Dairy Science & Technology · Dairy Engineering & Plant Operations · also Refrigeration and Air Conditioning, Refrigeration Engineering
Refrigeration systems, ice-bank and chilled-water plants, cold stores and the cold chain from farm to retail. The agricultural engineering track (refrigeration-cold-chain) gives the general theory.
In the exams: ICAR AICE subject 38 Unit 6; ICAR AIEEA-PG group 17 Unit III (refrigeration, compression systems, sizing a refrigeration machine)
Refrigeration is the backbone of the dairy industry. From the moment milk leaves the cow it must be cooled, and it stays cold — chilled or frozen — through collection, processing, storage, distribution and sale, except for the deliberate heat treatments in between. Refrigeration is also one of the largest energy users in a dairy, a source of safety hazards (ammonia, pressure), and a focus of climate policy because of refrigerant emissions. A dairy technologist must understand how refrigeration systems work, how they are sized, and how the cold chain is protected.
This manual covers the vapour compression cycle and coefficient of performance; refrigerants and the rules that govern them; compressors, condensers, evaporators and expansion devices; multi-pressure and ammonia systems; vapour absorption refrigeration and heat pumps; ice-bank and chilled-water systems; bulk milk coolers; cooling load calculation for cold stores; psychrometry and air conditioning; insulation and vapour barriers; hardening rooms and frozen storage; and refrigerated transport and cold-chain monitoring. General refrigeration theory is treated in the agricultural engineering track's refrigeration-cold-chain, chilling and freezing of foods in general in the food technology track's chilling-freezing-frozen-foods, milk cooling rules in market-milk-processing, ice cream freezers in dairy-processing-equipment and frozen-dairy-desserts; this manual focuses on dairy systems.
ICAR AICE-JRF/SRF Dairy Engineering (subject 38, Unit 6) covers the vapour compression system and its components, theoretical and actual cycles, effect of operating parameters on COP, multi-pressure commercial systems, vapour absorption, heat pumps, design and construction of cold storage and air-conditioning systems, cooling loads, psychrometry, equipment selection, insulation, vapour barriers and ice-bank tanks. ICAR AIEEA-PG Dairy Technology (group 17, Unit III) names refrigeration and air-conditioning, compression systems, accessories and calculation of the size of a refrigeration machine. Values in this manual are representative; equipment data, refrigerant property tables and national safety and environmental rules govern real design.
Almost all dairy refrigeration uses the vapour compression cycle. A refrigerant circulates through four components. In the evaporator, low-pressure liquid refrigerant boils at a low temperature, absorbing heat from the milk, water, glycol or air being cooled. The compressor draws in the low-pressure vapour and compresses it to a higher pressure and temperature. In the condenser, the hot, high-pressure vapour gives up heat to cooling water or air and condenses to liquid. The expansion device then reduces the pressure of the liquid, part of which flashes to vapour and cools, and the cold mixture returns to the evaporator. The cycle moves heat from the cold space to the warm surroundings, using work input at the compressor. It is represented on a pressure–enthalpy (p–h) diagram, on which the refrigerating effect (the enthalpy gained per kilogram of refrigerant in the evaporator), the compressor work and the heat rejected in the condenser can be read. Real cycles include superheating of vapour before the compressor (to protect it from liquid) and subcooling of liquid after the condenser (which increases the refrigerating effect), as well as pressure drops and compressor inefficiencies.
Refrigeration performance is measured by the coefficient of performance (COP): COP = refrigerating effect ÷ work input, or in plant terms, cooling capacity ÷ compressor power. A plant giving 200 kW of cooling with 50 kW of compressor power has a COP of 4. The ideal (Carnot) COP for refrigeration, which no real cycle can exceed, depends only on the evaporating and condensing temperatures in kelvin: COP Carnot = T evaporator ÷ (T condenser − T evaporator). For evaporation at −10 °C (263.15 K) and condensation at 35 °C (308.15 K), it is 263.15 ÷ 45 ≈ 5.85. COP falls as the evaporating temperature is lowered or the condensing temperature is raised, so plants should evaporate at the highest temperature that does the job and condense at the lowest practical temperature (clean condensers, good cooling water or air). Capacity is expressed in kilowatts or in tons of refrigeration (TR), where one ton of refrigeration equals 3.517 kW (12,000 British thermal units per hour, historically the rate of melting one short ton of ice in 24 hours); 100 kW is about 28.4 TR. The refrigerant mass flow needed is the cooling load divided by the refrigerating effect per kilogram.
