Chiller Troubleshooting

Troubleshooting of a chiller panel on a system running on ammonia refrigerant

Chiller Troubleshooting

In this article a bespoke chiller with ammonia refrigerant is having troubleshooting carried out to identify the faults. Read below to have a look into the various issues that commonly occur on this machine…

News Article No.19

Chiller Troubleshooting of Low Oil Level

There are times when this machine experiences a low load condition. This is because another chiller is also available as well as free cooling.

Building Management System

The BMS regulates the sequencing of the chillers, the pump sets, the water system valves and the free cooling. We found after troubleshooting that, especially in the winter, the BMS can be slow to react. This causes the chiller to continue running with low load.

Slide Valve at 0%

During a low load condition, the compressor has been designed to run with the slide valve at 0% to prevent too many start ups. These start ups would cause a higher amount of electricity to be used because of the amps being drawn to get the compressor going. Although the compressor uses star/ delta contactors which are a soft start, the repeated starts would still draw a lot more amps. Therefore, the compressor continuing to run at 0% is a lot more efficient.

Oil Pump and Oil Level

When this condition occurs, however, the oil pump continues to suck and then discharge the oil. Eventually, the level in the oil separator drops to below the oil level switch and the alarm is triggered. The photo shows this switch which is a float on a stem. The circuit is completed when the float is at the top of the stem. When the float slides down the stem- this triggers the alarm.

Stainless steel oil level switch removed from the chiller showing the float and the stem
The switch protects the compressor

The Repair

Specialist ammonia resistant personal protective equipment, or PPE is donned by our engineers. Then, ammonia grade oil is pumped in so as to complete the circuit of the switch. After a start up, the same amount of oil has to be taken back out at the other end of the system using the dead man’s valve. This is a valve on the bottom of the economizer which shuts by itself using a spring.

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Chiller Troubleshooting of Oil Filter Differential

When this alarm occurs, you can see the discharge pressure remain at 8 bar and the pressure after the oil filter slowly drop off to less than 6 bar. When interrogating the program, the oil pressure differential trip out is set to 2 bar. After this differential is exceeded, a timer starts to prevent spurious trip outs occurring if the differential pressure is exceeded momentarily. If the differential pressure continues to be exceeded after the timer has timed out- the machine goes into a fault condition which needs a manual reset.

Chiller Troubleshooting Inspection

When removing the cover for the compressor enclosure, the discharge oil pipe can be seen going into the oil filter. An oil pressure transducer can be seen on the far side of the filter.

Contaminants

After the above troubleshooting, it was found that with this kind of system, the oil becomes contaminated with sludge and debris from around the system. Over time this builds up behind the oil filter and so causes the trip out.

The Repair

The repair requires the oil filter to be valved off on either side. Then, full length PPE suitable for the handling of ammonia is required along with beathing apparatus. Each of our engineers carries a portable ammonia alarm and state of the art ammonia handling equipment. We also produce our own ammonia oil filters which we keep on the shelf at Head Office. Give our Technical Support Desk a bell for further assistance.

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Chiller Troubleshooting of Slide Valve Potentiometer

This is a 4-20mA sensor which detects the position of the slide valve. 4mA being shut and 20mA being open. We attended a call out to troubleshoot the alarm message of “slide valve failed to reach the closed position” There is a spring on the end of the slide valve which is used to push it to the closed position when the compressor is off. We decided to activate the drain and fill solenoids which are used to open and close the slide valve. Still the same fault persisted. Having decided that the slide valve was indeed at its closed position, we decided to calibrate the closed and also the open position of the valve.

The Repair

The compressor was valved off and our engineer donned the necessary PPE. The sensor stem was removed from the end of the spring which is attached to the slide valve. A lock nut is available to hold the stem at the required position. This was found to have vibrated loose and so causing the problem. The correct length was set on the stem and the lock nut was re tightened. After rebuilding the end of the compressor, the valves were opened and the compressor checked for leaks. Then, the system was run tested using the manual load and unload buttons on the controller. A good read back was recorded and the machine went into seamless operation.

