Weak liquor from the generator is passed through another pressure- reducing valve to the absorber. 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. 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. Table 2. A modified system of the ammonia— water absorption cycle has been developed for small domestic refrigerators. 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.
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. 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. Cooled Heat surface sink — P type 15 V d. 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, R and ammonia R 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. 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 agreed that the production of these chemicals would be phased out by and alternative fluids developed.
From Table 3. 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. Table 3. 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 , as agreed under the Montreal Protocol. A separate European Com- munity decision has set the following dates. The most infamous greenhouse gas is carbon dioxide CO2 , which once released remains in the atmosphere for years, so there is a constant build-up as time progresses. The main cause of CO2 emission is in the generation of electricity at power stations.
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 recover y losses and energy consumption. It is a term which should be calculated for each refrigeration plant. Figures 3. 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. 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.
Hydrocarbons such as propane and butane are being successfully used as replacement and new refrigerants for R12 systems. They obviously have flammable characteristics which have to be taken into account by health and safety requirements.
However, there is a market for their use in sealed refrigerant systems such as domestic refrigeration and unitary air-conditioners. Figure 3. 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 glide can be used to advantage in improving plant performance, by correct design of the heat exchangers. A problem associated with blends is that refrigerant leakage results in a change in the component concentration of the refrigerant. Compressor manufacturers should be consulted with regards to changing the specified oil for a particular compressor.
Refrigerants 35 Table 3. The moisture in the air will be absorbed into the oil and will lead to contamination of both refrigerant and oil. With hermetic compressors this can lead to motor winding failure.
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.
In the UK and most of Europe, it is illegal to dispose of refrigerant in any other way than through an authorized waste disposal company. The UK legislation expects that anyone handling refrigerants is competent to do so and has the correct equipment and containers. Disposal must be through an approved contractor and must be fully documented.
Severe penalties may be imposed for failure to implement these laws. Compressors may be of the positive displacement or dynamic type. The general form of positive displacement compressor is the piston type, being adaptable in size, number of cylinders, speed and method of drive. It works on the two-stroke cycle see Figure 4. As the piston descends on the suction stroke, the internal pressure falls until it is lower than that in the suction inlet pipe, and the suction valve opens to admit gas from the evaporator.
At the bottom of the stroke, this valve closes again and the compression stroke begins. When the cylinder pressure is higher than that in 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 Discharge inlet outlet a b Figure 4. Construction at first was double acting, but there was difficulty in maintaining gas-tightness at the piston rod, so the design evolved further into a single-acting machine with the crankcase at suction inlet pressure, leaving only the rotating shaft as a possible source of leakage, and this was sealed with a packed gland.
The resulting component parts were heavy and cumbersome. To take advantage of larger-scale production methods and provide interchangeability of parts, modern compressors tend to be multicylinder, with bores not larger than mm and running at higher shaft speeds. Machines of four, six and eight cylinders are common. This construction gives a large number of common parts — pistons, connecting rods, loose liners and valves — through a range of compressors, and such parts can be replaced if worn or damaged without removing the compressor body from its installation.
Compressors for small systems will be simpler, of two, three or four cylinders see Figure 4. The smallest compressors have spring steel reed valves, both inlet and outlet in the cylinder head and arranged on a valve plate Figure 4.
Above a bore of about 40 mm, the port area available within the head size is insufficient for both inlet and outlet valves, and the inlet is moved to the piston crown or to an annulus surrounding the head. The outlet or discharge valve remains in the central part of the cylinder head. In most makes, both types of valve cover a ring of circular gas ports, and so are made in annular form and generally termed ring plate valves Figure 4. Ring plate valves are made of thin spring steel or titanium, limited in lift and damped by light springs to assist even closure and lessen bouncing.
These may be arranged on a spring- loaded head, which will lift and relieve excessive pressures. Some makes also have an internal safety valve to release gas pressure from the discharge back to the suction inlet. An alternative valve design uses a conical discharge valve in the centre of the cylinder head, with a ring plate suction valve surrounding it.
This construction is used in compressor bores up to 75 mm. Valve and cylinder head design is very much influenced by the need to keep the volumetric clearance q.
Such variations require capacity reduction devices, originally by speed control when steam driven or in the form of bypass ports in the cylinder walls. The construction of multicylinder machines gives the opportunity to change the working swept volume by taking cylinders out of service with valve-lifting mechanisms.
