Dairy Science & Technology · Dairy Engineering & Plant Operations · also Evaporation and Drying, Concentration and Dehydration Engineering
Evaporators and dryers for milk and whey: design, energy economy, operation and powder engineering.
In the exams: ICAR AICE subject 38 Unit 2; ICAR AIEEA-PG group 17 Unit II (evaporation, vacuum pan, drum dryer, spray dryer, fluidisation); GATE XL-U Section 4
Evaporation and drying turn milk and whey into concentrates and powders that can be stored for months and shipped around the world. They are also the most energy-intensive operations in most dairies. Good engineering here determines not only cost but product quality — heat damage, solubility, bulk density, instant properties — and safety, because dryers handle combustible dust and hot air.
This manual covers the principles of evaporation and boiling-point elevation; vacuum pans; falling-film evaporators; multiple-effect evaporation and steam economy; thermal and mechanical vapour recompression; drying fundamentals and psychrometrics; spray dryers with their atomisers, chambers and multistage designs; drum dryers; fluid-bed dryers and agglomeration; freeze drying and freeze concentration; powder separation by cyclones and bag filters; and energy and safety. The products made with this equipment — evaporated and condensed milks, milk powders, whey powders — are covered in concentrated-dried-milks and whey-dairy-byproducts, and membrane pre-concentration in membrane-processing-dairy-ingredients; general heat-transfer theory is in dairy-fluid-flow-thermodynamics.
ICAR AICE-JRF/SRF Dairy Engineering (subject 38, Unit 2) asks for evaporation techniques and evaporators, drying fundamentals and types of dryers, spray dryers and equipment for separation and product recovery, with freeze drying and freeze concentration in Unit 1. ICAR AIEEA-PG Dairy Technology (group 17, Unit II) names principles of evaporation and drying, vacuum pans, atmospheric drum dryers, spray dryers and fluidisation. GATE XL Food Technology includes evaporation and drying under food engineering. Operating values are representative; equipment suppliers' data and plant trials govern real design.
Evaporation concentrates milk, whey and other dairy liquids by boiling off water. It is the main step before drying and the way evaporated and sweetened condensed milk, concentrated whey and many ingredient streams are made. Because milk proteins and lactose are heat-sensitive, dairy evaporators operate under vacuum so that the liquid boils at a low temperature. Water boils at 100 °C at atmospheric pressure (101.3 kPa) but at about 70 °C at around 31 kPa absolute and at about 45 °C at around 10 kPa. Operating at reduced pressure lowers the product temperature, reduces browning, protein denaturation, fouling and cooked flavour, and allows a cascade of effects at progressively lower pressures and temperatures. The vacuum is created and maintained by condensing the vapour from the last stage in a condenser cooled by water or a cooling tower, and by removing non-condensable gases (air leaking in and gases released from the product) with vacuum pumps or steam ejectors.
The design of an evaporator rests on mass and energy balances. The solids balance gives the amount of water to evaporate: if 10,000 kg of skim milk at 9 per cent solids is concentrated to 45 per cent, the concentrate is 900 ÷ 0.45 = 2,000 kg and the water evaporated 8,000 kg. The energy balance gives the heat needed: heat supplied by condensing steam = heat to raise the feed to boiling + latent heat to evaporate the water (the latent heat of vaporisation of water is about 2,257 kJ/kg at 100 °C and somewhat higher, about 2,330 kJ/kg, at 70 °C). The heat transfer rate in a heating surface is Q = U × A × ΔT, where U is the overall heat transfer coefficient (kW/m² K), A the heating area and ΔT the temperature difference between condensing steam and boiling liquid. For example, evaporating 2,000 kg/h of water at 70 °C needs about 2,000 × 2,330 ÷ 3,600 ≈ 1,294 kW; with U = 2.5 kW/m² K and ΔT = 10 K, the area is about 52 m². U depends on the evaporator type, the liquid's viscosity and fouling, and falls as the product concentrates.
