Troubleshooting operations

The expression troubleshooting may sound quite dramatic. However, what is understood here as troubleshooting is in fact any intervention that changes the operational parameters in order to obtain the desired result. An operator of a spray dryer is exposed every day to situations where he has to decide what to do if, for instance, the final moisture content of the product is suddenly too high. This is also a kind of mini-troubleshooting intervention. On the other hand there are troubleshooting operations requiring great effort in analyzing the situation and proposing a strategy for tackling the problem. This may be the case if one of the more sophisticated properties of the powder is deviating from the specification, due to lack of dryer capacity, formation of serious deposits in the dryer, bacteriological problems etc.

It is impossible to give exact instructions for each case. However, the paragraph below presents at least some guidelines for the most typical problems. The first approach to most problems involves:

- careful study of the production documentation around the time when the problem appeared, especially:
- log-sheets to find out any deviations from usual operating conditions,
- maintenance log-books to find out what changes or maintenance work on the dryer or its components have been conducted,
- laboratory analysis records to find out if any change of product quality has been detected,
- check the calibration of the control instruments which may have some connection to the problem,
- check independently the laboratory results,
- conduct laboratory analyses of the properties, which may have some connection to the problem, but are not part of the daily routine analyses,
- interview the staff individually about observations and opinions.

12.1.

Lack of capacity

It is not unusual that lack of capacity appears gradually or suddenly on an installation, which has been in successful operation for many years. There is only one reason, which may cause the lack of capacity: the specified amount of heat for evaporation is not available. This again may be caused either by lower T (too low inlet or too high outlet temperature) or lack of drying air. The following diagnostic steps are recommended:

- conduct air flow measurement as described in section 6.6, preferably water evaporation test and cyclone pressure drop measurements. If the air flow is too low, check the damper positions and pressure drops over all components in the system, i.e. air filter, air heater, cyclones etc.,

- check the inlet temperature: the cause of low inlet temperature can be too low steam pressure. If the steam pressure is correct, the problem may be caused by loose fins in the steam heater or by malfunction of condensate traps,

- check the outlet temperature: the cause of too high outlet temperature (when obtaining the same moisture content as before) may be either poor atomization or wrong air flow pattern (air disperser adjustment),

- poor atomization is due to either malfunction of the atomizing device, or changes of feed properties,

- check rpm and direction of rotation of the rotary atomizer and the performance of the liquid distributor,

- check atomizing pressure, sizes and condition of nozzle inserts of pressure nozzle system,

- check the properties of the feed, especially viscosity. Too high a viscosity can be caused by increased heat treatment (pasteurization) conditions, increase of total protein content of the processed milk, increased solids content or higher homogenization pressure in case of fat-containing products.

With rotary atomizer operations, the pressure on the top point of the feed line should be positive - negative pressure can cause pulsation of feed supply.

Pressure nozzles require a daily check. Spray nozzle performance has a critical influence on the operating costs as well as on the quality of the product. It is easy to control a single nozzle plant by comparison of atomizing pressures of successive operations, while malfunction of one nozzle in a multi-nozzle assembly is less apparent. A worn nozzle exhibits a dramatic increase of flow rate and consequently larger droplet size. The performance of nozzles can be

checked by following methods:

- on each start-up on water, adjust the inlet temperature for operation with one nozzle. Under constant feed rate exchange the nozzles one by one and check the pressure. A difference >10% requires attention,

- simultaneously with the previous step check the atomizing cloud visually (preferably against the light source). Correct nozzle operations exhibit a uniformly transparent cloud while streaky sections indicate an erosion problem. A difference in spraying angle indicates the same,

- clean the nozzle inserts carefully manually after each operation. Do not use hard tools but wood, plastic materials and brushes. Do not use cleaning solutions, especially acid which may cause corrosion,

- check the conditions of inserts frequently under the microscope.

Many modern plants are nowadays equipped with nozzle test stands, where most of the above mentioned checks can be performed.

Some typical examples of capacity troubleshooting actions are given below:

Case I: An old dryer, which has been in operation for 20 years, has during the last several months exhibited a gradual capacity decrease, which finally resulted in only 60% of specified capacity. A check of cyclone pressure drop indicated that the dryer had too low an air flow and this was confirmed by a water evaporation test. Further investigation of pressure drops in the system revealed a high pressure drop across the main air filter, and visual inspection confirmed that the filter material was dirty. Mounting of clean filters cured the problem.

Case II: After an overhaul a dryer exhibited lack of capacity, as a higher outlet temperature than previously was required to achieve the same moisture content as before. A check of the cyclone pressure drop confirmed that the air flow was as specified. Investigation of feed properties did not give any indication of increased viscosity. Powder properties in comparison with samples of earlier produced powder showed slightly higher mean particle size and higher

content of occluded air. Thus the attention was directed to the atomization. It was found that the rotary atomizer which operated with a curved vane wheel was rotating in the wrong direction. During the overhaul, the phases on the atomizer motor were incorrectly connected.

