Category Archives: Chiller Basics

Showing how do chillers work - brass expansion valves inside machine

How do Chillers Work? Expansion Valves

In this article we are going to explore: how do chillers work- expansion valves. They are also referred to as a metering device, as they control the flow of refrigerant into the evaporator. Read below for an insight into the 4 main types that are used on chillers…

How do Chillers Work - Electronic Expansion Valves

This is a refrigerant control which uses a mechanical valve operated by a step motor. A step motor opens the valve according to the number of steps in between the closed and the open positions. This step motor is controlled by a program in the chiller controller…

On Board Comparator

Every engineer has a comparator app on his phone and a set of comparator sliders in his tool case. The controller has sensors fitted to the evaporator and an in built comparator for the refrigerant which runs through the system. The comparator is a scale which compares the pressure of the refrigerant to the corresponding saturation point. Saturation point is another way of describing the boiling point of the liquid refrigerant.


The above algorithm is needed to decide the position of the step motor. A pre programmed amount of superheat (the temperature above saturation) is programmed into the controller. This prevents compressor failure due to liquid refrigerant slug back.


Because the above is controlled by a step motor and a microprocessor, the superheat is closely controlled to as low as 1°C. A low superheat value is one of the factors which contributes to a machine running more efficiently.

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How do Chillers Work - Solenoid Expansion Valves

This is a very simple expansion valve. It is just a solenoid valve fitted into the liquid line which is operated by a solenoid coil. The control of this valve is very similar to the above. The difference is that the program works out how long to keep the valve open and how long to keep it shut.

How do high side float expansion valves work when welded into steel ammonia chillers pipework
Refrigerant control on flooded system

How do Chillers Work - High Side Float Expansion Valves

The photo shows this type of valve as used on an ammonia chiller. This kind of chiller is a flooded system where all of the refrigerant is stored in its liquid phase on the low side of the system. As load increases: more refrigerant boils off in the evaporator- it is sucked into the compressor- discharged into the condenser and then arrives as a liquid at the float valve. When the chamber in this valve fills, it lifts a float inside which is made of steel. The float operates a switch which sends volts to the liquid pipe solenoid valve. When this valve opens, it lets the liquid in the float chamber to pass into the evaporator. Having done this, the float drops back down and the valve closes again. The closing of the valve maintains the pressure difference between the high side and the low side of the system.

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How do Chillers Work - Thermostatic Expansion Valves

There are 4 forces acting on a TEV: the power element v spring and liquid line pressure v suction pressure. The expansion valve has been designed to balance these forces.

Power Element

A bulb is fitted to the suction pipe near to the compressor. This bulb is charged with the same refrigerant that is running in the chiller system. It is connected to the power element using a capillary tube (photo at the top of the page) When the temperature rises in the evaporator, liquid refrigerant boils off in the bulb, this forces a diaphragm to open in the power element.


A needle located on the outside of this diaphragm pushes the spring down. This opens the expansion valve and lets more refrigerant through.

Liquid Line Pressure

The liquid line is the inlet to the expansion valve and carries the refrigerant arriving from the condenser.

Suction Pressure

The pressure drop on the far side of the expansion valve is maintained by the sucking action of the compressor.

Mass Flow Rate

The above allows for a high mass flow rate when the evaporator warms and a lower mass flow rate when the evaporator cools.

Liquid Line Solenoid Valve

To prevent refrigerant passing into the evaporator during off cycles, a liquid line solenoid valve is often used. This type of system usually pumps the evaporator down before the compressor stops. Shortly before the compressor starts back up again, the liquid line solenoid valve opens, the pressure increases in the evaporator and then the compressor starts.

Equalizing Line

This is usually a copper pipe which connects the expansion valve to the far side of the evaporator. It compensates for the pressure drop across the evaporator. It also detects the pressure of the suction pipe at the same location of the sensing bulb. This allows the TEV to accurately adjust the superheat.

Hot Gas Bypass

Some chillers, like laser chillers, require a very close control of setpoint. Sometimes the deadband can be as narrow as 0.2°C. If the compressor were to stop and start all the time to achieve this- it would fail very quickly. Therefore, a hot gas bypass can be used to allow the compressor to carry on running when the chiller achieves setpoint. A solenoid valve opens which allows the discharge hot gas to bypass the expansion valve and return to the suction. It can open and close as many times as needed to achieve a very close control, while at the same time allowing the compressor to run normally.

How do Chillers Work - Expansion Valve Flash Gas

This is the term that describes that portion of the liquid refrigerant which, in any expansion valve, instantly evaporates after it passes through. When this happens, the absorption of latent heat cools the remaining liquid refrigerant in the evaporator to the required saturation temperature.

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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 thermostatic expansion valves at the Danfoss website

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.


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.


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.


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


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.


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.


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.


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.


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

How does a grey centrifugal chiller work showing the main system components

How do Chillers Work?