Dissolved solids raise the boiling point of a liquid above that of pure water at the same pressure. For dilute milk the boiling-point elevation is small (a fraction of a degree), but it increases as solids rise, reaching a degree or more in concentrates of around 50 per cent solids and more in sugar-rich concentrates such as sweetened condensed milk. It reduces the useful temperature difference in each effect and must be included in design. As milk concentrates, its viscosity rises steeply (especially above about 40–50 per cent solids), heat transfer falls and the risk of fouling increases; whey proteins denature progressively, and calcium phosphate becomes supersaturated. These changes limit the maximum concentration achievable — usually around 45–55 per cent solids for skim milk and somewhat higher for whole milk — and affect the powder properties after drying. Concentrate viscosity also determines how well it can be atomised in the spray dryer.
The vacuum pan is the oldest type of dairy evaporator: a closed, usually spherical or cylindrical vessel with a steam jacket and often internal steam coils, connected to a condenser and vacuum system. Milk or other product is drawn in and boiled in batches at reduced pressure; vapour rises through an entrainment separator (to catch droplets of product) to the condenser. Vacuum pans are simple, flexible and suited to small volumes, viscous products and batch processes, and are still used for sweetened condensed milk, khoa and other traditional concentrates, dulce de leche, and products where a final 'striking' (finishing) step needs control. Their disadvantages are long residence time at elevated temperature, which causes more heat damage and browning than continuous evaporators, low steam economy because they are single-effect, and difficulty in automating. The concentration end-point is judged by sampling for total solids, density or viscosity.
Condensers for vacuum pans and evaporators are either direct-contact (barometric or jet condensers, in which cooling water sprays into the vapour and the mixed water leaves through a tall barometric leg or pump) or surface condensers (shell-and-tube or plate units that keep the condensate separate from the cooling water). Surface condensers are preferred where the condensate is to be recovered as clean water ('cow water'). Non-condensable gases must be removed continuously, or they blanket heat transfer surfaces and raise the pressure; steam-jet ejectors or liquid-ring vacuum pumps do this. Entrainment separators — baffles, cyclonic separators or mesh pads — prevent product droplets from being carried over into the condenser, which would lose product and contaminate the condensate.
A vacuum pan run starts with drawing an initial charge of preheated milk under vacuum, then feeding further milk continuously as water boils off, keeping the level above the heating surfaces to avoid scorching. Steam pressure in the jacket and coils is controlled to maintain steady boiling without foaming, which can carry product into the condenser; antifoam practices and correct vacuum levels help. For sweetened condensed milk, sugar syrup is drawn in during the later stage. As the end-point approaches, samples are tested by refractometer, hydrometer or viscometer, and the batch is 'struck' (discharged) when the target solids are reached, then cooled and, for SCM, seeded with lactose. Care of the vacuum pan includes regular cleaning of heating surfaces, checking the condenser and vacuum system for leaks, inspecting sight glasses and valves, and keeping the entrainment separator clean.
The falling-film evaporator is the standard continuous evaporator for milk and whey. Product is pumped to the top of a bundle of long vertical tubes (or between plates in plate evaporators) and spread by a distribution system so that it flows down the inner surface of each tube as a thin film. Steam condenses on the outside of the tubes; the film boils, and vapour and liquid travel down together and are separated at the bottom in a vapour separator. Because the film is thin and moves quickly, heat transfer is high even with small temperature differences, and the residence time in each pass is short (seconds to a minute), so heat damage is low. Falling-film evaporators can operate with temperature differences of only a few degrees, which allows many effects and efficient vapour recompression.
The most important design detail is even distribution: every tube must receive enough liquid to keep its whole surface wetted. If the film breaks, dry spots form on the hot wall, product burns on, fouling accelerates and the tube may block. Distribution plates, nozzles or perforated trays are used, and a minimum wetting rate (liquid flow per metre of tube perimeter) must be maintained, sometimes by recirculating part of the product over the same effect. Rising-film (climbing-film) evaporators, in which vapour carries the liquid upwards, and forced-circulation evaporators, which pump viscous product through heated tubes at high velocity, are alternatives for special cases, while scraped-surface evaporators handle very viscous products. Fouling — deposits of denatured whey protein and calcium phosphate — forms mainly in the hotter first effects and the finishing stages; it lowers U, raises steam use, and harbours thermophilic bacteria. Evaporators therefore run for limited periods (commonly up to about a day) before being cleaned in place with alkali and acid.
