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In its simplest form, the refrigeration cycle consists of just 4 basic components to complete the circuit:
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. 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. 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. 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. A cold room condenser. Chiller condensers on a roof. Air conditioning condensers.3. The Restriction (Metering Device).A capillary tube metering device. A thermostatic metering device. An electronic 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. 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. Many different refrigerants. In many different sizes. A sight glass to see the refrigerant in a system.Before we go any further, it’s important to understand what refrigeration actually is:
The important part of this definition is the removed heat. Something that you perceive as ‘Cold’ is lacking ‘Heat’. 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. 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:Here we can see the 4 basic components in the circuit.The Refrigeration Cycle – Flow Direction:Shows the direction of flow of the refrigerant – Starting at the Compressor in a clockwise direction. The Refrigeration Cycle – Transfer of Heat:Shows the transfer of heat energy. Heat is absorbed by the evaporator and rejected by the condenser.The heat removed from the air flowing over the evaporator makes it colder. The evaporator fan then blows this colder air back in to the space being cooled.The heat removed is then rejected by the condenser which is outside of the space being cooled, and usually physically outside in the open air. The fan blows ambient air over the hot condensing coils. This cools and condenses the refrigerant but heats up the air blown over the condenser. That’s why when you stand in front of a condenser it’s usually blowing hot air at you. The Refrigeration Cycle – Pressures:By dividing the system vertically as above, we can see that at all points to the left of the line – the refrigerant is at low pressure, and at all points to the right of the line – the refrigerant is at high pressure.The Refrigeration Cycle – Refrigerant State:By dividing the system horizontally as above, we can see that at all points above the line – the refrigerant is a gas, and at all points below the line – the refrigerant is a liquid.In the middle of both the condenser & evaporator, where the change of state of the refrigerant happens, the refrigerant is present in both liquid & gaseous states, and is referred to as a vapour. The Refrigeration Cycle – Complete:In this final diagram of the refrigeration cycle we have introduced 3 new terms: Superheated, Saturated & subcooled.
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:
And there it is. The refrigeration cycle in its most basic and understandable terms. For further information on the process click here. |