The ring plate suction valve which is located at the crown of a loose liner can be lifted by various Discharge valves Valve plate Gasket Suction valves Compressor body Figure 4. Typically, an annular piston operates push rods under the valves. In this way a multicylinder machine see Figure 4. Capacity may be reduced by external bypass piping see Chapter 9. The compressor speed may be reduced by two-speed electric motors or by electronic variation of the motor speed, down to a lower limit dictated by the inbuilt lubrication system.
Many high- speed industrial machines are still driven by steam turbines and this gives the opportunity for speed control within the limits of the prime mover. Refrigerants having high discharge temperatures mainly ammonia require the use of water-cooled cylinder heads. Oil coolers are needed under some working conditions which will be specified by the manufacturer.
These may be water cooled or take refrigerant from the system. Detectors may need to be fitted to warn against this condition. Lubrication Incoming gas may contain particles of dirt from within the circuit, especially on a new system.
Suction strainers or traps are provided to catch such dirt and will be readily accessible for cleaning on the larger machines. All but the smallest compressors will have a strainer or filter in the lubricating oil circuit. Strainers within the sump are commonly of the self-cleaning slot disc type.
Larger machines may also have a filter of the fabric throwaway type, as in automobile practice. Reciprocating compressors operate with a wet sump, having splash lubrication in the small sizes but forced oil feed with gear or crescent pumps on all others. A sight glass will be fitted at the correct working oil level and a hand pump may be fitted to permit the addition of oil without stopping or opening the plant, the sump being under refrigerant gas pressure. At the moment of starting, the oil will be diluted by this refrigerant and, as the suction pressure falls, gas will boil out of the oil, causing it to 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.
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 of the electric motor or other prime mover.
The usual form of shaft seal for open drive compressors comprises a rotating carbon ring in contact with a highly polished metal facing ring, the assembly being well lubricated. The carbon ring is spring- loaded to maintain contact under all working crankcase pressures, and to allow for slight movement of the shaft. Compressors 45 When first started, a refrigeration system will operate at a higher suction temperature and pressure than normal operating conditions and consequently a higher discharge pressure, taking considerably more power.
Electrical protection and safety devices must take this into account and power factor correction should be fitted on large motors.
See also Chapter 8 on maximum operating pressure expansion valves. Recent developments in electronic motor power and speed controls have provided the means to reduce the power input at normal speed to balance this reduced load requirement, and also to modulate both power and speed as a method of capacity reduction.
It is improbable that electronic speed control will be economical for motors above kW. There is a need for small compressors to be driven from low- voltage d. Typical cases are batteries on small boats and mobile homes, where these do not have a mains voltage alternator. It is also possible to obtain such a supply from a bank of solar cells. This requirement has been met in the past by diaphragm compressors driven by a crank and piston rod from a d.
The advent of suitable electronic devices has made it possible to obtain the mains voltage a. The wide use of small refrigeration systems has led to the evolution of methods of avoiding shaft seals, provided that the working fluid is compatible with the materials of electric motors and has a high dielectric strength. The semi-hermetic or accessible-hermetic compressor Figure 4. Suction gas passes through the motor itself to remove motor waste heat. Induction motors only can be used, with any starting switches outside the crankcase, since any sparking would lead to decomposition of the refrigerant.
Electrical leads pass through ceramic or glass seals. Small compressors will be fully hermetic, i. Compressors 47 repair or maintenance. The application of the full hermetic compressor is limited by the amount of cooling by the incoming cold gas, heat loss from the shell, and the possible provision of an oil cooler.
The failure of an inbuilt motor will lead to products of decom- position and serious contamination of the system, which must then be thoroughly cleaned.
Internal and external motor protection devices are fitted with the object of switching off the supply before such damage occurs. Larger models have eight or more blades and do not require inlet or outlet valves. The blades are held in close contact with the outer shell by centrifugal force, and sealing is improved by the injection of lubricating oil along the length of the blades.
Rotating vane machines have no clearance volume and can work at high pressure ratios. Figure 4. Ltd Larger rotating vane compressors are limited in application by the stresses set up by the thrust on the tips of the blades, and are used at low discharge pressures such as the first stage of a compound cycle. Smaller compressors, up to kW cooling capacity, are now available for the full range of working pressures. These also incorporate a spring-loaded safety plate to relieve excess pressure if liquid refrigerant enters see Figure 4.
