Refrigeration Intro

Refrigeration IntroRAC400SRefrigerationDefinition: the process of cooling of bodies or fluids to temperatures lower than those available in the surroundings at a particular time and placeCooling is a heat transfer process down a temperature gradient, it can be a natural, spontaneous process or an artificial processRefrigeration is not a spontaneous process, as it requires expenditure of energy

Refrigeration [2]

Methods of producing low temperaturesSensible cooling by cold mediumIf a substance is available at a temperature lower than the required refrigeration temperature, then it can be used for sensible cooling by bringing it in thermal contact with the system to be refrigeratedEndothermic mixing of substancesLow temperatures can be obtained when certain salts are dissolved in water. This is due to the fact that dissolving of these salts in water is an endothermic processMethods of producing low temperatures [2]Phase change processesRefrigeration is produced when substances undergo endothermic phase change processes such as sublimation, melting and evaporation.Expansion of Liquidsin refrigeration systems which use expansion of liquids to produce low temperatures (e.g. vapour compression refrigeration systems), the inlet state of the liquid is always such that the outlet falls into the two phase region.Expansion of gasesBy throttlingExpansion of gases through a turbineMethods of producing low temperatures [3]Thermoelectric RefrigerationThermoelectric refrigeration is a novel method of producing low temperatures and is based on the reverse Seebeck effectAdiabatic demagnetizationMagnetic refrigeration is based on the magnetocaloric effect. There are some materials that raise their temperatures when adiabatically magnetised, and drop their temperature when adiabatically demagnetised. Temperature very near the absolute zero may be obtained by adiabatic demagnetization of certain paramagnetic salts.Applications of refrigerationThe major applications of refrigeration can be grouped into following four major equally important areas.Food processing, preservation and distributionChemical and process industriesSpecial ApplicationsComfort air-conditioningRefrigeration for Food PreservationA major use of refrigeration is in the preservation, storage and distribution of perishable foods.The edibility of foodstuffs is prolonged by lowering the temperature, since this slows chemical reactions and breakdown by bacteria. Some products can be frozen, and when they are in the solid state all movement in the individual cells will cease, inhibiting further reactionsThe decision whether to just chill or to freeze solid depends on the type of product and the length of time it must be stored. Freezing results in some structural change, since ice crystals are formed inside the cells, and the final foodstuff may be of different texture when thawed out.As a general rule, foods which are not to be frozen are handled and stored at a temperature just above their freezing point, providing this does no damage (exceptions are fruits such as bananas and lemons). Produce which is to be frozen must be taken down below the freezing point of the constituents. Since foodstuffs contain salts and sugars, the freezing process will continue down to 18C and lowerTwo categories of foodLiving food (vegetables and fruit) must be kept alive and enzyme activity must be retarded by refrigeration.Non-living food (red meat, poultry and fish) must be protected from enzyme action (hydrolysis and oxidation of animal fats) and microbial decay (by refrigeration and airtight packaging).A distinction must be drawn between the cooling process and subsequent storage.

Pre-storage treatmentCooling and freezing cannot improve a product, and the best that can be achieved is to keep it near to the condition in which it entered the cooling process. This means that only the best produce should be used, and this should be as fresh as possible.All foods must be clean on entry. Some, such as fish, leaf vegetables and some fruits, may be washed and left wet. Fish will tend to dry out and lose its fresh appearance, so it is packed wet or given a sprinkling of ice chips to keep the surface moist.Other products, especially the meats, must be dry, or bacteria will live on the moisture and make the skin slimy.Potatoes will start to sprout after a long period in storage. This can be checked by spraying the freshly lifted tubers with a chemical sprout depressant.Certain fruits, notably grapes and dates, may have some surface contamination or infestation when first picked, and they are fumigated with sulphur dioxide or some other gas. They must, of course, then be thoroughly ventilated before going into storage.Pre-coolingIf warm produce is taken into a cold store, moisture will evaporate from its 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.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 dry all the timeWet products can be pre-cooled in chilled water, or by the addition of flake ice. Ice is also used with fish and leaf vegetables to help maintain freshness in transit to storage. Leaf vegetables can be cooled by placing them in a vacuum chamber and so evaporating surface water at low pressure.

