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Plant and Equipment | Milk Dryers: Dryer Design
Milk Dryers: Dryer Design M Skanderby, GEA Niro A/S, Soeborg, Denmark ª 2011 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by V. Westergaard, Volume 2, pp 871–889, ª 2002, Elsevier Ltd.
Introduction This article focuses on spray dryers. Although other means of drying are possible, this is by far the most common in the dairy industry. By definition, spray drying is the transformation of a product from a fluid state into a dried form by spraying the liquid feed into a hot drying medium. The feed can be a solution, a suspension, or a paste, depending on the characteristics of the dairy product to be dried. The dried product is a powder consisting of single particles or agglomerates, all depending on the chemical composition and physical properties of the feed as well as on dryer design and operation.
Drying Principles A spray dryer operates in the following way: The feed is pumped from the product feed tank to the atomizing device that is situated in the air disperser at the top of the drying chamber. The drying air is drawn from the atmosphere via a filter by a supply fan and is passed through the air heater to the air disperser. As the atomized droplets meet the hot air, evaporation takes place cooling the air at the same time. After the drying of the atomized feed in the chamber, the majority of the dried product falls to the bottom for further processing. The fines, which are the particles with a small diameter, will remain entrained in the air. Therefore, the air has to pass through powder collectors like cyclones or bag filters. The air passes from the powder collector to the atmosphere via the exhaust fan. The two fractions of powder are collected, for example, in a pneumatic system for conveying and cooling. After separation in a cyclone, the powder is bagged off. A conventional spray dryer consists of the following main components (Figure 1): 1. 2. 3. 4. 5. 6.
Drying chamber Hot air system and air distribution Feed system Atomizing device Powder separation system Pneumatic conveying and cooling system
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7. Integrated fluid bed 8. Fluid bed after-dryer/cooler
Drying Chamber Various designs of the drying chamber are available on the market. The most common one is a cylindrical chamber with a cone of 40–60 , enabling the powder to leave the chamber by gravity. The chamber is also found with a flat bottom in which case a scraper or suction device is needed for removing the powder fraction from the chamber. Horizontal box-type drying chambers are also used, and they, too, operate with a forced (i.e., scraper or screw) powder removal system (Figure 2). Generally, it can be concluded that chambers with a cone for gravity discharge of the powder give the best flexibility for adapting various drying processes like integrated fluid beds or belts to the plant and therefore offer the greatest possibility for drying different products. The tendency in modern designs of drying chamber is to avoid any object inside the chamber that can obstruct the air flow. In the chamber of the TALL FORM, the emphasis has been put on designing a plant with a laminar air flow and a special air outlet system, where the diameter of the cone is bigger than the diameter of the cylindrical part thus forming a ring duct termed ‘bustle’. This minimizes the cyclone fraction by the low velocity of the exhaust air. This chamber is especially suited for infant milk formulae or protein products dried from low-solid content feed. The drying chamber should always be equipped with inspection doors and overpressure vents to withstand a pressure of 1.6 mbar(g). Other safety equipment such as fire extinguishing equipment in the form of water or steam nozzles is always standard in a modern dryer. Drying chambers are usually insulated, either with removable air-filled sandwich panels (see Figure 3) or with 80–100 mm mineral wool covered with a stainlesssteel plate. The advantage of the removable panels is that inspection for cracks in the chamber wall is possible. Furthermore, the risk of having wet insulation material, which can foster bacterial development or cold spots on the chamber wall, is eliminated.
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Ring-formed fluid bed (compact drying chamber) 4
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Figure 1 Spray drying plant. 1, Drying chamber; 2, Hot air system and air distribution; 3, Feed system; 4, Atomizing device; 5, Powder separation system; 6, Pneumatic conveying and cooling system; 7, Integrated fluid bed; 8, Fluid bed after-dryer/ cooler.
Integrated Static Fluid Bed In an attempt to improve the drying efficiency, a static fluid bed is integrated in the drying chamber. The secondary drying air, typically 25% of the main drying air, is introduced into a plenum chamber below a perforated plate, through which the drying air is distributed. This type of dryer can be operated in such a way that the primary particles reach a moisture level higher than that obtained by using the VIBRO-FLUIDIZER. A specially designed and patented perforated plate, the BUBBLE PLATE (see Figure 4), provides an air– powder mixture that ensures optimal drying without attrition and powder penetration into the clean-air plenum. Furthermore, the BUBBLE PLATE has a more sanitary finish than the other types of perforated plates. The static fluid bed is available in two configurations: fluid bed (compact drying chamber) • Ring-formed Circular fluid bed (multistage drying (MSD) chamber) •
The ring-formed back-mix bed is placed at the bottom of a conventional chamber cone around the exhaust duct placed in the center. The powder is discharged continuously from the static fluid bed by overflowing an adjustable powder weir, thus maintaining a certain level of fluidized powder. When the powder leaves the drying chamber it may be cooled in a pneumatic conveying or VIBRO-FLUIDIZER system. The resulting powder will consist of single particles. For fat-containing products, cooling should be done in a vibrating fluid bed that is also used when agglomerated powders are produced. In this case, the cyclone fraction is returned to the atomizer device for agglomeration (see Figure 5). Circular fluid bed (multistage drying (MSDTM) chamber)
To improve the dryer efficiency and the powder properties even further, the multistage dryer MSD has been designed (see Figure 6). The dryer operates with three drying stages, each adapted to the moisture content prevailing during the drying process. In the preliminary drying stage, the concentrate is atomized by co-current nozzles or a rotary atomizer placed in the hotdrying air duct. Air enters the dryer vertically through the air disperser, ensuring optimal mixing of the atomized droplets with the drying air. The particles reach a moisture content of 6–15%, depending upon the type of product. The fluid bed is supplied with air at a sufficient velocity and temperature for the second-stage drying. The drying air from the preliminary drying stage and the integrated fluid bed leaves the chamber from the top passing through powder separators. This type of dryer offers a perfect choice if the aim is to produce an agglomerated product. Owing to the velocity of the primary drying air, a venturi is formed around the atomizing device, thus sucking in secondary air with powder entrained so that agglomeration is facilitated, that is, attrition between the primary spray particles and the fines powder. When the powder has reached a certain moisture content it is discharged via a rotary valve into a VIBROFLUIDIZER for the final drying and subsequent cooling. The powder exhibits a coarse powder structure originating from the natural agglomeration taking place in the chamber.
Hot Air System and Air Distribution Air Filtration System Until a few years ago, no special requirements were placed on filtration of the process air for the spray drying process. Today, however, very strict requirements are presented by local authorities to ensure a cleaner
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Drying air Cooling air Concentrate/product Fines Figure 2 Different types of drying chambers. 1, TALL FORM dryer type seen in Japan. It is equipped with a low-velocity air disperser, and, although it is used for milk, it is not very suitable for this product; 2, TALL FORM dryer with high-temperature primary drying air. Secondary air is sucked into the drying chamber during the drying operation. The ‘mix’ air temperature is similar to that of a normal spray dryer; 3, Conventional TALL FORM DRYER chamber used predominantly for baby food and protein products; 4, Conventional drying chamber with conical bottom; 5, Box dryer for one-stage drying only – poor economy and normally seen only in the United States; 6, Multistage MSD drying chamber with integrated fluid bed; 7, Conventional COMPACT drying chamber with integrated fluid bed; 8, Flat-bottom spray drying chamber for one-stage drying only. No longer seen on new installations; 9, FILTERMAT drying chamber in a special design for very sticky products.
operation and a higher level of food safety. Common for the different standards are as follows: air should be prefiltered and supplied by a separate • The fan to the fan/filter/heater room, which must be under
•
pressure to avoid the entry of unfiltered air (Figure 7). As an alternative to a fan room, complete ducting of all air flows is fully acceptable. Filtration degree and filter position depend on the final temperature of the process air as follows: – For air to be heated above 120 C only coarse filtration up to 90% (filter class EU7/F7) is needed. The filter should be placed on the pressure side of the fan.
– For air to be heated below 120 C or not heated at all, the filtration must be 95% (filter class EU/F9) or above, and the filter must be placed after the heater/ cooler. Some countries and companies have even stricter requirements demanding a filtration of up to 99.995% (filter class EU13-14/ H13-14).
Air Heating System The drying air can be heated in different ways: indirect (steam/oil/gas/hot oil/electricity) or direct (gas).
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Figure 3 Removable insulation panels for spray drying chambers.
Figure 4 BUBBLE PLATE.
Indirect heating
A steam heater is a simple radiator. The temperature to be obtained depends on the steam pressure available. The air heater consists of rows of finned tubes housed in an insulated metal case. The heat load is calculated from the quantity and specific heat of the air. The heater size depends on the heat transfer properties of the tubes and fins and is usually about 50 kcal C1 h1 m3 for an air velocity of 5 m s1. To avoid corrosion of the tubes in the air heater, use of stainless-steel tubes is recommended. In indirect oil and gas heaters, drying air and combustion gases have separate flow passages. The combustion
gases pass through galvanized tubes, which act as the heat transfer surface for the drying air. The combustion chamber is made of heat-resistant steel. Heaters of this type will have an efficiency of about 85% in the range of 175–250 C (see Figure 8). Hot oil liquid phase air heaters are used either alone or to boost the inlet drying air temperature when the steam pressure is not high enough. The heater system consists of a heater, which can be gas- or oil-fired, and an air heat exchanger. Between these two components, a special food-grade oil or heat transfer fluid, which does not crack at high temperatures, is circulated at high speed.
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Air
Figure 7 Filtration of process air.
