Refrigeration and Air Conditioning 3 E Part 2 potx

24 Refrigeration and Air-Conditioning 5. Compatibility with materials of construction, with lubricating oils, and with other materials present in the system 6. Convenient working pressures, i.e. not too high and preferably not below atmospheric pressure 7. High dielectric strength (for compressors having integral electric motors) 8. Low cost 9. Ease of leak detection 10. Environmentally friendly No single working fluid has all these properties and a great many different chemicals have been used over the years. The present situation has been dominated by the need for fluids which are environmentally friendly. This is dealt with in Chapter 3. 2.7 Total loss refrigerants Some volatile fluids are used once only, and then escape into the atmosphere. Two of these are in general use, carbon dioxide and nitrogen. Both are stored as liquids under a combination of pressure and low temperature and then released when the cooling effect is required. Carbon dioxide is below its critical point at atmospheric pressure and can only exist as ‘snow’ or a gas. Since both gases come from the atmosphere, there is no pollution hazard. The temperature of carbon dioxide when released will be – 78.4°C. Nitrogen will be at – 198.8°C. Water ice can also be classified as a total loss refrigerant. 2.8 Absorption cycle Vapour can be withdrawn from an evaporator by absorption (Figure 2.11) into a liquid. Two combinations are in use, the absorption of ammonia gas into water and the absorption of water vapour into lithium bromide. The latter is non-toxic and so may be used for air- conditioning. The use of water as the refrigerant in this combination restricts it to systems above its freezing point. Refrigerant vapour from the evaporator is drawn into the absorber by the liquid absorbant, which is sprayed into the chamber. The resulting solution (or liquor) is then pumped up to condenser pressure and the vapour is driven off in the generator by direct heating. The high-pressure refrigerant gas given off can then be condensed in the usual way and passed back through the expansion valve into the evaporator. Weak liquor from the generator is passed through another pressure- reducing valve to the absorber. Overall thermal efficiency is improved The refrigeration cycle 25 Low-pressure refrigerant gas Absorber Pressure reducing valve Weak liquor High-pressure refrigerant gas Generator Pump Strong liquor Expansion valve Condenser High-pressure refrigerant liquid Evaporator Expansion valve Generator Evaporator Absorber Pump Condenser (b) Figure 2.11 Absorption cycle. (a) Basic circuit. (b) Circuit with heat interchange by a heat exchanger between the two liquor paths and a suction-to- liquid heat exchanger for the refrigerant. Power to the liquor pump will usually be electric, but the heat energy to the generator may be any form of low-grade energy such as oil, gas, hot water or steam. (a) 26 Refrigeration and Air-Conditioning (Solar radiation can also be used.) The overall energy used is greater than with the compression cycle, so the COP (coefficient of performance) is lower. Typical figures are as shown in Table 2.2. Table 2.2 Energy per 100 kW cooling capacity at 3°C evaporation, 42°C condensation Absorption Vapour compression Load 100.0 100.0 Pump/compressor (electricity) 0.1 30.0 Low-grade heat 165 – Heat rejected 265.1 130.0 The absorption system can be used to advantage where there is a cheap source of low-grade heat or where there are severe limits to the electrical power available. A modified system of the ammonia– water absorption cycle has been developed for small domestic refrigerators. 2.9 Steam ejector system The low pressures (8–22 mbar) required to evaporate water as a refrigerant at 4–7°C for air-conditioning duty can be obtained with a steam ejector. High-pressure steam at 10 bar is commonly used. The COP of this cycle is somewhat less than with the absorption system, so its use is restricted to applications where large volumes of steam are available when required (large, steam-driven ships) or where water is to be removed along with cooling, as in freeze-drying and fruit juice concentration. 