Your refrigeration system – the heat transfer principle
Refrigeration is one of the most widely misunderstood topics amongst the DIYer community yet we have a slew of articles and explanations covering this subject. So what makes it so difficult to understand?
Many articles written by specialists rely on the reader’s own understanding of the fundamentals and the refrigeration cycle, even its most rudimentary form can go over one’s head if by design we leave out the basics. As an example, explaining power dissipation in a circuit would be near impossible if the reader did not have a firm grasp of Ohm’s Law.
In refrigeration we need to understand that first and foremost by compressing a gas there will be heat generated. Compressing a bicycle tyre pump into your hand is a really great example. By releasing this pressure suddenly the gas cools down.
Fluids used in a refrigeration plant, called the refrigerant, boils at a much lower temperature than water at the same pressure. E.g. water boils at 100 degrees C (212F), the refrigerant may boil at 4 to 10 degrees C (40F~50F).
A refrigeration system works on the heat transfer principle – cold to warm, warm to cold. Evaporated (fluid to gas) and condensed (gas to fluid). Taking warm air from inside and dumping it outside. Simple and reliable. But just what makes it tick?
The refrigeration system has five basic parts:
- The compressor,
- the condenser (outside),
- the metering device, an expansion valve,
- the evaporator (inside),
- the thermostat.
The first four all form part of a closed loop. The thermostat is a temperature controlled switch.
The heart of the refrigeration system: The compressor
At the heart of the system is the compressor which forces the refrigerant into the condenser coils which are usually outside the building – the higher pressure (called the high side) causes the refrigerant temperature to rise to between 40 and 50 degrees C (120F~140F) before reaching the condenser. As the refrigerant passes through the condenser coils a fan passes the cooler outside air over these same coils – heat transfer takes place, cooling the refrigerant causing the refrigerant to condense and become cooler. The compressed warm refrigerant then passes through the expansion valve releasing pressure to become colder still before passing through the (low side) evaporator coils which then absorbs heat, cooling down the area in which the evaporator unit is placed before being sucked back into the compressor to process the next cycle. This closed loop system therefore collects heat inside and dissipates this heat outside. Both air-conditioners and domestic refrigerators use the same principle.
Refrigerants are used based on their boiling temperatures. They cannot be mixed. Different applications use different refrigerant. Certain refrigerants, such as Freon R-22 is gradually been phased out and in some countries illegal to sell (or buy). R-410A is now one of the replacement types for R-22. To purchase refrigerant you will need to be a licenced refrigerant engineer, installer or technician.
The BTU in your refrigeration system
The BTU or British Thermal Unit is the amount of energy needed to raise or lower the temperature of one pound of water by one degree Fahrenheit at one Atmospheric pressure and is equal to about 1055 Joules of energy. The BTU is used in metric countries as well. As a unit of power 1000BTU/Hr is equal to 273.071W. 12 000BTU/Hr is therefore equal to about 3.5kW. Now you know why air conditioners are expensive to run.
Easy Maintenance to your air conditioner or refrigerator
One of the most common causes of failure to your refrigeration system is by NOT cleaning the evaporator or condenser coils at least once per year (radiator fins). This causes the compressor to work harder than it should. Another cause of premature failure of an a/c system is running the air conditioner at settings which would be impossible to achieve or leaving the doors or windows open in a conventional system (not water evaporators, which work best when well ventilated for humidity reasons). On a hot day it is not uncommon to find users setting the air-conditioner to the lowest possible setting – sometimes to as low as 16 degrees C (60F). An under spec’d air-con is another cause for premature failure – often even the installers get it wrong.
Do not transport your domestic refrigerator on it’s side unless you plan to leave it for at least 24 hours before plugging in and switching on.There are numerous reasons for this, one being to allow the refrigerant to settle.
Modern refrigeration techniques have mainly involved changes to the refrigerant and producing higher efficiency systems to cut down on running costs. Although the principles remain the same, inverter driven compressors have become more common today as technology has improved, the modern variable speed compressor no longer goes through a stop/start process which although costing more, lowers operating costs. The refrigeration system therefore still relies on tried and trusted techniques rather than a complete metamorphosis.