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AMBO UNIVERSITY HACHALU HUNDESSA CAMPUS MECHANICAL ENGINEERING DEPARTMENT AMBO , ETHIOPIA Refrigeration Refrigeration and air conditioning and air conditioning By By: : Sisay Sisay Bekele(MSc) Bekele(MSc) 2021 2021
UNIT 1 1. Introduction 1.2 Basic Concepts in Refrigeration, 1.3 Applications of Refrigeration & Air Conditioning
1. INTRODUCTION 1.1 Definition The term ' refrigeration' may be defined as the process of removing heat from a substance under controlled conditions. It also includes the process of reducing and maintaining the temperature of a body below the general temperature of its surroundings.
In other words, the refrigeration means a continued extraction of heat from a body whose temperature is already below the temperature of its surroundings. For example, if some space (say in cold storage) is to be kept at - 2°C (271K), we must continuously extract heat which flows into it due to leakage through the walls and also the heat which is brought into it with the articles stored after the temperature is once reduced to - 2°C(271K)
Thus in a refrigerator, heat is virtually being pumped from a lower temperature to a higher temperature. According to Second Law of Thermodynamics, this process can only be performed with the aid of some external work. It is thus obvious that supply of power (say electric motor) is regularly required to drive a refrigerator. Theoretically, a refrigerator is a reversed heat engine or a heat pump which pumps heat from a cold body and delivers it to a hot body. The substance which works in a beat pump to extract heat from a cold body and to deliver it to a hot body is called a refrigerant.
The refrigeration system is known to the man since the middle of nineteenth century. The scientists, of the time, developed a few stray machines to achieve some pleasure. But it paved the way by inviting the attention of scientists for proper studies and research. They were able to build a reasonably reliable machine by the end of nineteenth century for refrigeration jobs. But with the advent of efficient rotary compressors and gas turbines, the science of refrigeration reached the present height. Today it is used for the manufacture of ice and similar products. It is also widely used for the cooling of storage chambers in which perishable foods, drinks and medicines are stored. The refrigeration has also wide applications in submarine ships, aircraft and rockets.
1.2 Basic Concepts in Refrigeration 1.2.1 Temperature, Work And Heat The temperature scale now in general use is the Celsius scale , based nominally on the melting point of ice at 0°C and the boiling point of water at atmospheric pressure at 100°C (by strict definition, the triple point of ice is 0.01°C at a pressure of 6.1 mbar). The law of conservation of energy tells us that when work and heat energy are exchanged there is no net gain or loss of energy. However, the amount of heat energy that can be converted into work is limited. As the heat flows from hot to cold a certain amount of energy may be converted into work and extracted. It can be used to drive a generator, for example.
The minimum amount of work to drive a refrigerator can be defined in terms of the absolute temperature scale. Figure 1.1 shows a reversible engine E driving a reversible heat pump P; Q and W represent the fl ow of heat and work. They are called reversible machines because they have the highest efficiency that can be visualized, and because there are no losses, E and P are identical machines.
Ideal Heat Engine, E, Driving An Ideal Refrigerator (Heat Pump), P
The arrangement shown results in zero external effect because the reservoirs experience no net gain or loss of heat. If the efficiency of P were to be higher, i.e. if the work input required for P to lift an identical quantity of heat Q2 from the cold reservoir were to be less than W, the remaining part of W could power another heat pump. This could lift an additional amount of heat. The result would be a net fl ow of heat from the low temperature to the high temperature without any external work input, which is impossible.
The relationship between Q1,Q2 and W depends only on the temperatures of the hot and cold reservoirs. The French physicist Said Carnot (1796–1832) was the first to predict that the relationship between work and heat is temperature dependent, and the ideal refrigeration process is known as the Carnot cycle In order to find this relationship, temperature must be defied in a more fundamental way. The degrees on the thermometer are only an arbitrary scale.