Because COP depends on the temperature lift between evaporator and condenser, the biggest efficiency gains come from narrowing that lift: raising evaporating temperatures where the product allows (for example, using 1–2 °C ice water rather than glycol at −5 °C when milk only needs to reach 4 °C), keeping evaporators and condensers clean and adequately sized, using floating head pressure control so that condensing pressure falls in cool weather instead of being held artificially high, and removing non-condensable gases. Variable-speed drives on compressors, fans and pumps match energy use to load; good part-load control avoids short cycling. Heat recovery and thermal storage add further savings. Plants track specific energy use (kilowatt-hours of refrigeration electricity per tonne of product) and COP over time to detect deterioration.
A good refrigerant has suitable pressures at the operating temperatures, a high latent heat, chemical stability, compatibility with materials and oil, low toxicity and flammability, and low environmental impact. Ammonia (R717) has excellent thermodynamic properties, high latent heat, low cost, no ozone depletion potential and no global warming potential, and its smell reveals leaks at low concentrations; it is toxic and flammable in certain concentrations in air and attacks copper and its alloys, so it is used in steel systems in plant rooms of large dairies and cold stores, under strict safety rules. Carbon dioxide (R744) is non-toxic, non-flammable and has a very low global warming potential; it works at high pressures and is used in transcritical systems for retail refrigeration and in cascade systems with ammonia for low temperatures. Hydrocarbons such as propane (R290) and isobutane (R600a) are efficient and climate-friendly but flammable, so they are used in small charges in self-contained units such as display cabinets and small chillers. Synthetic halocarbon refrigerants dominate small and medium commercial equipment.
Chlorofluorocarbons (CFCs, such as R12) were phased out under the Montreal Protocol because their chlorine destroys stratospheric ozone; hydrochlorofluorocarbons (HCFCs, such as R22) have lower ozone depletion potential but are also being phased out, with timetables differing between developed and developing countries. Hydrofluorocarbons (HFCs, such as R134a and R404A) do not deplete ozone but have high global warming potentials — R134a around 1,400 times that of carbon dioxide and R404A several thousand times — and are being phased down under the Kigali Amendment (2016) to the Montreal Protocol, with national rules such as the European Union's fluorinated gas regulation and India's national phase-down commitments and cooling action plan. Replacements include natural refrigerants (ammonia, carbon dioxide, hydrocarbons) and hydrofluoroolefins (HFOs, such as R1234yf and R1234ze) with very low global warming potential. For dairies, this means preferring natural refrigerants for new installations, reducing refrigerant charge (for example with secondary coolants), detecting and repairing leaks, and recovering refrigerant at end of life.
Compressors raise the refrigerant pressure and are the main energy users of a refrigeration plant. Reciprocating (piston) compressors are robust and common in small and medium plants; screw compressors, with two meshing helical rotors, dominate large ammonia plants in dairies and cold stores because they handle large volumes, run efficiently at full load and can be controlled by slide valves or variable-speed drives; scroll compressors are compact and quiet for small units; centrifugal compressors suit very large water chillers. Capacity control (unloading, slide valves, multiple compressors, variable-speed drives) matches output to load and saves energy. Compressors need oil for lubrication and sealing, so oil separators, oil coolers and oil return systems are important, especially with ammonia.
Condensers reject the heat absorbed in the evaporator plus the compressor work. Air-cooled condensers are simple and use no water but run at higher condensing temperatures in hot weather; water-cooled shell-and-tube or plate condensers use cooling-tower water; evaporative condensers, common in large dairy ammonia plants, spray water over coils carrying the refrigerant while air is blown through, combining the low condensing temperature of water cooling with low water use. Condensers must be kept clean and their water treated to prevent scale and Legionella growth. Evaporators absorb heat from the medium being cooled. Direct-expansion evaporators feed liquid refrigerant through a metering device so that it evaporates completely in the coil; flooded evaporators keep the surfaces covered with liquid refrigerant, giving better heat transfer, with a separator to return liquid; pumped liquid-overfeed systems circulate more liquid than evaporates. Forms include shell-and-tube and plate evaporators for chilling water or glycol, coils with fans (air coolers) for cold rooms, and the plates or coils of bulk milk coolers and ice banks. Air coolers below 0 °C accumulate frost and need defrosting (by hot gas, electric heaters or water), which must be managed to avoid temperature rises and energy waste.
