Lesson 1. Basic refrigeration cycle and concepts, standard rating of refrigerating machines

Module 1.Fundamentals of refrigeration

Lesson 1

1.1 Introduction

The American Society of Heating, Air conditioning and Refrigerating Engineers (ASHARE) defines refrigeration as the science of providing and maintaining temperatures below the immediate surrounding temperatures. Thus it is a process of removing heat from a medium. In other words, the term “refrigeration” is used to denote maintenance of a system or body at a temperature lower than that of its surroundings. In other words it is the process of cooling a substance below the initial temperature of the substance.

1.2 What is Air Conditioning?

Air conditioning refers the control of environmental conditions of air depending on the use of air conditioning. Air-conditioning signifies the control of an atmospheric environment either for human or to carry out industrial or scientific process efficiently. The purpose of air-conditioning is to supply sufficient volume of condition air having a specific amount of water vapour at a required temperature within a selected space. The space may be a small compartment such as a research laboratory, computer laboratory or big area like cinema hall, shopping centre, air port etc. Thus, air conditioning refers the control of temperature, relative humidity, quality of air and distribution of air depending upon the application of air conditioning. The environmental air conditioned in terms of temperature, humidity, purity of air results in greater comfort to occupants when applied to public places, offices and factories. Air conditioning designed for industrial purpose has several benefits including better control of product quality and efficiency.

1.3 Importance of Refrigeration in Dairy Industry

Refrigeration is a basic requirement for the processing and storage of milk and milk products as majority of dairy products are perishable in nature. The need of refrigeration is indicated below.

1. Chilling of milk at producers’ level by employing bulk milk coolers and at milk chilling centers is the first requirement in dairy industry. Immediate cooling of milk to about 2-3 ºC is very important to reduce the multiplications of micro-organisms and to get low bacterial count in the milk and milk products.

2. Processing of milk using either batch pasteurizer or HTST plant requires chilled water or any other cooling medium for cooling of milk.

3. Manufacture of many products requires refrigeration. e.g. butter, ice-cream etc.

4. Storage of milk and milk products requires maintaining low temperature in the cold storages depending on the type of product to be stored. e.g. milk is stored at around 3-4 ºC while ice-cream is stored at -30 ºC temperature.

5. Transportation of many products requires refrigerated vehicles to maintain the quality of products.

Low temperature storage is required for distribution of products as well as at the consumers’ level.

Thus, refrigeration is a basic requirement from production to consumption level for dairy and food products. The concept of cold chain of transportation has been accepted in order to supply quality products to the consumers.

1.4 Methods of Refrigeration

1.4.1 Natural methods

1. Use of water: If a material is cooled below the initial temperature by using water, the process is called refrigeration. This method has limitations as desired value of low temperature water may not be available and it is not possible to reduce the temperature of the product/material to a required level.

2. Use of water added with salt: When salts ( e.g. sodium chloride) are added in water, the temperature of the water falls and if this salt water is used for cooling , the process is called refrigeration. This is not practical as the drop in temperature is very small.

3. Use of naturally harvested ice: If naturally available ice is used for cooling, the process is called natural refrigeration. But, ice is not available every place and it can not meet the requirement of low temperature.

4. Use of ice and salt mixture: When salts (e.g. sodium chloride) are mixed in ice, it is possible to achieve temperature below 0 ºC. In many towns, ice-cream is made using hand freezers employing ice and salt mixture for freezing of ice-cream.

5. Evaporative cooling with water: When ambient air passes through a spray of water or a wet grass pad, evaporation of part of water takes place by using sensible heat of the same air. As the sensible heat of the air decreases, the dry bulb temperature of the air decreases. The vapour produced due to evaporation of water goes with the air and hence total enthalpy of the air remains constant. This process is called adiabatic process. This method of air cooling is more effective when the relative humidity of the air is less. This principle of air cooling is employed in air coolers/dessert coolers. The lowest possible temperature of the air achieved in evaporative cooling is up to the wet bulb temperature of air. Another limitation of this method of cooling is higher humidity in the room area. The knowledge of psychrometry is necessary to understand the process of evaporative cooling which may not have much significance in air conditions, but the understanding of the process is important to understand the process of spray drying, cooling of water in cooling tower etc.

1.4.2 Artificial methods

The artificial method of refrigeration, which is known as vapour compression refrigeration system, is commonly used in dairy plants, food factories, air conditioning systems etc. The basic working principle is the same in all such systems. It may be house hold freeze, small capacity air conditioner or a big capacity refrigeration plant of any dairy/food factory. The basic cycle of the vapour compression refrigeration system is discussed in lesson 2.

1.5 Unit of Refrigeration

Capacity of refrigeration system is expressed as ton of refrigeration (TR). A ton of refrigeration is defined as the quantity of heat to be removed in order to form one ton (2000 lbs.) of ice at 0 ºC in 24 hrs, from liquid water at 0 ºC. This is equivalent to 12600 kJ/h or 210 kJ/min or 3.5 kJ/s (3.5 kW).

1 TR = 12600 kJ/h or 210 kJ/min or 3.5 kW.

The capacity of refrigeration plant required in any dairy/food plant can be estimated based on the cooling load requirement of the plant.
Last modified: Thursday, 18 October 2012, 9:53 AM