Figure 5 Compact spray dryer with VIBRO-FLUIDIZER as agglomerator/instantizer (CDI).
as boosters instead of, for example, hot oil liquid phase air heaters. Direct heating
Direct gas heaters are used only when the combustion gas can be allowed to come into contact with the product. They are, therefore, not common in the dairy industry. Direct gas heaters are inexpensive, they have a high efficiency, and the obtainable temperature can be as high as 2000 C. When a plant is designed with an air heater with direct combustion, it is necessary to calculate the amount of vapor resulting from the combustion (44 mg kg1 dry air C1), as this will increase the humidity of the drying air. The outlet temperature has therefore to be increased to compensate for this increase in humidity and to maintain the relative humidity. The heater system can be designed with separate heaters for each consumption point or with fewer heaters, of which some supply two or more consumption points. By mixing warm air from the main air heater with cold air, the entire dryer can be run with only one heater. Figure 6 Multistage spray dryer (MSD).
Air Distribution System
The main advantage of a hot oil liquid phase heater is that it is an open, pressureless system. Electrical air heaters have for many years been used mainly for laboratory and pilot plant spray dryers. This heater has low investment costs, but previously the operation costs were considered to be too high for commercial production. However, as the price of electricity in certain parts of the world can be very low during off-peak periods, it is becoming more common to use electrical heaters
Drying-air distribution is one of the most vital functions in a spray dryer. There are various systems depending on the plant design and the type of product. The most common system is where the air disperser is situated on top of the dryer ceiling, and the atomizing device is placed in the middle of the air disperser, thus ensuring an optimal mixing of the air and the atomized droplets. In cylindrical vertical dryers, the whole ceiling may be perforated, thus creating a plug-flow air stream – numerous nozzles are situated in the perforated plate to
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Indirect steam-heated air heater
Cold air in Hot air out
Oil Combustion air
Figure 8 Indirect air heaters.
ensure that the air is cooled by the concentrate. This system, however, operates with a low air velocity, and it makes fines return complicated. It is therefore not suitable for all dairy products. It should be noted that an air disperser should have the ability to guide the air and the atomized droplets in the right direction to avoid deposits in the drying chamber. Two different types of air dispersers are currently used in spray dryers for food and dairy products:
Concentrate
Drying air
Cooling air
Rotary air stream
The air enters tangentially into a spiral-shaped distributor housing (see Figure 9), from where the drying air is
Figure 9 Ceiling air disperser with adjustable guide vanes.
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led radially and downward over a set of guide vanes provided for adjustment of air rotation. This type of air disperser is used for rotary atomizers and nozzle atomizers placed in the center of the air disperser and is used in conventional drying chambers.
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Plug flow air stream
The air enters radially through one side (see Figure 10) and is distributed through a specially designed air guiding arrangement, which ensures a uniform air flow pattern in the entire air disperser area. This enables a very precise, laminar, high-velocity plug flow, which is required in TALL FORM dryers or multistage dryers. This type of air disperser is used for nozzle atomizers only, and has excellent possibilities for nozzle position adjustment and thereby the adjustment of agglomeration structure.
Feed System The feed system (see Figure 11) is the link between the evaporator and the spray dryer, and comprises the following: 1. Feed tanks 2. Water tank
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3 Figure 11 Feed system. 1, Feed tanks; 2, Water tank; 3, Concentrate pump; 4, Preheating system; 5, Filter; 6, Homogenizer/high-pressure pump; 7, Feed line, including return line for CIP.
3. 4. 5. 6. 7.
Concentrate pump Preheating system Filter Homogenizer/high-pressure pump Feed line, including return line for cleaning-in-place (CIP)
Feed Tanks
Fines
Cooling air
Drying air
If feed tanks are used, the use of two tanks is recommended so as to change from one to the other at least once an hour to avoid the risk of bacteria growth. One is therefore in use while the other one is being cleaned. The size of each tank should correspond to 15–30 min of the feed capacity of the dryer. The feed tanks are very often omitted, and the last-stage evaporator is designed as a buffer tank under vacuum. The evaporator then operates as a ‘slave’ to the dryer, because the level switches in the evaporator buffer tank control the inlet feed to the evaporator. Water Tank The water tank is used during the start and stop of the plant, and during the run if there is a sudden shortage of concentrate. It is used only when the evaporator is used as a buffer tank. As an alternative, a direct water supply to the feed line is often used. Concentrate Pump
Concentrate
Figure 10 Plug flow air disperser.
If a rotary atomizer is used, the most common feed pump is the mono type, as it has lower energy consumption and can handle concentrates of high viscosity. In plants equipped with homogenizers for the production of whole milk powder, the homogenizer is used as a feed pump. In plants equipped with high-pressure nozzles, a high-pressure pump is used – often combined with a homogenizer.
Plant and Equipment | Milk Dryers: Dryer Design
Preheating System Preheating of the concentrate to a higher temperature than that coming from the evaporator is advantageous, not only from a bacteriological point of view. It also produces a decrease in viscosity, which together with the applied calories results in a capacity increase of the spray dryer and an improved solubility of the powder produced. The heating can be either indirect or direct. Indirect preheaters may be of the following types: 1. Spiral-tube heat exchanger 2. Plate heat exchanger 3. Scraped-surface heat exchanger Spiral-tube heat exchanger
The spiral-tube heater (see Figure 12), often with corrugated tubes, is able to heat a concentrate with high solid content to a higher temperature without frequent scaling and cleaning owing to high product velocity and a low T throughout the heater. Furthermore, this type of heater has no moving parts; hence, maintenance costs are minimized.
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temperatures are required. They can operate continuously for 20 h and are cleaned together with the remaining feed system. The disadvantages are high cost of maintenance as well as big variation in holding time. Direct preheaters may be of the following types: steam injection (DSI) • Direct Lenient steam injection (LSI) • Direct steam injection
In the DSI unit, steam is introduced into the milk concentrate via a nozzle, producing relatively big steam bubbles resulting in a superheating of some parts of the concentrate, which leads to protein denaturation. Lenient steam injection
In the LSI unit, steam is mixed into the concentrate by a dynamic mixer. Very small steam bubbles are created, and superheating/denaturation is avoided. Therefore, a much higher steam pressure can be used. The LSI unit can be used in combination with the spiral-tube heat exchanger if temperatures above 80 C are required in the concentrate. Filter
Plate heat exchanger
A plate heat exchanger system is inexpensive, but if the concentrate should be heated to >70 C, if the solid content is >46%, or if a 20 h run is aimed at, it is necessary to have two interchangeable heaters allowing one to be cleaned while the other is being used. Steam or warm water can be used as the heating medium. Scraped-surface heat exchanger
In the scraped-surface heater, the heat transfer surface is continuously scraped off by a fast-rotating scraper made of food-grade synthetic material to avoid any product adherence. The scraped-surface heater is especially suitable for products with high solid content and when high
An in-line filter is always incorporated in the feed system after the heater to avoid lumps, etc., passing to the atomizing device. Homogenizer/High-Pressure Pump If whole milk powder is to be produced, it is recommended that a homogenizer be incorporated to reduce the free-fat content in the final powder. A two-stage homogenizer is preferred; the first stage is operated at 50–100 bar g, and the second stage at 25–50 bar g. Usually the homogenizer and feed pump are combined in one unit. If nozzle atomization is used, then a higher pressure (up to 250 bar g for the nozzles þ 150 bar g for homogenizing) is required, and a combined homogenizer/ high-pressure pump is chosen. Temperatures of 80 C are needed to produce a whole milk powder with a good coffee stability. In view of calcium phosphate precipitation – which is abrasive – the pistons should be made of a ceramic material. Feed Line
Figure 12 Spiral-tube heat exchanger.
The feed pipe should be of stainless steel and, of course, of the high-pressure type if atomization is to be carried out by means of nozzles. The dimensions of the pipe should be such that the feed velocity is 1.5 m s1. In a feed system, a return pipe should also be included for the cleaning solution, so that the entire equipment can be cleaned thoroughly.
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Atomizing Device The aim of atomizing the concentrate is to provide a very large surface from which evaporation can take place. The smaller the droplets, the bigger the surface and the easier the evaporation; and thus a better thermal efficiency of the dryer is obtained. The ideal from the point of view of drying would be a spray of drops of the same size, which would mean that the drying time for all particles would be the same to obtain an equal moisture content. As mentioned previously, air distribution and atomization are the factors key to the successful utilization of the spray dryer. Atomization is directly responsible for many distinctive advantages offered by spray drying: First, the very short drying time of the particles; second, a very short particle retention time in the hot atmosphere and a low particle temperature (wet bulb temperature); and, finally, the transformation of the liquid feed into a powder with long-storage stability ready for packing and transport. In summary, the primary functions of atomization are
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a high surface-to-mass ratio resulting in a • tohighcreate evaporation rate create particles of the desired shape, size, and • todensity
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1. Nozzle body 2. Orifice insert 3. Swirl chamber 4. End plate 5. Screw pin
