maintenance visit is condenser transducer calibration. Transducer readings are not linear, so care will be taken to achieve an accurate calibration. A password will be entered into the controller to gain access to the required menu. Then, each of the transducers will be adjusted. The result of this means that accurate diagnosis can be carried out and the correct subcooling readings can be recorded.

The above is just part of the service that we provide to you- the customer! Having the capability to do anything and to extend the life of your chillers is part of what we call the MAXIMUS ADVANTAGE™ Any Chiller- Any Problem- Any Part- Any Refrigerant- Anywhere.

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6 green Bitzer scroll compressors being maintained in a chiller

Scroll Chiller Compressor Maintenance

Here at Maximus Chillers, we carry out scroll chiller compressor maintenance to extend the lifespan and reduce downtime to your critical plant. This planning ahead is central to how we do things- we resolve small problems before they become big problems. Having the capability to do anything is part of what we call the MAXIMUS ADVANTAGE™ Any Chiller- Any Problem- Any Part- Any Refrigerant- Anywhere.

How they Work

The refrigerant vapour passes in between two scrolls (spirals) One of these is fixed, the other orbits backwards and forwards against the fixed scroll using a swing link. This creates a series of crescent shaped gas pockets in between the scrolls. These gas pockets get smaller in size as the refrigerant travels from the suction at the edge, down the spirals to the discharge at the centre. It then leaves downwards through a port. There are check valves to prevent back feeding of refrigerant during off cycles when other compressors on the same system are still running.

Advantages

There are several gas pockets occurring at any one time through the scroll, therefore giving a smooth and continuous compression cycle. Other advantages are low internal friction resulting in a quiet operation and low vibration levels. These low vibration levels help to lower the occurrence of leaks around the compressor.

Oil Level during Scroll Chiller Compressor Maintenance

The correct oil level is noted on our detailed Tick Sheets for all of the compressors. This record helps with the diagnosis when things start to go wrong. We can look for a pattern where the level starts to drop off in the compressors on a particular system, then look at the other readings which may be the cause.

Oil Samples during Scroll Chiller Compressor Maintenance

At periodic intervals, we take oil samples which we analyse in our laboratory at Head Office in Droylsden. We look for white metal, coloured metal, refrigerant composition, acid and sludge. This means that we can notice, then resolve system issues before a costly compressor failure occurs.

Oil Changes during Scroll Chiller Compressor Maintenance

This kind of compressor is fitted with a service port for the draining and filling of oil. The correct procedure is followed due to system pressure being present inside. We then carry out the oil changes using our specialist oil handling tools. Our skilled engineers carry the correct PPE for the work being carried out. We also we provide Risk Assessment Method Statements for each job.

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Crank Case Heaters

In the photo you can see the silver band of the crank case heaters at the bottom of the compressors. They keep the oil at the required temperature so as to not cause compressor wear on start up. The heaters also have the effect of ensuring that the refrigerant cannot condense into liquid during cold weather. As a compressor cannot compress a liquid, compressor failure would occur. A normally closed contact on the delta compressor starter contactor usually runs the heater: it drops out when the compressor starts. The function of all of these heaters is checked on each maintenance visit.

Discharge Temperature

It is critical that the discharge temperature is not too high, as this is the cause of the refrigerant breaking down into acid and sludge as previously mentioned. The acid rots the insulation on the copper windings inside the compressor. When this has occurred, an electrical failure will result in the compressor, causing the fuses to blow and a fault condition on the chiller. Compressor swap out is the cure which is expensive and inconvenient. We take the discharge temperature readings during the maintenance so as to fault find the cause.

Suction Pressure

Adequate suction pressure is needed to ensure good oil return to the compressor and prevent low pressure trips. As the refrigerant entrains the oil around the system, a good mass flow rate is needed, or the oil will just ‘pool’ in the bottom of the evaporator. Poor oil return will result in a seized compressor. Therefore, particular attention is taken to the low pressure gauge by our engineers. System adjustments or recommendations are made to ensure the seamless operation of your plant. If system overhaul is required, a Quote will be submitted at the end of the visit.