Case III: During commissioning of a new dryer, the maximum obtainable inlet air temperature was about 20°C lower than specified. The steam pressure and temperature were correct. Closer investigation indicated that the fins on the steam heater were loose.

Case IV: An old dryer suddenly required 6°C higher outlet temperature to obtain the specified powder moisture. Nothing unusual was found during the whole checking procedure as outlined above. The closer inspection of the system revealed that the reason was leaking steam heaters.

Generally the lack of capacity due to lack of main air flow is relatively seldom and easy to find and cure. On the other hand, a lack of air appears quite often on fluid bed dryers, both with fluid beds integrated in the dryer chamber base and externally mounted ones. This, however, does not result in a too distinct capacity loss - if any at all - but can cause fluidization problems leading, in the extreme, to plugging of the fluid beds. This problem develops slowly and is caused by a gradual, partial blocking of the holes in the perforated plate in combination with the air flow being controlled by keeping a constant pressure below the plate. Consequently the best way to avoid this problem is to keep a constant air flow. This can be done by controlling the air flow frequently by means of Pitot-tube measurements (see section 6.5) and adjusting the damper accordingly. A better way is an air flow meter installed in the air inlet duct, combined with automatic damper adjustment for constant air flow.

Fig. 12.1 shows measured values of static fluid bed air rates and plenum pressures during the period of 10 days between CIP. The air flow was controlled by a flow meter and kept practically constant (56,860-57,380 kg/h), i.e. the fluidizing velocity was also constant (0.955-0.961 m/s). The third curve on the graph shows what the air flow would be if the plenum pressure is kept constant i.e. 152 mm WG as it was after CIP. The fluidizing velocity would gradually drop from 0.957 to 0.848 m/s.

**Şek.12.1. Demonstration of the effect of a dirty plate on fluid bed air rate**

Obviously a fluid bed, which is supposed to operate with powder of higher moisture content, is exposed to higher risk of deposits and blockages than a fluid bed operating with dry powder. Therefore an integrated fluid bed is also affected faster by powder deposits than an externalone. However, such deposit creations do not take place that much during normal operation, but rather during plant start-up and shut-down.

12.2.

Product quality

In a factory producing the highest quality products, small adjustments of the plant operating conditions are part of the daily routine in order to compensate for small changes in external factors, mainly air humidity and milk composition. Factors influencing individual properties are described in chapter 10. The most important product properties, i.e. moisture content, insolubility index, scorched particles and bulk density are usually detected by process control in 1 - 2 hour intervals. The key person to decide what action to take to re-establish the desired target powder specification is the operator.

The task of the technologist is to follow the variations in incoming milk and powder quality and to recommend the operating conditions for the operators. However, it is not unusual that suddenly, without any obvious reason, quality exhibits a serious deterioration. A more extensive investigation must then be conducted following the guidelines presented in the introduction to this chapter and using the guidelines for all involved individual properties as described in chapter 10. One of the external conditions, which can fluctuate quite extensively and which may have considerable influence on both powder quality and capacity, is the ambient air humidity. It is surprising how little attention is paid in practice to this factor. A humidity-meter placed in the main air intake can eliminate the necessity for a powder moisture adjustment, thus enabling a preventive correction before a deviation appears.

Fig. 12.2 shows levels of air humidity during a two week period (February 1992) in New Zealand (North Island). It is interesting to observe that the daily variations are between 3 - 7 g/kg. The maximum humidity in this period was 21 and minimum 11 g/kg. This would mean that for a single stage spray dryer operating at a constant outlet temperature, the moisture fluctuations would be in the range of 0.86% (see Fig. 10.1) or alternatively it would be necessary to adjust the outlet temperature within the range of 4.3°C.

**Şek.12.2. An example of ambient humidity fluctuations**

Fig. 12.3 demonstrates the average month’s values and maximum month’s values of air humidity on the European continent near Munich. The daily variations cannot be seen on this graph, but the difference between average and maximum values indicates that they are of the same order of magnitude as shown on the previous figure.

Another factor often ignored, but with a great influence on the powder quality, is the variation of the composition of the processed milk.

**Şek.12.4. An example of seasonal variation in protein content**

Fig. 12.4 demonstrates the variations of total protein content in non-fat solids of milk in a region on the North Island of New Zealand. Such variations are extreme and seldom seen in other parts of the world. In Europe the maximum

value is about 42% protein in nonfat solids and the variations are smaller. The influence of protein variations on viscosity is shown in Fig. 9.7. Generally, milk with up to about 40% protein can be processed by adjusting the operating parameters, mainly the concentrate solids content correspondingly, but above this level, protein standardization is advisable.

12.3.

Deposits in the system

A build-up of powder deposits in a spray dryer is obviously undesirable due to:

- product loss,

- downgrading of the powder quality,

- jeopardizing a smooth operation, with the need for more frequent dryer shut-down,

- requiring more frequent dryer cleaning (washing) and thus increased down-time,

- initiating stainless steel corrosion,

- increasing risk of spontaneous combustion and fire.