Example 1.1 Heat is to be removed at a temperature of -5°C and rejected at a temperature of 35°C. What is the Carnot or Ideal COP?
In the process where there is steady flow, the factor Pv will not change appreciably and the difference in enthalpy will be the quantity of heat gained or lost. Enthalpy may be expressed as a total above absolute zero, or any other base which is convenient. Tabulated enthalpies found in reference works are often shown above a base temperature of -40°C, since this is also -40° on the old Fahrenheit scale. In any calculation, this base condition should always be checked to avoid the errors which will arise if two different bases are used.
If a change of enthalpy can be sensed as a change of temperature, it is called sensible heat . This is expressed as specific heat capacity, i.e. the change in enthalpy per degree of temperature change, in kJ/(kg K). If there is no change of temperature but a change of state (solid to liquid, liquid to gas, or vice versa) it is called latent heat.
This is expressed as kJ/kg but it varies with the boiling temperature, and so is usually qualified by this condition. The resulting total changes can be shown on temperature–enthalpy diagram ( Figure 1.3) .
Example 1.2 The specific enthalpy of water at 80°C, taken from 0°C base, is 334.91kJ/kg. What is the average specific heat capacity through the range 0–80°C? Example 1.3 If the latent heat of boiling water at 1.013 bar is 2257 kJ/kg, the quantity of heat which must be added to 1 kg of water at 30°C in order to boil it is:
1.2.3 Boiling Point The temperature at which a liquid boils is not constant, but varies with the pressure. Thus, while the boiling point of water is commonly taken as 100°C, this is only true at a pressure of one standard atmosphere (1.013 bar) and, by varying the pressure, the boiling point can be changed ( Table 1.1 ). This pressure–temperature property can be shown graphically (see Figure 1.4 ).
Figure 1.4 Change Of State With Pressure And Temperature
The boiling point of a substance is limited by the critical temperature at the upper end, beyond which it cannot exist as a liquid, and by the triple point at the lower end, which is at the freezing temperature. Between these two limits, if the liquid is at a pressure higher than its boiling pressure, it will remain a liquid and will be subcooled below the saturation condition, while if the temperature is higher than saturation, it will be a gas and superheated. If both liquid and vapour are at rest in the same enclosure, and no other volatile substance is present, the condition must lie on the saturation line.
At a pressure below the triple point pressure, the solid can change directly to a gas (sublimation) and the gas can change directly to a solid, as in the formation of carbon dioxide snow from the released gas. The liquid zone to the left of the boiling point line is subcooled liquid. In refrigeration the term Saturation is used to describe the liquid/vapour boundary, saturated vapour being represented by a condition on the line and superheated vapour below the line.
1.2.4 General Gas Laws Many gases at low pressure, i.e. atmospheric pressure and below for water vapour and up to several bar for gases such as nitrogen, oxygen and argon, obey simple relations between their pressure, volume and temperature, with sufficient accuracy for engineering purposes. Such gases are called ‘ ideal ’ . Boyle’s Law states that, for an ideal gas, the product of pressure and volume at constant temperature is a constant: pV =constant
Boyle ’ s and Charles ’ laws can be combined into the ideal gas equation: PV/T=(a constant) The constant is mass x R , where R is the specific gas constant, so: pV =mRT Example 1.6 What is the volume of 5kg of an ideal gas, having a specific gas constant of 287J/(kg K), at a pressure of one standard atmosphere and at 25°C?
1.2.6 Dalton’s Law Dalton’s Law of partial pressures considers a mixture of two or more gases, and states that the total pressure of the mixture is equal to the sum of the individual pressures, if each gas separately occupied the space. Example 1.7 A cubic meter of air contains 0.906 kg of nitrogen of specific gas constant 297J/(kg K),0.278 kg of oxygen of specific gas constant 260 J/(kg K) and 0.015 kg of argon of specific gas constant 208J/(kg K). What will be the total pressure at 20°C?