Chiller Troubleshooting of Ammonia Leak Detection System

This device is stand alone from the chiller and has its own uninterruptable power supply. This UPS is needed so that when there is a power failure to the building, the alarm would still function. There are 2 toxic gas sensors and 2 fire detectors fitted in different locations on the chiller.

Out of Calibration

When we took over the contract 5 years ago, the chiller had been in fault for some time with a leaking shaft seal. After troubleshooting the problem, we found that the small amount of ammonia leaking from the shaft seal and onto the sensor had knocked it out of calibration. This is because the sensor needs to be calibrated to a zero ammonia atmosphere as a reference point.

The Repair

After changing the shaft seal, we decided to change all 4 of the sensors and plug the detection system into our laptop for calibration. The benchmark was set for each sensor, then each sensor was bump tested to the required parts per million of ammonia to ensure that the detector trips when it should. All read backs were found to be okay, so we completed the calibration certificate which the customer keeps in his chiller file.

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Hit the Tags at the top of the page to navigate your way to our extensive library of further reading on this subject.

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Showing how do chillers work with pipework of black centrifugal compressor

How do Chillers Work? Compressors

How do chillers work, this time focusing on compressors. Read below and we will explore the subject in detail…

The compressor can be seen as the heart of a chiller system. As explained in the last article in our Chiller Basics series: it enables the principle of the pressure temperature relationship to take place and circulates the refrigerant.

Evaporator

The sucking action of the compressor drops the pressure in the evaporator. This lower pressure, corresponding to a lower temperature, enables the refrigerant to boil off, absorbing latent heat from the process. The refrigerant, laden with heat energy, is then sucked into the compressor.

Condenser

The refrigerant is discharged from the compressor at a high pressure, relative to a high temperature, this is sufficiently above atmospheric temperature to enable the latent heat in the refrigerant to exchange into the surrounding air.

How do Chillers Work - Mass Flow Rate of Compressors

Compressors enable the refrigerant to flow around the system. The higher the pressure of the refrigerant, the higher the mass and so the higher the mass flow rate. In low temperature systems that run on a vacuum, a very large compressor is needed to achieve the required mass flow rate and so to cool the process.

Below are the 4 main types of compressors and how they work…

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How do Chillers Work - Scroll Compressors

This compressor is more often found on smaller chillers. Scroll is another way of describing a spiral. There are two spirals, one inside the other. One of them is fixed and the other one orbits. This creates crescent shaped gaps in between the two spirals. The volume of these crescent shapes gets smaller towards the middle of the spirals and so compresses the refrigerant. The refrigerant is then discharged through a port at the middle of the scrolls.

Maintenance

Scroll compressors are meant to be maintenance free as they are hermetically sealed with no moving parts on the outside. However, the correct amount and sufficient quality of lubricating oil must be maintained to prolong its life. A lack of routine maintenance by a chiller company causes frequent compressor failures and also allows acid to accumulate in the system. This acid causes further compressor failures to occur. Give us a bell and we will put together a tailor made maintenance schedule to prolong the life of your plant.

How do Chillers Work - Reciprocating Compressors

Small

These compressors can be small and used for low capacity applications. The motor is usually inside the compressor with the cylinder heads being removable. This is described as being a semi hermetic compressor.

Large

Reciprocating compressors can also be very large and often arranged in multiple compressor systems for high capacity applications. They are open drive when used with ammonia refrigerant which means that the motor is external from the compressor. This is because ammonia corrodes the copper windings of the motor.

How They Work

The circular motion of the crankshaft is converted into the linear motion of the connecting rods and pistons running up and down inside the cylinders- see a video of how they work. Another way of describing back and forth linear motion is reciprocating. The refrigerant is sucked into the cylinder on the down stroke and then the suction inlet valves are closed. The trapped refrigerant is compressed on the up stroke and then the discharge valves are opened when the piston reaches the top of the cylinder.

