Module 2. Drying

Lesson 14


Pneumatic Conveying and Cooling System

A pneumatic conveying system is established when powder has to be conveyed from one place to another. Products with high fat content require more air (5 times powder) than skimmilk (4 times powder). It is however not recommended to convey powders with high fat content than 30% otherwise blocking may occur. in the ducts

Air of 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/sec.).

A pneumatic conveying system is usually established in connection with a spray dryer of conventional design. Ambient air is used thus providing a dual advantage, namely both conveying and cooling of the powder. In order to get the full benefit of the cooling effect it is recommended that there is a lock between the chamber outlet and the duct, which will prevent warm moist air from entering the conveying system. This is usually done by means of a rotary valve. Pulsations in the powder flow as a result of deposits falling from the wall should also be avoided. This can be done using an air conveyor with a perforated plate through which cold air is blown. This will simultaneously have a cooling effect.

The conveying air duct will be passed via the outlet of the main cyclones picking up powder from here. The air/powder stream is passed to a cyclone separating the particles from the air. At the base of the cyclone the powder should be shifted after which it is ready to be bagged off. If climatic conditions are so that a suitable powder temperature cannot be obtained due to high air temperatures, or the humidity in the air is so high that the powder may pick up moisture from the air, cooling and dehumidifying equipment, where the cooling is done by circulating chilled water at 0-1°C, can be established. The temperature to which the air should be cooled depends on the product and ambient conditions, but it is usually around 8oC. Water will then in most cases condense, and it is necessary to include a section for removing this water which is usually done by passing the air over a set of labyrinths. At the outlet the air will be free from water droplests, but the relative humidity will be 100%. To avoid the risk of further condensation and development of water droplets, which will be picked up by the powder thus increasing the moisture content, the air is heated in a subsequent heater to 15-20 °C reducing the relative humidity.

The cooling can also be done by a refrigerator with direct expansion in coils in the airstream. A pneumatic conveying system is cheap and can handle big quantities of powder, and it will brake up any tendency of agglomeration resulting in powder with maximum bulk density. For powders with agglomerates the pneumatic conveying system can naturally not be used.

Fluid Bed After Drying/Cooling

In order to improve the drying economy the drying is divided in two or more steps. The first step is done in a spray drying chamber transforming the liquid into powder particles and evaporating the main portion of the water. The evaporation of moisture from a particle will become more difficult and require more time, as the residual moisture content approached 0%.

The subsequent drying is done in a fluid bed. The fluid bed drying technology has proved especially suited; 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 the evaporation takes place.

In a fluid bed the drying air is introduced to the powder through a special perforated plate. The fluid bed can be vibrating, or stationary. The fluid bed offers at the same time a very efficient and lenient tool for cooling of fat-containing and agglomerated products.

Instrumentation and Automation

In order to control the drying process and at any time to be able to record the drying parameters the installation should include instrumentation and control equipment. This is usually placed together with a mimic diagram in a control panel which in many factories is placed in a separate control room, partly to keep the panel and instruments dry, but also because the operators here can be in a place with reduced noise level.

A control panel, for a modern spray dryer should contain instruments for all relevant processing parameters, incl. Inlet drying air temperature for the main chamber and fluid beds, as well as outlet air temperature. It is an advantage. If all temperatures are recorded on a printer enabling the operator to see the trend of the temperature development, and also to go back and find the reason why a powder has been downgraded in the laboratory. An ammeter and hour counter for the atomizer or high-pressure pump are also necessary.

The aim of a spray dryer control system is the maintenance of the desired dried product quality, irrespective of what disturbances occur within the drying operation and variations in feed supply. The most effective product parameter to control is the moisture content. Outlet air temperature is the parameter controlled. This temperature represents product quality, i.e. bulk density, colour, flavor, activity as well as moisture content.

Spray dryers can be controlled either manually or automatically. Manual control is applied to small plants (Laboratory, pilot- plants or small industrial sizes). Manual control can be applied to large industrial units, but the demands of continuous operation on operating personnel, the maintenance of constant product quality over lengthy durations of productions makes automatic control (semi or full) a virtual necessity.

Control is accomplished by maintaining a set outlet temperature through varying (a) the feed rate to the dryer, hereafter denoted control system A, or (b) inlet drying air temperature, hereafter denoted control system B. Control system A is the more widely used of the two systems. By applying automatic control to the outlet air temperature, product moisture can be held within very narrow limits.

