In this article we will discuss about:- 1. Meaning and Applications of Refrigeration 2. Principle of Refrigeration 3. Unit 4. Effect 5. Types.
Meaning and Applications of Refrigeration:
Refrigeration is the process of removing heat continuously from the system to be cooled to maintain the temperature lower than the atmospheric temperature.
i. Food Preservation – Milk storage, butter storage, dairy products storage, storages of vegetables, fruits, and meat and poultry products
ii. Fish Storage (-16°C): Preservation of fish from the time it is caught until the ship returns to the port requires proper attention
ADVERTISEMENTS:
iii. Household refrigerator
iv. Comfort air conditioning
v. Industrial air conditioning
vi. Printing notes
ADVERTISEMENTS:
vii. Laboratory, textile industries
viii. Chemical processes
ix. Separation of gases
x. Condensation of gases
ADVERTISEMENTS:
xi. Low-pressure storage in liquid form
xii. Cold treatment of metals
xiii. Manufacturing of drugs
xiv. Blood plasma and antibiotics are manufactured using this method called freeze drying. Freeze drying is the process of removal of water by sublimation at low pressure and temperature which is less damaging to the human tissues than driving the vapor at high temperature.
ADVERTISEMENTS:
xv. Ice skating rinks
xvi. Ice manufacturing
Principle of Refrigeration:
In the process of refrigeration, the available heat, with the system to be maintained at low temperature, is continuously removed and transferred to the surrounding which is at high temperature.
According to the second law of thermodynamics (Clausius theorem), the removal of heat from low temperature in order to supply heat at high temperature is only possible by supplying external work to the system under operation. Hence, a refrigerator needs external power for removing heat continuously from the cabinet to maintain temperature lower than the surrounding.
ADVERTISEMENTS:
The basic mechanism of refrigeration is shown in Fig. 6.1 in which T1 and T2 are the maximum and minimum temperature bodies, respectively; R is the refrigerator; Q1 is the heat supplied to the hot body and Q2 is the heat removed from the low-temperature body; and WR is the work required to produce low temperature.
Units of Refrigeration:
The capacity of a refrigeration system is expressed in terms of tons of refrigeration. In SI system, 1 ton of refrigeration = 210 kJ/min = 3.5 kW
A ton of refrigeration is defined as the amount of heat extracted by the refrigerating machine to produce 1 US ton of ice at 0°C in 24 hours.
1 US ton = 2000 lbs.
Taking latent heat of fusion at 0°C = 334.4 kJ/kg
Heat extracted to form 1 ton of ice at 0°C in 24 hours
= 2000 × 334.4/2.204 × 60 × 24
= 210.72kJ/min = 210 kJ/min or 211 kJ/min
Refrigerating Effect:
The rate at which heat is extracted by the refrigerating machines from the system to be cooled is known as refrigerating effect.
Coefficient of Performance:
The performance of any refrigerating system is represented by coefficient of performance (COP).
The COP of a refrigerating system is defined as the ratio of heat extracted by the refrigerating system to the work required by the system.
COP = Q2/W
Where, Q2 is the heat extracted in kW and W is the work supplied in kW.
Types of Refrigeration System:
i. Working Fluid of a Refrigeration System:
In a refrigerating system, a working fluid known as refrigerant is used to remove heat continuously from the system to be cooled at a temperature less than that of the surrounding. A refrigerant never leaves the plant but it is circulated over and over again.
The working fluid changes its thermodynamic properties. The refrigerant has specific properties. It evaporates at low temperature and pressure by absorbing heat from the system to be cooled and condenses at high temperature and pressure rejecting heat to the atmosphere.
There are two main processes involved:
(a) The refrigerant absorbs heat by changing liquid phase to vapor phase. Thus, the latent heat is absorbed.
(b) The refrigerant rejects heat by changing vapor phase to liquid phase. Thus, the latent heat is released.
ii. Electrolux System:
The electrolux system has a unique feature that it operates without the use of a pump, i.e., without the use of mechanical energy. The unit is 100% heat-operated and could be successfully used for domestic use. The circulation of fluids occurs by virtue of density variation and the refrigerant is evaporated in the presence of hydrogen.
The schematic diagram of a simple electrolux system is presented in Fig. 6.23. In this cycle, ammonia acts as a refrigerant and water as an absorbent. The generator is heated by means of a simple flame, produced by the burning of fuel. The liquid is separated from the vapor by special designing of the outlet of evaporator. Due to heat, a mist of liquid droplets goes up to the separator.
The vapor travels to the condenser and the liquid drains into the absorber. The liquid ammonia after condensation flows down to the evaporator, where it receives heat from the system to be cooled and vaporizes. In the absorber, the weak solution from separator is mixed with the incoming vapor. The concentrated liquid then flows back to the generator and the cycle is repeated again.
In evaporator and absorber, hydrogen is present in addition to the ammonia and water. Hydrogen exerts partial pressure which in combination with the partial pressure of ammonia and water results a total pressure in evaporator and absorber which equals the pressure of ammonia and water in condenser and generator.
Thus, liquid ammonia is evaporated at low temperature because of the low partial pressure available for ammonia in evaporator. In condenser, the condensation of ammonia takes place at high temperature where no hydrogen is present and the heat is rejected to the atmosphere. The total pressure throughout the system remains the same.
The circulation in the system takes place by a vapor lift pump. Once the heat is added to the generator, a mist of vapor with water goes up near the outlet of the separator which separates water and circulates further by gravity. The U-bend is provided after the separator and condenser to entrap the liquid which acts as liquid seal which prevents hydrogen from escaping.
The main use of this system has been in household refrigerators. Even though ammonia is toxic, it became popular due to its simplicity. The chance of leakage is minimum and also the quantity of ammonia is so small that there is little danger.
iii. Refrigerator using Vapor Compression System:
Figure 6.24 shows a vapor compression refrigerator. The principle of vapor compression system has been used for working of a refrigerator. It consists of a hermitically sealed compressor, air-cooled condenser; a capillary tube to work as expansion device and a coiled evaporator fitted in the freezing compartment of the refrigerator and connected to the suction side of the compressor.
The delivery side of the compressor is connected to the condenser which in turn connected to the capillary tube. The liquid refrigerant passes to the evaporator coil where it absorbs its heat. The heat is continuously extracted by the items kept inside the refrigerator and rejected in the condenser to the atmosphere.
This will keep the item of refrigerator at the required lower temperature. The required low temperature is maintained in the refrigerator by thermostat switches on and off the compressor motor by a relay. One of the most common refrigerants with the vapor compression system is dichlorodifluoromethane, popularly known as Freon 12 or R12. The temperature maintained in the evaporator is about 7°C and that in the condenser is about 38°C.
iv. Refrigerator using Vapor Absorption System:
This type of refrigerator is shown in Fig. 6.25. It consists of an absorber, a pump, heat exchanger, generator-cum-separator, condenser, expansion device, and a coiled tube evaporator. Dry saturated ammonia vapor is dissolved in water kept in an absorber. The strong solution is passed to the generator-cum-separator at high pressure.
The ammonia vapor is driven out from the separator which passes to the condenser at high pressure, where it condenses. High-pressure ammonia liquid at low temperature is passed to the evaporator coil placed in the freezing compartment, where it evaporates. Low-pressure ammonia vapor from the evaporator coil is passed again to the absorber where it is absorbed by dissolving in water. This process is repeated again and again.