Chiller Controls

Open chiller controls panel showing PLC, relays, contactors and wiring

Chiller Controls

Chiller controls can be remotely operated and monitored, but in this article, we will be looking at chillers operating in local.

Each chiller has a panel where the lead or the lag chiller can be switched from. They have N+1 redundancy built in, so one of two chillers will normally be in standby with the other one running. The chillers in the photo are equipped with kilowatt hour meters because the customer wants to monitor their efficiency. He has targets to meet and wants to gauge the effect that our maintenance has in reducing his energy costs.

Condenser Pressure

The condenser pressure control is external and stand alone from the panel.

Transducer

A transducer is fitted to the discharge pipe near to the compressor. This gives a 0 to 5vdc control signal to the fan speed controller which is bolted to the frame. There is a minimum and a maximum value on the transducer, so the FSC is programmed to work out the pressure from the voltage.

Fan Speed Controller

415v on three phases are the input to the FSC. It uses solid state thyristors to regulate the output to the fans. This is according to the demand received by the transducer. Solid state means that all the parts are electronic with no moving parts. Fan speed controllers are really good at extending the life of the fans. This is because all of the fans operate together- smoothly and reliably.

Chiller Controls Digital Inputs

There are three essential digital inputs to the controls of any chiller. All of them have a volt signal out to them, which returns back to the panel. If there is a fault- the volts drop out.

LP Switch

This protects the chiller from a low pressure condition. Compressor and evaporator failure would result, so this device is set below the running pressure of the system, but high enough to offer protection.

HP Switch

If the head pressure control mentioned above were to fail, this device would save the chiller from damage from excessive pressure in the system. Components or the pressure relief valve can blow causing a catastrophic refrigerant leak.

Flow Switch

This device detects a lack of flow in the water system. Serious system failure would result if this part is not maintained properly. It needs to be periodically tested and adjusted at regular intervals.

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Chiller Controls Analogue Inputs

The essential analogue inputs on a chiller are the Water In and the Water Out sensors. These are usually NTC (negative temperature coefficient) that is to say: if you hold one in between your fingers and warm it up- the resistance will start to drop off. They usually read in kilo ohms which can be read on a standard multi meter. The program looks at these two sensors and using an algorithm, it calculates the loading requirement of the compressor. They can read incorrectly, so a sensor offset function is available in the software for adjustment. This is just one of the many checks and procedures that we carry out during our maintenance visit.

Chiller Controls Relays

In the photo you can see wires from the various devices around the chiller, wired into a row of relays. These, in turn, are wired into the white relay board at the top. This relay board has several expansion boards linked into it which are held together with an electrical ribbon. Next, the relay board is wired into the PLC... 

Chiller Controls PLC

The reason for these steps in between a device and the PLC is for protection. Sensitive electrical components can be blown due to an earth shortage. At each stage there is a volt drop from 240v to 24v and then to 5vdc.The programmable logic controller is the nerve centre of the chiller. This is where all the inputs go to and where all the digital outputs are sent from. The controller on this chiller is Beijer Electronics- it comes blank from the factory. User keys to operate the chiller are positioned below the display. It can be programmed to run most chillers and indeed it is often seen in factories running anything. A laptop plugs into it and the software for the chiller is uploaded. On one visit, we found a fault with this controller. We bubble wrapped it and took it to our electronics laboratory at Head Office. The issue was easy to resolve- it was just dust tracking across the back of the PCB and so corrupting the program.

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Chiller Controls Digital Outputs

The main digital outputs on this chiller are:

Compressor Run Signal

240v is sent to the compressor starter contactors. There are three: Star (lower amps for a soft start) then a timer switches over to Delta (higher amps for a more powerful running of the compressor) On the other end of the compressor windings is the Line contactor. This contactor runs with both the Star and the Delta contactors.

Float Valve

This is a camber where the level of refrigerant which is coming in from the condenser is detected. The level is transmitted to the PLC, where the program sends a signal to the expansion valve. It opens to the correct degree according to the load on the chiller.

Slide Valve

The compressor can run at 0% with the slide valve shut. When load is sensed from the Water In and Water Out sensors by the controller- the slide valve opens up. The position of the slide valve is detected by a potentiometer. This is calibrated from a minimum to a maximum position. The signal is 4-20 mA which the controller translates into the position of the slide valve.

MAXIMUS ADVANTAGE™

Whatever the problem with the controls, we can find a solution to resolve it. With years of industry experience and a fast supply chain, we offer a service that is second to none. Being able to retrofit is part of what we call the MAXIMUS ADVANTAGE™ Any Chiller- Any Problem- Any Part- Any Refrigerant- Anywhere.

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Grey shell & tube chiller evaporator being maintained with centrifugal compressor above

Shell & Tube Chiller Evaporator Maintenance

With shell & tube chiller evaporator maintenance to be carried out- it was time to sign the permit to work and get going. There are 5 chillers in total, but 1 of the chillers had been dismantled by the previous company and is unlikely to run again. This left 4 flooded systems to be maintained while the factory was still in production. Therefore, it had been decided in the Method Statement that one chiller would be done at a time. The plant had been designed with N+1 in mind, as it is critical not to loose the process- causing a factory shutdown. This means that so long as 2 of the chillers are running, there will be no loss in production. With 1 chiller out of service and another being worked on, this left 3 chillers in operation. If 1 were to fail, then there would still be the minimum of 2 left running.

Hire Chillers

In the unlikely event that 2 of the 3 chillers left running were to fail, an action plan was in place to lift in 1 MW of hire chillers.

Pump Sets and Hoses

The hire chillers would come with 2 engineers to pipe the pump sets into the onsite water system. This water system has been designed with this in mind so ‘stab ins’ have been built into it with shut off valves for the connection of temporary hoses.

