Refrigeration and Air Conditioning PDF
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Preface Refrigeration and its application is met in almost every branch of industry, so that practitioners in other fields find that they have to become aware of its principles, uses and limitations. This book aims to introduce students and professionals in other disciplines to the fundamentals of the subject, without involving the reader too deeply in theory. The subject matter is laid out in logical order and covers the main uses and types of equipment. In the ten years since the last edition there have been major changes in the choice of refrigerants due to environmental factors and an additional chapter is introduced to reflect this. This issue is on-going and new developments will appear over the next ten years. This issue has also affected servicing and maintenance of refrigeration equipment and there is an increased pressure to improve efficiency in the reduction of energy use. This edition reflects these issues, whilst maintaining links with the past for users of existing plant and systems. There have also been changes in packaged air-conditioning equipment and this has been introduced to the relevant sections. The book gives worked examples of many practical applications and shows options that are available for the solution of problems in mechanical cooling systems. It is not possible for these pages to contain enough information to design a complete refrigeration system. The design principles are outlined. Finally, the author wishes to acknowledge help and guidance from colleagues in the industry, in particular to Bitzer for the information on new refrigerants. T.C. Welch October 1999 1 Fundamentals 1.1 Basic physics – temperature The general temperature scale now in 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.) On the ° Celsius scale, absolute zero is – 273.15 C. In the study of refrigeration, the Kelvin or absolute temperature scale is also used. This starts at absolute zero and has the same degree intervals as the Celsius scale, so that ice melts at + 273.16 K and water at atmospheric pressure boils at + 373.15 K. 1.2 Heat Refrigeration is the process of removing heat, and the practical application is to produce or maintain temperatures below the ambient. The basic principles are those of thermodynamics, and these principles as relevant to the general uses of refrigeration are outlined in this opening chapter. Heat is one of the many forms of energy and mainly arises from chemical sources. The heat of a body is its thermal or internal energy, and a change in this energy may show as a change of temperature or a change between the solid, liquid and gaseous states. Matter may also have other forms of energy, potential or kinetic, depending on pressure, position and movement. Enthalpy is the sum of its internal energy and flow work and is given by: H = u + Pv In the process where there is steady flow, the factor Pv will not 2 Refrigeration and Air-Conditioning 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 a temperature–enthalpy diagram (Figure 1.1). Sensible heat of gas Latent heat of e melting Latent heat of boiling ur373.15 K at er mp Sensible heat of liquid 273.16 K e T Sensible heat of soild 334 kJ 419 kJ 2257 kJ Enthalpy Figure 1.1 Change of temperature (K) and state of water with enthalpy Example 1.1 For water, the latent heat of freezing is 334 kJ/kg and the specific heat capacity averages 4.19 kJ/(kg K). The quantity of ° heat to be removed from 1 kg of water at 30 C in order to turn it ° into ice at 0 C is: 4.19(30 – 0) + 334 = 459.7 kJ Example 1.2 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: Fundamentals 3 4.19(100 – 30) + 2257 = 2550.3 kJ Example 1.3 The specific enthalpy of water at 80°C, taken from ° 0 C base, is 334.91 kJ/kg. What is the average specific heat capacity ° through the range 0–80 C? 334.91/(80 – 0) = 4.186 kJ/(kg K) 1.3 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 1.2). Table 1.1 Pressure (bar) Boiling point (°C) 0.006 0 0.04 29 0.08 41.5 0.2 60.1 0.5 81.4 1.013 100.0 Critical temperature Liquid e curve Pressur Solid Boiling point Gas Triple point Temperature Figure 1.2 Change of state with pressure and temperature 4 Refrigeration and Air-Conditioning The boiling point 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. The gas under this line is superheated gas. 1.4 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 Example 1.4 A volume of an ideal gas in a cylinder and at atmospheric pressure is compressed to half the volume at constant temperature. What is the new pressure? p V =constant 1 1 =p V 2 2 V 1 = 2 V 2 × so p =2 p 2 1 × =2 1.013 25 bar (101 325 Pa) =2.026 5 bar (abs.) Charles’ Law states that, for an ideal gas, the volume at constant pressure is proportional to the absolute temperature: Fundamentals 5 V = constant T Example 1.5 A mass of an ideal gas occupies 0.75 m3 at 20°C and ° is heated at constant pressure to 90 C. What is the final volume? T V = V × 2 2 1 T 1 = 0.75 × 273.15 + 90 273.15 + 20 =0.93 m3 Boyle’s and Charles’ laws can be combined into the ideal gas equation: × pV = (a constant) T × The constant is mass R, where R is the specific gas constant, so: pV = mRT Example 1.6 What is the volume of 5 kg of an ideal gas, having a specific gas constant of 287 J/(kg K), at a pressure of one standard ° atmosphere and at 25 C? pV = mRT mRT V = p × 5 287(273.15 + 25) = 101 325 = 4.22 m3 1.5 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 metre of air contains 0.906 kg of nitrogen of specific gas constant 297 J/(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 208 J/(kg K). What will be the total pressure at 20 C? 6 Refrigeration and Air-Conditioning pV =mRT V =1 m3 so p =mRT × × For the nitrogen p =0.906 297 293.15 = 78 881 Pa N × × For the oxygen p =0.278 260 293.15 = 21 189 Pa O × × For the argon p = 0.015 208 293.15 = 915 Pa A ————— Total pressure =100 985 Pa (1.009 85 bar) 1.6 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: × area thermal conductivity Conductance = thickness × A k = L and the heat conducted is × Q = conductance (T – T ) f 1 2 Example 1.8 A brick wall, 225 mm thick and having a thermal conductivity of 0.60 W/(m K), measures 10 m long by 3 m high, and has a temperature difference between the inside and outside faces of 25 K. What is the rate of heat conduction? × × × 10 3 0.60 25 Q = f 0.225 = 2000 W (or 2 kW) Fundamentals 7 Thermal conductivities, in watts per metre kelvin, for various common materials are as in Table 1.2. Conductivities for other materials can be found from standard reference works [1, 2, 3]. Table 1.2 Material Thermal conductivity (W/(m K)) Copper 200 Mild steel 50 Concrete 1.5 Water 0.62 Cork 0.040 Expanded polystyrene 0.034 Polyurethane foam 0.026 Still air 0.026 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 very 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. With so many variables, expressions for convective heat flow cannot be as simple as those for conduction. The interpretation of observed data has been made possible by the use of a number of groups which combine the variables and which can then be used to estimate convective heat flow. The main groups used in such estimates are as shown in Table1.3. A typical combination of these numbers is that for turbulent flow in pipes: (Nu) = 0.023 (Re)0.8 (Pr)0.4 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 [1, 4–8]. Example 1.9 A formula for the heat transfer coefficient between forced draught air and a vertical plane surface ([1], Chapter 3, Table 6) gives: ′ h = 5.6 + 18.6V 8 Refrigeration and Air-Conditioning Table 1.3 Number Sign Parameters Reynolds Re Velocity of fluid Density of fluid Viscosity of fluid Dimension of surface Grashof Gr Coefficient of expansion of fluid Density of fluid Viscosity of fluid Force of gravity Temperature difference Dimension of surface Nusselt Nu Thermal conductivity of fluid Dimension of surface Heat transfer coefficient Prandtl Pr Specific heat capacity of fluid Viscosity of fluid Thermal conductivity of fluid What is the thermal conductance for an air velocity of 3 m/s? ′ × h =5.6 + 18.6 3 =61.4 W/(m2 K) 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 ‘U’ factor, is given by: R =R + R + R t i c o U = 1/R t where R = total thermal resistance t R =inside convective resistance i R =conductive resistance c R = outside convective resistance o Example 1.10 A brick wall, plastered on one face, has a thermal conductance of 2.8 W/(m2 K), an inside surface resistance of 0.3 (m2 K)/W, and an outside surface resistance of 0.05 (m2 K)/W. What is the overall transmittance? R =R + R + R t i c o 1 = 0.3 + + 0.05 2.8 =0.707 Fundamentals 9 U = 1.414 W/(m2 K) Typical overall thermal transmittances are: Insulated cavity brick wall, 260 mm thick, sheltered exposure on outside 0.69 W/(m2K) Chilled water inside copper tube, forced draught air flow outside 15–28 W/(m2K) Condensing ammonia gas inside steel tube, thin film of water outside 450–470 W/(m2K) Special note should be taken of the influence of geometrical shape, where other than plain surfaces are involved. The overall thermal transmittance, U, is used to calculate the total heat flow. For a plane surface of area A and a steady temperature ∆ difference T, it is × × ∆ Q = A U T f If a non-volatile fluid is being heated or cooled, the sensible heat ∆ will change and therefore the temperature, so that the T across the heat exchanger wall will not be constant. Since the rate of ∆ temperature change (heat flow) will be proportional to the T at any one point, the space–temperature curve will be exponential. In a case where the cooling medium is an evaporating liquid, the temperature of this liquid will remain substantially constant throughout the process, since it is absorbing latent heat, and the cooling curve will be as shown in Figure 1.3. T A Cooled mediu m ∆Tmax c∆hRTaantgee of temperature ∆T min T B In Out Cooling medium Figure 1.3 Changing temperature difference of a cooled fluid
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