Refrigeration system
What is Refrigeration?
Refrigeration is a branch of science that deals with the transfer of heat from a low-temperature region to a high-temperature region, in order to maintain the desired region at a temperature below that of the surroundings. In the refrigeration process, heat is continuously removed from a low-temperature region to a high-temperature medium by using a low boiling point refrigerant. External power is required to carry out.
What is Refrigerant?
The refrigerant is a heat-carrying medium, which undergoes the Thermodynamic cycle of refrigeration (i.e., compression, condensation, expansion, and evaporation). A refrigeration system absorbs the heat from a low-temperature medium and discards the absorbed heat to a high temperature environment.
Refrigerant Basic Characteristic:
- Low Freezing point.
- Low condensing pressure.
- Low boiling point.
- High latent Heat of vaporization.
- Non-toxic.
- Non-explosive.
- Non-inflammable.
- Non-corrosive.
Components of refrigeration:
1. The compressor
Compression is the first step in the refrigeration cycle, and a compressor is a piece of equipment that increases the pressure of the working gas. Refrigerant enters the compressor as a low-pressure, low-temperature gas, and leaves the compressor as a high-pressure, high-temperature gas.
2. The condenser
The condenser, or condenser coil, is supplied with high-temperature high-pressure, vaporized refrigerant coming off the compressor. The condenser removes heat from the hot refrigerant vapor gas vapor until it condenses into a saturated liquid state. After condensing, the refrigerant is a high-pressure, low-temperature liquid, at which point it’s routed to the loop’s expansion device.
3. The expansion device
The job of a system’s expansion device is to create a drop in pressure after the refrigerant leaves the condenser. This pressure drop will cause some of that refrigerant to quickly boil, creating a two-phase mixture.
This rapid phase change is called flashing, and it helps tee up the next piece of equipment in the circuit, the evaporator, to perform its intended function.
4. The evaporator
Refrigerant enters the evaporator as a low temperature liquid at low pressure, and a fan forces air across the evaporator’s fins, cooling the air by absorbing the heat from the space in question into the refrigerant.
After doing so, the refrigerant is sent back to the compressor, where the process restarts.
Type Of Refrigeration system
1. Gas refrigeration system
2. Vapor compression refrigeration system
3. Vapor absorption refrigeration system
4. Steam refrigeration system
1. Gas refrigeration system:
The gas such as air is used as a refrigerant. It transfers only its sensible heat and does not undergo a Change of phase. The gas power cycles used earlier can be used as gas refrigeration by simply reversing the direction of process involved in these cycles. The reversed Carnot cycle, reversed Brayton cycle and reversed Stirling cycle are some gas refrigeration cycles. The main advantages of air refrigeration is its free availability, light weight and eco-friendly.
Refrigeration Cycles used in gas refrigeration
- Reverse Carnot cycle
- Reverse Brayton Cycle
1.1. Reverse Carnot cycle
Carnot engine is reversed, then the cycle works as a refrigeration cycle. The reversed Carnot cycle will theoretically have the maximum possible coefficient of performance.
A reversed Carnot cycle using air as a working medium is shown in p-V and T-S diagrams in Fig. 4 (a) and (b), respectively. The cycle consists of four reversible processes in sequence.
Process 1–2 Isentropic expansion of air from higher temperature TH to lower temperature TL
Process 2–3 Heat removal from cold space in isothermal manner at temperature TL,
Process 3–4 Isentropic compression of air from low-temperature TL to high-temperature TH
Process 4–1 Heat rejection isothermally to a medium at temperature TH
The refrigeration effect = Heat absorbed by air during isothermal process 2–3 at temperature TL, RE =TL(S3-S2)
Heat rejected at temperature TH, QH = TH(S3-S2)
Net work input to cycle: Wnet = QH-QL
Coefficient of performance:
1.2. Brayton Refrigeration Cycle: Bell Coleman Cycle
The Bell Coleman refrigeration cycle is the reverse of the closed Brayton power cycle. The schematic and T-s diagrams of the reverse Brayton cycle are shown in Fig. 5
The refrigerant gas (may be air) enters the compressor at the state 1 and is compressed to the state 2. The gas is then cooled at constant pressure in a heat exchanger to the state 3. During cooling, the gas rejects heat to the surroundings and approaches the temperature of the warm environment. The gas is then expanded in an expander to the state 4, where it attains a temperature, that is well below the temperature of the cold region. The refrigeration effect is achieved through the heat transfer from the cold region to gas as it passes from state 4 to 1 in a heat exchanger and the cycle completes.
