Many people are surprised to learn that HVAC and refrigeration systems don&#;t actually &#;create&#; cold air. They work through heat transfer: by removing heat from one area and transferring it to another area. This is accomplished through one of four types of refrigeration systems.
A mechanical-compression refrigeration system is composed of four main parts: compressor, condenser, expansion valve, and evaporator. As the refrigerant flows through the compressor, its pressure rises. In the next step, the refrigerant is vaporized and condensed into liquid, giving off heat in the process. After passing through the condenser, the refrigerant experiences a pressure and temperature drop when it moves through the expansion valve. Finally, the refrigerant reaches the evaporator where it vaporizes; this is where the actual cooling process takes place. The evaporator draws heat from the area being cooled and cool dry air is released into the atmosphere. The cycle restarts when the vaporized refrigerant returns to the compressor.
This is the most common type of refrigeration system, especially in commercial applications. Mechanical-compression refrigeration is used both in air-conditioning as well as commercial and industrial refrigeration (walk-in freezers, reach-in refrigerators and freezers, vending machines, etc.).
Absorption refrigeration works similarly by expanding and compressing a refrigerant. Instead of a mechanical compressor, an absorption system uses different types of refrigerants that other substances are attracted to &#; that will &#;absorb&#; the refrigerant. For example, many of these systems use ammonia as the refrigerant and water as the absorbent. When the water absorbs the ammonia from the low-pressure side of the system, heat is released. The mixture then travels to the desorber where heat (usually from a generator) separates the ammonia from the water through evaporation. Finally, just as with mechanical-compression refrigeration systems, the high-pressure refrigerant gas is turned into a liquid and sent to the evaporator to transfer heat.
There are several advantages of absorption refrigeration:
Low boiling point of ammonia allows cooling temperatures of up to -60 degrees, making these systems more suited to industrial refrigeration applications.
Quieter, more reliable system since it&#;s not mechanical.
Ideal for areas where electric grids are unreliable as the system relies on natural absorption and a generator.
Better for the environment as ammonia is a natural refrigerant and doesn&#;t harm the ozone.
Evaporative cooling does not use the traditional cooling cycle described above. Often called &#;swamp coolers,&#; evaporative cooling systems suck in warm outdoor air and blow it over water-soaked pads to cool it. The heat is absorbed by the water and evaporated, as cool dry air is blown into the area being cooled. Compared with traditional refrigerant systems, they are more affordable and easier to install.
Evaporative cooling systems are only suitable for air-conditioning applications as they are not capable of drastic temperature reductions. They&#;re also more suitable for warm, dry climates like those in the southwest and primarily in smaller homes. In most cases, a business would not rely solely on an evaporative cooling system, but they may supplement their current HVAC system with one to increase airflow and humidity to make the indoor environment more comfortable for employees and customers.
Thermoelectric refrigeration systems work completely differently than other types of refrigeration systems. Instead of water or a refrigerant, it uses a thermocouple and an electrical current. The thermocouple is two separate metal wires that are connected on each end. When the electrical current is applied to the thermocouple, it has the effect of one end becoming hot and one cold. The hot end is placed outside of the area to be cooled, and the cool end is placed near the interior. Like other types of cooling systems, it works through heat transfer and drawing warm air out of the room through the cool end of the thermocouple.
The main advantage of thermoelectric refrigeration is its versatility. The technology is used in many consumer goods, such as portable coolers, but it can also scale to industrial and manufacturing applications. These systems are most commonly used in small, difficult to access spaces like electronic cabinets and server rooms.
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Why Choose RSI Out of State Students Scholarships Military StudentsOne of the first things every HVAC/R student learns is that air conditioning units don&#;t create cool air. What they actually do is remove heat out of a given area.
All refrigeration systems work by transferring heat around, moving it from a designated area to somewhere else, thus cooling that designated area off and reversing the natural flow of heat with the application of energy. How this happens, however, varies among the four different types of refrigeration systems.[1]
While the four types of refrigeration systems have many similarities, they have just as many differences. To help you get ahead of the curve, we&#;ve broken down each one in some summaries below.
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The most widely used refrigeration cycle method is mechanical compression. It has applications in both air conditioning and commercial and industrial refrigeration.
