Refrigeration
Methods of Refrigeration: Removing undesirable heat from one item, substance, or area and transferring it to another is known as refrigeration, sometimes known as chilling. The temperature can be reduced by removing heat, which can be accomplished by the use of ice, snow, cooled water, or mechanical refrigeration.
Methods of Refrigeration
There are three types of refrigeration techniques: thermoelectric, cyclic, and non-cyclic.
Non-cyclic refrigeration
These techniques allow for the sublimation of dry ice or the melting of ice to achieve refrigeration. These methods are utilised in laboratories and workplaces for small-scale refrigeration or portable coolers.
Due to its constant melting point of 0 °C (32 °F), ice is a powerful cooling agent. Ice requires 333.55 kJ/kg (or around 144 Btu/lb.) of heat to melt. Foods kept at this temperature or a little higher have a longer shelf life. Dry ice, or solid carbon dioxide, is another type of refrigerant. At a temperature of -78.5 °C (-109.3 °F), it sublimes directly from the solid to vapor phase since it has no liquid phase at normal atmospheric pressure. For keeping items at low temperatures throughout the sublimation process, dry ice works well.
Cyclic refrigeration
This consists of the thermodynamic power cycle and the refrigeration cycle, where heat is transferred from a low-temperature area or source and rejected to a high-temperature sink with the aid of external effort. Heat is given to the engine during the power cycle from a high-temperature source, with some of the heat being utilized to generate work and the remainder being rejected to a low-temperature sink. This complies with the second rule of thermodynamics. A refrigeration cycle explains the alterations that the refrigerant experiences as it travels through a refrigerator, alternately absorbing and rejecting heat. It is used to describe the “process” of refrigerant flow through an HVACR unit, whether it be a packaged or split system, when it comes to HVACR work.
Naturally, heat moves from hot to cold. Heat is transferred from a lower temperature heat source to a higher temperature heat sink in order to chill a dwelling area or storage area. Insulation is utilized to lessen the effort and energy needed to lower the temperature in the cooled room and keep it there. Said Carnot mathematically outlined the refrigeration cycle’s operation as a heat engine in 1824. Although absorption heat pumps are employed in a small number of applications, the reverse-Rankine vapor-compression refrigeration cycle is the most used form of refrigeration system.
The following categories apply to cyclic refrigeration:
- Vapor cycle
- Gas cycle
The following categories apply to cyclic refrigeration:
- Vapor compression refrigeration
- Vapor absorption refrigeration
Vapor-compression cycle
Most home refrigerators as well as several huge commercial and industrial refrigeration systems employ the vapor-compression cycle.
An analysis of the cycle’s thermodynamics may be done using the diagram. A circulating refrigerant, such Freon, enters the compressor during this cycle as a vapor. The vapor is compressed at constant entropy from point to another point , and it leaves the compressor superheated. The superheated vapor goes via the condenser, which first cools and eliminates the superheat before condensing the vapor into a liquid by removing more heat at constant pressure and temperature from point through and on to another point . The expansion valve, also known for the throttle valve, is where the liquid refrigerant travels between points . Here, its pressure suddenly drops, triggering flash evaporation and auto-refrigeration of, generally, less than half of the liquid refrigerant.
The chilly air (from the refrigerated compartment) is then blasted by a fan through the evaporator coil or tubes, where the cold liquid-vapor combination subsequently passes before being totally vaporized. The thermodynamic cycle is finished when the refrigerant vapor that results goes back to point at the compressor inlet. The reasoning above is based on a perfect vapor-compression refrigeration cycle and ignores phenomena that may occur in practice, such as frictional pressure loss in the system, a small amount of thermodynamic irreversibility during the compression of the refrigerant vapor, or (if any) less-than-ideal gas behavior.
The venerable “Perry’s Chemical Engineers’ Handbook” provides more details on the construction and operation of vapor-compression refrigeration systems.
Vapor absorption cycle
The vapor absorption cycle using water-ammonia systems was well-liked and widely used in the early years of the 20th century, but after the invention of the vapor compression cycle, it lost a lot of significance due to its low coefficient of performance (which was only about one fifth that of the vapor compression cycle). Nowadays, the vapor absorption cycle is only employed when waste heat, solar heat, or a lack of energy are all available.
