They are widely used to cool buildings, industrial plants and even electronic equipment, as they are among the most efficient devices for heat dissipation and because they can reach very low outlet fluid temperatures. But how do compression chillers work?

Basically, the chiller, or compression chiller, is a machine that uses a mechanical compressor to compress and pump a refrigerant fluid through a cooling cycle.

Let us look in detail at how this is done, along with some basics for its operation.

1. Foreword: difference between absorption and compression chillers

Let us assume that both compression and absorption chillers have the purpose of removing heat from water, air or other fluids. The difference naturally lies in the cooling mechanism used for this purpose.

• In compression chillers, an important role is played by the mechanical compressor: as we shall see, this component is intended to compress the refrigerant gas. In order to remove heat from the fluid, the gas completes an actual cycle over and over again.
• In contrast, an absorption chiller uses a mixture of water and salt as a refrigerant. Evaporation of the water causes heat to be removed from the process fluid. The water itself is then absorbed by the salt solution. Subsequently, the salt solution is heated to release the evaporated water, which is then condensed and reused in the cooling cycle. There is therefore no presence of a compressor.

In general, most industries use compression chillers to refrigerate their fluids. However, absorption chillers or chillers have their own particular use: trigeneration.

2. The two circuits for operating the compression chiller

Let us now turn to the operation of compression chillers.

This type of heat dissipation device operates by means of a refrigerant gas: the process fluid to be cooled (water, water and glycol, or other) comes into contact with this gas and gives it heat. In turn, the refrigerant will have to dispose of the heat “received”: this through the thermal and mechanical work of the various components of the chiller.

It is therefore clear that there are two circuits to be considered:

• that of the fluid to be cooled, peculiar to each process, but which must always dispose of the heat in order to be reused in the production line;
• and that of the refrigerant gas, which ‘takes’ heat from the fluid and must dispose of it in order to fulfil its purpose again: this is the compression refrigeration cycle.

Let us see how both work in detail.

3. The fluid circuit to be cooled

As a starting point, we consider the process fluid that has heated up as a result of its use in industrial lines. Therefore, it must be cooled by the compression chiller.

• The hot fluid arrives inside the machine through the hydraulic system and comes into contact with the cold refrigerant in the heat exchanger (or evaporator).
• The cooled fluid leaving the chiller is sent to the point of use: the industrial process lines or the air conditioning system.
• At the same time, the refrigerant, which has warmed up in the meantime, is sent to the compressor.

However, this last step will become clearer after considering the next chapter.

4. The refrigerant gas circuit: the compression refrigeration cycle

Instead, here is how the compression refrigeration cycle, which characterises the refrigerant gas (or ‘fluid’), works.

1. The gas, heated and in a liquid state, is fed into the compressor: in this component, through a system of disks, it transforms to a gaseous state, increasing in pressure and temperature.
2. The refrigerant fluid is then passed to the condenser: i.e. a radiator in which outside air, via an electronic fan, is pushed towards the inside of the machine. The gas condenses and simultaneously releases heat.
3. Next, the refrigerant passes through an expansion or lamination valve: it loses pressure, expands and decreases its temperature.
4. At this point, in the heat exchanger or evaporator, contact is made between the coil, in which the fluid to be cooled circulates, and the refrigerant gas: the hot fluid transfers heat to the gas, which then heats up again.
5. The superheated refrigerant gas is sent back to the compressor to be compressed again, and so the cycle begins again.

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5. The mechanical compressor: the 'characterising' element of the compression chiller

From what has been said so far, it is clear which is one of the main components of this system and which makes the refrigerant ‘do its job’: the mechanical compressor. So much so, in fact, that we very often refer to the refrigeration of process fluids (which does not take place via the aforementioned absorption) as “mechanical cooling”.

This is in contrast to other forms of process fluid cooling:

• evaporative,
• adiabatic,
• dry (or air) cooling.

Two screws make up the interior of the compressor. They are positioned to progressively move the refrigerant from the suction area to the discharge area: the volume decreases, the gas is compressed. This is, as can be seen, purely mechanical work.

6. Water- or air-cooled compression chillers: differences

As we have seen, passing through the condenser, the refrigerant gas condenses and gives up heat to the system. It is then necessary to understand how the system disposes of this heat.

So, the difference between these two types of chillers, or chiller, is quickly stated:

• In water chillers, the heat dissipation medium is water. This is then sent to a cooling technology, such as an evaporative tower (which also provides the recirculation water);
• In air chillers, on the other hand, hot air from the cooling cycle is simply drawn in by a fan and dissipated into the environment.

Of course, both systems have advantages and criticalities.

A water system is undoubtedly more efficient and uses less electricity. In addition, it can reach temperatures even far below zero, but naturally requires the support of technologies such as a cooling tower. In addition, the availability of the water resource must be taken into account. On the other hand, an air cooler only uses energy: in some regions of the world, it is practically an obligatory choice.

7. Refrigerant gases: which are used and which 'work best'

A refrigerant often used in chillers is R410A gas: a mixture of two gases (R32 and R125) which is an energy-efficient gas but also very safe in the event of leaks, as it is non-toxic and non-flammable. Also often used is the refrigerant R513A, which has a lower GWP (Global Warming Potential) while maintaining good performance.

Both of these refrigerants are fluorinated gases: consequently, according to current regulations, the refrigeration circuits of chillers that use them must be maintained by refrigeration technicians with a regular licence.

The trend is, of course, to use refrigerant gases with an increasingly limited greenhouse effect, also in line with the European Commission’s goal of climate neutrality by 2050.

8. Operation of compression chillers: manufacturers' secrets

We have seen some basics of how compression chillers work, through a series of components and the right choice of refrigerant gas.

Here, however, are some of the “secrets of chiller builders”…

• Condenser: the skill of chiller manufacturers lies here first and foremost in ensuring that the gas is condensed in the best possible way. This is achieved through uniform distribution throughout the structure of this component.
• Motor-fan unit: preferably an EC type, ‘electronically controlled’. It can best manage the number and speed of blade revolutions to optimise the use of electricity.
• Chillers with free-cooling: in chillers equipped with this device, and when the environmental and/or design conditions allow it, the fluid can be cooled by the action of the fans alone, resulting in very low electricity consumption. By activating this mode, the chiller therefore works as a dry cooler.

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