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8 Methods of Producing Low Temperatures

8 Methods of Producing Low Temperatures

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Published by: PRASAD326 on Feb 23, 2009
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8Methods of producingLow Temperatures
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 coldmedium, 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
Express clearly the working principles of various methods to produce lowtemperatures (
Section 8.2
8.1. Introduction
Refrigeration is defined as “the process of cooling of bodies or fluids to temperatures lower than those available in the surroundings at a particular time and place”. It should be kept inmind that refrigeration is not same as “cooling”, even though both the terms imply a decreasein temperature. In general, cooling is a heat transfer process down a temperature gradient, itcan be a natural, spontaneous process or an artificial process. However, refrigeration is not aspontaneous process, as it requires expenditure of exergy (or availability). Thus cooling of ahot cup of coffee is a spontaneous cooling process (not a refrigeration process), whileconverting 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 more difficult process than heating, this is in accordance with thesecond 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 processesinvolve reducing the temperature of a system from its initial value to the required temperaturethat is lower than the surroundings, and then maintaining the system at the required lowtemperature. The second part is necessary due to the reason that once the temperature of asystem is reduced, a potential for heat transfer is created between the system andsurroundings, and in the absence of a “perfect insulation” heat transfer from the surroundingsto the system takes place resulting in increase in system temperature. In addition, the systemitself may generate heat (e.g. due to human beings, appliances etc.), which needs to beextracted continuously. Thus in practice refrigeration systems have to first reduce the systemtemperature and then extract heat from the system at such a rate that the temperature of thesystem remains low. Theoretically refrigeration can be achieved by several methods. Allthese methods involve producing temperatures low enough for heat transfer to take placefrom 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 system to berefrigerated. For example, a building can be cooled to a temperature lower than the2
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. Theenergy absorbed by the substance providing cooling increases its temperature, and the heattransferred during this process is given by:(8.1)
 Where m is the mass of the substance providing cooling, c
is its specific heat and
T is thetemperature rise undergone by the substance. Since the temperature of the cold substanceincreases during the process, to provide continuous refrigeration, a continuous supply of thecold substance should be maintained, which may call for an external refrigeration cycle.
8.2.2. Endothermic mixing of substances
This is one of the oldest methods known to mankind. It is very well-known that lowtemperatures can be obtained when certain salts are dissolved in water. This is due to the factthat dissolving of these salts in water is an endothermic process, i.e., heat is absorbed fromthe solution leading to its cooling. For example, when salts such as sodium nitrate, sodiumchloride, calcium chloride added to water, its temperature falls. By dissolving sodiumchloride in water, it is possible to achieve temperatures as low as –21
C, while with calciumchloride a temperature of –51
C could be obtained. However, producing low temperature byendothermic mixing has several practical limitations. These are: the refrigeration effectobtained is very small (the refrigeration effect depends on the heat of solution of thedissolved substance, which is typically small for most of the commonly used salts), andrecovery of the dissolved salt is often uneconomical as this calls for evaporation of water from the solution.
8.2.3. Phase change processes
Refrigeration is produced when substances undergo endothermic phase change processessuch as sublimation, melting and evaporation. For example, when ice melts it produces arefrigeration effect in the surroundings by absorbing heat. The amount of refrigeration produced and the temperature at which refrigeration is produced depends on the substanceundergoing phase change. It is well-known that pure water ice at 1 atmospheric pressuremelts at a temperature of about 0
C and extracts about 335 kJ/kg of heat from thesurroundings. At 1 atmospheric pressure, dry ice (solid carbon dioxide) undergoessublimation at a temperature of –78.5
C, yielding a refrigeration effect of 573 kJ/kg. Bothwater ice and dry ice are widely used to provide refrigeration in several applications.However, evaporation or vaporization is the most commonly used phase change process in practical refrigeration systems as it is easier to handle fluids in cyclic devices. In thesesystems, the working fluid (refrigerant) provides refrigeration effect as it changes its statefrom liquid to vapor in the evaporator.For all phase change processes, the amount of refrigeration produced is given by:
(8.2)where Q is the refrigeration produced (heat transferred), m is the mass of the phase changesubstance and
is the latent heat of phase change. If the process is one of evaporation,3
Version 1 ME, IIT Kharagpur

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