The properties of refrigerant fluids at the pressures and temperatures of interest to refrigeration engineers exhibit considerable deviation from the ideal gas laws. It is therefore necessary to use tabulated or computer- based information for thermodynamic calculations.
1.2.6 Heat Transfer Heat will move from a hot body to a colder one, and can do so by the following methods: 1.Conduction:Direct from one body touching the other, or through a continuous mass 2.Convection: By means of a heat-carrying fluid moving between one and the other 3. Radiation: Mainly by infrared waves (but also in the visible band,e.g. solar radiation), which are independent of contact or an intermediate fluid. Conduction through a homogeneous material is expressed directly by its area, thickness and a conduction coefficient. For a large plane surface, ignoring heat transfer near the edges:
Thermal conductivities, in watts perimeter Kelvin, for various common materials are as in Table 1.2. Conductivities for other materials can be found from standard reference works.
Convection requires a fluid, either liquid or gaseous, which is free to move between the hot and cold bodies. This mode of heat transfer is complex and depends firstly on whether the flow of fluid is ‘ natural ’ , i.e. caused by thermal currents set up in the fluid as it expands, or ‘ forced ’ by fans or pumps. Other parameters are the density, specific heat capacity and viscosity of the fluid and the shape of the interacting surface.
The calculation of every heat transfer coefficient for a refrigeration or air conditioning system would be a very time- consuming process, even with modern methods of calculation. Formulas based on these factors will be found in standard reference works, expressed in terms of heat transfer coefficients under different conditions of fluid flow.
Where heat is conducted through a plane solid which is between two fluids, there will be the convective resistances at the surfaces. The overall heat transfer must take all of these resistances into account, and the unit transmittance ,or ‘Unvalued is given by: Rt= Ri+Rc+Ro U =1/Rt where R t =total thermal resistance R i =inside convective resistance R c = conductive resistance RO =outside convective resistance
Figure 1.5 Changing temperature difference of a cooled fluid
Figure 1.6 Temperature change. (a) Refrigerant cooling fluid. (b) Fluid cooling refrigerant. (c) Two fluids
In practice, many of these values will vary. A pressure drop along a pipe carrying boiling or condensing fluid will cause a change in the saturation temperature. With some liquids, the heat transfer values will change with temperature. For these reasons, the LMTD formula does not apply accurately to all heat transfer applications.
If the heat exchanger was of infinite size, the space– temperature curves would eventually meet and no further heat could be transferred. The fluid in Example 1.10 would cool the water down to 3°C. The effectiveness of a heat exchanger can be expressed as the ratio of heat actually transferred to the ideal maximum:
The metals used in refrigeration and air-conditioning systems, such as steel, copper and aluminum, quickly oxidize or tarnish in air, and the emissivity figure will increase to a value nearer 0.50. Surfaces will absorb radiant heat and this factor is expressed also as the ratio to the absorptivity of a perfectly black body. Within the range of temperatures in refrigeration systems, i.e. 70°C to 50°C (203–323 K), the effect of radiation is small compared with the conductive and convective heat transfer, and the overall heat transfer factors in use include the radiation component. Within this temperature range, the emissivity and absorptivity factors are about equal.
The exception to this is the effect of solar radiation when considered as a cooling load, such as the air- conditioning of a building which is subject to the sun’s rays. At the wavelength of sunlight the absorptivity figures change and calculations for such loads use tabulated factors for the heating effect of sunlight. Glass, glazed tiles and clean white-painted surfaces have a lower absorptivity, while the metals are higher.
1.2.7 Transient Heat Flow A special case of heat fl ow arises when the temperatures through the thickness of a solid body are changing as heat is added or removed. This non-steady or transient heat fl ow will occur, for example, when a thick slab of meat is to be cooled, or when sunlight strikes on a roof and heats the surface. When this happens, some of the heat changes the Temperature of the first layer of the solid, and the remaining heat passes on to the next layer, and so on.