1 Figure 13 High-pressure nozzle ‘Delavan’.
To comply with these requirements many atomization techniques have been used in spray dryers. However, the most common ones can be summarized as follows:
• • •
pressure nozzles using pressure forces two-fluid nozzles using kinetic forces rotating discs using centrifugal forces
Pressure Nozzle Atomization The basic function of pressure nozzles is to convert the pressure energy supplied by the high-pressure pump into kinetic energy in the form of a thin film, the stability of which is determined by the properties of the liquid such as viscosity, surface tension, density, and quantity per unit of time, and by the medium into which the liquid is sprayed. Most of the commercially available pressure nozzles are designed with a swirl chamber giving the liquid a rotation, so that it will leave the orifice as a hollow cone (see Figure 13). Capacity can usually be assumed to be directly proportional to the square root of pressure retain: Capacity ðkg h – 1 Þ ¼ K
pffiffiffi P
As a rule of thumb, higher viscosity, liquid density, and surface tension, and lower pressure will result in bigger particles. Typically, a feed rate of 1000–1500 kg h1 per
nozzle is used in industrial dryers. The advantages when using high-pressure nozzles are as follows: with a low level of occluded air • Powder with a high bulk density • Powder flowability, especially for whole milk • Improved to form less deposits in the drying chamber • Tendency when difficult products are produced • Ability to produce big particles Two-fluid nozzle or pneumatic atomization
The energy available for atomization in two-fluid atomizers is independent of liquid flow and pressure. The necessary energy (kinetic) is supplied by compressed air. Two-fluid atomization is the only successful nozzle method for producing very small particles, especially from highly viscous liquids. It is not normally used in the drying of milk products. However, it is often used in secondary systems such as lecithin application to the powder. Rotary atomization
In rotary atomizers the liquid is accelerated continuously to the wheel’s edge by the centrifugal forces produced by the rotation of the wheel. The liquid is distributed centrally, then extended over the wheel surface in a thin sheet and discharged at a high speed at the periphery of
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Viscosity of the liquid
Droplet size varies directly with viscosity, and bigger particles are obtained when the viscosity of the feed is higher. To ensure an optimal atomization, the viscosity is therefore normally kept as low and as constant as possible, often by heating the concentrate prior to atomization. Regarding droplet size distribution, it becomes broader with increased viscosity – an effect sometimes used when bulk density of the powder is to be increased. Rotary atomizer has been known and used in the dairy industry for many years; its main advantages are as follows: in throughput • Flexibility Ability to handle quantities • Ability to handle large highly concentrates • Different wheel designsviscous giving different powder • characteristics to handle products containing crystals • Ability Ability handle higher solid content in the feed; • therefore,to better economy
Figure 14 Rotary atomizer with direct drive.
the wheel. The degree of atomization depends upon peripheral speed, properties of the liquid, and feed rate (Figure 14). To select an optimal atomizer wheel, the following factors should be taken into consideration: Liquid feed rate
Droplet size varies directly with feed rate at a constant wheel speed and will increase with increased feed rate. Peripheral speed
The peripheral speed depends on the diameter of the wheel and the wheel speed, and is calculated as follows: Vp ¼
DN 1000 60
where Vp is the peripheral speed (m s1), D the diameter of the wheel (mm), and N the speed of the wheel (rpm). Peripheral speed is widely accepted as the main variable for the adjustment of droplet size. However, it has been shown that droplet size does not necessarily remain constant when equal peripheral speeds are produced in wheel designs of various diameter–speed combinations, as there is a tendency for bigger wheels to produce bigger particles, all other things being equal. However, in the choice of wheel diameter, one should rather look at the reliability of the atomizer, as the differences in spray characteristics are negligible.
To decide whether to use a pressure nozzle or rotary wheel is therefore a question of achieving the demanded properties of the final dried product, given the properties of the feed.
Powder Separation System As the drying air leaving the chamber will contain a small proportion of the powder (10–30%), it is necessary to clean it by separating the powder particles. This powder fraction is usually referred to as the ‘fines’, as they normally represent the smallest particles. The most widely used separators in the milk powder industry are
• Cyclone filter • Bag CIP-able bag filter • Wet scrubber • Combinations of the above • Cyclone Cyclone has some obvious advantages, such as high efficiency, if constructed properly. It is easily maintained, as there are no moving parts. Furthermore, it is easy to clean if the construction is with a fully welded center cyclone. The operation theory is based on a vortex motion where the centrifugal force is acting on each particle and therefore causes the particle to move away from the cyclone axis toward the inner cyclone wall. However, the movement in the radial direction is the result of two opposing forces where the centrifugal force acts to move
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compromise is sought with bigger sizes at the expense of high efficiency. Thus, the cyclones have become bigger and bigger and are now constructed with diameters of up to 4.0 m. When designing a cyclone, various key factors should be taken into account to obtain the highest efficiency. This is achieved if
Air
cyclone diameter 3 exit duct diameter
Air with powder
cyclone height 10 exit duct diameter Inside view of cyclone
Powder
Figure 15 Cyclone.
the particle to the wall, while the drag force of the air acts to carry the particles into the axis. As the centrifugal force is predominant, separation takes place. Powder and air pass tangentially into the cyclone at equal velocities. The mixture swirls in a spiral form down to the base of the cyclone separating the powder out to the cyclone wall. Powder leaves the bottom of the cyclone via a locking device. The clean air spirals upward along the central axis of the cyclone and leaves the cyclone at the top (see Figure 15). The centrifugal force that each particle is exposed to is given by the following equation: C¼
m Vt 2 r
where C is the centrifugal force, m the mass of the particle, Vt the tangential air velocity, and r the radial distance to the wall from any given point. From this equation it can be concluded that the higher the particle mass, the better the efficiency. Also, the shorter distance the particle has to travel, the better the efficiency; that is, the closer the particle is to the wall, the better the efficiency, because the velocity is the highest and the radial distance is short. However, time is required for the particles to travel to the cyclone wall, so a sufficient air residence time should be taken into consideration when designing a cyclone. From the above equation, it is evident that small cyclones (diameter <1 m) will have the highest efficiency, a fact that is generally accepted. However, as the big-tonnage dryers in operation in dairy industry today would require many cyclones, a
Increased air throughput (velocity Vt) and increased pressure drop will also increase the efficiency, but the energy requirement will also increase simultaneously, so in general the upper limit is 175–200 mm WG for skim milk powder. For whole milk, 140–160 mm WG is the maximum so that deposits and final blocking can be avoided. To determine a cyclone’s efficiency, the following terms have to be defined: particle diameter • critical cut size • overall cyclone efficiency • Critical particle diameter is defined as the particle size that will be completely removed from the air flow (100% collection efficiency). However, as there is no sharply defined point where a particle size is 100% separated or 100% lost, the critical particle diameter is not very valuable. Cut size is defined as the size for which 50% collection is obtained and is a much more useful parameter for stating the efficiency of cyclones. To determine a cyclone’s cut size, grade efficiency curves are constructed by systematically operating the cyclone with a uniform particle size dust (see Figure 16). Overall cyclone efficiency is the one obtained when handling a product of definite size distribution. Knowing the grade efficiency curve of the cyclone and the particle size distribution of the powder passing to the cyclone, the overall efficiency can be calculated, that is, the powder loss can be predicted. Another method for determining cyclone efficiency is by a simple powder loss measurement at the exit of the cyclone. A very small fraction of the outgoing air is passed through a high-efficiency minicyclone or through microdust filters. The amount of powder collected is directly proportional to the powder loss, which will mainly be a result of with low solid content or feed containing air • Feed High outlet air temperature • Low particle density (e.g., as a result of the above) • Leaking product on account of old-fashioned • nonadjustable rotaryoutlet valves
Plant and Equipment | Milk Dryers: Dryer Design 15 C
105 D
Theoretical curve E Theoretical critical particle diameter
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Actual critical particle diameter
Actual curve
Cut size
Collection efficiency (%)
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60 80 100 Particle size (µm)
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Figure 16 Critical particle diameter vs. grade efficiency curves for a cyclone.
cyclone • Blocked Changes in drying parameters resulting in a decrease • in mean particle size
blowing compressed air through the filter bags from the inner side. This powder is collected at the bottom via a rotary valve (see Figure 17).
Average powder loss from a normal, high-efficient cyclone should not exceed 250 mg Nm3 when spray drying skim milk.
Wet Scrubbers
Bag Filters However, local authorities in general conclude that 250 mg Nm3 is too high, thus requiring a final cleaning of the air. This is usually done by using bag filters consisting of numerous bags or filters arranged in such a way that all bags receive almost equal quantities of air. The direction of the air is from the outside, through the filter material, to the inner part of the bag from where the cleaned air enters an exhaust manifold. With a correct selection of filter material high efficiencies can be achieved, and collection of 1 m particles has been reported by manufacturers. The collected powder is automatically shaken off by
The wet scrubber is based on the venturi scrubber principle. The droplet separator is designed according to the well-known cyclone principles, however, with a modified outlet, resulting in a minimum liquid level, thereby minimizing bacterial growth, and a design ensuring deaeration, thus avoiding foam building. The principle of venturi wet scrubbers is as follows (see Figure 18). The outlet air from the spray dryer containing powder particles is accelerated to a high velocity in the venturi inlet, where the liquid also is injected through full-cone nozzles. Due to the different velocities of the air/particles and the liquid droplets, they will collide, and the powder will dissolve in the liquid droplets. Passing through the subsequent diffuser this process will continue simultaneously with a certain pressure recovery of the air–droplet mix.
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Compressed air
Air
Powder Air with powder
Powder
Air with powder
Powder
Figure 17 Bag filter.
Recirculation of water
Air with powder
Air + vapor
Scrubbing liquid recirculation
According to the above description of the principle, water is recycled by means of a centrifugal pump. The flow is controlled by a valve. The level is kept constant in the separator by a tank with an adjustable float simultaneously ensuring addition of water to make up for the evaporation taking place in the scrubber. The evaporation takes place owing to the high temperature of the air from the dryer, which is 90–95 C for example, being cooled to the wet-bulb temperature (45–50 C), at the same time evaporating the water (see Figure 19). As the temperature of the water continues to be around 40–45 C, bacterial growth must be expected after some time, and a CIP-able system is therefore recommended. The scrubbing liquid is used as animal feed.
CIP-Able Bag Filters
Figure 18 Sanitary wet scrubber.