The mechanism used to increase the pressure of the refrigerant vapor differs between the absorption cycle and the compression cycle. The compressor is replaced in the absorption system by an absorber that disperses the refrigerant in a suitable liquid, a liquid pump that increases pressure, and a generator that, upon the addition of heat, pushes the refrigerant vapor from the high-pressure liquid. The liquid pump has some work to do, but for a given amount of refrigerant, it is far less than what the compressor has to do during the vapor compression cycle. A proper blend of refrigerant and absorbent is utilized in an absorption refrigerator. The most popular combinations are water (absorbent) and lithium bromide (refrigerant), as well as ammonia (refrigerant) and water (absorbent).
Gas cycle
The refrigeration cycle is known as a gas cycle when the working fluid is a gas that is compressed and expanded but does not change phase. Most frequently, this working fluid is air. The hot and cold gas-to-gas heat exchangers in gas cycles are equivalent to the condenser and evaporator in a vapor compression cycle since condensation and evaporation are not envisaged in gas cycles.
Because the gas cycle uses the reverse Brayton cycle rather than the reverse Rankine cycle, it is less effective than the vapor compression cycle. Due to this, the working fluid does not absorb and emit heat at a steady rate. The cooling effect in the gas cycle is equal to the product of the gas’s specific heat and its temperature increase at the low temperature side. Therefore, a gas refrigeration cycle would need a high mass flow rate and be bulky for the same cooling load. Air cycle coolers are no longer often employed in terrestrial cooling equipment due to their reduced efficiency and greater mass. On gas turbine-powered jet aircraft, however, the air cycle machine is particularly prevalent because compressed air is easily accessible from the compressor portions of the engines. The cooling and ventilation systems on these jet aircraft also pressurize the cabin.
Thermoelectric refrigeration
The Peltier effect is used in thermoelectric cooling to produce a heat flux at the intersection of two types of materials. This effect is frequently used to cool electrical parts and tiny instruments, as well as in portable coolers for camping.
Magnetic refrigeration
A cooling method based on the magnetocaloric effect, a characteristic of magnetic materials, is known as magnetic refrigeration, sometimes known as adiabatic demagnetization. A paramagnetic salt, such as cerium magnesium nitrate, is frequently used as the refrigerant. The electron shells of the paramagnetic atoms serve as the case’s active magnetic dipoles. The refrigerant is exposed to a high magnetic field, which causes the refrigerant’s numerous magnetic dipoles to align and lowers the entropy of these degrees of freedom. The heat generated by the refrigerant as a result of its loss of entropy is subsequently absorbed by a heat sink. The magnetic field is subsequently turned off and thermal contact with the heat sink is severed, insulating the system. As a result, the refrigerant’s temperature drops below that of the heat sink. This enhances the heat capacity of the refrigerant.
Applications have thus far been restricted to cryogenics and research since few materials have the necessary characteristics at ambient temperature.
Other methods
Air cycle machines used in aero planes, vortex tubes used for spot cooling when compressed air is available, and thermoacoustic refrigeration, which uses sound waves in a pressurized gas to promote heat transfer and heat exchange, are further ways of refrigeration.
Unit of refrigeration
The cooling capacity of residential and commercial refrigerators may be expressed in kJ/s or Btu/h. In the US, commercial refrigerators are often rated in tones of refrigeration, although elsewhere they are measured in kW. One short tone of water may be frozen at 0 °C (32 °F) in 24 hours using one tone of refrigeration capacity. Considering that
Heat of fusion, or latent heat of ice, is equal to 333.55 kJ/kg 144 Btu/lb.
A short tone weighs 2000 pounds.
Heat extracted is equal to 2000(144)/24 hours, 288000 BTUs, 12000 BTUs each hour, and 200 BTUs every minute.
200 Btu/min refrigeration is 3.517 kJ/s or 3.517 kW.
The rate of heat removal necessary to freeze a metric tone (i.e., 1000 kg) of water at 0 °C in a 24-hour period is 1 tone of refrigeration, which is a considerably less frequent term. A tone of refrigeration requires 13,898 kJ/h or 3.861 kW based on the heat of fusion being 333.55 kJ/kg. One tone of refrigeration is 10% greater than one tone of refrigeration, as can be shown.