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Compressor Holding Down Bolts

There are usually four of these on each compressor. They fit though a steel sleeve which, in turn, fits through a rubber mount. The whole assembly is bolted into the frame of the chiller. The function is to securely hold the compressor in place and to dissipate vibration. We check the tightness is correct with our torque wrenches and change the rubbers periodically. A compressor rattling around uncontrollably, especially during start up and stopping causes catastrophic leaks around the compressor. This usually leads to the whole, or the most part of the refrigerant charge being lost to atmosphere.

Motor Protection Module

A resistance sensor embedded in the compressor windings plugs into this module on the S1 and S2 terminals. The fault feedback to the controls is on the M1 and M2 terminals. The power supply to the module is on the L and N terminals. When the windings start to get hot, the module detects this after a pre determined level of resistance is reached. The M2 terminal opens and volts drop out to the relay board, then a lower volt control signal drops out to the controller. We check the sensor resistance and compare it to a chart to ensure it is within the allowable range on each visit.

Wire Tightening during Scroll Chiller Compressor Maintenance

We waggle the motor connectors and check for any cracking to the wires which may lead to an earth sort out. We tighten the wiring on the compressor starter contactors and check the condition of the contactor contacts. These become ‘splattered’ causing an uneven amount of amps to the compressor. This volt drop causes the contactors to fail and will eventually cause compressor motor failure. We change the contactor contacts from our range of parts at Head Office.

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Green Bitzer screw chiller compressor with oil separator removed on bench in our workshop

Screw Chiller Compressor Maintenance

Open Drive Screw Chiller Compressor Maintenance

Open drive screw chiller compressor maintenance involves changing the shaft seal at intervals, or if it leaks. This kind of compressor is used with ammonia as this refrigerant corrodes the copper windings and the insulation. It is also used with most large HFC (hydrofluorocarbon) and HFO (hydrofluoroolefin) chillers.

Semi Hermetic Screw Chiller Compressor Maintenance

Because all of the components are internal, this kind of compressor needs less maintenance. It is most often used with smaller chillers running on HFC and HFO refrigerants. This is the compressor type featured in the photo.

Matched Helical Rotors

This kind of compressor design uses a matched pair of helical rotors. These are accurately machined so as to trap, then compress the refrigerant as it travels along the screw. Oil injection is used to create a seal between the rotors. The two rotors are different in shape: the male rotor is driven by the motor and usually has 4 lobes. The female rotor meshes with male and usually has 6 interlobe spaces. The cylinder casting around the rotors is equally important as it seals in the vapour along the screw. Both rotors are helixes with the male rotor moving more rapidly. This compressor design provides a continuous pumping action, rather than pulsating as with a reciprocating compressor. Another advantage of this kind of compression is that there is very little vibration. Indeed, you can place a coin, on its side, on top of the compressors we look after and it does not fall over. This lack of vibration helps to prevent refrigerant leaks around the compressor.

Single Screw with Gate Rotors

This kind of compressor design uses one main rotor, meshing with 2 star gate rotors. These are at right angles with the main rotor. The main rotor usually has 6 grooves.

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Screw Chiller Compressor Maintenance and Reliability

Both of these compressor designs are very reliable with a long bearing life. A maintenance free lifespan of 30 years for the bearings is not uncommon. It is quite common that the compressor will outlive the chiller. In the unlikely event of bearing wear, a characteristic is for there to be undue noise from the compressor at part load which goes away at full load. Higher oil temperature and an unsatisfactory oil analysis are also indicators.

Slide Valve

The capacity is seamless as it is regulated with a slide valve. A spring returns the valve to the unloaded position and a gear type oil pump gives above discharge pressure to load it. The oil pump is not for lubrication, it is just to give the valve enough force to slide with the discharge pressure acting against it. A slide valve potentiometer is fitted to a sliding rod on the end of the valve. It translates the movement along this rod into usually 4-20mA. This signal feeds back to the controller which converts it into a percentage loading reading.