2.10 Air cycle Any gas, when compressed, rises in temperature. Conversely, if it is made to do work while expanding, the temperature will drop. Use is made of the sensible heat only (although it is, of course, the basis of the air liquefaction process). The main application for this cycle is the air-conditioning and pressurization of aircraft. The turbines used for compression and expansion turn at very high speeds to obtain the necessary pressure ratios and, consequently, are noisy. The COP is lower than with other systems [15]. The normal cycle uses the expansion of the air to drive the first stage of compression, so reclaiming some of the input energy (Figure 2.12). The refrigeration cycle 27 Air inlet Cooling air out Compressor Heat exchanger Expander Cold air to process Cooling air in Fan Figure 2.12 Air cycle cooling 2.11 Thermoelectric cooling The passage of an electric current through junctions of dissimilar metals causes a fall in temperature at one junction and a rise at the other, the Peltier effect. Improvements in this method of cooling have been made possible in recent years by the production of suitable semiconductors. Applications are limited in size, owing to the high electric currents required, and practical uses are small cooling systems for military, aerospace and laboratory use (Figure 2.13). Cooled surface Heat sink P type – + 15 V d.c. N type Figure 2.13 Thermoelectric cooling 3 Refrigerants [73] 3.1 Background The last decade has seen radical changes in the selection and use of refrigerants, mainly in response to the environmental issues of ‘holes in the ozone layer’ and ‘global warming or greenhouse effect’. Previously there had not been much discussion about the choice of refrigerant, as the majority of applications could be met by the well- known and well-tested fluids, R11, R12, R22, R502 and ammonia (R717). The only one of these fluids to be considered environmentally friendly today is ammonia, but it is not readily suited to commercial or air-conditioning refrigeration applications because of its toxicity, flammability and attack by copper. This chapter is about the new refrigerants and the new attitude needed in design, maintenance and servicing of refrigeration equipment. 3.2 Ideal properties for a refrigerant It will be useful to remind ourselves of the requirements for a fluid used as a refrigerant. • A high latent heat of vaporization • A high density of suction gas • Non-corrosive, non-toxic and non-flammable • Critical temperature and triple point outside the working range • Compatibility with component materials and lubricating oil • Reasonable working pressures (not too high, or below atmospheric pressure) • High dielectric strength (for compressors with integral motors) • Low cost • Ease of leak detection • Environmentally friendly Refrigerants 29 No single fluid has all these properties, and meets the new environmental requirements, but this chapter will show the developments that are taking place in influencing the selection and choice of a refrigerant. 3.3 Ozone depletion potential The ozone layer in our upper atmosphere provides a filter for ultraviolet radiation, which can be harmful to our health. Research has found that the ozone layer is thinning, due to emissions into the atmosphere of chlorofluorocarbons (CFCs), halons and bromides. The Montreal Protocol in 1987 agreed that the production of these chemicals would be phased out by 1995 and alternative fluids developed. From Table 3.1, R11, R12, R114 and R502 are all CFCs used as refrigerants, while R13B1 is a halon. They have all ceased production within those countries which are signatories to the Montreal Protocol. The situation is not so clear-cut, because there are countries like Russia, India, China etc. who are not signatories and who could still be producing these harmful chemicals. Table 3.2 shows a comparison between old and new refrigerants. Table 3.1 Typical uses of refrigerants before 1987 Typical application Refrigerants recommended Domestic refrigerators and freezers R12 Small retail and supermarkets R12, R22, R502 Air-conditioning R11, R114, R12, R22 Industrial R717, R22, R502, R13B1 Transport R12, R502 It should be noted that prior to 1987, total CFC emissions were made up from aerosol sprays, solvents and foam insulation, and that refrigerant emissions were about 10% of the total. However, all the different users have replaced CFCs with alternatives. R22 is an HCFC and now regarded as a transitional refrigerant, in that it will be completely phased out of production by 2030, as agreed under the Montreal Protocol. A separate European Com- munity decision has set the following dates. 1/1/2000 CFCs banned for servicing existing plants 1/1/2000 HCFCs banned for new systems with a shaft input power greater than 150 kW 1/1/2001 HCFCs banned in all new systems except heat pumps and reversible systems 1/1/2004 HCFCs banned for all systems 1/1/2008 Virgin HCFCs banned for plant servicing 30 Refrigeration and Air-Conditioning Table 3.2 Comparison of new refrigerants Refrigerant Substitute ODP GWP Cond. Sat. type/no. for temp. temp. at 26 at 1 bar bar (°C) abs °C HCFC (short term) R22 R502, R12 0.05 1700 63 – 41 HFCFC/HFC service-blends (transitional alternatives) R401A R12 0.03 1080 80 – 33 R401B R12 0.035 1190 77 – 35 R409A R12 0.05 1440 75 – 34 HFC–Chlorine free (long-term alternative) R134A R12, R22 0 1300 80 – 26 HFC–Chlorine free–blends–(long-term alternatives) R404A R502 0 3750 55 – 47 R407A R502 0 1920 56 – 46 R407B R502 0 2560 53 – 48 R407C