The majority of household air conditioning units have a 1 to 5 tone refrigerated capacity range.
The 4 Main Refrigeration Cycle Components
All of us have been there. You are happily greeted by a wall of chilly air as soon as you enter on a hot day. For that break, you may thank the refrigeration cycle. Although there are many different ways to heat and cool things, the fundamental purpose is still the same and is utilized in some way by numerous different processes and businesses. The purpose of a refrigeration cycle is to absorb and reject heat, to put it simply. You can only remove heat, as any HVAC instructor would forcefully point out, thus you can’t create cold. By directing heat away from the region you wish to chill, the refrigeration cycle—also referred to as a heat pump cycle—can be used to achieve that goal. This is done by cycling through a cycle of compression and expansion to alter the pressure of the working refrigerant (air, water, synthetic refrigerants, etc.).
Of course, that’s not the whole image, but it conveys the general notion. Let’s now discuss the tools used to carry out that task. There are undoubtedly other aspects in the majority of loops, but most people concur that a basic cycle consists of the following four core components:
- The compressor
- The condenser
- The expansion device
- The evaporator
The compressor
Compression is the first step of refrigeration, and a compressor is the piece of equipment used to increase the working gas’s pressure. Low-pressure, low-temperature refrigerant enters the compressor and exits as high-pressure, high-temperature gas.
Types of compressors
Due to the variety of mechanical methods that may be employed to create compression, a variety of compressor designs are now used in HVAC and refrigeration systems. There are other instances, but a few well-liked options include:
- Reciprocating compressors
- Scroll compressors
- Rotary compressors
The condenser
The condenser, sometimes referred to as the condenser coil, is one of the two different types of heat exchangers utilized in a basic refrigeration loop. High-temperature, high-pressure, vaporized refrigerant from the compressor is used to fuel this component. Until it condenses into a saturated liquid state, also known as condensation, the condenser removes heat from the heated refrigerant vapor gas vapor.
When the refrigerant condenses, it becomes a high-pressure, low-temperature liquid, at which time it is directed towards the expansion device of the loop.
The expansion device
There are several distinct designs for these parts. Fixed orifices, thermostatic expansion valves (TXV) or thermal expansion valves , and the more sophisticated electronic expansion valves (EEVs) are common layouts. However, regardless of arrangement, an expansion device’s duty is to lower pressure once the refrigerant exits the condenser. Some of the refrigerant will start to boil fast as a result of the pressure reduction, resulting in a two-phase mixture.
Flashing is the name for this abrupt phase shift, which sets up the evaporator, the next piece of machinery in the circuit, to carry out its intended purpose.
The evaporator
The evaporator, like the condenser, is called for its primary purpose and is the second heat exchanger in a typical refrigeration circuit. It serves as the “business end” of a refrigeration cycle since it absorbs heat in a similar manner to how air conditioning works. This happens when air is forced through the evaporator’s fins by a fan. The refrigerant, which enters the evaporator as a low temperature liquid at low pressure, absorbs the heat from the room in question and cools the air as a result.
The process then resumes when the refrigerant is returned to the compressor. And that’s basically how a refrigeration loop operates. Give us a call if you have any questions concerning the refrigeration cycle or any of its parts, including how they function. Since over a century ago, we have been assisting clients in maximizing the performance of their HVAC and refrigeration equipment.
Home and consumer use
Safer refrigerators for residential and consumer use became feasible with the development of synthetic refrigeration systems, which are primarily based on the chemical chlorofluorocarbon (CFC). These CFC refrigerants, as well as subsequent hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC), are referred to as “Freon,” a trademark of the DuPont Corporation. These refrigerants, which were created in the late 1920s, were thought to be safer than the routinely employed refrigerants of the time, such as methyl format, ammonia, methyl chloride, and Sulphur dioxide. The goal was to supply residential refrigeration equipment while protecting the tenants’ life. These CFC refrigerants provided the solution.
