*Refrigeration And Air Conditioning Basics*
05 August 2011
Refrigeration is one of the technologies that have a "magic box" quality. Hidden from view, mysterious components make funny noises and somehow make air cold. I intend to pull back the curtain and show the basic processes most commonly used to remove thermal energy and familiarize the reader with the basic components of a refrigeration system. The information to be presented applies equally to refrigerators, freezers, and both home and automotive air conditioners. I will not be discussing propane refrigerators, or thermoelectric chillers.
First, the basics: heat is a form of energy and therefore cannot be created or destroyed. It can, however, be moved from one place to another. This is what refrigeration does.
The most common systems, the ones we'll be discussing, are vapor-compression units. The principle behind this design is the property of the refrigerant (the working fluid) to alternately absorb and release heat energy. This is done through the phenomenon of latent heat. Latent heat is best described as that quantity of heat energy absorbed or released by a substance while undergoing phase change (liquid to vapor or vice versa) with no detectable change in temperature. A good example is boiling water: it reaches the boiling point (212F/100C at sea level) and goes no higher regardless of heat input. It will boil faster, but boiling water doesn't get any hotter. Pressure is important, too. The boiling water analogy should be familiar to those living at altitude, where the lower atmospheric pressure causes water to boil at a lower temperature.
The refrigerant is a volatile chemical, usually a fluorocarbon, with a boiling point below the expected lowest temperature it is expected to function at when at the pressures encountered in the system. Few refrigerants are toxic, but all can suffocate by displacing air. Most aren't flammable, but many decompose to toxic compounds when exposed to high heat. When suddenly released from a system, it flashes rapidly to vapor, absorbing a lot of heat. This can cause severe frost burns, so keep clear of it.
In order to explain the refrigeration cycle, we must first be familiar with the major components:
The compressor - the heart of the system, this device is simply a pump. It circulates the refrigerant, discharging it at high pressure.
The condenser - one of two heat exchangers, this component provides a large surface area to allow the condensing refrigerant to give up its heat to outside air. There is sometimes a fan blowing across it to aid in heat transfer.
The expansion valve or orifice tube - a restriction in the line, variable or fixed, which aerosolizes the liquid refrigerant to aid in evaporation.
The evaporator - the other heat exchanger, in which the refrigerant evaporates and absorbs latent heat from the air to be cooled. There is also normally a fan to aid in heat transfer.
The refrigeration cycle is a continuous process, but is best visualized as a series of steps:
At the compressor, low pressure vapor from the evaporator is compressed to a high pressure vapor.
This high pressure, high temperature vapor gives up latent heat at the condenser, becoming a liquid.
This liquid is aerosolized at the expansion valve, entering the evaporator as a fine mist.
In the evaporator, the aerosolized refrigerant absorbs latent heat as it vaporizes.
The cycle repeats constantly.
To clarify, heat is absorbed by the vaporizing refrigerant at the evaporator and released by the condensing refrigerant at the condenser. This makes the evaporator cold and the condenser hot.
These systems are robust and reliable, not having been improved on in many years. When failures happen, they are frequently due to leaks or electronic controls.
Leaks commonly occur at the evaporator as it is usually wet from condensed air moisture. This can lead to corrosion and thus leaks. Vibration points, such as near an improperly mounted compressor, can lead to fatigue cracks in lines. Poorly installed fittings or faulty o-rings can leak, too. Repairs to refrigeration components require specialized tools and skills beyond the scope of this article.
Electronic failures are more common and, thankfully, easier to fix. The thermostat is a thermal switch to turn the system on and off. Mechanical ones can be readily adjusted and repaired, but the newer electronic models are difficult to service and are usually better replaced when faulty. The condenser fan is normally switched on and off with the compressor via a large relay called a contactor. After the contactor there are sometimes large capacitors to aid in starting the electric motors in the compressor and fan. The evaporator fan, sometimes called the blower, provides air circulation through the cooled system. It, too, has a starting capacitor and contactor, but is usually on a separate circuit from the compressor and condenser fan. When any of these fail, please open the breaker and verify the line is dead before attempting to troubleshoot. While the thermostat carries a low voltage control current, the major components operate at house current or higher and can kill or injure you.
In automotive applications, the systems are much smaller and the compressor is almost always driven by a belt connected to the engine crankshaft. There is rarely a thermostat, rather manual controls. There are also pressure switches to shut the system down on low or high pressure, a cancel switch to release the compressor clutch at wide-open throttle, and sometimes an evaporator thermal switch to prevent ice buildup.
In frost-free refrigerators, there is a solenoid driven three way valve that runs the system in reverse, heating up the evaporator coils at set intervals to melt ice off of them. Ice buildup would insulate the evaporator and restrict airflow across it.
Hopefully, this short discussion has removed some of the mystery surrounding refrigeration and air conditioning.
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