Expansion devices reduce pressure and control refrigerant flow to the evaporator. Capillary tubes (fixed restrictions) are used in small hermetic units; thermostatic expansion valves (TXVs) adjust flow to keep a set superheat at the evaporator outlet; electronic expansion valves give finer control over a wider range; float valves control the liquid level in flooded evaporators and receivers. Other components include liquid receivers (storing refrigerant), filter-driers (removing moisture and dirt in halocarbon systems), sight glasses, accumulators or suction separators (protecting compressors from liquid), pressure and temperature sensors, safety valves, and purgers that remove non-condensable gases (air) from ammonia systems, which otherwise raise condensing pressure and power use.
Refrigeration plants need planned maintenance: oil changes and analysis, checking compressor vibration and temperatures, cleaning condensers and evaporator coils, treating condenser water, calibrating sensors and safety cut-outs, testing relief valves and gas detectors, checking insulation of suction lines, and leak testing. Common faults and their signs include low refrigerant charge (high superheat, low suction pressure, bubbles in sight glasses), dirty condensers or non-condensables (high discharge pressure and power), iced or dirty evaporators (low suction pressure, poor cooling), failed expansion valves (flooding or starving of evaporators), and liquid slugging of compressors (noise and damage). Logged operating data — pressures, temperatures, currents — allow faults to be diagnosed early. Leak detection is both an efficiency and a regulatory matter for high-GWP refrigerants and a safety matter for ammonia.
A dairy needs refrigeration at several temperatures: chilled water or glycol near 0 °C for milk cooling, cold rooms around 2–5 °C, and frozen storage and ice cream hardening at −25 to −40 °C. Running all loads from one compressor at the lowest evaporating temperature would be very inefficient, and compressing from very low to condensing pressure in one stage gives high compression ratios, high discharge temperatures and low efficiency. Multi-pressure systems therefore use different evaporating pressures for different loads, with back-pressure regulators or separate compressor suction groups. For low temperatures, two-stage compression is used: a low-stage (booster) compressor raises vapour from the low-temperature evaporator to an intermediate pressure, where it is cooled (intercooled) in a flash intercooler or by liquid injection, and a high-stage compressor raises it to condensing pressure. Intercooling and economising (flashing some liquid at the intermediate pressure to subcool the rest) improve efficiency. Cascade systems use two separate circuits with different refrigerants, such as carbon dioxide in the low-temperature circuit and ammonia in the high-temperature circuit, linked by a cascade heat exchanger.
Large dairies and cold stores typically have central ammonia plants: screw compressors in an engine room, evaporative condensers on the roof, a high-pressure receiver, and either flooded or pumped-overfeed evaporators, with liquid separators at each temperature level. To keep ammonia out of processing areas, many dairies use secondary refrigerants — chilled water, ice water, glycol or brine — produced in the plant room and pumped to process coolers, plate heat exchangers, tanks and air coolers. This also reduces the ammonia charge. Ammonia safety requires ventilated machine rooms with gas detectors and alarms, emergency ventilation and shutdown, pressure relief to safe locations, trained operators, protective equipment, emergency procedures, and compliance with national rules on pressure equipment and hazardous substances. Ammonia leaks can be detected early by smell, but high concentrations are dangerous, and incidents in food plants have caused injuries and deaths.
Choosing a secondary coolant involves its freezing point, viscosity at operating temperature (which sets pumping power and heat transfer), specific heat, corrosiveness, toxicity and cost. Water is best where temperatures allow; propylene glycol is used where food contact might occur, ethylene glycol where it cannot, and brines (calcium chloride) in some older plants despite corrosion. Carbon dioxide can also serve as a volatile secondary coolant, pumped as liquid and evaporating in the coolers, which gives high heat transfer with small pipes. Using secondary circuits reduces the primary ammonia or synthetic refrigerant charge, limits leak risks in occupied and process areas, and simplifies compliance, at the cost of an extra heat-exchange step and pumping energy.
