Methods Of Producing Low Temperatures

Lesson
8 Methods of producing Low Temperatures
1 Version 1 ME, IIT Kharagpur

The specific objectives of the lesson :
In this lesson the basic concepts applicable to refrigeration is introduced. This chapter presents the various methods of producing low temperatures, viz. Sensible cooling by cold medium, Endothermic mixing of substances, Phase change processes, Expansion of liquids,Expansion of gases, Thermoelectric refrigeration, Adiabatic demagnetization. At the end of this lesson students should be able to: 1. Define refrigeration (Section 8.1) 2. Express clearly the working principles of various methods to produce low temperatures (Section 8.2)

8.1. Introduction
Refrigeration is defined as “the process of cooling of bodies or fluids to temperatures lower than thoseavailable in the surroundings at a particular time and place”. It should be kept in mind that refrigeration is not same as “cooling”, even though both the terms imply a decrease in temperature. In general, cooling is a heat transfer process down a temperature gradient, it can be a natural, spontaneous process or an artificial process. However, refrigeration is not a spontaneous process, as it requiresexpenditure of exergy (or availability). Thus cooling of a hot cup of coffee is a spontaneous cooling process (not a refrigeration process), while converting a glass of water from room temperature to say, a block of ice, is a refrigeration process (non-spontaneous). “All refrigeration processes involve cooling, but all cooling processes need not involve refrigeration”. Refrigeration is a much moredifficult process than heating, this is in accordance with the second laws of thermodynamics. This also explains the fact that people knew ‘how to heat’, much earlier than they learned ‘how to refrigerate’. All practical refrigeration processes involve reducing the temperature of a system from its initial value to the required temperature that is lower than the surroundings, and then maintaining thesystem at the required low temperature. The second part is necessary due to the reason that once the temperature of a system is reduced, a potential for heat transfer is created between the system and surroundings, and in the absence of a “perfect insulation” heat transfer from the surroundings to the system takes place resulting in increase in system temperature. In addition, the system itselfmay generate heat (e.g. due to human beings, appliances etc.), which needs to be extracted continuously. Thus in practice refrigeration systems have to first reduce the system temperature and then extract heat from the system at such a rate that the temperature of the system remains low. Theoretically refrigeration can be achieved by several methods. All these methods involve producing temperatureslow enough for heat transfer to take place from the system being refrigerated to the system that is producing refrigeration.

8.2. Methods of producing low temperatures
8.2.1. Sensible cooling by cold medium If a substance is available at a temperature lower than the required refrigeration temperature, then it can be used for sensible cooling by bringing it in thermal contact with the systemto be refrigerated. For example, a building can be cooled to a temperature lower than the 2 Version 1 ME, IIT Kharagpur

surroundings by introducing cold air into the building. Cold water or brine is used for cooling beverages, dairy products and in other industrial processes by absorbing heat from them. The energy absorbed by the substance providing cooling increases its temperature, and theheat transferred during this process is given by:
Q = mc p ( ΔT )

(8.1)

Where m is the mass of the substance providing cooling, cp is its specific heat and ΔT is the temperature rise undergone by the substance. Since the temperature of the cold substance increases during the process, to provide continuous refrigeration, a continuous supply of the cold substance should be maintained, which...