Maintenance

Reciprocating compressors need regular maintenance and strip downs at pre prescribed intervals. Failure for this to be carried out results in internal components becoming worn or loose. This imbalance in the compressor causes further wear and damage to take place. Eventually something like a piston comes loose and a compressor smash up occurs. The damage is usually limited due a low oil pressure condition occurring from the location of where the part came loose. Sometimes, however, parts of the connecting rods stay attached to the crankshaft which flail around inside the compressor. An example of this is when a compressor we rebuilt had carried on running on the last one of its 8 cylinders. The rest of the internal compressor components were reduced to small, triangle shaped pieces of metal in the sump. Our maintenance saves money!

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How do Chillers Work - Screw Compressors

A pair of matched helical rotors are machined to a high tolerance. One of them is driven by a motor and the other one interlocks with it. The gap between the rotors gets smaller as the refrigerant continues down the screw and so compression is achieved. Oil is injected into the screw to provide a seal, to lubricate and to cool the rotors.

Maintenance

This type of compressor is very reliable and can quite often out live the chiller. The components around the screw, however, require regular maintenance and replacement to enable the compressor to function effectively. Some of these components include the loading solenoids and coils, also the slide valve and its potentiometer which senses its position. When these components are not working as they should, the compressor cannot match the required load and so excessive electricity consumption occurs. The inspection, adjustment and replacement of these components is another way of how our maintenance saves money.

How do Chillers Work - Centrifugal Compressors

These compressors are mostly found on very high capacity systems used for the cooling of large industrial processes, district cooling and the cooling of large facilities. The compressor in the photo, however, is a small centrifugal compressor used for the cooling of a building.

How They Work

The refrigerant is sucked into the middle of the impellor. Then, centrifugal force pushes it out to the tip of the impellor edge and so providing the compression. Unlike the compressors above, these compressors are not positive displacement and so they have a lower compression ratio. That is to say: a lower difference in pressure between the suction and the discharge.

Maintenance

Regular oil and filter changes are required to prolong the life of the bearings. When this kind of compressor fails due to worn bearings and damaged internal components, the rebuild is extremely expensive. Read more about our centrifugal compressor remanufacturing facility

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Hit the Tags at the top of the page to navigate your way to our extensive library of further reading on this subject.

Read more about types of compressors on the Carel website


Oil return solenoid removed during industrial chiller service

Industrial Chiller Service

On an industrial chiller service visit, the ammonia alarm was found to have been triggered.

News Article No.10

Industrial Chiller Service Faults

The onsite engineer had fault found the chiller and silenced the alarm, the yellow light was still flashing. The red light was lit on the panel and the red LED was illuminated on the ammonia alarm console. He had reported a smell of ammonia to us over the phone. This gave us a priority of getting to site, as many other alarms of this nature are often spurious. Our engineer attended site within an hour and confirmed that the fault finding was correct as described by the onsite engineer. The chiller is containerized in design as it is situated outside. All around the chiller are door panels for access to the various system components.

Breathing Apparatus and PPE

He donned his mask and full length ammonia resistant PPE before opening one of the panel doors. This was to ensure that he did not get overwhelmed by the refrigerant when he opened the door. He started with the door into the storage area of the containerized chiller. A strong blast of ammonia came out in his face- lucky for the PPE!

Localising the Fault

Working his way around the chiller, our engineer found more and more hazardous door openings! Eventually he found the culprit: one of the two flanges were leaking on the oil return solenoid. The refrigerant vapour was coming out in its usual white form. The oil return pipe feeds off the oil pot which is a chamber that the oil sinks into from the refrigerant economizer. This vessel was valved off and the other end of the pipe valved off too.

Pinpointing the Fault

Now that the ammonia refrigerant leak had started to calm down- it was possible to see through the white vapour to exactly which of the flanges was leaking. It was the right one as seen in the picture. It consists of an ‘o’ ring made of ammonia resistant rubber material.