Vapour absorption refrigeration replaces the mechanical compressor with a 'thermal compressor' driven by heat. Refrigerant vapour from the evaporator is absorbed into a liquid absorbent in the absorber; the solution is pumped (using little work) to the generator, where heat drives the refrigerant out again at high pressure; the refrigerant vapour is condensed, expanded and evaporated as in a vapour compression cycle, while the weak solution returns to the absorber. Two pairs are common: lithium bromide–water, in which water is the refrigerant, for chilled water above 0 °C (air conditioning and process cooling); and ammonia–water, in which ammonia is the refrigerant, for temperatures below 0 °C. Absorption systems use heat such as low-pressure steam, hot water, exhaust heat from engines and turbines, or solar heat, so they are attractive where waste heat is available, such as in dairies with combined heat and power plants. Their COP (cooling ÷ heat input) is low compared with vapour compression — commonly well below 1 for single-effect machines and somewhat above 1 for double-effect machines — but the energy is cheap or free waste heat. They have few moving parts and run quietly.
A refrigeration system rejects at its condenser all the heat absorbed in the evaporator plus the compressor work, and in a dairy that heat can be used instead of wasted. Desuperheaters recover the high-temperature heat from compressor discharge gas, and condenser heat recovery preheats boiler feed water, cleaning water and process water. A heat pump is a refrigeration system designed to deliver useful heat: its heating COP is heat delivered ÷ work input, which equals the cooling COP plus one (since heat delivered = cooling + work); a system with a cooling COP of 4 has a heating COP of 5. High-temperature heat pumps, often using ammonia, carbon dioxide or hydrocarbons, can raise water to temperatures useful for CIP and pasteuriser hot water sets, recovering heat from refrigeration or from warm effluent. Because dairies need both cooling and heating at the same time, integrating them with heat pumps can cut fuel use and emissions considerably.
Milk cooling and many process coolers need large cooling capacity for short periods: at milking times, at reception peaks, or when a pasteuriser starts. An ice bank stores refrigeration as ice built on evaporator coils immersed in a water tank, using a relatively small compressor running for many hours (often at night, when electricity may be cheaper and condensing temperatures lower). When cooling is needed, water is pumped from the tank over the ice and through the process cooler, and the melting ice absorbs heat at a steady, near-freezing temperature, giving ice water at about 0.5–1 °C. Because the latent heat of fusion of ice is about 334 kJ/kg, a modest mass of ice stores a lot of cooling: cooling 10,000 L of milk (about 10,300 kg) from 35 to 4 °C needs about 10,300 × 3.9 × 31 ≈ 1,245 MJ, the latent heat of roughly 3,700 kg of ice. Ice-bank control measures ice thickness on the coils so that the bank is fully charged without bridging between coils. Ice banks are widely used in farm bulk coolers, village milk chilling centres and dairy plant process cooling.
Chilled-water systems produce water at a few degrees above freezing in plate or shell-and-tube evaporators (water chillers) and circulate it to plate heat exchangers that cool milk, cream and products, to jacketed tanks and to air-handling units. Water cannot be used below about 1 °C without freezing risk, so for lower temperatures plants use glycol solutions (propylene glycol, which is preferred where incidental food contact is possible, or ethylene glycol) or brines, whose concentration sets the freezing point. Secondary coolant systems need pumps, insulated pipework, buffer tanks, expansion tanks and control of flow and temperature; they keep primary refrigerant out of process areas and make it easy to distribute cooling around a plant. Chilled-water temperature is controlled to the process need, since colder water costs more energy. In milk cooling, a regenerative section of a plate cooler using incoming cold milk or well water followed by a chilled-water or glycol section is a common, energy-efficient arrangement.
Ice banks are one form of thermal energy storage; others include chilled-water storage tanks and phase-change materials. Storage lets a plant run its refrigeration when electricity is cheaper, when ambient temperature (and so condensing temperature) is lower, or when renewable power is available, and then meet peak cooling loads without large compressors. It also provides a reserve of cooling during power cuts, which is valuable at rural milk chilling centres. Designing storage requires the daily cooling-load profile, the tariff structure and the available charging hours.
