Refrigeration and Air Conditioning 3 E Part 6 pdf

12 Liquid chillers. Ice. Brines. Thermal storage 12.1 Distributed cooling A building or process having a large number of separated cooling loads could have a refrigeration system for each of these loads. It will usually be more convenient to concentrate the cooling into one plant. The cooling effect of a central refrigerating system can be distributed by a heat-transferring liquid or secondary refrigerant. Where the working temperatures are always above 0°C, such as in air-conditioning, water is commonly used. At temperatures below this, non-freezing liquids are used. 12.2 Liquid chillers The preferred secondary refrigerant will be water, if this can possibly satisfy the temperature requirement, i.e. if the load temperature is sufficiently above 0°C that water can be circulated without risk of freezing. The greatest demand for chilled water is in air-conditioning systems (see also Chapters 23–28). For this duty, water is required at a temperature usually not lower than 5°C and, for this purpose, the evaporator will be of the shell-and-tube type, operating with refrigerant temperatures close to freezing point. A very wide range of factory- built package chillers is available and models are mainly one-piece units with integral water-cooled condensers as shown in Figures 4.18 and 13.2. Other types may have air-cooled or evaporative condensers, and so require refrigerant pipe connections on site to these condensers. Sizes range from 14 kW to 35 000 kW, most installations being within the range 100–1500 kW. At water temperatures close to freezing, and with evaporators Liquid chillers. Ice. Brines. Thermal storage 145 which are vulnerable to ice damage, it is important to have adequate safety controls, to check the calibration of these frequently and to avoid interference by unauthorized persons. Nearly all troubles from packaged water chillers arise from a failure of safety controls. Several types of controls are in use, frequently three or more on the same equipment, but there should never be less than two of them: 1. Water flow switch, to stop the machine if flow stops in the chilled water circuit 2. Refrigerant low-pressure cut-out 3. Water outlet low-temperature cut-out 4. Back pressure regulation valve (see Section 9.8) 5. Hot gas bypass valve, to keep the evaporating temperature up above freezing point. Most packaged water chillers are large enough to have capacity control devices in the compressor. The main control thermostat will unload the compressor as the water temperature approaches a lower safe limit, so as to keep the water as cold as possible without risk of freeze damage. In all but the smallest installations, two or more chillers will be used, or one chiller with two separate circuits. This arrangement gives some continuity of the service if one machine is off-line for maintenance or another reason, gives better control and provides economy of running when loads are light. If water is required below 5°C, the approach to freezing point brings considerable danger of ice formation and possible damage to the evaporator. Some closed systems are in use and have either oversize heat exchange surfaces or high-efficiency-type surfaces. In both of these, the object is to improve heat transfer so that the surface in contact with the water will never be cold enough to cause ice layers to accumulate. 12.3 Baudelot coolers and ice bank coils Water can be cooled safely to near freezing point, using evaporators which have the refrigerant inside, with space for ice to form on the outside of the surface without causing damage. Two types are used: 1. Baudelot coolers (see Figures 7.7 and 7.8a). The evaporator stands above a collection tank, and the water runs down the outside surface in a thin layer. Evaporator construction can be pipe coils or embossed plates. The latter are now usually of stainless steel, to avoid corrosion troubles. Evaporators may be flooded or dry expansion. During operation, a Baudelot cooler 146 Refrigeration and Air-Conditioning may sometimes build up a thin layer of ice, but this does no damage to the evaporator, and should melt off again when the load changes. 2. Pipe coils within a water tank (see Figures 7.1c, 7.5 and 7.6). Both flooded and dry expansion evaporators are in use. Water is circulated by pumps and/or special agitators. This type of water chiller may be operated without formation of ice, or ice may be allowed to accumulate intentionally (see below). Water chillers of these two types are not usually made as single packages with their condensing unit, owing to the bulk of the system and subsequent difficulty of installation. 