Vanes

The chilled water is close control with a critical 6°C to 8°C set point. The chillers in the photo are very good at achieving this close control as they use vanes on the inlet to the suction. These vanes can be adjusted from anywhere between fully closed, with a small amount of bypassing, to fully open.

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Buffer Vessel

The hire chillers, on the other hand would not be close control. They would be multiple system chillers with various stages of loading on each chiller. One 600 kW chiller and another 400 kW. Each of the chillers onsite are 1.3 MW, but as they never run at 100% the onsite maintenance manager agreed to the proposal. It is unlikely that this plan would be needed, but planning for it is critical. A 10,000 ltr buffer vessel was decided upon to make the hire chillers close control. The chillers discharge into the buffer vessel, then the water mixes around before returning back to the chillers. The process water too being discharged into the buffer vessel before returning to the process. The position of the inlet and outlet on both water systems being strategically decided upon to ensure a steady exchange of heat and a smooth water out temperature back to the process.

Valve off and Drain Down

The Water In and Water Out pipes were valved off and the contents of the evaporator drained down. There was no glycol or chemicals in the water system which made the job easy. Otherwise, the water in the evaporator would have to have been pumped into vessels for disposal.

End Plate Removal during Shell & Tube Chiller Evaporator Maintenance

The lift and shift team were on site to assist with the removal of the end plate. After the bolts were removed, they attached their lifting eyes and straps, then lifted the end plate to the side.

Evaporator Cleaning 

We have in our stores a specialist evaporator cleaning tool. After a period of time, the evaporator tubes become fouled with impurities picked up from the water system. Also, contaminants get in from the air into the water tower. The strainer has some effect, but thermal insulation builds up on the water side of the tubes. It prevents latent heat being absorbed into the fridge system from the process. After the cleaning process, the efficiency of the copper tubes was increased to the same level as when the chiller was new. This is part of what we call the MAXIMUS ADVANTAGE Any Chiller- Any Problem- Any Part- Any Refrigerant- Anywhere. 

Reassembly after Shell & Tube Chiller Evaporator Maintenance

The seal was replaced from our range of seal sizes, on the shelf in our stores. The reassembly being the reverse of the strip down, the lift and shift team aligning the end plate for refitting on guide runners. It was then tightened to the correct torque setting. A little at a time, the water system valves were opened until full flow and water system pressure was achieved. No leaks occurred, so it was time to go on to the next phase of the job…

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Run Up after Shell & Tube Chiller Evaporator Maintenance

After bringing this chiller back online, it went through a timer, then loaded up steadily to 68%. The refrigerant system readings had been taken previously, so they were compared to the readings taken after the job. A lot more heat transfer being achieved with more latent heat being absorbed from the process.

Remaining Chillers

The above mentioned process was repeated to the remaining chillers one at a time. No issues were encountered and the whole job proceeded steadily to completion.

Compressor Loading

The load from the factory is very steady, so each time another chiller evaporator was cleaned, the row of chillers would unload slightly. They were running at 72% on arrival, then 68%, then 63% then 59%. This was demonstrated to the customer as a visual way of proving how we had improved the efficiency of his plant. Compressor unloading uses less electricity, so lower carbon emissions and a lower electricity bill are the direct result of our maintenance. Also, kilowatt hour meters are linked to the maintenance manager's computer for monitoring.

Shell & Tube Evaporator Chiller Maintenance Tidiness

As with all of our jobs, the work location of the chillers was left in a better condition than when we arrived. Any waste and unused items being recycled or returned to the stores on site or the stores at our Head Office. The chillers being wiped down and the end plates re painted. With regular maintenance, we at Maximus Chillers will take years off the life of your plant. It is not unusual for us to keep this kind of plant running for more than 50 years.

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Green chiller panel with doors open, showing contactors and PCBs during water chiller maintenance

Water Chiller Maintenance

Maximus chillers provides water chiller maintenance to all kinds of machines. Some of which have kilowatt hour meters fitted. This means we can monitor the reduction in energy consumption, directly as a result of our maintenance being carried out.

Efficient Water Chiller Maintenance

Maintenance is carried out to ensure every aspect of your chiller is running efficiently. We believe our maintenance checklist is the best in the industry. It looks in detail at the running conditions of the plant, component adjustments and the parameters in the controls. The checklist is used to assess if there are problems that are starting to occur before a safety shutdown happens. Below are some of the issues that we keep on top of to increase the efficiency of your plant.

Shell and Tube Insulation

The build up of dirt acts as an insulator in shell and tube heat exchangers. These are used for the evaporation and the condensing of the refrigerant.

The Evaporator

In the evaporator, should the tubes be fouled, there will be a reduction in latent heat absorbed into the system. This will cause the plant to stay on longer and use considerably more energy. Should the tubes become considerably fouled, the chiller will malfunction and eventually system shutdown will occur.

The Condenser

In a shell and tube condenser, the reverse of the above will occur. Tube fouling, acting as an insulator, will prohibit the rejection of heat from the system. The head pressure control will open the condenser controls to try and assist in heat rejection. Heavy fouling will cause an increase in the consumption of energy. Eventually a safety shutdown will occur causing loss of production.

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Compressor Loading during Water Chiller Maintenance

If the loading of the reciprocating compressors is faulty, the plant will not be able to effectively match the load.

Over Loading

Too much loading may occur, causing the water chillers to achieve set point too quickly. The plant will then go through the off cycle. This means that the plant will have to load back up, using more energy than matching the load continuously.

Under Loading

Should the plant be unable to load up to the required level to match the load, this will cause the water temperature to creep up and the lag chillers being called for to match the load. More chillers running than necessary dramatically increases energy consumption.

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