Passing through the separator, the air and the liquid are separated. The air leaves through the central duct and the liquid through the bottom outlet for further processing or recycling depending on the system selected.
Common for all powder separators is the pressure drop across the cyclones, bag filters/scrubbers, or combinations thereof. In a continued effort to comply with the authority’s demand for reduced powder emission and the powder producer’s demand for lower energy consumption figures and reduced space requirements, an optimized powder recovery system has been developed – the CIP-able bag filter – which replaces the cyclones as well as the bag filter.
Plant and Equipment | Milk Dryers: Dryer Design
Air out
Air in
Milk in
Water in
Powder out Figure 19 Wet scrubber recycled with water.
Based on almost 10 years of research, development, and testing of a CIP-able bag filter by GEA Niro, the SANICIP filter has reached a point where it is setting the standard for almost all dryers. The SANICIPTM bag filter
The SANICIPTM bag filter (see Figure 20) is of the reverse-jet type. It consists of a cylindrical bag housing with a spiral-shaped air inlet, a clean-air plenum on top, and a conical bottom with fluidized powder discharge. During operation, the product collected on the outside of the filter material is removed by a compressed-air jet stream from the inside of each bag. The bags are cleanblown individually, resulting in a very even discharge of the powder. The air supply system for the fluidizing bottom has a multiple purpose: During production, the cone of the bag house is first heated by the warm air, which subsequently is used for fluidizing the powder in the bottom. This ensures an even powder flow out of the bag house. During standstill, the air is used for the heating of the cone alone and is in a closed loop. The filter bags are made from a special three-layer gradient polyester material, which is heat-treated to give a special dust-releasing surface. Each bag is supported on a stainless-steel cage and is easily dismountable. In the SANICIP filter, a special reverse-jet air nozzle positioned above each bag (see Figure 21) is
Figure 20 SANICIP CIP-able bag filter.
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230 Plant and Equipment | Milk Dryers: Dryer Design
thereby avoiding discoloring/denaturation. The water is recirculated. 4. The shell CIP is performed by means of standard retractable CIP nozzles. The water is recirculated. Normal acid and caustic are used as CIP agents. The CIP is followed by bag drying. Estimated time for complete CIP and dry out is 10 h. Advantages of the SANICIP filter as follows: pressure loss across the bag filter and, thus, across • Low the entire exhaust system; that is, reduced energy con-
Figure 21 Reverse-jet air nozzle.
used. Compressed air is blown into the bag through this nozzle. A jet is formed that draws into the bag air from the clean-air plenum as well, thereby saving compressed air. The CIP system of the bag house is divided into the following main items: 1. The internal bag CIP system cleans the bag from the inside toward the powder side (outside). Clean water is injected into the inside of the bag through the reversejet nozzle and the water is atomized by compressed air. Powder that has penetrated into the bag material is forced out toward the powder side by the water spray. No recirculation of water in this step. 2. The clean-air plenum CIP cleans the clean-air plenum of the bag filter above the hole plate. No recirculation of water in this step. 3. The hole plate CIP cleans the bottom side of the hole plate and the snap ring area of the bag using a specially designed nozzle, also with a dual purpose: During the process, the nozzle is purged with compressed air to keep the hole plate free of deposits,
• • • • • •
sumption and noise emission Designed for optimum air-to-cloth ratio and powder load (owing to one bag being blown at a time) Higher yield from raw materials owing to the absence of second-grade products Designs with 4 or 6 m bags to suit specific building requirements Reduced space requirements for new installations Easy replacement of cyclones for retrofits without major building changes Short dry-out time, as compared with other CIP-able bag filters
The pros and cons of all the above-mentioned powder recovery tools are listed in Table 1.
Final Drying and Cooling of Powder Pneumatic Conveying and Cooling System A pneumatic conveying system is established when powder has to be conveyed from one place to another. The conveying medium is air, and the quantity is determined by the product. Products with a high fat content require more air (5 times the powder) than that required by skimmed milk (4 times the powder). It is, however, not recommended that powders with
Table 1 Comparison of powder separators
Emission Pressure loss – exhaust system (including ducts, etc.) Auxiliaries Cleaning Hygroscopic products Use of separated product Maintenance
Sanitary conditions
Cyclone
Cyclone þ bag filter
Cyclone þ wet scrubber
SANICIPTM
20–400 mg Nm3 280 mm WG
5–20 mg Nm3 340 mm WG
max. 20 mg Nm3 340 mm WG
5–20 mg Nm3 170 mm WG
None Suitable for CIP Insensitive First grade
Compressed air Difficult Sensitive First and second grade
Liquid circulating system Suitable for CIP Insensitive Not recommended
Compressed air Suitable for CIP Insensitive First grade
Minimal
Servicing of compressed air system and change of bags
Minimal
Good
Relatively good
Less good
Servicing of compressed air system and change of bags Good
Plant and Equipment | Milk Dryers: Dryer Design
a fat content higher than 30% be conveyed, as blocking may occur in the ducts. Air at any temperature may be used, and the powder temperature will naturally follow the air temperature. If hot air is used there will be a drying effect. This will, however, be minimal, as the residence time is short (air velocities of 20 m s1). A pneumatic conveying system is inexpensive and can handle large quantities of powder, but it will destroy any agglomerates, resulting in a powder with maximum bulk density. The powder is separated from the conveying air in a cyclone. Fluid Bed After-Dryer/Cooler In modern dryers, pneumatic conveying and cooling systems are replaced by a VIBRO-FLUIDIZER, which is designed also as an after-dryer, that is, drying is divided into two or more steps. The first step is done in the spray drying chamber transforming the liquid into powder particles and evaporating the main portion of water. The subsequent drying is done in a fluid bed (see Figure 22). The fluid bed drying technology has proved
especially suitable, as the residence time in the fluid bed is so long that the moisture from the center of the particle can reach the surface from where evaporation takes place. The VIBRO-FLUIDIZER is a horizontal box divided into an upper and a lower section by a perforated plate welded to the side wall of the box (see Figure 23). For drying, or alternatively cooling, warm or cold air is introduced into the air plenum chamber, which is distributed evenly over the whole area of the perforated plate. The perforation and amount of air are determined by the air velocity necessary for the fluidizing of the powder; however, special care must be taken to avoid attrition of agglomerates. The temperature and area are determined according to the required evaporation duty. The hole size in the perforated plate is chosen to give an air velocity high enough to fluidize the powder on the plate. The air velocity should be so high that the fines powder becomes airborne and leaves the fluid bed with the air, and is returned to the atomizing zone for agglomeration. The fluid bed can also be designed as a static back-mix bed integrated in the drying chamber.
Powder in
Air out
Drying air in Cooling air in
Powder out
Figure 22 VIBRO-FLUIDIZER.
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232 Plant and Equipment | Milk Dryers: Dryer Design
Figure 23 Construction detail of a sanitary VIBRO-FLUIDIZER.
Fines Return System Agglomeration means getting smaller particles to adhere to each other to form a powder consisting of bigger conglomerates/agglomerates, which are essential for easy reconstitution in water. By means of a fines return system, the cyclone fraction(s) is(are) conveyed back to the atomizer mist, the static fluid bed, or the VIBROFLUIDIZER, depending on the required degree of agglomeration. Fines return systems consist of the following: blowers (the quantity of air is depen• High-pressure dent on the amount of fines – typically, 1 kg of air can convey 3–5 kg of powder)
valves (devices to discharge powder • Blow-through from cyclones and/or bag filters into the conveying
• •
line) Conveying line/diverter valves to convey the fines powder to the desired destination – typically a 76–102 mm (3–4 inch) pipe Fines introduction to the atomization zone
The aim is to bring the fines as close as possible to the atomizer wheel. In modern dryers, fines are introduced from above through the air disperser (FRAD system) via four fines pipes situated just above the atomizer cloud. Deflector plates at the end of each fines pipe ensure a correct introduction and distribution of the fines (see Figure 24).
Cooling air
Fines Concentrate
Figure 24 Fines return for rotary atomizer FRAD.
Fines
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Cooling air Concentrate Fines Fines
Concentrate Cooling air
Drying air
Cooling air
Rotary air steam
Plug flow air steam
Figure 25 Fines return for nozzle atomizer.
For nozzle atomization, the fines return is an integral part of the nozzle unit with the fines duct in the center surrounded by nozzles at the periphery (see Figure 25), provided the dryer is designed for rotary air flow or is with vertical air flow.
See also: Analytical Methods: Sampling; Sensory Evaluation. Dehydrated Dairy Products: Milk Powder: Physical and Functional Properties of Milk Powders; Milk Powder: Types and Manufacture; Infant Formulae. Milk Protein Products: Milk Protein Concentrate; Whey Protein Products. Plant and Equipment: Evaporators; Milk Dryers: Drying Principles. Rheology of Liquid and Semi-Solid Milk Products.
Conclusion Spray drying plants are designed today to fulfill many requirements, including low energy consumption, high final-product quality, reduced space requirements, and a high degree of environmental protection – a challenge taken up by the designers and suppliers of the dryers.
Further Reading Masters K (1991) Spray Drying Handbook. Essex: Longman Scientific & Technical. Pı´secky´ J (1997) Handbook of Milk Powder Manufacture. Copenhagen: Niro A/S. Westergaard V (1994) Milk Powder Technology. Evaporation and Spray Drying. Copenhagen: Niro A/S. www.niro.com.
Introduction This article focuses on spray dryers. Although other means of drying are possible, this is by far the most common in the dairy industry. By definition, spray drying is the transformation of a product from a fluid state into a dried form by spraying the liquid feed into a hot drying medium. The feed can be a solution, a suspension, or a paste, depending on the characteristics of the dairy product to be dried. The dried product is a powder consisting of single particles or agglomerates, all depending on the chemical composition and physical properties of the feed as well as on dryer design and operation.