How do chillers work? This is a question we get asked quite a lot from our subscribers. So, in this article I am going to explain the science and also talk you round the main chiller components…

The Basic Refrigeration Cycle

Weather it is the large centrifugal chiller like the one in the photo, or the refrigerator in your kitchen- most cooling systems work according to the basic refrigeration cycle which involves vapour compression. Two scientific principals are at work: latent heat and the pressure temperature relationship. Four chiller components are needed: evaporator, compressor, condenser and expansion valve.

Sensible heat

Heat energy that can be sensed is called sensible heat. Imagine a pan of water with a flame under it and a thermometer resting in it. You are watching the flame licking up the bottom of the pan and you are also watching the thermometer going up, obvious- right? The heat energy from the flame going into the pan is sensible heat.


Now imagine if we pumped a liquid through a heat exchanger at 0°C with warm water being pumped through the other side of the heat exchanger. Some heat energy would exchange into the cold liquid and warm it up, for example, by 6°C. This is sensible heat.

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Latent Heat

Latent is the Latin word for hidden. Something less obvious happens when a pan of water gets to its boiling point of 100°C. You continue watching the flame licking up the bottom of the pan, but the temperature stops going up, strange- right? You carry on watching for a good while and the thermometer still does not go up? Then, eventually- the water boils into steam and the thermometer starts going up again. All of the heat energy that was going into the pan while the thermometer was not going up was latent heat. Latent heat was being absorbed into the water to cause it to change its state into steam. All of this hidden heat caused the liquid atoms to shake apart and become a gas.


Now let’s look at the above example again, but this time including latent heat. At Step 5, a different liquid is pumped through the heat exchanger at 0°C which is also its boiling point. The amount of sensible heat, in our example, is still 6°C. Between the outside of the bubble and Step 1, there is 3°C of sensible superheat above the boiling point. Between Step 4 and Expansion, there is 3°C of sensible subcooling below the boiling point. The latent heat is inside of the bubble which, as you can see, has a considerable size. We measure sensible and latent heat energy using kJ/kg.

How does a chiller work on a PH diagram

The Pressure Temperature Relationship

How we produce the above liquid at 0°C is the second scientific principle...

Higher Pressure

If you increase the pressure of a substance- the atoms are pushed together and so they get hot. The higher the pressure- the higher the temperature.

Lower Pressure

If you decrease the pressure of a substance- the atoms spread apart and so they go cool. The lower the pressure- the lower the temperature.


R134a is a refrigerant that has a pressure of 1.91 barg, relative to a temperature of 0°C.

Refrigerant Saturation Point

The above temperature of 0°C is also the boiling point of R134a. This boiling point can also be called the saturation point because no more heat can be added to the liquid before it boils. The refrigerant liquid is full of heat- or saturated.

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How do Chillers Work with Evaporators? 

This is a heat exchanger which is in between two other chiller components: the expansion valve and the compressor. The expansion valve is a restriction in the chiller system and so there is a pressure drop into the evaporator. The compressor sucks from the evaporator and so maintains this pressure drop. This is the pressure temperature relationship: the lower the pressure- the lower the temperature. Liquid refrigerant flows through the evaporator at 1.91 barg and 0°C. It boils off absorbing latent heat, then it superheats, in our example, by 3°C above its saturation point. The evaporator has absorbed heat energy from the water on the other side of the heat exchanger.

How do Chillers Work with Compressors?  

A bicycle pump is a compressor- notice how it gets hot when you inflate a tyre. This is the pressure temperature relationship: the higher the pressure- the higher the temperature. Therefore, the compressor also adds heat energy into the system. The refrigerant is sucked into the compressor from the evaporator as a cold, low pressure gas at 1.91 barg. It is then compressed into a hot, high pressure gas which is discharged from the compressor at 8 barg.

How do Chillers Work with Condensers? 

The hot, high pressure gas being discharged from the compressor is cooled down with fans which suck air through the fins of this heat exchanger. The gas goes through its latent heat phase again, but this time condensing from a gas and into a liquid. It is then subcooled, in our example, by 3°C below its saturation point into a hot, high pressure liquid. The heat energy absorbed in the evaporator and the heat energy added to the system by the compressor is rejected into the surrounding air.

How do Chillers Work with Expansion Valves?

The hot, high pressure liquid at 8 barg arrives from the condenser at the inlet of the expansion valve. The expansion valve could be seen as being a tap which is partially closed. This restriction causes the refrigerant to back up behind the expansion valve inlet. The refrigerant that gets through the valve and into the evaporator expands into a cold, low pressure liquid/ vapour mix. The vapour is called ‘flash gas’ and is as a result of the refrigerant expanding. Vapour is another word for a gas.

How do Chillers Work with Set Point?

This basic refrigeration cycle continues until the setpoint is achieved and the controller stops the compressor. After some time, the water warms up by a couple of degrees and the controller starts the compressor back up.

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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 vapour compression refrigeration on Wikipedia