Bulk milk coolers (BMCs) on farms and at village collection centres cool milk immediately after milking and hold it cold until collection, which is the most effective single step in keeping raw milk quality high (see market-milk-processing for cooling rules). They are insulated stainless steel tanks with an agitator, outlet valve, thermostat and refrigeration unit. In direct-expansion coolers, refrigerant evaporates in plates or channels welded to the tank bottom and lower walls; they cool quickly and efficiently but need enough compressor capacity for the peak load. In ice-bank coolers, an ice bank in a separate tank or around the inner tank is built up between milkings and ice water is circulated over the inner tank, allowing a smaller compressor and coping with power cuts. International standards for bulk milk coolers (such as ISO 5708) define performance classes by how fast they cool milk from a set starting temperature to about 4 °C under stated conditions, and specify energy use and construction. The cooling capacity needed is the heat to be removed divided by the allowed time: 2,000 L of milk (about 2,060 kg) cooled from 35 to 4 °C in 3 hours needs about 2,060 × 3.9 × 31 ÷ 10,800 ≈ 23 kW, before allowing for losses.
In India and other countries with smallholder dairying, bulk milk coolers at village dairy cooperative societies and private collection centres, typically of a few thousand litres, have greatly improved milk quality by cooling milk within a short time of collection instead of transporting it warm. Their performance depends on reliable electricity (many sites use diesel generators or, increasingly, solar-powered coolers with thermal storage), on proper cleaning, on adding warm milk in amounts the cooler can handle, and on maintenance of the refrigeration unit and agitator. Cleaning is critical because a poorly cleaned cooler contaminates every load. Monitoring systems record milk temperature and send alerts and data to the dairy, supporting quality-based payment. Smaller alternatives include can coolers, immersion coolers and pre-coolers that use well water through a plate heat exchanger before the milk enters the tank.
The refrigeration capacity of a cold store or cold room is chosen from a cooling load calculation that adds several components. Transmission load is heat conducted through walls, roof and floor: Q = U × A × ΔT, where U is the overall heat transfer coefficient of the construction, A the area and ΔT the difference between outside and inside temperatures (with solar gain added for sunlit surfaces). Product load is the heat removed from product brought in warmer than the store: sensible heat (mass × specific heat × temperature drop), plus latent heat of freezing for products frozen in the store and sensible heat below freezing. Infiltration (air change) load is the heat carried by warm, moist outside air entering through doors, often the largest single component in busy stores; it includes both sensible heat and the latent heat of moisture that freezes as frost on coils. Internal loads include people working in the store, lights, forklifts, and the evaporator fan motors themselves. Defrost heat adds load. The total is usually increased by a safety factor (commonly around 10 per cent) and divided by the planned compressor running time per day to give the required capacity.
Transmission through 100 m² of wall insulated with 150 mm of polyurethane foam (thermal conductivity about 0.022 W/m K) has U ≈ 0.022 ÷ 0.150 ≈ 0.15 W/m² K, ignoring surface resistances; with 40 °C outside and 2 °C inside, Q ≈ 100 × 0.147 × 38 ≈ 560 W. Product load for 20,000 kg per day of dairy products entering at 10 °C and cooled to 4 °C, with specific heat 3.9 kJ/kg K, is 20,000 × 3.9 × 6 = 468,000 kJ per day, or about 5.4 kW averaged over 24 hours (though in practice product arrives in batches and the peak may be higher). Infiltration depends on door size, opening frequency and duration, temperature and humidity difference, and the use of air curtains, strip curtains, rapid-roll doors and vestibules, which can reduce it greatly. The calculation shows designers and operators where to act: better insulation for transmission, pre-cooling products before storage, and door discipline for infiltration.
Psychrometry, the study of moist air, explains several problems in dairy plants (its fundamentals are covered in evaporation-drying-dairy). In cold rooms, warm humid air entering through doors cools below its dew point, so water condenses on cold surfaces and freezes on evaporator coils as frost, reducing their efficiency and requiring defrost; condensation dripping from ceilings and pipes onto product or packaging is a serious hygiene hazard, because condensate can carry Listeria and other contaminants. In processing halls, steam and hot water raise humidity; without adequate ventilation and dehumidification, condensation forms on cold pipes and ceilings, moulds grow and structures deteriorate. Relative humidity in cold stores is managed for product quality: too dry air dries unwrapped cheese and butter surfaces, too humid air promotes mould growth on cheese and labels. Cheese ripening rooms are controlled precisely for both temperature and humidity.