Drying Principles A spray dryer operates in the following way: The feed is pumped from the product feed tank to the atomizing device that is situated in the air disperser at the top of the drying chamber. The drying air is drawn from the atmosphere via a filter by a supply fan and is passed through the air heater to the air disperser. As the atomized droplets meet the hot air, evaporation takes place cooling the air at the same time. After the drying of the atomized feed in the chamber, the majority of the dried product falls to the bottom for further processing. The fines, which are the particles with a small diameter, will remain entrained in the air. Therefore, the air has to pass through powder collectors like cyclones or bag filters. The air passes from the powder collector to the atmosphere via the exhaust fan. The two fractions of powder are collected, for example, in a pneumatic system for conveying and cooling. After separation in a cyclone, the powder is bagged off. A conventional spray dryer consists of the following main components (Figure 1): 1. 2. 3. 4. 5. 6.
Drying chamber Hot air system and air distribution Feed system Atomizing device Powder separation system Pneumatic conveying and cooling system
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7. Integrated fluid bed 8. Fluid bed after-dryer/cooler
Drying Chamber Various designs of the drying chamber are available on the market. The most common one is a cylindrical chamber with a cone of 40–60 , enabling the powder to leave the chamber by gravity. The chamber is also found with a flat bottom in which case a scraper or suction device is needed for removing the powder fraction from the chamber. Horizontal box-type drying chambers are also used, and they, too, operate with a forced (i.e., scraper or screw) powder removal system (Figure 2). Generally, it can be concluded that chambers with a cone for gravity discharge of the powder give the best flexibility for adapting various drying processes like integrated fluid beds or belts to the plant and therefore offer the greatest possibility for drying different products. The tendency in modern designs of drying chamber is to avoid any object inside the chamber that can obstruct the air flow. In the chamber of the TALL FORM, the emphasis has been put on designing a plant with a laminar air flow and a special air outlet system, where the diameter of the cone is bigger than the diameter of the cylindrical part thus forming a ring duct termed ‘bustle’. This minimizes the cyclone fraction by the low velocity of the exhaust air. This chamber is especially suited for infant milk formulae or protein products dried from low-solid content feed. The drying chamber should always be equipped with inspection doors and overpressure vents to withstand a pressure of 1.6 mbar(g). Other safety equipment such as fire extinguishing equipment in the form of water or steam nozzles is always standard in a modern dryer. Drying chambers are usually insulated, either with removable air-filled sandwich panels (see Figure 3) or with 80–100 mm mineral wool covered with a stainlesssteel plate. The advantage of the removable panels is that inspection for cracks in the chamber wall is possible. Furthermore, the risk of having wet insulation material, which can foster bacterial development or cold spots on the chamber wall, is eliminated.
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Ring-formed fluid bed (compact drying chamber) 4
3 1
2
5 6
7
8
Figure 1 Spray drying plant. 1, Drying chamber; 2, Hot air system and air distribution; 3, Feed system; 4, Atomizing device; 5, Powder separation system; 6, Pneumatic conveying and cooling system; 7, Integrated fluid bed; 8, Fluid bed after-dryer/ cooler.
Integrated Static Fluid Bed In an attempt to improve the drying efficiency, a static fluid bed is integrated in the drying chamber. The secondary drying air, typically 25% of the main drying air, is introduced into a plenum chamber below a perforated plate, through which the drying air is distributed. This type of dryer can be operated in such a way that the primary particles reach a moisture level higher than that obtained by using the VIBRO-FLUIDIZER. A specially designed and patented perforated plate, the BUBBLE PLATE (see Figure 4), provides an air– powder mixture that ensures optimal drying without attrition and powder penetration into the clean-air plenum. Furthermore, the BUBBLE PLATE has a more sanitary finish than the other types of perforated plates. The static fluid bed is available in two configurations: fluid bed (compact drying chamber) • Ring-formed Circular fluid bed (multistage drying (MSD) chamber) •
The ring-formed back-mix bed is placed at the bottom of a conventional chamber cone around the exhaust duct placed in the center. The powder is discharged continuously from the static fluid bed by overflowing an adjustable powder weir, thus maintaining a certain level of fluidized powder. When the powder leaves the drying chamber it may be cooled in a pneumatic conveying or VIBRO-FLUIDIZER system. The resulting powder will consist of single particles. For fat-containing products, cooling should be done in a vibrating fluid bed that is also used when agglomerated powders are produced. In this case, the cyclone fraction is returned to the atomizer device for agglomeration (see Figure 5). Circular fluid bed (multistage drying (MSDTM) chamber)
To improve the dryer efficiency and the powder properties even further, the multistage dryer MSD has been designed (see Figure 6). The dryer operates with three drying stages, each adapted to the moisture content prevailing during the drying process. In the preliminary drying stage, the concentrate is atomized by co-current nozzles or a rotary atomizer placed in the hotdrying air duct. Air enters the dryer vertically through the air disperser, ensuring optimal mixing of the atomized droplets with the drying air. The particles reach a moisture content of 6–15%, depending upon the type of product. The fluid bed is supplied with air at a sufficient velocity and temperature for the second-stage drying. The drying air from the preliminary drying stage and the integrated fluid bed leaves the chamber from the top passing through powder separators. This type of dryer offers a perfect choice if the aim is to produce an agglomerated product. Owing to the velocity of the primary drying air, a venturi is formed around the atomizing device, thus sucking in secondary air with powder entrained so that agglomeration is facilitated, that is, attrition between the primary spray particles and the fines powder. When the powder has reached a certain moisture content it is discharged via a rotary valve into a VIBROFLUIDIZER for the final drying and subsequent cooling. The powder exhibits a coarse powder structure originating from the natural agglomeration taking place in the chamber.
Hot Air System and Air Distribution Air Filtration System Until a few years ago, no special requirements were placed on filtration of the process air for the spray drying process. Today, however, very strict requirements are presented by local authorities to ensure a cleaner
218 Plant and Equipment | Milk Dryers: Dryer Design
1
2
3 4
5
7
8
6
9
Drying air Cooling air Concentrate/product Fines Figure 2 Different types of drying chambers. 1, TALL FORM dryer type seen in Japan. It is equipped with a low-velocity air disperser, and, although it is used for milk, it is not very suitable for this product; 2, TALL FORM dryer with high-temperature primary drying air. Secondary air is sucked into the drying chamber during the drying operation. The ‘mix’ air temperature is similar to that of a normal spray dryer; 3, Conventional TALL FORM DRYER chamber used predominantly for baby food and protein products; 4, Conventional drying chamber with conical bottom; 5, Box dryer for one-stage drying only – poor economy and normally seen only in the United States; 6, Multistage MSD drying chamber with integrated fluid bed; 7, Conventional COMPACT drying chamber with integrated fluid bed; 8, Flat-bottom spray drying chamber for one-stage drying only. No longer seen on new installations; 9, FILTERMAT drying chamber in a special design for very sticky products.
operation and a higher level of food safety. Common for the different standards are as follows: air should be prefiltered and supplied by a separate • The fan to the fan/filter/heater room, which must be under
•
pressure to avoid the entry of unfiltered air (Figure 7). As an alternative to a fan room, complete ducting of all air flows is fully acceptable. Filtration degree and filter position depend on the final temperature of the process air as follows: – For air to be heated above 120 C only coarse filtration up to 90% (filter class EU7/F7) is needed. The filter should be placed on the pressure side of the fan.
– For air to be heated below 120 C or not heated at all, the filtration must be 95% (filter class EU/F9) or above, and the filter must be placed after the heater/ cooler. Some countries and companies have even stricter requirements demanding a filtration of up to 99.995% (filter class EU13-14/ H13-14).
Air Heating System The drying air can be heated in different ways: indirect (steam/oil/gas/hot oil/electricity) or direct (gas).
Plant and Equipment | Milk Dryers: Dryer Design
219
Figure 3 Removable insulation panels for spray drying chambers.
Figure 4 BUBBLE PLATE.
Indirect heating
A steam heater is a simple radiator. The temperature to be obtained depends on the steam pressure available. The air heater consists of rows of finned tubes housed in an insulated metal case. The heat load is calculated from the quantity and specific heat of the air. The heater size depends on the heat transfer properties of the tubes and fins and is usually about 50 kcal C1 h1 m3 for an air velocity of 5 m s1. To avoid corrosion of the tubes in the air heater, use of stainless-steel tubes is recommended. In indirect oil and gas heaters, drying air and combustion gases have separate flow passages. The combustion
gases pass through galvanized tubes, which act as the heat transfer surface for the drying air. The combustion chamber is made of heat-resistant steel. Heaters of this type will have an efficiency of about 85% in the range of 175–250 C (see Figure 8). Hot oil liquid phase air heaters are used either alone or to boost the inlet drying air temperature when the steam pressure is not high enough. The heater system consists of a heater, which can be gas- or oil-fired, and an air heat exchanger. Between these two components, a special food-grade oil or heat transfer fluid, which does not crack at high temperatures, is circulated at high speed.
220 Plant and Equipment | Milk Dryers: Dryer Design
Air
Figure 7 Filtration of process air.