The warm sections of long evaporator runs, typically at temperatures in the range where thermophilic spore-forming bacteria grow (roughly 45–70 °C), can support their growth in deposits and biofilms. Species of Geobacillus and Anoxybacillus are important examples. Their spores survive pasteurisation and drying and appear in powders, where many buyers set strict limits for thermophilic and heat-resistant spores. Controls include limiting run length, preheating to destroy vegetative cells before the warm zones, designing out stagnant areas, adequate cleaning, and monitoring spore counts along the plant. This is a case where process engineering and microbiology meet directly.
In a single-effect evaporator, about one kilogram of steam is needed to evaporate roughly one kilogram of water, because the latent heat released by condensing steam is similar to that needed to evaporate water. In a multiple-effect evaporator, the vapour boiled off in the first effect is used as the heating steam for the second effect, which operates at a lower pressure and temperature so that the vapour can still condense there; the vapour from the second heats the third, and so on. Each kilogram of fresh steam therefore evaporates roughly as many kilograms of water as there are effects, minus losses for heating the feed, boiling-point elevation and heat losses. Steam economy (kilograms of water evaporated per kilogram of steam) is the key performance measure: for example, a three-effect evaporator might have an economy around 2.7, so evaporating 8,000 kg of water needs about 8,000 ÷ 2.7 ≈ 2,960 kg of steam. Dairy plants often use four to seven effects, limited by the total temperature difference available between the first-effect steam and the last-effect boiling temperature.
Effects can be fed forward (product flows in the same direction as the vapour, from the highest to the lowest pressure), backward (product flows against the vapour) or in mixed arrangements. Forward feed is common in dairying because the most dilute product is heated most and the most concentrated, viscous product boils at the lowest temperature, which reduces heat damage and fouling of the concentrate. The available temperature difference — from the first-effect steam temperature (limited by product heat sensitivity) to the last-effect temperature (limited by condenser cooling) — is shared among the effects; more effects means a smaller ΔT per effect and therefore more heating area per effect. The design therefore balances steam savings against capital cost of area. Preheating the feed with vapour from later effects and recovering heat from condensate improve efficiency further. Modern plants combine multiple effects with vapour recompression, described in the next chapter.
Consider concentrating 10,000 kg/h of skim milk from 9 to 45 per cent solids, which removes 8,000 kg/h of water. A single-effect evaporator would need roughly 8,000 kg/h of steam plus the steam to heat the feed. A three-effect evaporator with an economy of about 2.7 needs about 2,960 kg/h; a five-effect with TVR might reach an economy several times higher, and an MVR unit uses very little steam but several hundred kilowatts of electricity. These differences translate directly into fuel cost and carbon emissions. Further savings come from preheating the incoming milk with vapour or condensate from later effects, recovering heat from the hot concentrate and condensate, and using the warm condensate as clean process water after polishing. The condensate from the first effect (from boiler steam) returns to the boiler; the vapour condensates from later effects ('cow water') are recovered separately.
Vapour leaving an effect is at a lower pressure and temperature than the steam that heated it, so it cannot heat the same effect directly. Recompressing it raises its pressure and condensing temperature so that it can. In thermal vapour recompression (TVR), a steam-jet thermocompressor uses high-pressure motive steam to entrain and compress part of the vapour from an effect; the mixture is used as heating steam for the first effect. TVR has no moving parts and is cheap, and it improves steam economy roughly as much as adding one or more extra effects, depending on the steam pressure available and the compression ratio. The surplus vapour that is not recompressed passes on to the next effect. TVR is widely used in dairy evaporators, often on the first effect or across the first few effects.
In mechanical vapour recompression (MVR), an electrically driven fan or centrifugal compressor recompresses the vapour from the evaporator by a few degrees of saturation temperature and returns it as heating medium to the same evaporator. Because almost all the latent heat is recycled, fresh steam is needed only for start-up and a small make-up, and the energy input is mainly electricity for the compressor. MVR evaporators therefore have very low energy cost per kilogram of water evaporated, especially where electricity is cheap or renewable, and they reduce cooling water demand because little vapour goes to the condenser. Their design needs small temperature differences (because compressors raise the temperature only a few degrees), so they require large heating areas and suit falling-film evaporators. Many modern dairy evaporators are MVR units with a small TVR or steam-heated finisher for the final concentration. The choice between multiple effects with TVR and MVR depends on the relative prices of steam and electricity, capacity, available space and capital.