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Industrial Chiller Service in Local

The above mentioned chiller runs in local in a lead/ lag configuration with the adjacent chiller. That is to say- there is no wire or modem to a remote location. A panel is available in between the two chillers to sequence the switch over between the two. When the chiller tripped out due to the fault, the other chiller was supposed to have been enabled. This did not happen, so our engineer investigated the situation. The sequencer panel sends out a 24v fault feedback signal to each chiller. This, in turn, goes through a relay and back to the sequencer panel if all is good. When a fault occurs, the volts drop out to the relay in the chiller and a relay drops out in the sequencer. When the relay drops out in the sequencer, a normally closed contact makes and brings a red light on. This was not happening, so our engineer followed it through with his multimeter. He found a blown 1 Amp control fuse in the chiller, he replaced it and it blew again. After some careful research he found that there was an earth leakage due to the ingress of water into a safety switch. This switch was nothing to do with the above, but it blew the whole control circuit. Having reinstated the fuse, he found that the panel switched over satisfactorily in local.

Advantages

This kind of operation method has an advantage in its simplicity. There are no complicated BMS systems for the chiller to be integrated into. A sequencer panel is easy to construct and maintain- keeping the costs down to the end user.

Disadvantages

The disadvantage of this kind of system is that the first thing the factory usually notices is that they are loosing the process. The water temperature getting too high is the first alarm signal. With this site, however, there is a permanent onsite engineer on hand. He is experienced with the first checks to carry out and can often get the plant running with no problem.

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Standing Pressure during Industrial Chiller Service

The standing pressure was taken into account on the return visit to fit the oil return solenoid valve seal. Because the valve is on the low side of the system, when the chiller is off, the pressure is higher than when it is on. Therefore, so long as the seal pressure tests to this pressure, then all will be good when the system is running. That is assuming that the valve seals work satisfactorily at a lower temperature range. There are issues sometimes when a seal will be okay at ambient temperature but will leak when it becomes brittle at a colder temperature. This happens usually on an old seal and, indeed, this condition can be tested for when run testing the system.

Leak Testing during Industrial Chiller Service

On fitting the seal, our engineer donned full length ammonia resistant PPE and breathing apparatus. A little at a time, he introduced refrigerant into the area of the valve seal. Any residual air being purged through a valve.

Pressure and Temperature

The standing pressure of refrigerant is affected by temperature. That is to say- that the higher the temperature- the higher the pressure. On the day this job was carried out, the ambient temperature was 12°C and using an app on his phone, he calculated that the pressure should be 5.6bar. This is consistent with Charles’ Law of Constant Volume with a coefficient added for this particular refrigerant. If the pressure had been higher than this, it would indicate the presence of air in the system. Daltons’ Law of Partial Pressures states that all gasses in a vessel will act as if they are on their own, therefore, causing a higher pressure.

Run Testing during Industrial Chiller Service

After the pressure was built up to full standing pressure and the seal held satisfactorily, the system was then run tested to ensure, as stated above, that the seal performed well across the full temperature range during the operation of the plant.

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Mass Flow Rate during Industrial Chiller Service

The mass of refrigerant passing, which is measured by the second.

Suction Density

In this case of the oil return valve seal, we are looking at the density of the low side refrigerant as it passes into the suction port of the compressor. This is shown on the LP gauge near to the compressor. The higher the pressure of the refrigerant, the more refrigerant there is- so it has a higher mass flow rate. This system has a refrigerant saturation point of 1°C which corresponds to a pressure 3.4bar. That is a high mass flow rate for this kind of refrigerant. This is because this refrigerant is usually used in low temperature applications where the pressure of the refrigerant is below that of the atmosphere. In that condition, when a leak occurs on the low side of the system- air leaks in. Air bleed valves are available to remove this unwanted air from the system.

Compressor Loading

The bigger the compressor on a chiller- the higher the mass flow rate. Most compressors have loading solenoids, vanes, or a slide valve to regulate this.

Piston Displacement

Reciprocating compressors use loading solenoids to increase piston displacement. Usually, oil from the oil pump holds the piston valves open and so preventing compression on that cylinder. When more flow rate is needed- the loading solenoid de energizes- the piston valves drop and the cylinder comes into action. Therefore, increasing the mass of refrigerant through the compressor.

Vanes

Vanes are used on centrifugal compressors to increase the flow of refrigerant through the compressor. An actuator linked to a chain is used to open the vanes. The controls work out the correct position of the vanes for a given load condition.