These compressors require discharge valves. This type has been developed extensively for domestic appliances, packaged air-conditioners and similar applications, up to a cooling duty of 15 kW see Figure 4. Spring recess Sliding valve Discharge valve Discharge point Comp. Body cylinder Eccentric Rolling piston Figure 4. 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 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. 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, Balancing outlet end piston Female Male Thrust Sliding valve Suction site rotor rotor bearing actuating piston Control cylinder Oil pump Plain radial bearing, Unloading spring inlet end Discharge side Shaft seal Capacity control, gas return port Figure 4.
Gas sealing at these surfaces is effected by injecting a small amount of the liquid refrigerant. This obviates the need for the oil lubrication and cooling circuit, with its pumps, and leaves the compressor and the circuit oil-free [17]. In all screw compressors, the gas volume will have been reduced to a pre-set proportion of the inlet volume by the time the outlet port is uncovered, and this is termed the built-in pressure ratio.
At this point, the gas within the screws is opened to condenser pressure and gas will flow inwards or outwards through the discharge port if the pressures are not equal. The absorbed power of the screw compressor will be at its optimum only when the working pressure ratio is the same as that of the built-in one. This loss of efficiency is acceptable since the machine has no valves and no working parts other than the screws and sealing vanes.
The oil separation, cooling and filtering for a twin-screw compressor adds to the complexity of an otherwise simple machine. Some commercial screw compressors are available which have the oil- handling circuit built into the assembly, with a small loss of overall efficiency.
Gas enters from the surrounding enclosure Figure 4. Gas Suction opening Delivery opening Suction Fixed scroll chamber Orbiting scroll Compression chamber Suction stroke Delivery stroke Compression stroke b c Suction pocket seal-off position a d Figure 4.
Owing to the close manufacturing tolerances the scroll compressor is built only in hermetic enclosed models. The dynamic and gas pressure loads are balanced so that it is free of vibration. It is currently available in cooling capacities up to 60 kW, and is being made in larger sizes as development proceeds. Capacity control of these compressors is achieved by varying the compressor speed by means of an inverter motor.
The most common type is the centrifugal compressor. Suction gas enters axially into the eye of a rotor which has curved blades, and is thrown out tangentially from the blade circumference.
The energy given to gas passing through such a machine depends on the velocity and density of the gas. Since the density is already fixed by the working conditions, the design performance of a centrifugal compressor will be decided by the rotor tip speed.
To reduce this to a more manageable size, drives are geared up from standard- speed motors or the supply frequency is changed to get higher motor speeds. The drive motor is integral with the compressor assembly, and may be of the open or hermetic type. Gas may be compressed in two or more stages. The impellers are on the same shaft, giving a compact tandem arrangement with the gas from one stage passing directly to the next. The steps of compression are not very great and, if two-stage is used, the gas may pass from the first to the second without any intercooling of the gas.
Centrifugal machines can be built for industrial use with ammonia and other refrigerants, and these may have up to seven compression stages. With the high tip speeds in use, it is not practical to build a small machine, and the smallest available centrifugal compressor for refrigeration duty has a capacity of some kW.
Semi-hermetic compressors are made up to kW and open drive machines up to 21 kW capacity. PLE08, R. As a result, and apart from large-scale industrial plants, they are almost invariably built up as liquid-cooling, water-cooled packages with the condenser and evaporator complete as part of a factory- built package Figure 4.
The main refrigerant for packaged water chillers of the centrifugal type are R and Ra. Since centrifugal machines are too big to control by frequent stopping and restarting, some form of capacity reduction must be inbuilt.
The general method is to throttle or deflect the flow of suction gas into the impeller. There are no components which require lubrication, with the exception of the main bearings.
As a result, the machine can run almost oil- free. The pumping characteristic of the centrifugal machine differs from the positive displacement compressor since, at excessively high discharge pressure, gas can slip backwards past the rotor. This characteristic makes the centrifugal compressor sensitive to the condensing condition, giving higher duty and a better coefficient of performance if the head pressure drops, while heavily penalizing performance if the head pressure rises.
This will vary also with the angle of the capacity reduction blades. Excessive pressure will result in a reverse flow condition, which is followed a fraction of a second later by a boosted flow as the head pressure falls. The vapour surges, with alternate forward and reverse gas flow, throwing extra stress on the impeller and drive motor. Rating curves indicate the stall or surge limit.