A dairy choosing an evaporator considers capacity, product range, energy prices, site utilities and capital. Where steam is cheap and electricity expensive, multiple effects with TVR are attractive; where electricity is cheap or renewable, MVR dominates. Large plants often combine MVR for the bulk of the evaporation with a steam-heated finisher that reaches the final concentration at higher temperature difference. Plate evaporators are compact and suit smaller capacities, while tubular falling-film units handle large flows. Products with high viscosity at the end (sweetened condensed milk, high-solids whey) may need forced-circulation or scraped-surface finishers. The evaporator must also integrate with the dryer: concentrate solids, temperature and viscosity at the dryer feed determine powder properties and dryer capacity, so evaporator and dryer are designed together.
Most dairy dryers use hot air both to supply heat and to carry away water vapour, so the properties of moist air (psychrometrics) govern their design. Key properties are the humidity ratio or absolute humidity (kilograms of water vapour per kilogram of dry air), relative humidity (the partial pressure of water vapour divided by the saturation vapour pressure at the same temperature, times 100), dry-bulb temperature, wet-bulb temperature (the temperature reached by a wet surface evaporating into the air, which is close to the adiabatic saturation temperature), dew point, and enthalpy. Heating air at constant humidity raises its temperature and lowers its relative humidity, increasing its capacity to absorb water; during drying, air cools and gains humidity along a line of nearly constant enthalpy. A psychrometric chart or equations relate these quantities. The water removed by the air equals the dry air flow multiplied by the increase in humidity ratio between inlet and outlet: for example, removing 433 kg/h of water with air whose humidity rises from 0.010 to 0.060 kg/kg needs about 433 ÷ 0.050 = 8,660 kg/h of dry air.
When a wet material dries in a stream of air, it first passes through a short warming-up period and then a constant-rate period, during which the surface is fully wet, evaporation is limited by heat and mass transfer from the air, and the surface stays near the wet-bulb temperature. When the moisture content falls to the critical moisture content, the surface can no longer be kept wet by internal movement of water, and the falling-rate period begins: the drying rate decreases, the particle temperature rises towards the air temperature, and internal diffusion of water controls the rate. In spray drying, milk droplets lose most of their water in a very short constant-rate period while their temperature stays near the wet-bulb temperature, which is why particles can meet hot inlet air without scorching; as a dry crust forms, drying slows and particle temperature rises, so the final moisture removal must be gentle. Final moisture is governed by the equilibrium moisture of the powder at the outlet air conditions, which is why outlet air temperature and humidity control powder moisture.
The spray dryer turns concentrate into powder in seconds. Atomisation, breaking the concentrate into droplets typically tens of micrometres across, creates the huge surface area needed for fast drying and largely determines particle size and powder properties. Rotary (wheel or disc) atomisers throw liquid from the rim of a wheel spinning at high speed; droplet size depends on wheel speed and diameter, feed rate and viscosity, and they tolerate variations in feed rate and viscosity well, but the horizontal spray requires a wide chamber. Pressure nozzles force concentrate at high pressure through a small orifice with a swirl insert, producing a hollow-cone spray; droplet size depends on pressure, orifice size and viscosity, and nozzles give coarser, denser particles with less occluded air and suit tall, narrower chambers and agglomerated instant powders. Two-fluid nozzles use compressed air or steam to atomise and are used for small dryers and difficult products. Concentrate viscosity strongly affects atomisation, so it is kept low by controlling solids and temperature, and concentrate heaters just before the atomiser are common.
Hot air, heated indirectly by steam coils or directly or indirectly by gas burners, enters through an air distributor that creates a controlled flow pattern around the atomiser. Most dairy dryers use co-current flow, in which droplets and the hottest air travel in the same direction, so the wettest droplets meet the hottest air and are cooled by evaporation, while dry particles meet cooler air. Inlet temperatures are commonly in the range of about 180–250 °C, outlet temperatures around 70–95 °C, depending on product and dryer type. Chamber size is set by the residence time needed and the spray pattern, to avoid wet particles hitting walls. Powder leaves the chamber bottom and the air leaves through a duct to the powder separation system.