The network input to the cycle is Wnet=Wc — Wt
RE = Heat transfer from the cold region to the refrigerant gas=h1-h4
The coefficient of performance of the cycle is the ratio of the refrigeration effect to the net work input
2. Vapour Compression Refrigeration System:
A schematic of an ideal vapour compression refrigeration cycle and its T-S diagram are shown in Fig. 6.
The vapour compression refrigeration cycle consists of four processes discussed below:
Process 1–2 Isentropic compression of saturated vapour in the compressor,
Process 2–3 Constant pressure heat rejection in the condenser,
Process 3–4 Throttling of refrigerant in an expansion device, and
Process 4–1 Constant pressure heat absorption in evaporator.
Vapour Compression Cycle on Pressure-Enthalpy Diagram
1. Evaporator (w = 0): QL =h1 — h4
2. Compressor (g =0): Win = h2-h1
3. Condenser (w = 0): QH =h2-h3
4. Expansion valve (q =0, w=0): h4 = h3
The coefficient of performance of refrigerator can be expressed as
3. Vapour Absorption Refrigeration System:
A vapour absorption system operates with a condenser, a throttle valve, and an evaporator in the same way as vapour compression system, but the compressor is replaced by an absorber, pump and generator units as shown in Fig. 8.
Ammonia-water Absorption Refrigeration System:
Ammonia is used as refrigerant and water is used as absorbent. In the absorber, the ammonia vapour coming out the evaporator at the state 1 is absorbed by liquid water. The formation of this liquid solution is exothermic; thus heat is released. The solvency of ammonia in the water decreases as temperature increases. Thus the cooling is required in the absorber to absorb the energy released due to absorption of ammonia in the water. The strong ammonia — water solution is pumped to the generator through a heat exchanger, where it is preheated with the help of hot weak solution returning to the absorber. In the generator, the heat transfer from the source (solar energy) drives the ammonia vapour out of the solution (endothermic process), leaving a weak ammonia — water solution. The ammonia vapour liberated passes to the condenser at the state 2 through a rectifier. The condensed ammonia is expanded through an expansion valve and then enters the evaporator, where it absorbs heat from the low-temperature region.
4. Steam Jet Refrigeration
It uses water as refrigerant, which is quite safe like air. If the pressure exerted on the surface of water is reduced then saturation temperature also lowers and water starts evaporating at lower temperature due to reduced pressure. It is the basic principle for steam jet refrigeration system. It consists of an evaporator (flash chamber), a steam nozzle, an ejector and a condenser. The steam expands through a nozzle to form a high speed, jet, which draws (sucks) the water from the flash chamber into the ejector. Therefore, the pressure in the flash chamber gets reduced, thereby results into further formation of vapour (evaporation) in the (flash chamber). This evaporation extracts heat (latent heat which is higher at lower pressures) for phase change, thus reducing the temperature of water in the flash chamber. The cold water is used for refrigeration. The mixture of steam and water vapour is diffused in the diverging part of the ventury tube to the exhaust pressure and fed to the condenser. After condenser some of water is returned to flash chamber as a make-up water, and rest is used as feed water to the boiler. The steam jet refrigeration system uses waste steam returning to condenser.
Applications:
1. District Cooling
2. Electricity Production
3. Chemical & Petrochemical
4. Pharmaceutical
5. Food & Beverages
6. Data Centers
Conclusion:
Without the advent of refrigerators, unreliable methods of storing food would not only cause problems for taste but also health.
For the efficiency of a consumer refrigerator when you’re buying one, one with an energy star label will indicate high efficiency, as well as other appliances. Refrigeration has become and likely will likely for as long as imaginable be the prime way to store food.
In terms of future technology, although it seems like just sci-fi, there is research into “refrigerating” people, to keep their bodies in a halted state until undoing this process much, much, later in the future, making it a potential candidate idea for space travel.