Figure 5 Compact spray dryer with VIBRO-FLUIDIZER as agglomerator/instantizer (CDI).
as boosters instead of, for example, hot oil liquid phase air heaters. Direct heating
Direct gas heaters are used only when the combustion gas can be allowed to come into contact with the product. They are, therefore, not common in the dairy industry. Direct gas heaters are inexpensive, they have a high efficiency, and the obtainable temperature can be as high as 2000 C. When a plant is designed with an air heater with direct combustion, it is necessary to calculate the amount of vapor resulting from the combustion (44 mg kg1 dry air C1), as this will increase the humidity of the drying air. The outlet temperature has therefore to be increased to compensate for this increase in humidity and to maintain the relative humidity. The heater system can be designed with separate heaters for each consumption point or with fewer heaters, of which some supply two or more consumption points. By mixing warm air from the main air heater with cold air, the entire dryer can be run with only one heater. Figure 6 Multistage spray dryer (MSD).
Air Distribution System
The main advantage of a hot oil liquid phase heater is that it is an open, pressureless system. Electrical air heaters have for many years been used mainly for laboratory and pilot plant spray dryers. This heater has low investment costs, but previously the operation costs were considered to be too high for commercial production. However, as the price of electricity in certain parts of the world can be very low during off-peak periods, it is becoming more common to use electrical heaters
Drying-air distribution is one of the most vital functions in a spray dryer. There are various systems depending on the plant design and the type of product. The most common system is where the air disperser is situated on top of the dryer ceiling, and the atomizing device is placed in the middle of the air disperser, thus ensuring an optimal mixing of the air and the atomized droplets. In cylindrical vertical dryers, the whole ceiling may be perforated, thus creating a plug-flow air stream – numerous nozzles are situated in the perforated plate to
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Indirect steam-heated air heater
Cold air in Hot air out
Oil Combustion air
Figure 8 Indirect air heaters.
ensure that the air is cooled by the concentrate. This system, however, operates with a low air velocity, and it makes fines return complicated. It is therefore not suitable for all dairy products. It should be noted that an air disperser should have the ability to guide the air and the atomized droplets in the right direction to avoid deposits in the drying chamber. Two different types of air dispersers are currently used in spray dryers for food and dairy products:
Concentrate
Drying air
Cooling air
Rotary air stream
The air enters tangentially into a spiral-shaped distributor housing (see Figure 9), from where the drying air is
Figure 9 Ceiling air disperser with adjustable guide vanes.
222 Plant and Equipment | Milk Dryers: Dryer Design
led radially and downward over a set of guide vanes provided for adjustment of air rotation. This type of air disperser is used for rotary atomizers and nozzle atomizers placed in the center of the air disperser and is used in conventional drying chambers.
7
1
Plug flow air stream
The air enters radially through one side (see Figure 10) and is distributed through a specially designed air guiding arrangement, which ensures a uniform air flow pattern in the entire air disperser area. This enables a very precise, laminar, high-velocity plug flow, which is required in TALL FORM dryers or multistage dryers. This type of air disperser is used for nozzle atomizers only, and has excellent possibilities for nozzle position adjustment and thereby the adjustment of agglomeration structure.
Feed System The feed system (see Figure 11) is the link between the evaporator and the spray dryer, and comprises the following: 1. Feed tanks 2. Water tank
1
4
2
5
6
3 Figure 11 Feed system. 1, Feed tanks; 2, Water tank; 3, Concentrate pump; 4, Preheating system; 5, Filter; 6, Homogenizer/high-pressure pump; 7, Feed line, including return line for CIP.
3. 4. 5. 6. 7.
Concentrate pump Preheating system Filter Homogenizer/high-pressure pump Feed line, including return line for cleaning-in-place (CIP)
Feed Tanks
Fines
Cooling air
Drying air
If feed tanks are used, the use of two tanks is recommended so as to change from one to the other at least once an hour to avoid the risk of bacteria growth. One is therefore in use while the other one is being cleaned. The size of each tank should correspond to 15–30 min of the feed capacity of the dryer. The feed tanks are very often omitted, and the last-stage evaporator is designed as a buffer tank under vacuum. The evaporator then operates as a ‘slave’ to the dryer, because the level switches in the evaporator buffer tank control the inlet feed to the evaporator. Water Tank The water tank is used during the start and stop of the plant, and during the run if there is a sudden shortage of concentrate. It is used only when the evaporator is used as a buffer tank. As an alternative, a direct water supply to the feed line is often used. Concentrate Pump
Concentrate
Figure 10 Plug flow air disperser.
If a rotary atomizer is used, the most common feed pump is the mono type, as it has lower energy consumption and can handle concentrates of high viscosity. In plants equipped with homogenizers for the production of whole milk powder, the homogenizer is used as a feed pump. In plants equipped with high-pressure nozzles, a high-pressure pump is used – often combined with a homogenizer.
Plant and Equipment | Milk Dryers: Dryer Design
Preheating System Preheating of the concentrate to a higher temperature than that coming from the evaporator is advantageous, not only from a bacteriological point of view. It also produces a decrease in viscosity, which together with the applied calories results in a capacity increase of the spray dryer and an improved solubility of the powder produced. The heating can be either indirect or direct. Indirect preheaters may be of the following types: 1. Spiral-tube heat exchanger 2. Plate heat exchanger 3. Scraped-surface heat exchanger Spiral-tube heat exchanger
The spiral-tube heater (see Figure 12), often with corrugated tubes, is able to heat a concentrate with high solid content to a higher temperature without frequent scaling and cleaning owing to high product velocity and a low T throughout the heater. Furthermore, this type of heater has no moving parts; hence, maintenance costs are minimized.
223
temperatures are required. They can operate continuously for 20 h and are cleaned together with the remaining feed system. The disadvantages are high cost of maintenance as well as big variation in holding time. Direct preheaters may be of the following types: steam injection (DSI) • Direct Lenient steam injection (LSI) • Direct steam injection
In the DSI unit, steam is introduced into the milk concentrate via a nozzle, producing relatively big steam bubbles resulting in a superheating of some parts of the concentrate, which leads to protein denaturation. Lenient steam injection
In the LSI unit, steam is mixed into the concentrate by a dynamic mixer. Very small steam bubbles are created, and superheating/denaturation is avoided. Therefore, a much higher steam pressure can be used. The LSI unit can be used in combination with the spiral-tube heat exchanger if temperatures above 80 C are required in the concentrate. Filter
Plate heat exchanger
A plate heat exchanger system is inexpensive, but if the concentrate should be heated to >70 C, if the solid content is >46%, or if a 20 h run is aimed at, it is necessary to have two interchangeable heaters allowing one to be cleaned while the other is being used. Steam or warm water can be used as the heating medium. Scraped-surface heat exchanger
In the scraped-surface heater, the heat transfer surface is continuously scraped off by a fast-rotating scraper made of food-grade synthetic material to avoid any product adherence. The scraped-surface heater is especially suitable for products with high solid content and when high
An in-line filter is always incorporated in the feed system after the heater to avoid lumps, etc., passing to the atomizing device. Homogenizer/High-Pressure Pump If whole milk powder is to be produced, it is recommended that a homogenizer be incorporated to reduce the free-fat content in the final powder. A two-stage homogenizer is preferred; the first stage is operated at 50–100 bar g, and the second stage at 25–50 bar g. Usually the homogenizer and feed pump are combined in one unit. If nozzle atomization is used, then a higher pressure (up to 250 bar g for the nozzles þ 150 bar g for homogenizing) is required, and a combined homogenizer/ high-pressure pump is chosen. Temperatures of 80 C are needed to produce a whole milk powder with a good coffee stability. In view of calcium phosphate precipitation – which is abrasive – the pistons should be made of a ceramic material. Feed Line
Figure 12 Spiral-tube heat exchanger.
The feed pipe should be of stainless steel and, of course, of the high-pressure type if atomization is to be carried out by means of nozzles. The dimensions of the pipe should be such that the feed velocity is 1.5 m s1. In a feed system, a return pipe should also be included for the cleaning solution, so that the entire equipment can be cleaned thoroughly.
224 Plant and Equipment | Milk Dryers: Dryer Design
Atomizing Device The aim of atomizing the concentrate is to provide a very large surface from which evaporation can take place. The smaller the droplets, the bigger the surface and the easier the evaporation; and thus a better thermal efficiency of the dryer is obtained. The ideal from the point of view of drying would be a spray of drops of the same size, which would mean that the drying time for all particles would be the same to obtain an equal moisture content. As mentioned previously, air distribution and atomization are the factors key to the successful utilization of the spray dryer. Atomization is directly responsible for many distinctive advantages offered by spray drying: First, the very short drying time of the particles; second, a very short particle retention time in the hot atmosphere and a low particle temperature (wet bulb temperature); and, finally, the transformation of the liquid feed into a powder with long-storage stability ready for packing and transport. In summary, the primary functions of atomization are
5
4 3
a high surface-to-mass ratio resulting in a • tohighcreate evaporation rate create particles of the desired shape, size, and • todensity