In single-stage drying the chamber removes all the water, which requires a high outlet temperature and gives fine, dusty, heat-damaged powder. Two-stage drying finishes drying in an external vibrating fluid bed, allowing the chamber to run at a lower outlet temperature with the powder leaving it at higher moisture; this improves energy efficiency, reduces heat damage and allows agglomeration. Three-stage (multistage) dryers add an integrated static fluid bed in the cone of the chamber, where moist particles are agglomerated and partly dried, followed by an external fluid bed for final drying and cooling; they give agglomerated powders with good functional properties at high thermal efficiency and allow very low outlet temperatures. Fines collected by cyclones or bag filters are returned to the atomisation zone or the fluid bed to build agglomerates. The specific energy of a spray dryer — several megajoules per kilogram of water evaporated, well above the latent heat alone — is much higher than that of a multiple-effect or MVR evaporator, which is why plants concentrate as far as possible before drying.
What happens to a droplet in the first fractions of a second of drying fixes much of the powder's character. As water evaporates from the surface, solids concentrate there and a skin or crust forms; proteins and surface-active components accumulate at the surface, and fat may migrate there in whole milk. If the crust forms early and the interior still contains water and air, the particle may expand or form hollow centres with occluded air, lowering particle density and bulk density. Higher feed solids and lower inlet temperature generally give denser particles with less occluded air; air whipped into the concentrate before atomisation increases occluded air. Larger droplets give larger particles and better flowability but need longer drying. Outlet temperature and moisture determine whether the amorphous lactose is above or below its glass transition, and therefore whether particles stick to walls, agglomerate or remain free-flowing. Powder engineers therefore adjust concentrate solids, viscosity, atomiser settings, air temperatures and fluid-bed conditions together to meet specifications for moisture, bulk density, particle size, solubility and instant properties.
Drum (roller) dryers dry product as a thin film on the outside of internally steam-heated rotating cylinders. In double-drum dryers, two drums rotate towards each other and the feed is introduced into the nip between them, where the gap sets film thickness; in single-drum dryers, product is applied by dipping, splashing or applicator rolls. Steam inside the drums condenses at a pressure that gives a surface temperature well above 100 °C, the film dries within a fraction of a revolution, and a doctor blade scrapes off the dried sheet, which is then broken, milled and sieved. Capacity is proportional to drum surface area and the drying rate per square metre, which depends on steam pressure, film thickness, drum speed and feed solids. Vacuum drum dryers enclose the drums in a vacuum chamber to dry at lower temperatures for heat-sensitive products.
Drum dryers are simple, compact, energy-efficient per kilogram of water (heat is transferred by conduction directly from steam to film) and handle viscous or pasty feeds. Their main disadvantage for milk is heat damage: whey protein denaturation, Maillard browning and poor solubility, so roller-dried milk powder reconstitutes poorly, although its high free fat and cooked flavour are valued in milk chocolate. They are used today mainly for chocolate crumb-type products, some infant cereals, feed products, and starch-based and other non-dairy materials. Their properties and uses as milk powders are described in concentrated-dried-milks.
A fluid bed is a layer of particles through which air is blown upwards at a velocity high enough to support the particles, so that the bed behaves like a boiling liquid. Above the minimum fluidisation velocity the particles separate and mix vigorously, giving excellent heat and mass transfer between air and every particle. Velocity must stay below the terminal velocity of the particles, or they are carried out with the air. Dairy powders are cohesive and sticky when moist, so plain fluid beds do not fluidise them well; vibrating fluid beds, in which the perforated bed plate is vibrated, move the powder along and help fluidisation even at lower air velocities, and are used for drying, cooling, agglomeration and lecithination after the spray dryer. Static fluid beds integrated in the cone of a spray dryer receive moist powder directly from the chamber.
In fluid beds, moist, sticky particles collide and stick to form agglomerates, which are then dried and cooled to fix them. Agglomeration is promoted by returning fines to zones where particles are moist, by spraying liquid onto the bed, or by steam; the process is controlled through moisture, temperature, air velocity and residence time to reach the target particle size. Fluid beds are divided into sections at different air temperatures: drying sections with warm air and cooling sections with cool, dehumidified air, so that powder leaves cool and does not cake in bags. Lecithin dissolved in butter oil can be sprayed onto whole milk powder in a fluid bed to make it instant. Fluid beds also allow gentle final drying of heat-sensitive products and the post-crystallisation of lactose in whey powders. Air leaving fluid beds carries fines that are recovered by cyclones or bag filters and returned.