Such published data will include absorbed power and indicate any limitations of the application. Ratings of this sort may be standardized to certain conditions at the suction, which may not apply to a particular use and need to be interpreted. See also Chapter Refrigeration compressors are mechanical devices with component parts which slide together, so requiring lubrication to reduce friction, remove frictional heat and assist with gas sealing. Lubricants for general commercial systems are based on mineral oils, and the following properties are required of the lubricant selected: 1.
It must be compatible with the refrigerant, i. The mixture with the refrigerant in the lubrication circuit must provide adequate lubrication of the working parts. It must not solidify or throw out any solids such as waxes, within the working range, or clog strainers or driers. It must be free of water or other contaminants which will affect the system.
It must not be prone to foaming. It must be resistant to oxidation high flash-point. It must have a low vapour pressure. For hermetic and semi-hermetic compressors, it must have a high dielectric strength. A large variety of oils is available, and recommendations for any set of conditions, compressor type and refrigerant can be obtained from the refiners.
They are naphthene or paraffin-based oils. Synthetic lubricants have been developed for ultra-low- and high-temperature systems, especially for process heat pumps. To reduce the amount of this oil which will be carried around the circuit, an oil separator is frequently fitted in the discharge line see Figure 5.
The hot entering gas is made to impinge on a plate, or may enter a drum tangentially to lose much of the oil on the surface by centrifugal force. The oil return line will be controlled by a float valve, or may have Gas outlet to Delivery gas inlet condenser from compressor High-pressure cut-out connection Conical baffle Becoil demister unit Separator shell Baffle Oil return float valve Heater boss Mounting feet Oil reservoir Figure 5.
In either case, this metering device must be backed up by a solenoid valve to give tight shut-off when the compressor stops, since the separator is at discharge pressure and the oil sump at suction. On shut-down, high-pressure gas in the separator will cool and some will condense into liquid, to dilute the oil left in the bottom.
When the compressor restarts, this diluted oil will pass to the sump. In order to limit this dilution, a heater is commonly fitted into the base of the separator. For installations which might be very sensitive to accumulations of oil, a two-stage oil separator can be fitted. The second stage cools the gas to just above condensing temperature, and up to Even so, a small quantity will be carried over.
Sliding vane and screw compressors may have extra oil injected into the casing to assist with sealing, and this must be separated out and re-cooled. The two liquids will then pass to the expansion valve and into the evaporator.
Here, the refrigerant will change to a vapour but most of the oil will remain as a liquid, slight traces of the latter passing out as a low-pressure vapour with the suction gas. It is necessary to limit the build-up of liquid oil in the evaporator, since it would quickly concentrate, reducing heat transfer and causing malfunction.
Methods of limiting oil accumulation in the evaporator depend on the ease with which the liquids mix, and their densities. These properties see Table 5. The extent of miscibility and the consideration of liquid density divides the problem of oil separation and circulation into two distinct classes.
With ammonia, oil sinks to the bottom and does not go into solution with the refrigerant. Ammonia condensers, receivers and evaporators can be distinguished by the provision of oil drainage pots and connections at the lowest point.
Automatic drainage and return of the oil from these would have to depend on the different densities, and is very rarely fitted. The removal of oil from collection pots and low-point drains is a periodic manual function and is carried out as part of the routine maintenance.
The halocarbons are all sufficiently miscible with oil to preclude the possibility of separate drainage in this way. Evaporators containing a large body of R. Since the addition of outside heat for this distillation would be a direct waste of energy, the heat is obtained from the warm liquid passing from the condenser to the expansion valve.
This is termed the dry expansion circuit. The critical section of the circuit Figure 5. The principle of continuous fluid velocity means that the evaporator will be in a continuous circuit.
This does not imply that it has to be one pipe, since many pipes may be arranged in parallel to get the required heat transfer surface, providing the minimum velocity criteria are met. Under such conditions it may not be possible to maintain the minimum velocity to carry oil back to the compressor, and it will settle in the circuit. Arrangements must be made to increase or reverse the gas flow periodically to move this oil. The condition of the compressor oil is therefore a direct indication of the physical and chemical cleanliness of the system.