FreezingMost products will keep longer and fresher in the frozen condition, and this process is used for those whose sale value will support the extra process cost.

Fruit, vegetables and other foodsIt is well-known that some bacteria are responsible for degradation of food, and enzymatic processing cause ripening of the fruits and vegetables. The growth of bacteria and the rate of enzymatic processes are reduced at low temperature. This helps in reducing the spoilage and improving the shelf life of the food.It can be seen that the storage temperature affects the useful storage life significantly.In general the storage life of most of the food products depends upon water activity, which essentially depends upon the presence of water in liquid form in the food product and its temperature.

In case of fruits and vegetables, the use of refrigeration starts right after harvesting to remove the post-harvest heat, transport in refrigerated transport to the cold storage or the processing plantThe cold chain has proved to be very effective in reducing spoilage of food and in food preservation. Refrigeration helps in retaining the sensory, nutritional and eating qualities of the food. The excess crop of fruits and vegetables can be stored for use during peak demands and off-season; and transported to remote locations by refrigerated transport. In general, the shelf life of most of the fruits and vegetables increases by storage at temperatures between 0 to 10 C.

Meat industry applicationsIn the meat industry, the main applications of mechanical refrigeration are:Chilling of carcases directly after slaughter and dressingCooling of meat-handling rooms such as butcheriesChilled water and brine for cooling poultryChill storage of edible meats and offalChilling of brine and pickling vatsMeat and poultry freezingBeefAnimals when slaughtered, are at a body temperature of 39C. The carcase cools slightly as it is being dressed, but must be put into refrigerated chambers as soon as possibleA general rule for lean meat such as beef is that no part should be cooled below 10C for at least 10 hours after slaughter, although this limit may be varied by the local producer.The total time in this chiller stage will be about 24 hours for a beef sideChilled meat on the bone is stored at about 0C, up to the point of sale.The humidity of the surrounding air is also critical in the case of fresh meats too dry and the meat will lose weight and discolour, too humid and a slime will form on the surface.Pork and baconFresh pork has a shorter shelf life than beef, but is handled in the same way and at the same chill-room temperatures.A high proportion of pork is pickled in brine and smoked, to make ham or bacon. The original process was to immerse the meat in a tank of cold brine for a period. A quicker method is to inject the cold pickle with hypodermic needles into the cuts. Smoking is carried out at around 52C, so the cured bacon must be cooled again for slicing, packing and storage.PoultryPoultry is immersed in hot water just after slaughter, in order to loosen the feathers for the plucking process. The carcases are then eviscerated and chilled as soon as possible by cold air blast or using iced water in the form of a bath or spray.Larger birds may be reduced to portions, so the flesh must be cooled to about 0C to make it firm enough for cutting.Poultry may be chilled for the fresh chicken market, or frozen. Chilling and freezing are mainly by cold air blast.Storage of chilled poultry is at 1C. The shelf life is relatively short and the product will not remain in store for more than a couple of days.FishMost fish is still caught at sea and must be cooled soon after it is taken on board, and kept cold until it can be sold, frozen or otherwise processedThe general practice is to put the fish into refrigerated sea water tanks, kept down to 0C by direct expansion coils or a remote shell-and-tube evaporator. The sea water must be clean and may be chlorine dosed. At this condition, fish can be kept for up to four days.Fresh fish is stored and transported with layers of ice between and over the fish, cooling by conduction and keeping the product moist. Fish kept at chill temperatures in this manner can travel to the final point of sale, depending on the time of the journey.Dairy, brewing and soft drinks industriesIn the dairy industry as a whole, the main needs for mechanical cooling are:Cooling milk directly after it leaves the cow and before transport to a central creameryKeeping the raw milk cool after it enters the creameryChilled water for use in plate heat exchangers to cool milk and milk products directly after pasteurizingChilled water to wash butterChill temperature stores for milk, butter, cheese, yoghurt and other liquid milk productsFrozen storage for butter (and sometimes cheese)Continuous, plate and air blast freezers for ice-creamLow-temperature brine for lollipop freezingMilk comes from the cow at about 37C, and must be cooled within two hours to 4C or lower, and under hygienic conditions. At this temperature any micro-organisms present will not multiply at a dangerous rate and the milk can be transported to the creamery.Throughout the subsequent processes, milk and milk products will require to be re-cooled down to 4C or thereaboutsWhole milk for human consumption is pasteurized at 75C for a short time, and then re-cooled to 4C immediately.Pasteurization process:Raw milk at 4C is heated by the outgoing milk up to about 71C.This milk is finally heated by hot water up to the pasteurizing temperature of 75C (or hotter for UHT milk) and held for a few seconds.The milk is cooled by the incoming milk, down to about 10C.The final stage of cooling from 10C to 4C is by chilled water at 2C.Refrigeration load estimationRefrigeration loads are from two sources:To cool something down, i.e. reduce its enthalpyTo keep something cool, i.e. remove incoming and internally generated heatThe components of the total cooling load will be:Removal of heat, sensible or latent, from a productHeat conducted in through the surfaces of the room, tank, pipe, etc., from warmer surroundingsRadiant heat from outsideHeat convected from outside (air infiltration or ventilation), both sensible and latentInternal sources of heat lights, fan motors, machinery, personnel, etc. and heat generated by the productProduct coolingThe total amount of sensible and latent heat to be removed in cooling a product is given by:

29The rate of heat extraction, i.e. the product cooling load, will be:

Conducted heatConducted heat is that going in through cold store surfaces, tank sides, pipe insulation, etc. It is normally assumed to be constantHeat load must be estimated through all surfaces including piping, ducts, fan casings, tank walls, etc., where heat flows inwards towards the cooled system.Radiant heat is not a serious factor in commercial or industrial refrigeration systems, being confined to sunshine through refrigerated display windows (which should have blinds) and radiation into open shop display cabinets from lighting.

Convected heatWarm air will enter from outside mainly during the opening of doors for the passage of goods. This must be estimated on the basis of the possible use of the doors, and such figures are based on observed practice.Standard textbooks give data on which to base an estimate, and this can be summed up as

For V < 100 m3For V > 100 m3Internal heat sourcesThe main sources of internal heat are fan motors and circulating pumps. Where the motor itself is within the cooled space, the gross energy input to the motor is liberated as heat which must be removed.Personnel will give off about 120 W each.All lighting within the space must be included on the basis of the gross input. The usual 80-W lighting tube takes about 100 W gross.Where coolers are fitted with defrosting devices, the heat input from this source must be determined.Heat of respirationCertain stored foodstuffs are living organisms and give off heat as their sugar or starch reserves are slowly consumed. This is known as the heat of respiration, since the products consume oxygen for the process. The heat of respiration varies with the sugar or starch content of the product, the variety, and its temperature, and is between 9 and 120 W/t at storage temperatures.

Example 1A cold room is used to store frozen meat at -30C. Calculate the refrigeration load of the facility.Data:Internal volume of the room 1000 m3Surface area exposed to the exterior 700 m2Loading program: 25 000 kg meat at -20C per day is introduced to the room; an equal quantity of meat at -30C is removedThe specific heat of meat at that range of temperatures is 1.8 kJ /kg KThe exposed surface area is covered with polystyrene foam insulation, 25 cm thick; the thermal conductivity of the insulation is 0.04 W/m2 KAmbient temperature 25C.Air changes: 0.08333 ACH (assume dry air)Allow 1.2 kW for lights, fans, defrost, people, conveyors etc.

Example 2A cold room measures 35 m long by 16 m wide and is 5 m high inside. Insulation is 125 mm to walls and ceiling and 75 mm under the floor, of polystyrene having a thermal conductivity of 0.035 W/(m K). Inside it is at -10C, the ambient is 27C, and the ground temperature is 12C. The cold room has 12 lighting fittings labelled 280 W. The four evaporators each have three fan motors of 660 W gross per fan and 18 kW defrost heaters which operate alternately for 15 min twice during the shift. The fork-lift truck is rated 80 A at 24 V and will be in the store 20 min each hour during the 8-h working day. Two packers will be present for 10 min each hour. The cold room stores 700t of beef at 2C that arrives on at the beginning of each shift and is immediately loaded into the cold room and leaves hourly in batches of 100t at -10CWhat is the heat flow inwards?Estimate the infiltration air heat gain for the cold roomEstimate the average and peak loads for the shiftEstimate the required plant capacity for the shift