Industrial Chiller Service

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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Chiller Fault Finding & Diagnosis

To read more about chiller fault finding hit the Tag at the top of the page.

Read more about refrigerant mass flow rate at Science Direct | Click Here


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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Carel controller and tick sheet during chilling plant maintenance visit

Chilling Plant Maintenance Visit

On this chilling plant maintenance visit, particular attention was made to compressor loading. This was to ensure that the compressors are capable of operating at 100%. With summer now here- we want the plant capable of running at full capacity.

Controller Loading Timer

On start up, the controller goes through a timer, this is to prevent the compressor from loading up too quickly, achieving set point and going off. With available load, the compressor would start back up and go into a short cycling condition. With 5 minute intervals, the controller brings System 1 screw compressor on at 25%. Then System 2 screw compressor on at 25%. In stages, the controller loads up the compressors until it matches the load.

Compressor Loading Solenoid Coils

These are 24vac. The controller sends out a run signal through the solenoid coil which magnetises the lift valve inside.

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Compressor Loading Solenoid Valves

As the valve lifts, discharge pressure oil passes through a channel and pushes the slide valve open a 25% stage. There are 4 valves for the 4 stages.

Chilling Plant Maintenance Visit at 100%

The chilling plant being maintained on this visit was now running at 100% on both systems. The system readings can easily be read by following the menu in the Carrel controller. Superheat and subcooling readings were found to be within normal operating limits. Also, a good read back was recorded on the water system.

Compressor Unloading

At the end of the day, the three way valves on the air handlers closed down according to the BMS schedule. This meant that the water was diverted away from the heat exchangers in the air handlers. This return water had not picked up any heat, so the controller started unloading the compressors. It did this through 75% to 50% then 25% until the water system was down to setpoint.

Off Cycle at Chilling Plant Maintenance Visit

The BMS stops the chiller with the remote start/ stop signal. Should the BMS malfunction, the chiller would stay off most of the night anyway. The water system pump adds heat into the water system. Therefore, every so often enough load would be available to bring one system on at 25% for a short while.

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

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