Spray Dryer Control System


1. Outlet temperature control by feed rate regulation.
2. Inlet temperature control by air heater regulation.

The control system is illustrated in figure-I. The system consists of two control loops of quick response. Quick response loops are the desired control characteristics to prevent adverse drying conditions.

The air temperature in the exhaust duct is measured and transmitted to the temperature indicating controller (TIC) which counteracts any temperature deviation from the desired set points by varying the feed rate. The temperature of air to the dryer air disperser is measured and transmitted to a temperature indicating controller. Any deviation from the desired inlet air temperature is corrected by control of fuel and combustion air to the burner (oil and gas tired air heaters), steam pressure in a steam air heater, or power input to an electric air heater.

In the event of failure in the feed system (pipe or atomizer blockage, pump damage. pump control failure) where feed supply to the atomizer is drastically reduced or ceases, a safety system can be installed to prevent the outlet air temperature rising above a known safety level, since the system can be potentially dangerous for many products in as much that feed failure can lead to rapid use in outlet air temperature as the air heater continues to function. A built in safety system can shut down the air heater once a certain outlet air temperature is reached or water can be passed to nozzles positioned as safety measure, for example, in the dryer root.

Some safety systems switch feed over to water at the feed pump and operate on a two temperature level safety system. For a partial blockage of the feed system, the outlet air temperature will rise more slowly and on reaching the first level (say 300 ° F- 5000), feed supply is stopped and switched over to water. If the blockage is minor, the winter flow may dislodge the blockage and after a short time when the normal outlet temperature has been restored, so can the feed supply. If the blockage cannot be removed by water, or water cannot pass to the atomizer (in case of pump failure), the outlet air temperature will continue to rise to the secondary safe level after which the chamber is flooded by quench nozzles or the air heater is shut down or a combination of both. To draw attention to rise on outlet air temperatures, alarms can sound 20-40 °F. (10-20 °C) before any unsafe temperature level is reached.

The use of two level safety system gives the opportunity of restoring dryer conditions on water by using water to flush out a blocked feed or enable feed system failures to be rectified. If feed system difficulties can be overcome by a short operation on water feed, time will be saved by not having to shut down and restart the dryer.


1. Outlet temperature control by air heater regulation.

2. Feed rate held constant.

The control system is shown in figure 2 and is used particularly for dryers with nozzle atomization where wide variations in feed rate cannot be handled. The outlet air temperature is measured and transmitted to a temperature indicating controller (TIC). To compensate for any deviation from the desired outlet air temperature, the heat input to the dryer is adjusted by the controller through regulation to the combustion rate in the gas or oil air heater (if fitted) or steam pressure at the steam-air heater (if fitted).

From a theoretical view point, this is an inherently safe and acceptable as any increase in outlet temperature brings about a decrease in heat input to the dryer. From a practical view point however, the system was operational disadvantages of lengthy control large due to time lags in the heater control circuit. These time lags can increase outlet air temperature fluctuations. The system has inability to handle effectively variations in feed solids, but it can be improved by outlet temperatures cascaded on the inlet temperature controller which controls the heater.

Safety systems similar to these used in control system A are used. When using a two level safety system, as the outlet air temperature reaches first level, water is flushed in dryer and on reaching the second safety level, the heater is automatically shut down.

Feed System Conlrols

Systems for rotary atomizers

Control system A is adopted with direct feeding from a suitable positive displacement pump with variable speed control, a centrifugal pump throttled by a control valve, or a gravity feed constant heat tank with a control valve in the outlet pipe. Control system B is rarely considered.

Systems for nozzle atomizers

Both control systems are applicable but control system B is preferred. Control system B can be applied in two ways with the feed pump on a fixed manual control setting. The outlet temperature controller can be cascaded on to the inlet temperature controller as shown in figure 3, or linked directly to the heater. Pressure safety systems are incorporated to shut down the dryer if excessive pressure build up in the feed system due blocked nozzle(s). Control system A can be applied in two ways. The outlet temperature controller sets the pressure control loop in the feed system or is linked directly to the feed pump. It is usual for each nozzle in a multi nozzle assembly to be brought in manually during start up as the inlet temperature rises.