Fluid-bed performance depends on air velocity (above minimum fluidisation and below entrainment), air temperature and humidity in each section, bed depth and residence time (set by the weir height and vibration), and the stickiness of the powder. Too hot or humid air in the drying sections makes powder sticky and lumpy; too cold air in cooling sections can lead to condensation if it is not dehumidified. Plate perforation design affects air distribution and powder transport. Operators adjust these parameters to reach the target moisture and particle size, and monitor the bed for channelling, dead zones and deposits, which are both quality and fire risks.
Freeze drying (lyophilisation) removes water by sublimation from the frozen state, avoiding the high temperatures and liquid-phase reactions of other drying methods. The product is frozen, then placed in a chamber under a high vacuum, below the triple point of water (611 Pa, 0.01 °C), and heated gently so that ice sublimes directly to vapour, which is collected on a very cold condenser. Primary drying removes the frozen water by sublimation; secondary drying removes bound water by desorption at slightly higher temperature. The latent heat of sublimation (about 2,830 kJ/kg) must be supplied by conduction or radiation through the dried layer, which is slow, so freeze drying takes hours to days and is expensive in capital and energy. It is used in dairying for starter and probiotic cultures (with cryoprotectants such as skim milk, sugars and polyols to protect cells), for colostrum, high-value bioactive proteins and enzymes, and for some speciality products where structure, flavour and biological activity must be preserved.
Freeze concentration removes water as ice crystals rather than as vapour. The liquid is partly frozen under controlled conditions so that pure ice crystals grow in the remaining, increasingly concentrated solution; the crystals are then separated, commonly in wash columns where they are washed with melt water to recover entrained concentrate. Because no evaporation occurs, volatile aroma compounds are retained and heat damage is avoided, which makes freeze concentration attractive for high-quality concentrates such as fruit juices, coffee extracts and some dairy products. However, the maximum concentration is limited (viscosity rises and crystallisation becomes difficult at high solids), capital costs are high, and throughput is lower than evaporation, so it remains a niche process in the dairy industry. Block freeze concentration and progressive freeze concentration are simpler variants studied for small-scale use.
For starter and probiotic cultures, freeze drying is preceded by growing the cells to the right physiological state, concentrating them by centrifugation, and mixing them with a cryoprotectant medium that protects membranes and proteins during freezing and drying. Freezing rate affects ice crystal size and cell survival. After drying, the product is packed in moisture- and oxygen-proof containers, often under nitrogen, and stored cold; survival during storage depends on residual moisture, water activity and temperature. Spray drying of cultures at low outlet temperatures is cheaper and is used for some robust strains, but freeze drying remains the standard for sensitive organisms.
The exhaust air from spray dryers and fluid beds carries fine powder that must be recovered, both for yield and to meet emission limits. Cyclones are the classic separators: air enters tangentially into a cylindrical-conical body, spins, and the centrifugal force throws particles to the wall, from where they slide to the bottom outlet, while the cleaned air leaves through a central vortex finder at the top. Cyclone efficiency depends on particle size, density and the cyclone geometry and velocity: large particles are almost all collected, very fine ones partly escape, and the 'cut size' (the particle size collected with 50 per cent efficiency) characterises the cyclone. Efficiency is calculated as powder collected ÷ powder entering × 100. Higher velocity gives better separation but higher pressure drop and energy use, and can break agglomerates. Cyclones are robust and simple but rarely sufficient alone to meet modern emission limits.
Bag filters pass the air through fabric filter bags that retain fine particles, with very high efficiency. The collected powder cake is removed periodically by reverse air pulses (pulse-jet cleaning) and falls to the hopper. Modern sanitary bag filters for food powders are designed for cleaning in place, with washable, food-grade bags and housings, and the recovered fines are returned to the process. Bag filters must be protected against condensation (which wets and blinds the bags), fires and explosions (they contain fine dust and are a common site of incidents, so they are fitted with vents or suppression). Wet scrubbers, which wash the exhaust air with water or concentrate, are an alternative or final stage; they also recover heat but produce a liquid stream. Combinations — cyclones followed by bag filters, or integrated filters in the dryer — are chosen to balance recovery, emissions, hygiene and safety. Exhaust air heat recovery (air-to-air or air-to-liquid heat exchangers) improves overall energy efficiency.
