The Refrigeration Cycle

The Refrigeration Cycle is a simple but amazingly clever and useful process.

In its simplest form, the refrigeration cycle consists of just 4 basic components to complete the circuit:

• A Compressor
• A Condenser
• A Restriction
• An Evaporator

That’s it. Well, that’s almost it – we also need a refrigerant to cycle inside the circuit.

As the name suggests, the refrigeration process is a cycle.
We start at the compressor, go through the condenser, then through the restriction, then through the evaporator and finally back to the compressor where the cycle starts all over again.

So let’s have a brief look at each of the components in turn. Luckily, their names are quite self explanatory:

1. The Compressor.

The Compressor can be thought of as the heart of the process.
It acts like a pump to create the circulation by compressing the refrigerant gas, creating a pressure difference that drives the refrigerant around the circuit in a continuous cycle.

2. The Condenser.

The Condenser cools and condenses the refrigerant gas coming from the compressor in to a vapour and finally in to a liquid.

3. The Restriction.

The restriction restricts the liquid refrigerant flow and creates a pressure difference between itself and the evaporator. The restriction is more commonly referred to as a METERING DEVICE as it meters the amount of refrigerant entering the evaporator.

4. The Evaporator.

The Evaporator evaporates the liquid refrigerant in to a vapour and then in to a gas before it gets back to the compressor.

5. The Refrigerant.

You may have noticed that in this very brief and simplified introduction to the components, that we have already talked about the refrigerant being a GAS, VAPOUR and a LIQUID. It is this changing of state within the refrigerant that produces the refrigeration effect, and is the main principle of the refrigeration cycle – more on this a bit later.

Here are some examples of these components and what they look like:

1. The Compressor.

The Compressor is the heart of the refrigeration cycle and comes in a vast array of sizes.
In smaller systems it is usually found inside the outdoor unit, but in large applications of multiple compressors, they are usually found inside a plant room.

2. The Condenser.

The condenser is often referred to as the ‘outdoor unit’, and that’s usually where you will find it – outdoors, mounted on the floor, wall or roof. In most air conditioning and smaller refrigeration plants, the outdoor unit will house the compressor, condenser, various electronics and in some cases, the restriction (metering device) too.

3. The Restriction (Metering Device).

The vast majority of all modern refrigeration & air conditioning systems will use one of these 3 types of metering device.

Capillary tubes are simply a length of very narrow tube that causes a restriction to the flow of refrigerant.
They are most commonly found on small refrigerators like you have in your home.

Thermostatic metering devices, more commonly termed TEV’s or TXV’s (Thermostatic Expansion Valves), are very common throughout all refrigeration systems. They use a bulb which is partially filled with refrigerant and is strapped to pipe work exiting the evaporator. This bulb senses the temperature of the refrigerant leaving the evaporator, and through pressure can open and close to vary the amount of refrigerant entering the evaporator.

Electronic metering devices, more commonly termed EEV’s or EXV’s (Electronic Expansion Valves), are a more modern and accurate version of a TEV. They are electronically controlled through data provided by an electronic temperature sensor, and can open and close multiple times every second to allow very precise control of the amount of refrigerant entering the evaporator.

To help understand the job of the Restriction or Metering Device, it can be loosely compared to the nozzle on an aerosol spray can.

4. The Evaporator.

The Evaporator is often referred to as the ‘indoor unit’, and that’s usually where you will find it – indoors inside the room being cooled (or heated in the case of heat pump air conditioning). They are usually mounted at high level on a ceiling or wall.

The Evaporator & Condenser coils are basically the same type of construction.
A long length of pipe work surrounded by aluminium fins.
They are essentially heat exchangers, similar to the radiator in a car.

5. The Refrigerant.

There are many types of refrigerants and refrigerant blends. Different refrigerants have different properties to suit the application – Air Conditioning, Cold Rooms, Freezers etc.
Refrigerants are usually referred to by an ‘R’ number, for example R32, R410A, R422D, R507.
Propane (R290), Ammonia (R717), and CO² (R744) are also currently used as refrigerants.

