Refrigeration is the process of heat removal by lowering temperature beyond atmospheric temperature. For producing refrigerating effect various methods are used. We shall be discussing various refrigeration system in this article.
Two important terms used in refrigeration system is TR(Ton of Refrigeration)and COP ( Coefficient of performance), TR is used to define the capacity of refrigeration system while COP is a ratio of useful heating or cooling provided to work required.
TR : A ton of ice melting over 24 hrs. provides a ton of refrigeration each hour. 2,000 lbs. of ice at 144 BTUs per pound = 288,000 BTUs 288,000 ÷ 24 hrs. = 12,000 BTUs per hour, or 1 ton of refrigeration. (3024 Kcal/Hr or 3.5 Kw), for detail on Tonnage, COP, EER, SEER, ISEER and Star Rating For Air Conditioners, click here
COP : Coefficient of performance is a measure of efficiency of a refrigeration cycle/system is defined as Refrigeration effect / Energy Spent.
The different type of refrigeration system are :
- Vapor Compression Refrigeration.
- Vapor Absorption Refrigeration
- Air Refrigeration
- Steam Jet Refrigeration
- Non conventional Refrigeration.
1 Vapor Compression Refrigeration
Vapor compression refrigeration is most common and widely used refrigeration system. In Vapor compression system refrigerant is used to generate refrigeration. For Different type of refrigerant used in air conditioning click here,
Vapor compression system consist of majorly four equipment’s , Evaporator, compressor, condenser and expansion valve. In evaporator refrigerant is evaporated from liquid to vapor and produces refrigerating effect. The evaporated vapor goes to compressor and it is compressed to higher pressure level and then passed to condenser for cooling. Condenser cools refrigerant from vapor to liquid phase with the use of atmospheric temperature or cooling water temperature. The liquid refrigerant is passed to expansion valve where refrigerant of high pressure is expanded to low pressure and fed to evaporator for next cycle of refrigeration.
Vapor compression system is most reliable refrigeration system and used is household refrigerators, Air conditioners. The COP of vapor absorption system is high which makes is economical in wide range of operation.
Compressor used in vapor compression system can be centrifugal / Reciprocating/ Screw / scroll etc, Each type of compressor are having their own advantages and disadvantages, therefore before selection of refrigeration system IPLV and NPLV should be compared, For information regarding IPLV and NPLV, click here. Expansion valve used in refrigeration system can be capillary tube, thermostatic valve or float valve based on the refrigerant capacity required to be expanded. in Industrial application secondary refrigerant is used to transfer refrigeration load from one place to another place, For understanding primary and secondary refrigerant, click here
To improve operational and energy efficiency of vapor refrigeration system, other components are added like subcooler, super heater, economizer, Multistage compression system etc. are used. for guideline on efficient chiller operation, click here
2 Vapor Absorption refrigeration
Vapor absorption system utilizes heat for producing refrigeration in place of electricity. It consists of absorber, Generator, condenser expansion valve and evaporator. The refrigerant in evaporator evaporates and goes to absorber where it is absorbed by absorber fluid. absorber fluid is pumped to generator where refrigerant is separated and sent to condenser for condensation. condensed refrigerant is passed to expansion valve and this refrigeration cycle continues to produce refrigeration.
Vapor absorption system are of two types :-
- Lithium Bromide Absorption Refrigeration system.
- Electrolux (Ammonia Hydrogen refrigeration system)
Lithium Bromide Absorption Refrigeration system.
In lithium bromide absorption system water is used as refrigerant and LiBr is used as absorber. Water evaporation and condensation takes place in vaccum to produce refrigerating effect. This Li Br absorption system can be successfully designed for chilled water requirement of 5 Deg C, Now a days even chilled water systems can be designed with 2 Deg C or with the help of brine temperature can be reduced to -2 deg C.
Lithium bromide based refrigeration systems are very popular and used in various process industries, COP of vapor absorption system is lower than vapor compression system. The vapor absorption system can be designed in three different stages :-
|Single Effect Vapor Absoption System
|Double Effect Vapor Absorption system
|Tripple Effect Absorption system
|One Generator is used for refrigerant separation.
|Additional high Temperature generator is used than single effect vapor absorption system.(Two Generator is used)
|Additional Medium temperature and high Temperature generator is used for separation of refrigerant. ( Three generator is used)
|Hot water can be used above 75 deg C for chilled water generation
|Hot water can be used from 150 to 180 Deg C
|Hot water can be used from 190 to 225 Deg C
|Steam required upto 3.5 Kg/cm2
|Steam required from 3 to 10 Kg/cm2
|Steam Required from 10 to 25 Kg/cm2
|COP – 0.7 to 0.8
|COP 1.4 to 1.5
|COP upto 1.8
Electrolux (Ammonia Hydrogen refrigeration system)
Ammonia hydrogen refrigeration system is type of vapor absorption system where Ammonia is refrigerant and water is absorbent,.