2
1. Nozzle body 2. Orifice insert 3. Swirl chamber 4. End plate 5. Screw pin
1 Figure 13 High-pressure nozzle ‘Delavan’.
To comply with these requirements many atomization techniques have been used in spray dryers. However, the most common ones can be summarized as follows:
• • •
pressure nozzles using pressure forces two-fluid nozzles using kinetic forces rotating discs using centrifugal forces
Pressure Nozzle Atomization The basic function of pressure nozzles is to convert the pressure energy supplied by the high-pressure pump into kinetic energy in the form of a thin film, the stability of which is determined by the properties of the liquid such as viscosity, surface tension, density, and quantity per unit of time, and by the medium into which the liquid is sprayed. Most of the commercially available pressure nozzles are designed with a swirl chamber giving the liquid a rotation, so that it will leave the orifice as a hollow cone (see Figure 13). Capacity can usually be assumed to be directly proportional to the square root of pressure retain: Capacity ðkg h – 1 Þ ¼ K
pffiffiffi P
As a rule of thumb, higher viscosity, liquid density, and surface tension, and lower pressure will result in bigger particles. Typically, a feed rate of 1000–1500 kg h1 per
nozzle is used in industrial dryers. The advantages when using high-pressure nozzles are as follows: with a low level of occluded air • Powder with a high bulk density • Powder flowability, especially for whole milk • Improved to form less deposits in the drying chamber • Tendency when difficult products are produced • Ability to produce big particles Two-fluid nozzle or pneumatic atomization
The energy available for atomization in two-fluid atomizers is independent of liquid flow and pressure. The necessary energy (kinetic) is supplied by compressed air. Two-fluid atomization is the only successful nozzle method for producing very small particles, especially from highly viscous liquids. It is not normally used in the drying of milk products. However, it is often used in secondary systems such as lecithin application to the powder. Rotary atomization
In rotary atomizers the liquid is accelerated continuously to the wheel’s edge by the centrifugal forces produced by the rotation of the wheel. The liquid is distributed centrally, then extended over the wheel surface in a thin sheet and discharged at a high speed at the periphery of
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Viscosity of the liquid
Droplet size varies directly with viscosity, and bigger particles are obtained when the viscosity of the feed is higher. To ensure an optimal atomization, the viscosity is therefore normally kept as low and as constant as possible, often by heating the concentrate prior to atomization. Regarding droplet size distribution, it becomes broader with increased viscosity – an effect sometimes used when bulk density of the powder is to be increased. Rotary atomizer has been known and used in the dairy industry for many years; its main advantages are as follows: in throughput • Flexibility Ability to handle quantities • Ability to handle large highly concentrates • Different wheel designsviscous giving different powder • characteristics to handle products containing crystals • Ability Ability handle higher solid content in the feed; • therefore,to better economy
Figure 14 Rotary atomizer with direct drive.
the wheel. The degree of atomization depends upon peripheral speed, properties of the liquid, and feed rate (Figure 14). To select an optimal atomizer wheel, the following factors should be taken into consideration: Liquid feed rate
Droplet size varies directly with feed rate at a constant wheel speed and will increase with increased feed rate. Peripheral speed
The peripheral speed depends on the diameter of the wheel and the wheel speed, and is calculated as follows: Vp ¼
DN 1000 60
where Vp is the peripheral speed (m s1), D the diameter of the wheel (mm), and N the speed of the wheel (rpm). Peripheral speed is widely accepted as the main variable for the adjustment of droplet size. However, it has been shown that droplet size does not necessarily remain constant when equal peripheral speeds are produced in wheel designs of various diameter–speed combinations, as there is a tendency for bigger wheels to produce bigger particles, all other things being equal. However, in the choice of wheel diameter, one should rather look at the reliability of the atomizer, as the differences in spray characteristics are negligible.
To decide whether to use a pressure nozzle or rotary wheel is therefore a question of achieving the demanded properties of the final dried product, given the properties of the feed.
Powder Separation System As the drying air leaving the chamber will contain a small proportion of the powder (10–30%), it is necessary to clean it by separating the powder particles. This powder fraction is usually referred to as the ‘fines’, as they normally represent the smallest particles. The most widely used separators in the milk powder industry are
• Cyclone filter • Bag CIP-able bag filter • Wet scrubber • Combinations of the above • Cyclone Cyclone has some obvious advantages, such as high efficiency, if constructed properly. It is easily maintained, as there are no moving parts. Furthermore, it is easy to clean if the construction is with a fully welded center cyclone. The operation theory is based on a vortex motion where the centrifugal force is acting on each particle and therefore causes the particle to move away from the cyclone axis toward the inner cyclone wall. However, the movement in the radial direction is the result of two opposing forces where the centrifugal force acts to move
226 Plant and Equipment | Milk Dryers: Dryer Design
compromise is sought with bigger sizes at the expense of high efficiency. Thus, the cyclones have become bigger and bigger and are now constructed with diameters of up to 4.0 m. When designing a cyclone, various key factors should be taken into account to obtain the highest efficiency. This is achieved if
Air
cyclone diameter 3 exit duct diameter
Air with powder
cyclone height 10 exit duct diameter Inside view of cyclone
Powder
Figure 15 Cyclone.
the particle to the wall, while the drag force of the air acts to carry the particles into the axis. As the centrifugal force is predominant, separation takes place. Powder and air pass tangentially into the cyclone at equal velocities. The mixture swirls in a spiral form down to the base of the cyclone separating the powder out to the cyclone wall. Powder leaves the bottom of the cyclone via a locking device. The clean air spirals upward along the central axis of the cyclone and leaves the cyclone at the top (see Figure 15). The centrifugal force that each particle is exposed to is given by the following equation: C¼
m Vt 2 r
where C is the centrifugal force, m the mass of the particle, Vt the tangential air velocity, and r the radial distance to the wall from any given point. From this equation it can be concluded that the higher the particle mass, the better the efficiency. Also, the shorter distance the particle has to travel, the better the efficiency; that is, the closer the particle is to the wall, the better the efficiency, because the velocity is the highest and the radial distance is short. However, time is required for the particles to travel to the cyclone wall, so a sufficient air residence time should be taken into consideration when designing a cyclone. From the above equation, it is evident that small cyclones (diameter <1 m) will have the highest efficiency, a fact that is generally accepted. However, as the big-tonnage dryers in operation in dairy industry today would require many cyclones, a
Increased air throughput (velocity Vt) and increased pressure drop will also increase the efficiency, but the energy requirement will also increase simultaneously, so in general the upper limit is 175–200 mm WG for skim milk powder. For whole milk, 140–160 mm WG is the maximum so that deposits and final blocking can be avoided. To determine a cyclone’s efficiency, the following terms have to be defined: particle diameter • critical cut size • overall cyclone efficiency • Critical particle diameter is defined as the particle size that will be completely removed from the air flow (100% collection efficiency). However, as there is no sharply defined point where a particle size is 100% separated or 100% lost, the critical particle diameter is not very valuable. Cut size is defined as the size for which 50% collection is obtained and is a much more useful parameter for stating the efficiency of cyclones. To determine a cyclone’s cut size, grade efficiency curves are constructed by systematically operating the cyclone with a uniform particle size dust (see Figure 16). Overall cyclone efficiency is the one obtained when handling a product of definite size distribution. Knowing the grade efficiency curve of the cyclone and the particle size distribution of the powder passing to the cyclone, the overall efficiency can be calculated, that is, the powder loss can be predicted. Another method for determining cyclone efficiency is by a simple powder loss measurement at the exit of the cyclone. A very small fraction of the outgoing air is passed through a high-efficiency minicyclone or through microdust filters. The amount of powder collected is directly proportional to the powder loss, which will mainly be a result of with low solid content or feed containing air • Feed High outlet air temperature • Low particle density (e.g., as a result of the above) • Leaking product on account of old-fashioned • nonadjustable rotaryoutlet valves
Plant and Equipment | Milk Dryers: Dryer Design 15 C
105 D
Theoretical curve E Theoretical critical particle diameter
50
0
Actual critical particle diameter
Actual curve
Cut size
Collection efficiency (%)
100
227
B
A
20
10
20
40
60 80 100 Particle size (µm)
120
60 70 40 50 Particle size (µm)
80
140
Grade efficiency (%)
100 80 60 40 20
0
30
90
100
Figure 16 Critical particle diameter vs. grade efficiency curves for a cyclone.
cyclone • Blocked Changes in drying parameters resulting in a decrease • in mean particle size
blowing compressed air through the filter bags from the inner side. This powder is collected at the bottom via a rotary valve (see Figure 17).
Average powder loss from a normal, high-efficient cyclone should not exceed 250 mg Nm3 when spray drying skim milk.
Wet Scrubbers
Bag Filters However, local authorities in general conclude that 250 mg Nm3 is too high, thus requiring a final cleaning of the air. This is usually done by using bag filters consisting of numerous bags or filters arranged in such a way that all bags receive almost equal quantities of air. The direction of the air is from the outside, through the filter material, to the inner part of the bag from where the cleaned air enters an exhaust manifold. With a correct selection of filter material high efficiencies can be achieved, and collection of 1 m particles has been reported by manufacturers. The collected powder is automatically shaken off by
The wet scrubber is based on the venturi scrubber principle. The droplet separator is designed according to the well-known cyclone principles, however, with a modified outlet, resulting in a minimum liquid level, thereby minimizing bacterial growth, and a design ensuring deaeration, thus avoiding foam building. The principle of venturi wet scrubbers is as follows (see Figure 18). The outlet air from the spray dryer containing powder particles is accelerated to a high velocity in the venturi inlet, where the liquid also is injected through full-cone nozzles. Due to the different velocities of the air/particles and the liquid droplets, they will collide, and the powder will dissolve in the liquid droplets. Passing through the subsequent diffuser this process will continue simultaneously with a certain pressure recovery of the air–droplet mix.
228 Plant and Equipment | Milk Dryers: Dryer Design
Compressed air
Air
Powder Air with powder
Powder
Air with powder
Powder
Figure 17 Bag filter.
Recirculation of water
Air with powder
Air + vapor
Scrubbing liquid recirculation
According to the above description of the principle, water is recycled by means of a centrifugal pump. The flow is controlled by a valve. The level is kept constant in the separator by a tank with an adjustable float simultaneously ensuring addition of water to make up for the evaporation taking place in the scrubber. The evaporation takes place owing to the high temperature of the air from the dryer, which is 90–95 C for example, being cooled to the wet-bulb temperature (45–50 C), at the same time evaporating the water (see Figure 19). As the temperature of the water continues to be around 40–45 C, bacterial growth must be expected after some time, and a CIP-able system is therefore recommended. The scrubbing liquid is used as animal feed.