Air conditioning in dairies serves product protection, hygiene and comfort. High-hygiene areas such as filling rooms, cheese moulding and packing areas, and powder packing need filtered air (often high-efficiency particulate air filters) supplied at positive pressure so that air flows outwards to less clean areas, with controlled temperature and humidity to limit microbial growth and condensation. Air-handling units cool and dehumidify air by passing it over coils below its dew point (removing water as condensate), then reheat it if necessary to reach the desired relative humidity; desiccant dehumidifiers are used for very dry air, for example in powder packing. Packing rooms for butter and chocolate-coated ice cream products are kept cool to prevent softening. Air conditioning loads are calculated like cold-store loads, adding the latent heat of moisture removed. Worker comfort in cold areas also matters: people working in chilled and frozen rooms need protective clothing, rest breaks and safety measures against cold stress.
Cheese ripening rooms need precise and uniform temperature and humidity, with gentle air movement: too little air movement gives uneven conditions and surface moulds, too much dries rinds. Typical conditions depend on the variety, from cool rooms for long-ripened hard cheeses to warmer rooms for eye formation in Swiss types, and high humidity for surface-ripened and mould cheeses. Humidification and dehumidification are integrated with cooling, and air is filtered to limit mould spores. Butter cold stores are kept cold and stable, with control of odours, since butter readily absorbs smells from other products stored with it.
Insulation reduces heat flow into cold spaces and so reduces refrigeration load and running cost. Common materials are polyurethane (PUR) and polyisocyanurate (PIR) foams, with thermal conductivities commonly around 0.022–0.025 W/m K, used in prefabricated sandwich panels with steel facings for walls and ceilings of cold rooms and in spray or pipe insulation; expanded and extruded polystyrene (around 0.033–0.038 W/m K), used particularly for floors because of its compressive strength and low water absorption; and mineral wool, which is non-combustible but absorbs moisture and is less used in cold stores. Fire performance is an important consideration in choosing panels, since cold-store fires involving combustible insulation have been very destructive. The heat flow through a layer is proportional to its conductivity and inversely proportional to its thickness: U ≈ k ÷ thickness for a single layer (ignoring surface resistances and other layers), and layers in series add their resistances (thickness ÷ conductivity). Insulation thickness is chosen economically, balancing capital cost against energy saved, with thicker insulation for colder stores. Thermal bridges — places where steel structures, fixings or doors cross the insulation — must be minimised, since they leak heat and cause condensation.
Water vapour moves from the warm, humid outside towards the cold inside of a cold store, driven by the difference in vapour pressure. If it enters the insulation, it condenses and freezes there, destroying the insulation's effectiveness, adding weight and causing structural damage. A continuous vapour barrier (the steel facing of sandwich panels, or membranes and foils) must therefore be placed on the warm side of the insulation, with all joints, penetrations and panel seams sealed. Frozen stores have an additional problem: the cold can penetrate the ground beneath the floor and freeze the soil, which expands (frost heave) and can lift and crack the floor. Floors of freezer stores are therefore insulated and built over a heated layer (warm air ducts, glycol pipes or electric heating) or a ventilated void that keeps the soil above freezing. Door frames and thresholds are often heated to prevent ice build-up. Regular inspection for damaged panels, failed seals and ice formation keeps insulation effective.
Doors are the weak points of an insulated envelope. Cold-store doors are insulated to the same standard as walls, with good seals, heated frames in freezer stores, and fast operation; rapid-roll doors, automatic doors linked to forklift sensors, strip curtains and air curtains reduce the time and area open to warm air. Loading docks with dock shelters and levellers allow vehicles to be loaded without exposing products to outside air. Damaged door seals and doors left open are among the commonest causes of high energy use and temperature excursions.
Ice cream and other frozen dairy products are hardened immediately after filling, as described in frozen-dairy-desserts. Hardening tunnels and spiral freezers blow air at around −30 to −40 °C at high velocity over packaged products on conveyors, giving fast heat removal; plate hardeners press flat packages between refrigerated plates; and hardening rooms with strong air circulation are used for larger packs and smaller plants. These low temperatures require two-stage ammonia, cascade or other low-temperature systems, with large evaporator coils and defrost arrangements. Hardening capacity must match production, since slow hardening coarsens ice crystals. Butter for long-term storage, frozen cream and some cheeses may also be frozen in blast freezers before storage.