Evaporation and drying are among the largest energy users in a dairy. Removing water by multiple-effect or MVR evaporation needs only a small fraction of the energy that a spray dryer needs per kilogram of water, so the general rule is to remove as much water as possible before the dryer — by RO, evaporation and high concentrate solids — limited by concentrate viscosity and product quality. In the dryer, energy use is reduced by lowering outlet temperature (made possible by multistage drying), recovering heat from exhaust air to preheat inlet air, using indirect heating with efficient heat exchangers, insulating chambers and ducts, minimising air leaks and reducing the air-to-product ratio. The specific energy of a dryer is calculated as heat input ÷ water evaporated, for example 2,200 kW evaporating 1,000 kg/h of water is about 7.9 MJ per kilogram, several times the latent heat. Energy audits track steam, electricity and gas per tonne of powder and identify savings; renewable energy sources and heat pumps are increasingly used.
Spray dryers, fluid beds, cyclones and bag filters handle combustible dust and hot air, so fire and explosion safety is integral to their design and operation. Hazards arise from powder deposits self-heating on hot surfaces, smouldering nests igniting powder clouds, sparks, static electricity and hot work. Safety measures include preventing deposits through design and cleaning, controlling inlet and outlet temperatures, monitoring carbon monoxide in exhaust air as an early warning, explosion vents on chambers, cyclones and filters, explosion suppression and isolation systems, fire detection and extinguishing systems, and procedures for safe shutdown and response. Hazardous area classification (such as ATEX zoning in Europe) determines equipment requirements. Product-related risks — powders with high fat or sugar have lower ignition temperatures — are covered in more detail in concentrated-dried-milks. Operator training and regular inspection are as important as hardware.
Modern evaporators and dryers are controlled by programmable controllers and distributed control systems. In evaporators, key variables are feed flow, steam pressure or compressor speed, effect temperatures and pressures, and concentrate solids, measured in line by density or refractive index and used to adjust feed or steam. In spray dryers, the outlet air temperature is the main controlled variable: it is held constant by adjusting the concentrate feed rate (more feed cools the outlet) or the inlet temperature, which keeps powder moisture steady. Other variables include atomiser speed or nozzle pressure, air flows, fluid-bed temperatures, and exhaust humidity. Interlocks and alarms protect against overheating, loss of feed (which lets the outlet temperature rise sharply), high carbon monoxide and high differential pressure across filters. Data from these systems also support energy monitoring, traceability and continuous improvement.
A student uses this subject to do evaporator and dryer mass and energy balances, explain steam economy, TVR and MVR, use psychrometric relations, and compare atomisers and dryer configurations. It supports AICE Dairy Engineering Unit 2, AIEEA-PG group 17 Unit II and GATE XL food engineering.
An evaporator and dryer operator or engineer five years in runs falling-film evaporators and multistage spray dryers, sets concentrate solids and outlet temperatures for moisture and bulk density, plans run lengths and CIP to control fouling and thermophilic spores, tracks steam economy and dryer specific energy, and follows fire-safety routines. This subject gives the engineering behind those decisions.
A plant head or engineering director decides on evaporator and dryer investment (MVR or TVR, number of effects, dryer stages and capacity), energy strategy, water recovery and safety systems. These are among the largest capital and energy decisions in a dairy.
A professor or research director works on drying kinetics, particle formation, stickiness and glass transition, fouling and spore control, energy integration and novel drying and concentration methods. They examine AICE Dairy Engineering on these topics and advise industry on process design.
This manual does not yet give: detailed steam tables and psychrometric charts; full design procedures for multiple-effect evaporators, thermocompressors and MVR compressors; spray-dryer chamber sizing calculations; cyclone efficiency correlations; and specific national rules on dust explosions and emissions. Temperatures, pressures, economies and energy figures are representative examples, and supplier data govern real plants.
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