12.4 Ice manufacture Ice may be made and transported to where the cooling effect is required. The refrigeration energy available in this way will be mainly its latent heat of melting, 334 kJ/kg, as it changes back to water. Ice can be made as thin slivers on the surface of evaporator drums, and removed mechanically when the correct thickness has been formed. Either the drum or the scraper may rotate. This is a continuous process and the ice flakes fall directly onto the product or into a storage bin below the machine. Smaller units are made as packages with the bin integral and cooled by a few turns of the suction line or by a separate evaporator. Small pieces of ice can be formed in or on tubes or other prismatic shapes made as evaporator tubes, arranged vertically. Water is pumped over the surface to freeze to the thickness or shape required. The tube is then switched to ‘defrost’ and the moulded section thaws sufficiently to slide off, possibly being chopped into short pieces by a rotating cutter. The machine itself will be made as a package, and the smaller sizes will include the condensing unit. The manufacture of ice in large blocks is by the can method (see Figure 12.1), where a number of mould cans, filled with water, are immersed to just below the rim in a tank of refrigerated brine. The smallest block made in this way is 25 kg and will freeze in 8–15 h, using brine at –11°C. Blocks up to 150 kg are made by this method. When frozen, the moulds are lifted from the tank and slightly warmed to release the ice block from the sides of the moulds, when they can be tipped out. Blocks may go into storage or for direct use. Where the available water has a high proportion of solids, there are methods either of pretreating the water or, by agitating the water in the centre of the block (which freezes last), of removing the concentrated dirty water before it becomes solid. The core is then refilled with fresh water [30]. Liquid chillers. Ice. Brines. Thermal storage 147 Block ice can be made from sea water but the central core cannot be frozen. 12.5 Brines Where a secondary refrigerant fluid is to be circulated, and the working temperatures are at or below 0°C, then some form of non- freeze mixture must be used. These are collectively termed brines. Brines may be, as the name suggests, solutions of inorganic salts in water, and the two in general use are sodium chloride and calcium chloride. Of these, the former is compatible with most foodstuffs and can be used in direct contact or in circumstances where the brine may come into contact with the product. Calcium chloride has an unpleasant taste and cannot be permitted to contaminate foods. 12.6 Physical properties With any solution, there will be one concentration which remains liquid until it reaches a freezing point, and then will freeze solid. This is the eutectic mixture, and its freezing point is the eutectic point of the solute (see Figure 12.2). At all other concentrations, as Ice crane Ice moulds Ice tanks Agitator Ice tip Filling tank Submerging coil evaporator Thawing tank Ice store Suction separator Control panel Compressors Liquid receiver Ceiling mounted air cooler To condenser Figure 12.1 Can ice plant (Courtesy of Hall-Thermotank Products Ltd) 148 Refrigeration and Air-Conditioning Liquid Liquid + salt slush Liquid + ice – 21.1°C Eutectic point 23.3% 0102030405060708090100 Sodium chloride (% by weight) (a) Liquid Liquid + salt slush 0102030405060708090100 Calcium chloride (% by weight) (b) –51°C Eutectic point 29.6% Liquid + ice 0 –10 – 20 –30 –40 –50 –60 Temperature (°C) 0 –10 –20 –30 Temperature (°C) Figure 12.2 Eutectic curves. (a) Sodium chloride in water. (b) Calcium chloride in water the solution is cooled it will reach a temperature where the excess water or solute will crystallize out, to form a slushy suspension of Liquid chillers. Ice. Brines. Thermal storage 149 the solid in the liquid, until the eutectic point is reached, when it will all freeze solid. For economy of cost, and to reduce the viscosity (and so improve heat transfer), solutions weaker than eutectic are normally used, provided there is no risk of freezing at the evaporator. In salt brines, the water may be considered as the heat transfer medium, since the specific heat capacity of the salt content is low (see Figure 12.3). The specific heat capacity of the brine will therefore 0 5 10 15 20 25 30 Sodium chloride (% by weight) (a) 1200 1100 1000 4.0 3.5 3.0 2.5 Specific heat Specific density capacity at 15°C 0 5 10 15 20 25 30 1300 1200 1100 1000 4.0 3.5 3.0 2.5 Specific heat Specific density capacity at 15°C Calcium chloride (% by weight) (b) Figure 12.3 Density and specific heat capacity. (a) Sodium chloride. (b) Calcium chloride 150 Refrigeration and Air-Conditioning decrease as the salt concentration increases. At the same time, the specific mass will increase. Non-freezing solutions can also be based on organic fluids, principally the glycols, of which ethylene and propylene glycol are in general use. Where contact with food is possible, propylene glycol (see Figure 12.4) should be used. Liquid Liquid + slush ice Extremely viscous liquid 01520 3040 50 60 80 100 Propylene glycol (% by weight) (a) 1100 1050 1000 4.0 3.5 3.0 01020304050 Propylene glycol (% by weight) (b) 0 –10 –20 –30 –40 –50 Temperature (°C) Specific heat Specific density capacity at 10°C Figure 12.4 Propylene glycol in