CIP-Able Bag Filters
Figure 18 Sanitary wet scrubber.
Passing through the separator, the air and the liquid are separated. The air leaves through the central duct and the liquid through the bottom outlet for further processing or recycling depending on the system selected.
Common for all powder separators is the pressure drop across the cyclones, bag filters/scrubbers, or combinations thereof. In a continued effort to comply with the authority’s demand for reduced powder emission and the powder producer’s demand for lower energy consumption figures and reduced space requirements, an optimized powder recovery system has been developed – the CIP-able bag filter – which replaces the cyclones as well as the bag filter.
Plant and Equipment | Milk Dryers: Dryer Design
Air out
Air in
Milk in
Water in
Powder out Figure 19 Wet scrubber recycled with water.
Based on almost 10 years of research, development, and testing of a CIP-able bag filter by GEA Niro, the SANICIP filter has reached a point where it is setting the standard for almost all dryers. The SANICIPTM bag filter
The SANICIPTM bag filter (see Figure 20) is of the reverse-jet type. It consists of a cylindrical bag housing with a spiral-shaped air inlet, a clean-air plenum on top, and a conical bottom with fluidized powder discharge. During operation, the product collected on the outside of the filter material is removed by a compressed-air jet stream from the inside of each bag. The bags are cleanblown individually, resulting in a very even discharge of the powder. The air supply system for the fluidizing bottom has a multiple purpose: During production, the cone of the bag house is first heated by the warm air, which subsequently is used for fluidizing the powder in the bottom. This ensures an even powder flow out of the bag house. During standstill, the air is used for the heating of the cone alone and is in a closed loop. The filter bags are made from a special three-layer gradient polyester material, which is heat-treated to give a special dust-releasing surface. Each bag is supported on a stainless-steel cage and is easily dismountable. In the SANICIP filter, a special reverse-jet air nozzle positioned above each bag (see Figure 21) is
Figure 20 SANICIP CIP-able bag filter.
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230 Plant and Equipment | Milk Dryers: Dryer Design
thereby avoiding discoloring/denaturation. The water is recirculated. 4. The shell CIP is performed by means of standard retractable CIP nozzles. The water is recirculated. Normal acid and caustic are used as CIP agents. The CIP is followed by bag drying. Estimated time for complete CIP and dry out is 10 h. Advantages of the SANICIP filter as follows: pressure loss across the bag filter and, thus, across • Low the entire exhaust system; that is, reduced energy con-
Figure 21 Reverse-jet air nozzle.
used. Compressed air is blown into the bag through this nozzle. A jet is formed that draws into the bag air from the clean-air plenum as well, thereby saving compressed air. The CIP system of the bag house is divided into the following main items: 1. The internal bag CIP system cleans the bag from the inside toward the powder side (outside). Clean water is injected into the inside of the bag through the reversejet nozzle and the water is atomized by compressed air. Powder that has penetrated into the bag material is forced out toward the powder side by the water spray. No recirculation of water in this step. 2. The clean-air plenum CIP cleans the clean-air plenum of the bag filter above the hole plate. No recirculation of water in this step. 3. The hole plate CIP cleans the bottom side of the hole plate and the snap ring area of the bag using a specially designed nozzle, also with a dual purpose: During the process, the nozzle is purged with compressed air to keep the hole plate free of deposits,
• • • • • •
sumption and noise emission Designed for optimum air-to-cloth ratio and powder load (owing to one bag being blown at a time) Higher yield from raw materials owing to the absence of second-grade products Designs with 4 or 6 m bags to suit specific building requirements Reduced space requirements for new installations Easy replacement of cyclones for retrofits without major building changes Short dry-out time, as compared with other CIP-able bag filters
The pros and cons of all the above-mentioned powder recovery tools are listed in Table 1.
Final Drying and Cooling of Powder Pneumatic Conveying and Cooling System A pneumatic conveying system is established when powder has to be conveyed from one place to another. The conveying medium is air, and the quantity is determined by the product. Products with a high fat content require more air (5 times the powder) than that required by skimmed milk (4 times the powder). It is, however, not recommended that powders with
Table 1 Comparison of powder separators
Emission Pressure loss – exhaust system (including ducts, etc.) Auxiliaries Cleaning Hygroscopic products Use of separated product Maintenance
Sanitary conditions
Cyclone
Cyclone þ bag filter
Cyclone þ wet scrubber
SANICIPTM
20–400 mg Nm3 280 mm WG
5–20 mg Nm3 340 mm WG
max. 20 mg Nm3 340 mm WG
5–20 mg Nm3 170 mm WG
None Suitable for CIP Insensitive First grade
Compressed air Difficult Sensitive First and second grade
Liquid circulating system Suitable for CIP Insensitive Not recommended
Compressed air Suitable for CIP Insensitive First grade
Minimal
Servicing of compressed air system and change of bags
Minimal
Good
Relatively good
Less good
Servicing of compressed air system and change of bags Good
Plant and Equipment | Milk Dryers: Dryer Design
a fat content higher than 30% be conveyed, as blocking may occur in the ducts. Air at any temperature may be used, and the powder temperature will naturally follow the air temperature. If hot air is used there will be a drying effect. This will, however, be minimal, as the residence time is short (air velocities of 20 m s1). A pneumatic conveying system is inexpensive and can handle large quantities of powder, but it will destroy any agglomerates, resulting in a powder with maximum bulk density. The powder is separated from the conveying air in a cyclone. Fluid Bed After-Dryer/Cooler In modern dryers, pneumatic conveying and cooling systems are replaced by a VIBRO-FLUIDIZER, which is designed also as an after-dryer, that is, drying is divided into two or more steps. The first step is done in the spray drying chamber transforming the liquid into powder particles and evaporating the main portion of water. The subsequent drying is done in a fluid bed (see Figure 22). The fluid bed drying technology has proved
especially suitable, as the residence time in the fluid bed is so long that the moisture from the center of the particle can reach the surface from where evaporation takes place. The VIBRO-FLUIDIZER is a horizontal box divided into an upper and a lower section by a perforated plate welded to the side wall of the box (see Figure 23). For drying, or alternatively cooling, warm or cold air is introduced into the air plenum chamber, which is distributed evenly over the whole area of the perforated plate. The perforation and amount of air are determined by the air velocity necessary for the fluidizing of the powder; however, special care must be taken to avoid attrition of agglomerates. The temperature and area are determined according to the required evaporation duty. The hole size in the perforated plate is chosen to give an air velocity high enough to fluidize the powder on the plate. The air velocity should be so high that the fines powder becomes airborne and leaves the fluid bed with the air, and is returned to the atomizing zone for agglomeration. The fluid bed can also be designed as a static back-mix bed integrated in the drying chamber.
Powder in
Air out
Drying air in Cooling air in
Powder out
Figure 22 VIBRO-FLUIDIZER.
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232 Plant and Equipment | Milk Dryers: Dryer Design
Figure 23 Construction detail of a sanitary VIBRO-FLUIDIZER.
Fines Return System Agglomeration means getting smaller particles to adhere to each other to form a powder consisting of bigger conglomerates/agglomerates, which are essential for easy reconstitution in water. By means of a fines return system, the cyclone fraction(s) is(are) conveyed back to the atomizer mist, the static fluid bed, or the VIBROFLUIDIZER, depending on the required degree of agglomeration. Fines return systems consist of the following: blowers (the quantity of air is depen• High-pressure dent on the amount of fines – typically, 1 kg of air can convey 3–5 kg of powder)
valves (devices to discharge powder • Blow-through from cyclones and/or bag filters into the conveying
• •
line) Conveying line/diverter valves to convey the fines powder to the desired destination – typically a 76–102 mm (3–4 inch) pipe Fines introduction to the atomization zone
The aim is to bring the fines as close as possible to the atomizer wheel. In modern dryers, fines are introduced from above through the air disperser (FRAD system) via four fines pipes situated just above the atomizer cloud. Deflector plates at the end of each fines pipe ensure a correct introduction and distribution of the fines (see Figure 24).
Cooling air
Fines Concentrate
Figure 24 Fines return for rotary atomizer FRAD.
Fines
Plant and Equipment | Milk Dryers: Dryer Design
233
Cooling air Concentrate Fines Fines
Concentrate Cooling air
Drying air
Cooling air
Rotary air steam
Plug flow air steam
Figure 25 Fines return for nozzle atomizer.
For nozzle atomization, the fines return is an integral part of the nozzle unit with the fines duct in the center surrounded by nozzles at the periphery (see Figure 25), provided the dryer is designed for rotary air flow or is with vertical air flow.
See also: Analytical Methods: Sampling; Sensory Evaluation. Dehydrated Dairy Products: Milk Powder: Physical and Functional Properties of Milk Powders; Milk Powder: Types and Manufacture; Infant Formulae. Milk Protein Products: Milk Protein Concentrate; Whey Protein Products. Plant and Equipment: Evaporators; Milk Dryers: Drying Principles. Rheology of Liquid and Semi-Solid Milk Products.
Conclusion Spray drying plants are designed today to fulfill many requirements, including low energy consumption, high final-product quality, reduced space requirements, and a high degree of environmental protection – a challenge taken up by the designers and suppliers of the dryers.
Further Reading Masters K (1991) Spray Drying Handbook. Essex: Longman Scientific & Technical. Pı´secky´ J (1997) Handbook of Milk Powder Manufacture. Copenhagen: Niro A/S. Westergaard V (1994) Milk Powder Technology. Evaporation and Spray Drying. Copenhagen: Niro A/S. www.niro.com.