Liquid ammonia is fed to evaporator and vaporizes to produce cooling effect, Vapor ammonia is absorbed in absorber where weak solution of ammonia and water is already present, Water is used as absorbent here, is unsaturated and had capacity to absorb more ammonia, Due to absorption of ammonia, Strong solution is formed in absorber and then it is pumped to generator, External heat is applied to generator and ammonia is evaporated, During evaporation there is chances of water vapor to get evaporated so rectifier is used to trap water vapor and pure liquid ammonia is passed to condenser where vapor ammonia is cooled to liquid ammonia. Weak solution generated after separation of ammonia is sent to absorber to keep absorption operation for continuity of refrigeration system. COP of this system is low and in the range of 0.6 -0.65, therefore it is not very popular system of refrigeration.
3 Air Refrigeration
Air refrigeration is based on Bell-Coleman Cycle Or Reversed Brayton Cycle, In air refrigeration system air is used as working fluid, No phase change is involved in air refrigeration, It is used in air craft as high pressure air is readily available.
Air refrigeration consists of four elements, Air is compressed in air compressor and then cooled to atmospheric condition with the used of heat exchanger. Atmospheric cooled air is passed to expander or turbine where due to expansion effect cooled air is produced. as shown above.
There can be various configuration of air refrigeration system like :-
- Simple air refrigeration
- Bootstrap air refrigeration
- Boot Strap evaporative air refrigeration
- Regenerative air refrigeration
- Reduced ambient refrigeration
Detail discussion on type of air refrigeration system not part of this article so parking this topic, Performance curve of various air refrigeration is shown below.
4 Steamjet Refrigeration
Water is used as refrigerant in Steam jet or Ejector refrigeration system. This refrigeration system is based on the principle of boiling of liquid at lower temperature by reducing pressure on its surface. Steam ejector is used for steam jet refrigeration system.
Steam jet ejector usage high pressure steam to generate vacuum in flash chamber, Due to vacuum generation in flash chamber water evaporates and provides cooling effect. Water evaporation also takes away water from the chamber therefore water needs to be makeup to keep refrigeration system working. Steam of ejector is cooled at condenser and generally sent to boiler as boiler feed water.
Steam jet refrigeration is widely used in paper mills, breweries, food processing plants, gas plants etc. Because of water is used as refrigerant, it cannot be used for applications below 0°C.
5.Non conventional Refrigeration.
Some of the non conventional refrigeration systems are
- Votex tube refrigeration system.
- Thermo electric Refrigeration System
Votex tube refrigeration
Votex tube refrigeration system usages air as for cooling, Compressed air is passed through the nozzle as shown below. Air expands and acquires high velocity due to particular shape of the nozzle.
A vortex flow is created in the chamber and air travels in spiral like motion along the periphery of the hot side. This flow is restricted by the valve. When the pressure of the air near valve is made more than outside by partly closing the valve, a reversed axial flow through the core of the hot side starts from high-pressure region to low-pressure region. During this process, heat transfer takes place between reversed stream and forward stream.
Therefore, air stream through the core gets cooled below the inlet temperature of the air in the vortex tube, while air stream in forward direction gets heated up. The cold stream is escaped through the diaphragm hole into the cold side, while hot stream is passed through the opening of the valve. By controlling the opening of the valve, the quantity of the cold air and its temperature can be varied. COP of Votex tube refrigeration system is low capital cost solution with lower footprint requirement and maintaninance is simple and easy but this system is having lower COP and not economical.
Thermo electric Refrigeration System
Thermoelectric refrigeration uses a principle called the “PELTIER” effect to pump heat electronically, when an electrical current is applied across the junction of two dissimilar metals, heat is removed from one of the metals and transferred to the other. This is the basis of thermoelectric refrigeration. Thermoelectric modules are too expensive for normal domestic and commercial applications which run only on regular household current.
Thermo electric refrigeration system
Some example of thermoelectric refrigeration are water or beer cooler, Cooled car sheet, motherboard cooling etc