water. (a) Eutectic curve. (b) Density and specific heat capacity Liquid chillers. Ice. Brines. Thermal storage 151 The concentration of a solute has a considerable effect on the viscosity of the fluid and so on the surface convective resistance to heat flow. There is little published data on these effects, so applications need to be checked from basic principles. Industrial alcohol (comprising ethyl alcohol with a statutory addition of methyl alcohol to render it poisonous) may be used as a secondary refrigerant, either at 100% concentration or mixed with water. The fluid has a low viscosity and good heat transfer, but is now little used on account of its toxicity and the fire risk in high concentrations. Other non- freeze heat transfer fluids are used in specialist trades. 12.7 Brine circuits Brine may be pumped to each cooling device, and the flow controlled by means of shut-off or bypass valves to maintain the correct temperature (see Figure 12.5) Where a brine system services a multiple-temperature installation such as a range of food stores, the coolant may be too cold for some conditions, causing excessive dehydration of the product. In such cases, to cool these rooms the brine must be blended. A separate three-way blending valve and pump will be required for each room (see Figure 12.6). Storage tank Brine chiller Pump Coil 1 Coil 2 Coil 3 Figure 12.5 Brine circuit for separate rooms 152 Refrigeration and Air-Conditioning 12.8 Corrosion If brine circuits are open to the atmosphere, air may be entrained, with consequent oxidation, and the solution will become acid. This will promote corrosion and should be prevented as far as possible by ensuring that return pipes discharge below the tank surface. To reduce the effects of corrosion, inhibitors are added, typically sodium chromate in the salt brines and sodium phosphate in the glycols. These are alkaline salts and help to counteract the effects of oxidation, but periodic checks should be taken, and borax or similar alkali added if the pH value falls below 7.0 or 7.5 [1]. Brines are hygroscopic and will weaken by absorbing atmospheric moisture. Checks should be made on the strength of the solution and more salt or glycol added as necessary to keep the freezing point down to the required value. The preferred brine circuit is that shown in Figure 12.5, and having the feed and expansion tank out of the circuit, which is otherwise closed. This avoids entrainment of air and too much surface exposure. The same arrangement can be used with the divided storage tank shown in Figure 12.6, except that the tank will be enclosed, with a separate feed and expansion tank. 12.9 Thermal storage by frozen brines and ice Variations in cooling load can be provided from the latent heat of melting of ice or a frozen eutectic. Ice can be formed by allowing it Coil 1 Coil 2 Coil 3 Brine chiller Pump Cold side Pump Pump Pump Warm side Storage tank Figure 12.6 Brine circuit for rooms at different temperatures Liquid chillers. Ice. Brines. Thermal storage 153 to build up on the outside of evaporator coils in a tank. Brines are more normally held in closed tanks or plates, again with evaporator coils inside, the outside of the tank forming the secondary heat exchange surface. Eutectics can be formulated according to the temperature required (see Figure 7.8). A variation is to have a pumpable fluid such as one of the glycols, and to contain a eutectic solution within capsules in a storage tank. The capsules are in the form of plastic balls and the eutectic within may be formulated to suit any required thermal storage temperature. The capsules are frozen solid by pumping the glycol through a normal shell-and-tube cooler and then through the tank. When the stored cooling effect is to be used, the glycol flow is diverted to the load, and the capsules then melt again. This system has the advantage of avoiding the corrosion effects of salt brine, and can be used at almost any required storage temperature, depending on the eutectic temperature of the mixture within the capsules. A similar product is available for domestic use. Plastic containers hold a eutectic solution, and these are frozen down by placing these elements in the domestic deep-freeze cabinet. Once frozen, they can be used in picnic baskets, etc., for the short-term storage of cold foods, wines, ice cream, etc. The use of ice cubes to cool beverages by contact or immersion is well known. In commercial use, thermal storage has three main applications: 1. To handle a peak cooling load with a reduced size of refrigeration plant, typically to make ice over a period of several hours and then use ice water for the cooling of a batch of warm milk on a dairy farm. This is also used at main creameries, to reduce peak electricity loads. The available water is very close to freezing point, which is the ideal temperature for milk cooling. 