Interlocks are closely connected with the control and operation of dryer. Interlocks are installed to ensure safe start up, operation and shut down of the dryer. Some examples of typical interlocks are as follows

For prevention of drying chamber damage

An interlock ensures that an exhaust fan cannot be started before the supply fan. This is installed if there is a change of chamber collapse under the low negative pressure conditions caused by an exhaust fan operating alone. The interlock is usually over ridden when a chamber door is open. This allows the exhaust fan to operate alone for shut down and cleaning purposes. For example, for an "air sweep" chamber cleaning operation. An alternative system for prevention involves a vacuum switch mounted at the chamber ceiling in a powder free area. The switch is normally set to shut down the exhaust fan if chamber pressure reaches -4 in w.g. (-100 mm W.G.)

For prevention of wet chamber wall

An interlock ensures that feed cannot be passed to the rotary atomizer when not running. Furthermore the interlock will cause the feed to cease an atomizer failure.

For prevention of heater damage

An interlock ensures that the burner cannot be ignited unless the storage pump, burner fuel pump, combustion fan, and dryer fans are in operation. For indirect heaters, a differential pressure switch can be installed to check the correct air flow, and linked to the combustion and supply fans.

Precaution against burnt product

An interlock ensures that the main dryer fans cannot be started unless the necessary cooling fans are in operation. The fans cool potential hot areas in the chamber where build-up and scorching of product can take place.

Full automation of spray dryer and feed pretreatment

The full automation of the spray dryer as described above will maintain a uniform product quality as long as input feed compositions do not vary widely during operation. To reduce variations in feed solid concentration, attention must be paid to pretreatment section that supplies the spray dryer. The maintenance of constant feed solids to the spray dryer is best achieved by coupling the pretreatment section to the spray dryer and applying control system analysis to the two parts as one unit.

The coupling of an evaporator (pretreatment section) to the spray dryer has been achieved within the dairy industry. Notable advantages achieved, apart from control of feed conditions to the dryer include the elimination of intermediate feed tanks and great precision in the dosing of additives. Feed concentrate does not come into contact with ambient air.

The control parameter is the total solids in the feed concentrate leaving the evaporator and is based upon the continuous measurement of density, or continuous weighing. Systems are based upon hydrometer measurement, refractometers or electronic sensors but all have disadvantages to various degrees in accuracy, reliability or unsuitability to certain products. Continuous weighing techniques of a given volume, however, have been successfully applied. One of such measuring equipments is known as U-tube equipment.

The milk or steam supply at the evaporator is controlled by signals from the U-tube transmitter. Four possible control arrangements are shown in figure 5. System A has the evaporator as the dominating unit. (The evaporator is termed the master spray dryer is termed the slave). Systems B-D are inverse with the spray dryer playing the dominant role in the control.

In system A, signal from the total solids detector (i.e. U-tube equipment) control the input of raw feed to the evaporator. The steam pressure is held at a fixed setting. The variations in the amount of concentrate due to deposit formation in the evaporator are compensated by to the spray dryer inlet temperature.

In system B, the spray dryer inlet and outlet temperatures are fixed. The total solids in the concentrate are kept at the required value through alterations of feed input. The evaporative capacity of evaporator is changed through alteration to the steam pressure supplied to the thermo-compression stage which itself is controlled by a level controller in the last effect of the evaporator.

In system C, steam pressure regulation is governed by the solid content passing in the feed passing to the spray dryer. The raw feed input to the evaporator is governed by level control on the last effect of the evaporator.

In system D, steam is added to the finisher stage and is independent of steam supply to the main evaporator stages. The total solids control the steam pressure to the finishers, and the level controller regulates raw feed input to the evaporator. This system has faster response characteristics.

Cleaning-in-place (C.LP.) can be built into the control system to operate automatically on evaporator shut down. There are considerable advantages in establishing a fully programmed C.I.P. system for the spray dryer too. Full benefit of evaporator and spray dryer automation is achieved only when other sections of the factory are under automatic control, but the time is fast approaching when instrumentation for a whole factory incorporating spray drying plant is centralized, and the control room linked to those already established and publicized in the modern oil refining industry. Electronic instrumentation is now being applied more widely to spray dryers. More extensive usage appears inevitable with computers for process control of complete plants that include a spray drying stage.

Factors affecting bulk density of powder

Bulk Density – “ Mass of powder which occupies fixed volume” (mass per unit volume) it is also termed as packing density or apperent density, unit= gm/ml

This property is important because :

1. It influence packaging, transportation cost and storage space. It is functionally important. Low B.D powder requires larger velocity to pack so total volume during transportation will be more per unit wt. So transportation cost increase, so mfr desires heavy powder
2. Functional property also influenced by this basic property .
e.g reconstitusion is rapid by lighter powder as water can easily entre inside particle.

