Refrigeration

UNIT – IV

Chapter - 18

REFRIGERATION

 

REFRIGERATION – AN INTRODUCTION

Second Law of Thermodynamics

It states that, “Heat flows from a system at a lower temperature to a system at a higher temperature with the aid of external work”.

Refrigeration:

Refrigeration is defined as an art of producing and maintaining the temperature in a closed space below atmospheric temperature.

The American Society of Refrigeration Engineers defines Refrigeration as the process of removing heat continuously from a body to reduce its temperature lower than that of its surrounding atmosphere and maintaining it by continuously absorbing the heat.

Refrigerator: Refrigerator is an equipment used for producing and maintaining the temperature in a closed space below the surrounding atmospheric temperature.

Refrigerant: Refrigerant is the working fluid in a refrigerator. It absorbs heat at a lower temperature and rejects the heat at a higher temperature in the form of latent heat.

Refrigerating System: Refrigerating system keeps the temperature of the substance below the atmospheric temperature by removing heat from the substance using external source of power.

Refrigerated Space: Refrigerant continuously absorbs heat from the space inside the refrigerator to keep it cool at a temperature less than the atmospheric temperature. Finally, it rejects the heat to the atmosphere. The space inside the refrigerator is called the Refrigerated Space.

Put water on your skin, it makes you feel cool. As water evaporates, it absorbs heat from the body and creates that cool feeling. When we rub petrol on our skin, we feel even cooler. Yes, petrol evaporates at a lower temperature. If you place your wet hand in front of a fan, the moisture from the hand evaporates. This evaporation from the hand keeps the hand cool.

Similarly, the refrigerant evaporates, i.e., changes from liquid to vapour state at a very low temperature. Therefore, it creates freezing temperature inside the refrigerated space.

Applications of Refrigeration

 1. Refrigeration is used to preserve Fruits, Milk, Drinks, Food, etc., for a long period.

2. It is used in Water Coolers.

 3. It is used to preserve Medicines, Blood, etc. in hospitals.

 4. Refrigeration is used in industries for processing Lubricants, Chemicals, Rubber, etc.

 5. Refrigerating machines are used to produce ice in Ice Plants.

 

1. PROPERTIES OF REFRIGERANTS

a. Thermodynamic Properties

1. Low Boiling Point

A good refrigerant should have low boiling point at atmospheric pressure so that it can be used at low temperature which exists in the refrigeration system.

2. Low Freezing Point

A good refrigerant should have a low freezing point, because the refrigerant should not freeze at low evaporator temperature, causing the pipes to clog.

3.  High Latent Heat of Evaporation                                                                          A good refrigerant should have a high latent heat of evaporation so that a minimum quantity of refrigerant increases the refrigeration effect.

4. High Coefficient of Performance

A refrigerant should have a high C.O.P. so that energy spent in refrigeration will be less.

b. Physical Properties

1. Low Viscosity

A good refrigerant, both at vapour and liquid states, should have a low viscosity so as to improve the heat transfer.

2. Specific Heat: A good refrigerant should have low specific heat in the liquid state and high specific heat in the vapour state to increase the refrigerating effect.  3. Leakage: Moisture may leak in the refrigerating system. Therefore, if a refrigerant reacts with moisture, it cannot be used in a refrigerating system.

c. Chemical Properties

 1. Action with Water

Ammonia is highly soluble in water. Therefore, a wet cloth is put at the point of leak to avoid harm to human beings in ammonia refrigeration plants.

2. Action with Lubricating Oil

Lubricating oil is used to lubricate the parts of the compressor. The refrigerant should not react with the lubricating oil.

3. Chemical Stability: Refrigerant should not decompose under the operating temperature.

4. Odourless and Colorless

It must be odourless and colorless. It should not contaminate the food stuff.

d. Safe Working Properties

 1. Inflammability: A refrigerant should be non-inflammable when mixed with air or oil.

2. Non-Toxic

If a substance, when mixed with air in small percentage, is injurious to human beings, it is called toxic. Leakage of toxic refrigerator causes suffocation.

 3. Non-Corrosiveness: It should be non-corrosive to prevent corrosion of metallic parts.

 

2. LIST OF COMMONLY USED REFRIGERANTS

1. Ammonia (NH₃)

Ammonia is used in ice manufacturing and cold storage plants, i.e., in Vapour Absorption System. Ammonia is commercially employed where toxicity is secondary. Because of inflammability and toxicity, it is not used in domestic refrigeration and air-conditioning.

2. Freon-12

Freon-12 is a combination of Chlorine and Fluorine. It is colourless, odourless, non-toxic, non-flammable and non-corrosive to metals. It does not affect the stored food products. Freon-12 is used in Domestic Refrigerators, Water coolers, Air conditioners, etc.

3. Freon-22

It is superior to Freon-12. It is colourless, odourless, non-toxic, non-flammable and noncorrosive. It is used in Food freezing and Air conditioners in large capacity plants. The disadvantage of Freon-22 compared with Freon-12 is the discharge temperature which requires water cooling of the compressor head and cylinder.

4. Carbon Di-oxide (CO2): It is colourless, odourless, non-toxic, non-corrosive and non flammable. Due to low specific volume, the plant size is compact. e.g., Marine refrigerator.

5. Air: It is used in the aircraft where C.O.P., i.e., efficiency of operation is secondary.

 

3. TERMINOLOGY

1. Refrigerating Effect

Refrigerating Effect is defined as the rate at which heat is removed from the space to be cooled in a cycle. It is expressed in kJ / second or kW..

2. Ton of Refrigeration (TR)

Capacity of Refrigerator is defined as the rate at which heat can be absorbed from the cold body. It is expressed in Ton of Refrigeration (TR).

3. Unit of Refrigeration

It is expressed in terms of Ton of Refrigeration (TR). A Ton of Refrigeration is the amount of heat absorbed to produce 1 Tonne of ice at 0°C within 24 hours when the initial condition of water is also at 0°C. The value of 1 Tonne of refrigeration is 3.5 kW in S.I. units.

 Note: Air conditioners are also specified by the same unit TR.

4. Coefficient Of Performance (C.O.P.)

 It is defined as the ratio of amount of heat removed in a system to the external work done upon the working medium for the heat removal.

Therefore, C.O.P. = Q/W = Heat removed from a refrigerator in kW/Work         done upon the working medium in kW

C.O.P. is always more than 1. Higher the value of C.O.P., performance of the refrigerator is better. [C.O.P. should not be confused with the term Efficiency. Efficiency is always less than 1.]

 

4. TYPES OF REFRIGERATING SYSTEMS

1. Vapour Compression Refrigeration System

2. Vapour Absorption Refrigeration System