R22 0 1610 58 – 44 ISCEON 59 R22 0 2120 68 – 43 R410A R22, R13B1 0 1890 43 – 51 R411B R12, R22, 0.045 1602 65 – 42 R502 Halogen free (long-term alternatives) R717 ammonia R22, R502 0 0 60 – 33 R600a isobutane R114 0 3 114 – 12 R290 propane R12, R22, 0 3 70 – 42 R502 R1270 propylene R12, R22, 0 3 61 – 48 R502 3.4 Global warming potential (GWP) Global warming is the increasing of the world’s temperatures, which results in melting of the polar ice caps and rising sea levels. It is caused by the release into the atmosphere of so-called ‘greenhouse’ gases, which form a blanket and reflect heat back to the earth’s surface, or hold heat in the atmosphere. The most infamous greenhouse gas is carbon dioxide (CO 2 ), which once released remains in the atmosphere for 500 years, so there is a constant build-up as time progresses. The main cause of CO 2 emission is in the generation of electricity at power stations. Each kWh of electricity used in the UK produces Refrigerants 31 about 0.53 kg of CO 2 and it is estimated that refrigeration compressors in the UK consume 12.5 billion kWh per year. Table 3.3 shows that the newly developed refrigerant gases also have a global warming potential if released into the atmosphere. For example, R134a has a GWP of 1300, which means that the emission of 1 kg of R134a is equivalent to 1300 kg of CO 2 . The choice of refrigerant affects the GWP of the plant, but other factors also contribute to the overall GWP and this has been represented by the term total equivalent warming impact (TEWI). This term shows the overall impact on the global warming effect, and includes refrigerant leakage, refrigerant recovery losses and energy consumption. It is a term which should be calculated for each refrigeration plant. Figures 3.1 and 3.2 show the equation used and an example for a medium temperature R134a plant. Table 3.3 Environmental impact of some of the latest refrigerants Refrigerant ODP (R11 = 1.0) GWP (CO 2 = 1.0) R22 HCFC 0.05 1700 R134a HFC 0 1300 R404a HFC 0 3750 R407c HFC 0 1610 R410a HFC 0 1890 R411b HCFC 0.045 1602 R717 ammonia 0 0 R290 propane 0 3 R600a isobutane 0 3 R1270 propylene 0 3 Figure 3.1 Method for the calculation of TEWI figures TEWI = (GWP × L × n) + (GWP × m [1 – α recovery ] + (n × E annual × β) TEWI = TOTAL EQUIVALENT WARMING IMPACT Leakage Recovery losses Energy consumption direct global warming potential GWP = Global warming potential [CO 2 -related] L = Leakage rate per year [kg] n = System operating time [Years] m = Refrigerant charge [kg] α recovery = Recycling factor E annual = Energy consumption per year [kWh] β = CO 2 -Emission per kWh (Energy-Mix) indirect global warming potential 32 Refrigeration and Air-Conditioning Figure 3.2 Comparison of TEWI figures (example) 300 200 100 Medium temperature R134a +10% +10% Comparison with 10% higher energy consumption RL RL LL LL LL RL LL RL 10 kg 25 kg 10 kg 25 kg Refrigerant charge [m] RL = Impact of recovery losses LL = Impact of leakage losses TEWI × 10 3 E N E R G Y E N E R G Y E N E R G Y E N E R G Y t o –10°C t c +40°C m 10 kg // 25 kg L [10%] 1 kg // 2,5 kg Q o 13,5 kW E 5 kW × 5000 h/a β 0,6 kg CO 2 /kWh α 0,75 n 15 years GWP 1300 (CO 2 = 1) time horizon 100 years Example Refrigerants 33 One thing that is certain is that the largest element of the TEWI is energy consumption, which contributes CO 2 emission to the atmosphere. The choice of refrigerant is therefore about the efficiency of the refrigerant and the efficiency of the refrigeration system. The less the amount of energy needed to produce each kW of cooling, the less will be the effect on global warming. 3.5 Ammonia and the hydrocarbons These fluids have virtually zero ODP and zero GWP when released into the atmosphere and therefore present a very friendly environ- mental picture. Ammonia has long been used as a refrigerant for industrial applications. The engineering and servicing requirements are well established to deal with its high toxicity and flammability. There have been developments to produce packaged liquid chillers with ammonia as the refrigerant for use in air-conditioning in supermarkets, for example. Ammonia cannot be used with copper or copper alloys, so refrigerant piping and components have to be steel or aluminium. This may present difficulties for the air- conditioning market where copper has been the base material for piping and plant. One property that is unique to ammonia compared to all other refrigerants is that it is less dense than air, so a leakage of ammonia results in it rising above the plant room and into the atmosphere. If the plant room is outside or on the roof of a building, the escaping ammonia will drift away from the refrigeration plant. Isotherms Bubble Line Pressure Dew line C 1 t cm B 1 B C ∆t g D 1 D t cm ∆t g A A 1 Enthalpy ∆t g Temperature glide t cm Mean condensing temperature t om Mean evaporating temperature Figure 3.3 Evaporating and condensing behaviour of zeotropic blends [...]