Lubricating oil should be kept in tightly sealed containers to exclude atmospheric moisture. Oil drained from oil pots and drains is not used again unless it can be properly filtered and kept dry. If it takes on a white, emulsified appearance it is wet and should be drained and discarded. Overheating or an electrical fault in the winding of a hermetic or semihermetic compressor motor will produce contaminants, including the halogen acids, which can be detected by their acrid smell, litmus paper or other tests [18].
Eye goggles and rubber gloves should be worn when handling such suspect oil. If shown to be acid, the oil must be removed and carefully disposed of, and the system thoroughly cleaned out [19, 20]. In addition, the liquid is usually slightly subcooled. In nearly all cases, the cooling medium will be air or water. Calculate the condenser capacities in each case. What is the condenser duty if the cooling capacity is kW? This is of special note with twin-screw compressors, where a high proportion of the compressor energy is taken away in the oil.
This proportion varies with the exact method of oil cooling, and figures should be obtained from the compressor manufacturer for a particular application. An example is the condenser of the domestic refrigerator, which may also have some secondary surface in the form of supporting and spacer wires.
Above this size, the flow of air over the condenser surface will be by forced convection, i. The high thermal resistance of the boundary layer on the air side of the heat exchanger leads to the use, in all but the very smallest condensers, of an extended surface. This takes the form of plate fins mechanically bonded onto the refrigerant tubes in most commercial patterns. The ratio of outside to inside surface will be between 5 : 1 and 10 : 1.
Flow of the liquefied refrigerant will be assisted by gravity, so the inlet will be at the top of the condenser and the outlet at the bottom. Rising pipes should be avoided in the design, and care is needed in installation to get the pipes level. The flow of air may be vertically upwards or horizontal, and the configuration of the condenser will follow from this see Figure 6. Small cylindrical matrices are also used, the air flowing radially inwards and out through a fan at the top.
Forced convection of the large volumes of air at low resistance leads to the general use of propeller or single-stage axial flow fans. Where a single fan would be too big, multiple smaller fans give the advantages of lower tip speed and noise, and flexibility of operation in winter see Section 6.
In residential areas slower-speed fans may be specified to reduce noise levels. If the mass flow is reduced, the temperature rise must increase, raising the condensing temperature and pressure to give lower plant efficiency. In practice, the temperature rise of the air is kept between 9 and 12 K. The mass flow, assuming a rise of As an example of these large air flows required, the condenser for an air-conditioning plant for a small office block, having a cooling capacity of kW and rejecting kW, would need This cooling air should be as cold as possible, so the condenser needs to be mounted where such a flow of fresh ambient air is available without recirculation.
The large air flows needed, the power to move them, and the resulting noise levels are the factors limiting the use of air-cooled condensers. Materials of construction are aluminium fins on stainless steel tube for ammonia, or aluminium or copper fins on aluminium or copper tube for the halocarbons. Aluminium tube is not yet common, but its use is expected to increase. Air-cooled condensers must, of course, be used on land transport systems. They will also be used in desert areas where the supply of cooling water is unreliable.
Configurations may be straight, with return bends or headers, or coiled Figure 6. The double-pipe condenser is circuited in counterflow media flowing in opposite directions to get the most subcooling, since the coldest water will meet the outgoing liquid refrigerant.
Figure 6. This construction is a very adaptable mechanical design and is found in all sizes from mm to 1. Materials can be selected for the application and refrigerant, but all mild steel is common for fresh water, with cupronickel or aluminium brass tubes for salt water.
Some economy in size can be effected by extended surfaces on the refrigerant side, usually in the form of low integral fins formed on the tubes. On the water side, swirl strips can be fitted to promote turbulence, but these interfere with maintenance cleaning and are not much in favour. Some condensers have two separate water circuits double bundle, Figure 6. The main bundle rejects the unwanted heat. Where the mass flow of water is unlimited sea, lake, river or cooling tower , the temperature rise through the condenser may be kept as low as 5 K, since this will reduce the ln MTD with a lowering of head pressure at the cost only of larger water pumps and pipes.
Condensers and water towers 69 Hot gas Heat Main recovery condenser circuit water Main condenser water Liquid Figure 6.
Total duty at the condenser is kW. How much water flow is required? Example 6. The evaporator duty is 4. What is the water mass flow? This arrangement permits tube cleaning while the plant is operating. The supply of water is usually limited and requires the use of a cooling tower. Other possibilities are worth investigation; for example, in the food industries, large quantities of water are used for processing the product, and this could be passed first through the condensers if precautions are taken to avoid contamination.