Lubrication during Screw Chiller Compressor Maintenance

Pressures and temperatures are taken during the maintenance to ensure seamless operation. The oil sump is usually inside the base of the oil separator which is at discharge pressure. An oil return pipe is available from the oil sump to the suction side of the screw. Because of the pressure difference from discharge to suction, the oil naturally lubricates the compressor without any need of an oil pump. The oil lubricates the bearings and is injected with the refrigerant along the screw. This provides a seal between the rotors or gate rotors, it also lubricates the rotors to prevent excessive wear.

Oil Separator

The oil enters the oil separator after being discharged with the refrigerant from the compressor. This vessel is insulated so as to stop refrigerant condensing inside as it would in the condenser. An oil heater keeps the oil at the optimum temperature for the compressor. This heater also prevents liquid from forming in the oil separator during off cycles. A check valve on the outlet also prevents this from happening by stopping the migration of refrigerant from the condenser. As the oil sump is the oil supply to the compressor, a temperature sensor will make the program lock the compressor out, should the oil be too cold. This is usually because the main power supply to the chiller has been off during maintenance. The larger volume inside the oil separator slows the speed of the refrigerant so as to allow the oil to drop out. A common design is for the discharge to be directed to the top of the vessel, with a spiral going down to the sump. The oil falls out of the refrigerant vapour during this process. For additional oil recovery, the oil goes up through finer and finer layers of mesh. The oil sticks to this mesh and runs down into the sump.

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Oil Return

Small amounts of oil that have escaped the oil separator will end up in various vessels around the system. On smaller HFC systems there is less of a problem as the oil is entrained by the refrigerant, round the system and back to the compressor. In larger, flooded HFC systems, the oil mainly ends up in the bottom of the evaporator. For ammonia systems, the oil does not entrain with the refrigerant, so oil return devices must be used.

Eductor

This is a pot at the bottom of the vessel where the oil collects. At periodic intervals, discharge gas is blown across the top of the oil which has collected. This has the effect of picking it up and carrying it into the suction of the compressor.

Periodic Oil Changes

We at Maximus Chillers have the full range of refrigerant grade oil for all refrigerant types. It is part of what we call the MAXIMUS ADVANTAGE™ Any Chiller- Any Problem- Any Part- Any Refrigerant- Anywhere. It is critical that the correct oil is selected otherwise compressor failure will result. During each maintenance visit, we make a note of the compressor run hours and carry out oil changes at the correct intervals.

Oil Analysis

We take oil samples which we analyse in our laboratory for signs of compressor wear, oil quality and contaminants. This way, we can prevent untimely compressor failure. If one of your compressors were to fail, however, we have a remanufacturing facility and a lift and shift team to get the job done fast! 

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Industrial refrigeration oil change showing ammonia warning sign

Industrial Refrigeration Oil

An industrial refrigeration oil visit was arranged because of a return issue. Several fans had been down on the air cooled system. This causes the system to unload so as to prevent a fault occurring. The lower mass flow rate of the refrigerant caused oil to migrate to the low side of the system. Several vessels are available with various forms oil return. These vessels and the oil cooler were valved off. A two man team with the correct ammonia PPE and breathing apparatus carried out the job.

Industrial Refrigeration Oil Recovery

Only some of the oil was recovered from the plant. Therefore, the engineers decided to add new oil to get the system running. The only problem with doing this is that once running the machine, the oil has to be got back out. Therefore, the oil had to be adjusted a second time to resolve the oil return issue.

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Fan Decks

For this visit, scaffolding had been erected in accordance with the Risk Assessment. The old fan decks were lifted out by the two man team. A really good team was sent out: one of the men was more technical and the other more mechanical- complementing each other with their different skill sets. The new fans were an updated version and the wiring was different. The fault link had to be adapted, but the 0-10v input from the controller was the same. The controller reads the pressure from a transducer, then sends out a voltage which the fan turns into the corresponding speed.

System Testing

The fault link had been correctly modified, so the testing of the system was carried out. The machine loaded up to 62% and monitoring continued during the rest of the visit.

Any Chiller- Any Problem- Any Part- Any Refrigerant- Anywhere- The MAXIMUS ADVANTAGE™

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