As the name suggests, these types of systems transfer heat by mechanically compressing refrigerant into a low-pressure, cold liquid and expanding it into a high-pressure, hot gas. Refrigerants are substances that can boil at a wide range of temperatures with the application or removal of pressure.
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Like all liquids, they absorb heat when they boil into a gas and release it when they condense back into a liquid.
The basics of the process deal with moving the refrigerant from the low-pressure side of the closed system to the high-pressure side. The liquid refrigerant boils at 40°F on the low-pressure side and absorbs heat from the warm indoor air.
On the high-pressure side of the system, the heat travels from the 110°F vapor refrigerant to the cooler outdoor air, and the process repeats.[2]
Heat is also transferred in absorption refrigeration systems by compressing and expanding refrigerant.
These systems rely on the process of absorption and heat, instead of an electrically powered mechanical compressor, to move the refrigerant from the low-pressure side to the high-pressure side. Unlike mechanical-compression refrigeration systems, which typically use R-22 and R-410A for refrigerants, absorption refrigeration systems use refrigerants that other substances are attracted to and absorb.
For example, residential HVAC chillers often use ammonia as the refrigerant and water as the absorbent. The water is located in a component called the absorber, where it sucks ammonia from the low-pressure side of the system and removes heat while absorbing it. A pump sends the water-ammonia solution to a generator, which boils it, separating the ammonia from the water before sending it to the high-pressure side.[3]
In addition to the process of absorption, heat is also used to move the refrigerant throughout the system. The heat can derive from hot water, steam, natural gas or other fuel sources.[4]
Unlike the mechanical-compression and absorption refrigeration systems discussed above, evaporative cooling doesn&#;t use the traditional refrigeration cycle. Instead, these units, often called swamp coolers, cool warmer outdoor air by blowing it over water-soaked pads as it enters the home.
The water absorbs the heat from the air and evaporates. The cooler air is channeled into the home and the warm air out of it.
Evaporative coolers can reduce air temperature by 15° to 40°F but are best suited for dry climates, such as those in the southwestern U.S. They&#;re also less costly to install and use about a quarter of the energy of central air conditioners.[5][6]
Thermoelectric refrigeration systems are unique from the three other types of refrigeration in that no refrigerant or water is used. These systems use an electric current and a thermocouple.
A thermocouple is made up of two different metal wires that are united at both ends. Insulation separates the rest of the wires from each other. When the current is directed on the thermocouple, one end will become hot and the other cool.
Reversing the current&#;s direction has the effect of swapping the cold and hot junctions. The hot end will typically be placed outside of the area to be cooled with a heat sink attached to it to keep it the same temperature as the surrounding air.
The cold side, which is below room temperature, is placed in the area to be cooled, attracting heat out of the air.[7]
This type of refrigeration is generally used for small cooling loads that can be difficult to access, such as electronic systems.[1]
Refrigeration is only one part of an entire industry built around keeping indoor air comfortable and healthy called heating, air conditioning, ventilation, and refrigeration (HVAC/R).
A whole universe of science exists behind the ways refrigeration operates to keep the world cooler. Learn more facts about how refrigeration works.
Additional Sources
[1] Title: Fundamentals of HVAC; Authors: Carter Stanfield and David Skaves; Air-Conditioning, Heating, and Refrigeration Institute; Second Edition; Textbook page 144
[1] Title: Fundamentals of HVAC; Authors: Carter Stanfield and David Skaves; Air-Conditioning, Heating, and Refrigeration Institute; Second Edition; Textbook page 137-138
[2] Title: Fundamentals of HVAC; Authors: Carter Stanfield and David Skaves; Air-Conditioning, Heating, and Refrigeration Institute; Second Edition; Textbook page 138-141
[3] Title: Fundamentals of HVAC; Authors: Carter Stanfield and David Skaves; Air-Conditioning, Heating, and Refrigeration Institute; Second Edition; Textbook page 141-143
[4] http://www.brighthubengineering.com/hvac/-simple-vapor-absorption-refrigeration-system/
[5] Title: Fundamentals of HVAC; Authors: Carter Stanfield and David Skaves; Air-Conditioning, Heating, and Refrigeration Institute; Second Edition; Textbook page 142-144
[6] https://energy.gov/energysaver/home-cooling-systems/evaporative-coolers
[7] https://www.activecool.com/technotes/thermoelectric.html
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