Also, where ground water is present, it could be taken from a borehole and afterwards returned to the ground at some distance from the suction. Cooled water leaving the tower will be 3—8 K warmer than the incoming air wet bulb temperature. See also Chapters 24 and Cooled water from the drain tank is taken by the pump and passed through the condenser, which may be built up with the compressor as part of a compressor—condenser package condensing unit.
The warmed water then passes back to sprays or distribution troughs at the top of the tower and falls in the upgoing airstream, passing over packings which present a large surface to the air.
Evaporation takes place, the vapour obtaining its latent heat from the body of the water, which is therefore cooled see Figure 6. The mass flow of water over the condenser tubes must be enough to ensure wetting of the tube surface, and will be of the order of 80— times the quantity evaporated. The mass flow of air must be sufficient to carry away the water vapour formed, and a compromise must be reached with expected variations in ambient conditions.
An average figure is 0. What will be the evaporation rate, the approximate circulation rate, and the air mass flow? Evaporative condensers have a higher resistance to air flow than cooling towers and centrifugal fans are often used, ganged together to obtain the required mass flow without undue size. This arrangement is also quieter in operation than axial flow fans. Most types use forced draught fans Figure 6. Flexible - Read on multiple operating systems and devices.
Easily read eBooks on smart phones, computers, or any eBook readers, including Kindle. We cannot process tax exempt orders online. If you wish to place a tax exempt order please contact us. Add to cart. Sales tax will be calculated at check-out. Free Global Shipping. Description The use of refrigeration, either directly or as part of an air-conditioning system, is essential to almost every branch of industry.
The subject of psychrometry for analysing the various thermodynamic processes in air conditioning is particularly dealt with in considerable detail. This text incorporates such tables and charts so that the students are exposed to solving real-life design problems with the help of ASHRAE Tables.
Finally, the book highlights the features, characteristics and selection criteria of hardware including the control equipment. It also provides the readers with the big picture in respect of the latest developments such as thermal storage air conditioning, desiccant cooling, chilled ceiling cooling, Indoor Air Quality IAQ and thermal comfort. Besides the students, the book would be immensely useful to practising engineers as a ready reference.
Comprehensive introduction to refrigeration and heating. Covers principles, practices and servicing techniques. The text begins by reviewing, in a simple and precise manner, the physical principles of three pillars of Refrigeration and Air Conditioning, namely thermodynamics, heat transfer, and fluid mechanics.
Following an overview of the history of refrigeration, subsequent chapters provide exhaustive coverage of the principles, applications and design of several types of refrigeration systems and their associated components such as compressors, condensers, evaporators, and expansion devices.
Refrigerants too, are studied elaboratively in an exclusive chapter. The second part of the book, beginning with the historical background of air conditioning in Chapter 15, discusses the subject of psychrometrics being at the heart of understanding the design and implementation of air conditioning processes and systems, which are subsequently dealt with in Chapters 16 to It also explains the design practices followed for cooling and heating load calculations.
Each chapter contains several worked-out examples that clarify the material discussed and illustrate the use of basic principles in engineering applications. Each chapter also ends with a set of few review questions to serve as revision of the material learned. Download Refrigeration and Air Conditioning book written by C. Explains the functions and operations of refrigeration and air conditioning units through an analytical synthesis of the principles of thermodynamics, heat transfer and fluid mechanics.
Download Refrigeration and Air Conditioning book written by J. Now in its fourth edition, this respected text delivers a comprehensive introduction to the principles and practice of refrigeration. Inexperienced readers are provided with a comprehensive introduction to the fundamentals of the technology. With its concise style yet broad sweep the book covers most of the applications professionals will encounter, enabling them to understand, specify, commission, use and maintain these systems.
Many readers will appreciate the clarity with which the book covers the subject without swamping them with detailed technical or product specific information. New material in this edition includes the latest developments in refrigerants and lubricants, together with updated information on compressors, heat exchangers, liquid chillers, electronic expansion valves, controls and cold storage. Topics also covered include efficiency, environmental impact, split systems, retail refrigeration supermarket systems and cold rooms , industrial systems, fans, air infiltration and noise.
He has authored and co-authored papers and articles on compressors, applications and refrigerant changeover topics.
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