Before we go any further, it’s important to understand what refrigeration actually is:

The term refrigeration means cooling a space, substance or system to lower and/or maintain its temperature below the ambient one (while the removed heat is rejected at a higher temperature). In other words, refrigeration is artificial (human-made) cooling.

Wikipedia.

The important part of this definition is the removed heat. Something that you perceive as ‘Cold’ is lacking ‘Heat’.

Heat is relative – what do you consider hot?

One very important aspect to grasp when understanding the refrigeration cycle is that heat is relative.

We tend to think of heat in terms relative to our everyday experiences and situations.
At 30°C we think of it as being a BOILING HOT day!
When we take a dip in the 16°C sea on that hot day it feels positively FREEZING!
So with a difference of just 14°C, our perception of heat has gone from BOILING to FREEZING!

But when we look at those temperatures in relation to other temperatures, the reality is very different.

If we look at the temperature of the sun at 5,500°C, our 30°C HOT day, in relation, is positively chilly. And likewise, liquid nitrogen at -200°C, makes our FREEZING COLD 16°C sea seem BOILING HOT!

When we think of the term ‘BOILING’ we instantly think of water in a kettle boiling at 100°C. We instinctively associate boiling as being 100°C. But it is important to understand that this only happens with water, at sea level, where the atmospheric pressure is at 1 bar. If we were at the top of mount Everest, where the pressure is only 0.34 bar, our water would ‘boil’ at 71°C.

The effect of reducing the pressure to reduce the boiling temperature of water is brilliantly demonstrated by boiling water at room temperature by placing the water in a vacuum:

From this it is important to forget your connection of boiling = 100°C and think of boiling as being a CHANGE OF STATE from a liquid to a gas. Some refrigerants can ‘boil’ at -40°C.

This relationship between PRESSURE & TEMPERATURE is a key factor in the refrigeration cycle process.

The changing of state within the refrigerant, from a liquid to a gas, is achieved by manipulating its pressure.
Under high pressure the refrigerant remains in its liquid state, and when the pressure is reduced the liquid refrigerant begins to ‘boil’ and change in to a vapour or gas.

If we return to the refrigeration cycle with the aid of some diagrams, we can see how these pressure changes causing changes of state within the refrigerant actually happen.

The Refrigeration Cycle – Components:

The Refrigeration Cycle – Flow Direction:

The Refrigeration Cycle – Transfer of Heat:

The Refrigeration Cycle – Pressures:

The Refrigeration Cycle – Refrigerant State:

The Refrigeration Cycle – Complete:

• SUPERHEAT – Is an amount of heat added to refrigerant vapour beyond its boiling point. This ensures the refrigerant is in a gas state with no liquid present.
  
• SATURATED – Is when the refrigerant is a vapour with both liquid & gas present.
  
• SUBCOOLING – Is an amount of heat removed from the refrigerant below its condensing point. This ensures the refrigerant is in a liquid state with no gas present.

Superheat is important to ensure no liquid makes its way back to the compressor. Although we described the compressor as ‘acting’ like a pump earlier, it isn’t a pump. Pumps usually move liquids by way of an impeller, where as compressors, as the name suggests, compress the volume of the gas which raises both its temperature & pressure. Liquid’s can’t be compressed, and any liquid making its way back to the compressor can cause serious damage.

Subcooling is important as it ensures only pure liquid makes its way to the metering device. This maximises the capacity, efficiency and reliability of the system.

So, looking back at our completed refrigeration cycle diagram, let’s describe the process in full:

1. The refrigerant enters the compressor as a low pressure superheated gas.
2. The compressor compresses the gas, changing it to a high pressure superheated gas.
3. Inside the condenser the gas begins to cool and change state in to a vapour. Additional cooling inside the condenser causes the refrigerant vapour to condense in to a high pressure subcooled liquid.
4. As the high pressure liquid refrigerant passes through the metering device it enters a low pressure environment, causing it to flash off in to a vapour.
5. The refrigerant vapour enters the evaporator where it absorbs heat from the space being cooled, causing the refrigerant to boil. As it continues through the evaporator coil the vapour is superheated turning the refrigerant to gas before it enters the compressor and starts the cycle over again.

And there it is. The refrigeration cycle in its most basic and understandable terms.

For further information on the process click here.