... condensing temperatures in the latent heat of vaporization phase, referred to as the ‘temperature glide’ Figure 3. 3 shows the variation in evaporating and condensing temperatures To compare the performance between single component refrigerants and blends it will be necessary to specify the evaporating temperature of the blend to point A on the diagram and the condensing temperature to point B The temperature... safety requirements However, there is a market for their use in sealed refrigerant systems such as domestic refrigeration and unitary air- conditioners 3. 6 Refrigerant blends Many of the new, alternative refrigerants are ‘blends’, which have two or three components, developed for existing and new plants as comparable alternatives to the refrigerants being replaced They are ‘zeotropes’ with varying evaporating... centrifugal compressor will be decided by the rotor tip speed Owing to the low density of gases used, tip speeds up to 30 0 m/s are common At an electric motor speed of 29 00 rev/min, a singlestage machine would require an impeller 2 m in diameter To reduce this to a more manageable size, drives are geared up from standardspeed motors or the supply frequency is changed to get higher motor speeds The.. .34 Refrigeration and Air- Conditioning The safety aspects of ammonia plants are well documented and there is reason to expect an increase in the use of ammonia as a refrigerant Hydrocarbons such as propane and butane are being successfully used as replacement and new refrigerants for R 12 systems They obviously have flammable characteristics which have to be taken into account by health and safety requirements... and the compressed gas passes to the condenser Clearance gas left at the top of the stroke must re-expand before a fresh charge can enter the cylinder (see Suction inlet Discharge outlet (a) (b) Figure 4.1 Reciprocating compressor (a) Suction stroke (b) Discharge stroke Compressors 37 Figure 4 .2 and also Chapter 2, for theoretical and practical cycles on the Mollier chart and for volumetric efficiency)... Service and maintenance staff need to be familiar with safety procedures and what to do in the event of an emergency Health and safety requirements are available from manufacturers of all refrigerants and should be obtained and studied Safety codes are available from the Institute of Refrigeration in London, for HCFC/HFC refrigerants (A1 and A2), ammonia (B2) and hydrocarbons (A3) In the UK and most of Europe,... separate one stroke from the next, so that no extra inlet or outlet valves are needed The more usual form has twin meshing rotors on parallel shafts (see Figure 4.15) As these turn, the space between two grooves comes opposite the inlet port, and gas enters On further rotation, this pocket of gas is cut off from the inlet port and moved down the barrels A meshing lobe of the male rotor then compresses the... pocket, and the gas is finally released at the opposite end, when the exhaust port is uncovered by the movement of the rotors Sealing between the working parts is usually assisted by the injection of oil along the length of the barrels This extra oil must be separated from the discharge gas, and is then cooled and filtered before returning to the lubrication circuit (see Chapter 5) Plain radial bearing,... Screw compressors The screw compressor can be visualized as a development of the gear pump For gas pumping, the rotor shapes are modified to give maximum swept volume and no clearance volume where the rotors mesh together, and the pitch of the helix is such that the inlet and outlet ports can be arranged at the ends instead of at the side The solid portions of the screws slide over the gas ports to separate... foam To reduce this solution of refrigerant in the oil to an acceptable factor, heating devices are commonly fitted to crankcases, and will remain in operation whenever the compressor is idle 4.8 Shaft glands Motors Compressors having external drive require a gland or seal where the shaft passes out of the crankcase, and are termed open compressors They may be belt driven or directly coupled to the shaft . the discharge pipe, the discharge valve opens and the compressed gas passes to the condenser. Clearance gas left at the top of the stroke must re-expand before a fresh charge can enter the cylinder (see Suction inlet Discharge outlet (a) (b) Figure. each refrigeration plant. Figures 3. 1 and 3. 2 show the equation used and an example for a medium temperature R 134 a plant. Table 3. 3 Environmental impact of some of the latest refrigerants Refrigerant. attitude needed in design, maintenance and servicing of refrigeration equipment. 3. 2 Ideal properties for a refrigerant It will be useful to remind ourselves of the requirements for a fluid used