-roller dried have low BD than spray dried so spray dried is heavier. In roller dried BD mainly depends on fitness of milling and grinding range 0.3-0.5 gm/ml. Irregular shape particles will make avoids of air and so BD LESS.

Spray dried has BD higher: - because uniform shape so close packing of particles and loss void(free)space. partical density is less in case of spray dried.particle density of individiul particle, it depends on

I. Nature of milk

II. Amount of enterupted air

I. Whole milk 1.032 g/ml, skim milk 1.0362-1.04 g/ml this affect particle density

II. There is a void of air and air particles in spray dried while roller dried particle is compact so it has more particle density

There is less void left in between the particle of spray dried as small particle will fit in void of big particle so particle density are different

I. Nature of milk

II. Particle density

III. Void volume left

Instant powder

Random fusing of particle gives irregular shape so more void left and agglomerates are porous in nature (rewet powder). Straight thr‘ may so slight higher bulk density than conventional. So in straight thr’ we can use higher TS temperature history of particle different

In the first stage of drying the particle size is decided and final shape decided in second stage depending on temperature difference. So higher TS can be used to have high bulk density.

Depending on temperature difference of inlet air and drying particle temperature which decide how quick the crust is form. If skim forms rapidly, moisture within the particle is gradually escapeting. The vapour pressure developed inside the crust. As vapour goes out, its place will be taken by air. As high ts , amount of vapour escape is less so less volume will left for air entrapment. So high bulk density.

If vapour pressure inside the particle is very high and cause bulging effect.(expansion of particle size)so powder become higher so less B.D. B.D. of spray dried=0.05-0.8gm/ml

Rewetted powder process statting of powder itself. So its B.D.is expected lower bulk density then that of powder from which it is made.

During manufacturing of straight the powder we are removing the fines and thus it tends to decrease bulk density as it fines are present, they will be occupied in the gap in between the particle having irregular shape.

Measurement of bulk density

Bulk density in four terms

1. Fixed weight is taken and volume occupied is to be found out = gm/ml without tapping.

This is poured bulk density as render size of particle- poured bulk density varies widely

2. Loose B.D. = tapping for 10 times.

3. Tapped B.D. = tapping for 100 times.

4. Tapped to extreme density = tapping is done for 1250 times.

Normally 3rd type of tapping is done for more reliable and consistent B.D. no. Of tapping should be specified.

- Feed composition- high protein and low fat = (whipping)

- So foaming ability is more is SM

- Nature composition –more denatured WP –test foaming as WP give good whipping

- Feed concentration –high TS =less foaming and more viscosity also = less ent-air

- Feed temperature – high temp. low foaming

- Preheating- denaturation of WP = high denaturation – less foaming

- A.Tk – effect on viscosity- affect the amount of air to be incorporate. More air incorporated= more occulted air in powder unless de-aeration is done

- A.Tk=more viscosity =high air =low B.D.

- Type of atomizer- nozzle atomizer=less entrapped air than disc type, but disc type dcan use high concentrated milk so effect of type is compensated.

- A new atomizer- steam hustling (‘S’ vanes) reduces chances of air incorporation.

- Speed / pressure

- Higher the air in feed = higher air in the product= low B.D.

- Feed viscosity= coarse particle= larger size

o Better flow ability

o Lower outlet air temp.

o More moisture

o More interstitial air

- Outlet temp – lower temp =higher moisture

- Fine returns – to increase dispersibility and facilitate agglomeration high fines = more agglomeration

- B.D. is affected by two – main factors

1) P.D, 2) Interstitial air

- conventional spray drying process 11% Ts

- outlet = 90°c gives 0.39 gm/ml B.D

- sp.vol = 2.54 ml/g

- same process – but 34% Ts

thus high Ts = more BD BD sp.vol

0.55 1.81

0.66 1.51

0.46 2.19

-Same process Ts =47

-Same process outlet =113°c
-Thus outlet temp increase =B.D decrease

Straight through process

BD sp.vol

0.63 1.60

0.33 3.0

Rewet process - low B.D large particles

Limit – packaging is costly


Last modified: Friday, 2 November 2012, 11:19 AM