2. To run the refrigeration system at night, or other times when electricity is cheaper, to avoid premium electricity rates, or to avoid maximum demand charges. It is also in use in areas where the electricity supply is unreliable. Where the cold water is to be used at a higher temperature, such as in air-conditioning, the circuit will require three-way blending valves. 3. As hold-over cooling plates in transport (see Figure 7.8d and Chapter 20). [...]... weeks 3 6 weeks 6 10 months 3 5 weeks 8–12 days Indefinite *See also Section 18.2 Depending on variety, harvest time and other factors † 15 Cold store construction 15.1 Size and shape The purpose of a cold store is to provide an insulated and refrigerated enclosure suitable for the handling and storage of perishable goods, at some predetermined temperature The shape and size of the cold store will depend... consistency and texture, and they require special treatment (see Chapter 17) Once a product has been frozen, it must never be allowed to warm and then be re-cooled, or partial thawing may take place with slow re-freezing 14.5 Packing and handling Cold storage packing must contain and protect the product, while allowing the passage of cooling air to keep an even temperature Packages generally will be small... bottles (beer, soft drinks, wines) Instantaneous draught beer coolers These usually comprise a tank of constantly chilled water, through which the beverage flows in stainless steel piping Ice makers – cubes and flakes Cooled vending machines Soft ice-cream freezers Dehumidifiers, in which air is passed first over the evaporator to remove moisture, and then over the condenser to re-heat and lower the humidity... to freeze, surplus water will freeze out until the eutectic mixture is reached (see Section 13. 2) If freezing is not carried out quickly, these ice crystals will grow and pierce the cell walls; then when the product thaws out, the cells will leak and the texture will be spoiled This is of no great consequence with the meats, whose texture is changed by cooking, but will not be suitable for fresh fruits... Comprehensive tables will be found in standard works of reference [ 36 , 37 ] In the event of a product being encountered which cannot be found in general references, information can often be obtained from agricultural and other research establishments It will be noted that fruits and vegetables, with the exception of dried fruit and onions, are stored in high humidity to prevent drying out through the skin... Packaged vehicle cooler (Courtesy of Transfrig Ltd) Other packages A very large variety of self-contained refrigeration and air- conditioning packages are made, mainly for the consumer durable market and small domestic applications They include: 1 2 3 4 5 6 7 8 9 10 The domestic refrigerator and freezer Ice-cream conservators Retail display cold and freezer cabinets and counters Cooling trays for bottles... temperature These will be sized on the probable individual contents Where a low-temperature room is required as well as some at higher temperatures, it should be placed between them, to reduce heat gains Old stores, especially those occupying valuable land in city centres, were built several stories high Access was by lifts and most of the handling was by hand or with hand pallet trucks Such stores are occasionally... surface and may condense on the cold produce already there This will be of no consequence with wet products such as fish and leaf vegetables but cannot be permitted with meat or poultry  164 Refrigeration and Air- Conditioning For these meats, pre-cooling is carried out in a separate room under controlled conditions so that the product is reduced to near-final storage temperature, the surface remaining... distribution and sale, they need to mature Refrigeration of foods Cold storage practice 169 Table 14.1 Storage conditions for foodstuffs Products Temperature Humidity Life Apples Bananas, green ripe Beer, barrel Cabbage Carrots, young old Celery Cucumber Dairy products, milk cream cheese yoghurt Dried fruits Eggs, shell Fish, wet Fruit soft (berries) Grapefruit Grapes Lemons, green Lettuce Meats, bacon beef... condensing unit refrigerant gas to maintain a slight positive pressure for transit Suction and liquid interconnecting lines and wiring will have to be installed on site 13. 3 One-piece packages The true packaged unit will have all the parts of the system and will be factory tested in the complete state There are four basic types: 158 Refrigeration and Air- Conditioning Table 13. 1 Capacity, in kW, of water-cooled . have to be installed on site. 13. 3 One-piece packages The true packaged unit will have all the parts of the system and will be factory tested in the complete state. There are four basic types: 25°C 30 °C 35 °C 40°C. solution, and these are frozen down by placing these elements in the domestic deep-freeze cabinet. Once frozen, they can be used in picnic baskets, etc., for the short-term storage of cold foods, wines,. from the condenser, the compressor going with either (see Figure 13. 4). The unit will be designed as a complete system but the two parts are located separately and connected on site. On some small