Frozen dairy products are stored at about −18 °C or below, often −25 °C or colder for ice cream to preserve texture. Frozen stores need stable temperatures, since fluctuations cause ice recrystallisation, shrinkage and freezer burn (surface drying by sublimation through poor packaging). Good practice includes: adequate refrigeration capacity with standby; correctly sized and positioned air coolers giving even air distribution without direct blasts on products; scheduled defrosts that minimise temperature rise; airlocks, rapid-roll doors, strip curtains and air curtains to limit warm air entry; heated door frames; racking that allows air circulation; stock rotation by first-expiry, first-out; temperature mapping and continuous recording with alarms; and safety for workers, including cold-protective clothing, limited exposure, and systems to prevent people being trapped inside (door releases operable from inside, alarms). Energy use is controlled by keeping doors closed, maintaining door seals, cleaning coils, using variable-speed fans and compressors, and raising the store temperature only within quality limits.
The cold chain links farm, collection centre, plant, distribution centre, retailer and consumer, and any weak link shortens shelf life or creates safety risks. Raw milk is transported from chilling centres to plants in insulated road tankers without refrigeration, relying on pre-cooling and insulation to limit the temperature rise during the journey; tanker design, journey time and cleaning are controlled. Packaged chilled products are distributed in insulated vehicles with mechanical refrigeration units driven by the engine or by separate motors, with eutectic plates (which store cooling as phase-change material charged overnight) for urban delivery, or with cryogenic systems using liquid nitrogen or carbon dioxide. Vehicles must be pre-cooled before loading, loaded quickly with air circulation space, and kept closed. International carriage of perishable foods between signatory countries follows the ATP Agreement (Agreement on the International Carriage of Perishable Foodstuffs), which classifies and tests insulated and refrigerated vehicles. In many developing countries, gaps in the cold chain — insufficient refrigerated vehicles, unreliable power at retail, and long informal distribution — cause large losses of dairy products.
Cold-chain monitoring has moved from paper thermometer records to electronic data loggers and wireless sensors that record temperature (and sometimes humidity and location) through storage and transport, send alarms when limits are exceeded, and provide data for traceability and audits. Time-temperature indicators on packs show cumulative exposure (see dairy-packaging). At retail, open-front chilled cabinets are convenient but lose cold air and energy, and products at the front and top are warmer; closed-door cabinets maintain temperature better and use less energy; loading above the load line and blocking air curtains are common causes of warm product. Stock rotation by first-expiry, first-out, prompt unloading of deliveries, and regular checks of cabinet temperatures protect product. Consumers are the last link: advice on packs to refrigerate promptly and keep products cold matters. Shelf-life studies use the realistic temperature profile of the chain, including expected abuse, to set date marks. Monitoring data are used to find weak links and justify investment in better equipment and practices.
In many developing dairy markets, a large share of milk moves through informal channels without cooling, and organised cold chains are incomplete: village chilling centres may depend on unreliable power, distribution vehicles may be uninsulated, and retail outlets may lack refrigeration or switch it off at night. Solutions include village bulk milk coolers with backup power or solar energy, insulated tankers with short routes, eutectic or ice-pack distribution for small retailers, UHT and other ambient-stable products for areas without cold chains, and training and incentives for retailers to keep products cold. Government programmes and private investment in cold-chain infrastructure, together with monitoring data, aim to reduce losses and improve food safety.
A student uses this subject to explain the vapour compression cycle, calculate COP, tons of refrigeration, cooling loads, ice-bank capacity and bulk-cooler size, compare refrigerants and systems, and understand insulation and vapour barriers. It supports AICE Dairy Engineering Unit 6 and AIEEA-PG group 17 Unit III.
A refrigeration or utilities engineer five years in operates and maintains ammonia plants and chillers, manages defrost and condenser performance, checks ammonia safety systems, sizes coolers for new lines, troubleshoots temperature complaints in cold stores and vehicles, and reports energy use. This subject gives the theory and practice behind those tasks.
A plant head or engineering director decides on refrigeration plant investment and refrigerant choice under phase-down rules, heat recovery and heat pumps, cold-store capacity, distribution fleet and cold-chain monitoring systems, balancing quality, safety, energy and climate commitments.
A professor or research director works on natural refrigerant systems, heat integration, thermal storage, cold-chain modelling and losses, and teaches and examines AICE Dairy Engineering on refrigeration and cold storage.
This manual does not yet give: refrigerant property tables and worked p–h diagram cycle calculations; detailed compressor and heat exchanger selection procedures; national ammonia safety codes; exact phase-down schedules and GWP values under each regulation; full ISO 5708 performance classes; and detailed cold-chain loss statistics. Temperatures, conductivities, GWP values and capacities are representative and must be checked against current data and regulations.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
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