Slide Valve

The slide valve offers a seamless amount of loading, anywhere between 0% and 100% A slide valve potentiometer senses the position of the slide so that the controls can regulate the flow through the compressor. The screw compressor in this article uses a slide valve- on full load with the slide at 100% all readings were taken with a good read back. Another job done- another happy customer!

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Controls panel open during chilling plant maintenance

Chilling Plant Controls Maintenance

Prior to this chilling plant controls maintenance visit, another contractor had changed some of the settings and adjustments in the controllers. They did this while they were diagnosing a fault with the water system and the pumps. Therefore, this visit was to recommission the plant and to resolve the issues resulting from the adjustments.

Chilling Plant Flow Controls Maintenance

The flow controls were found to be set wrong. Therefore, our engineer adjusted the pumps, then various valves on the water system, a little at a time, while monitoring the controller. Full load and part load readings were taken until they came to within standard industry limits.

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Compressor Run On Time

A timer had been adjusted which made the compressor wait to stop after reaching 0%. In the meantime, some more heat would build up in the water system and the compressor slide valve would open up a little. The result was that the compressor would run for an hour with the slide valve shut most of the time. The oil pump carried on pumping during this time resulting in a head of oil building up in the discharge pipe- all the way back up to the oil separator. Then, a low oil level fault had occurred. This being confirmed by the sight glass on the oil separator. The compressor had enough oil charged into it to allow a start up. During the start up, a low oil level timer counts down. As soon as the compressor loaded and started pumping, the oil level returned to the correct level on the sight glasses. The timer was adjusted along with the dead band to ensure the chiller off cycled after achieving set point.

Slide Valve Potentiometer

The slide valve potentiometer has a configuration mode button. It can be pressed to set the 4mA or 0% position. This is the usual position of the slide valve as a spring and 2 drain valves return it to the start position. The slide valve can then be manually opened in the program. Then, the potentiometer button can be pressed to set the 20mA or 100% position. “Chattering” can occur on the fully closed position so a setting is available to only close the slide valve to 2%.

To read more about chiller control systems click the Tag at the top of the page.

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Read more about potentiometers on Wikipedia


R134a chilling plant maintenance showing screw compressor and oil separator with tick sheet and digital thermometer on top

R134a Chilling Plant Maintenance

We recently carried out R134a chilling plant maintenance at our customer’s factory in the North West. The chiller is 600kw with 2 single compressor systems. It has an ‘in house’ controller on it with occasional spurious trip outs. We are working with the problem so far, but an option if the problem persists is to fit a reliable, cheap, off the shelf controller. The factory requires a process water temperature of 6°C. The plant is around the middle of the lifespan and has been properly maintained.

Unlock Controller during R134a Chilling Plant Maintenance

The customer had accidentally locked the controller by pressing the wrong buttons. The machine still functioned as it should, but the customer was unable to modify User settings or look at the readings. Our engineers carry a book with an extensive list of information for any controller which has been built up over time. The procedure was followed to unlock the controller, then the settings were checked.

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R134a Chilling Plant Sensor Maintenance

The sensors can be offset to compensate for a sensor reading out slightly. No offsets were saved and the sensors all read to within a degree or two of our digital thermometer. Our engineer then checked the sensor locations to ensure they were fitted correctly and insulation had not deteriorated.

Fan Speed Controllers and Subcooling

During the R134a chilling plant maintenance, particular attention was paid to the fan speed controllers and the subcooling of the refrigerant. This is because of occasional spurious high pressure trips. All the wiring was tightened and the plugged connections were checked and tested. The controller sends a variable run signal to the fan speed controllers. This is worked out from the analogue input signal from the high pressure transducer. If the problem persists, we will have to look into fitting more reliable head pressure controls.

Compressor Slide Valve

The oil solenoids which push and pull the compressor slide valve were operating correctly. The controller sends volts to the solenoids to control the position of the valve. A slide valve potentiometer sends feedback so the controller can work out the percentage position of the valve.

To read more about chiller control systems click the Tag at the top of the page.

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Read more about Tetrafluoroethane R134a on Wikipedia


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