Powder deposits can occur in any place in the system, i.e. chamber, fluid bed, ducts, cyclones, bag filters, rotary valves etc. The creation of deposits depends on the combination of four main factors:

1. Poor adjustment of air/spray cloud mixing, usually related to the setting of the air disperser.

2. Too high a moisture content of the particles reaching the wall of the chamber, duct, cyclone, fluid bed etc. Two-stage process is therefore more exposed to the risk of deposits than single stage drying, and the higher the moisture content from the primary drying stage, the higher the danger of product build-up.

3. Certain chemical and physical properties of the product, i.e. hygroscopic and fat-containing powders tend to create more wall build-up than non-fat and non-sticky products.

4. Product type, i.e. non-agglomerated products especially in combination with two stage process are more likely to form deposits than agglomerated powders, due to their poor flowability.

Dry powder deposits occurring in relatively cool (i.e. close to outlet temperature) places in the system and reaching just a few mm in layer thickness during operation and disappearing during plant shut-down can be considered of no importance regarding powder quality or risk of fire. However, thick wet deposits on the chamber walls and cone present a potential risk of developing heat by the Maillard reaction, leading, under certain circumstances, to fire. Also deposits occurring around the hot edge of the air disperser, if discoloured and burnt, exhibit a high potential risk. Fire risk is discussed further in section 12.4.

If increased amounts of deposits occur in the drying chamber with a product, which has previously been produced without difficulties, it is advisable to compare all the present operation parameters, including moisture, with those from earlier operations and, if no difference or fluctuations are found, the calibration of the measuring instruments should be checked. The feed conditions have also to be checked whereby special attention must be paid to the viscosity and acidity. Excessive homogenisation pressure on fat containing products can also be the reason for high viscosity.

For a new spray dryer the first approach to solve the deposit problem in the chamber is to check whether the feed and atomization systems are operating correctly and to inspect the atomizer cloud (see also section 12.1). An air disperser for a rotary atomizer must have adjustment possibilities. Usually these are (see 4.2.4):

a) the height of the guide cone - lowering the cone increases the gap and thus the amount of inner air, passing down over the atomizer wheel edge,

b) outer vanes on the guide cone - controlling the rotation of the outer air and swirl in the drying chamber,

c) inner vanes on the guide cone - controlling the rotation of the inner air.

The spray cloud can best be observed if portholes are situated at the level of the atomizer facing a light source. It can be seen clearly on fat-containing or agglomerated products, but is more difficult with dusty powders. Under specified feed rate conditions the cloud should have a shape of a broad umbrella without any back flow (eddies). The water cloud is much steeper with the diameter of the lower part about three wheel diameters.

Adjusting air dispersers requires experience and should preferably be conducted by the equipment manufacturer.

A question often asked is whether it is possible to have a deposit-free system. Before answering this question, let us define the meaning of a deposit-free system and what the limitations are. A suitable definition is an installation, which can operate for a long period of time without the necessity of any cleaning between programmed shut-downs, and operates without any adverse effects on powder quality or safety. The achievement of such a goal requires elimination of the factors responsible for formation of powder deposits:

1. The deposits, which are observed after the operation, i.e. usually after 20 hours run, can have been created during just a few minutes of operation when operating conditions are out of control.

2. In most cases, the deposits are not created under normal operating conditions, but during the start and shut-down of the dryer. These are the most critical phases and very difficult to control even by experienced operators, if the plant is equipped only with simple control instruments, which just indicate inlet and outlet temperatures and amperes for the load of an atomizer or nozzle pressure.

Powder deposits can be created even during start-up on water because normally there is powder from the previous run circulating, which will be wetted. Such deposits, when touched, feel like sandpaper.

The most critical phase of an operation is the switching from product to water under a shutdown operation. At the moment when water reaches the atomizer, the feed rate must be cut down to about half. It happens very often that the water reaches the atomizer at a rate equivalent to the full product rate, and consequently wetting of the walls is the result. This cannot be seen immediately on the outlet air temperature because, due to air retention time in the chamber, the reaction is first seen with about half a minute’s delay. Moreover, part of this water reaches the walls without being evaporated and thus does not influence the outlet air temperature. A very helpful procedure for tracing the conditions during this critical phase is to place a thermometer close to the wall (at about half of the cylinder height). The temperature measured is very often well below the outlet temperature as measured by the thermometer in the control room. An example of such investigation is shown in Fig. 12.5, where the manually measured wall temperature is indicated by symbols, while the other parameters were obtained from the dryer’s computer. The shut-down procedure was in this case automatic using the outlet air temperature to control feed rate. The best way to solve start-up and shut-down problems is automatic control of feed rate based on in-line measurement of the total solids content of the feed.

**Şek.12.5. An example of poor shut-down procedure**

3. A similar effect can occur due to variations of ambient air humidity, as shown in Fig. 12.2. It can rise during a couple of hours quite considerably resulting in higher powder moisture if the outlet air temperature is kept constant. Measurement of the ambient air humidity is helpful to correct the outlet temperature before any problem will appear.

4. Type of drying process is of course of primary importance. Two stage drying has many advantages, mainly better powder quality, higher powder output and better economy. On the other hand, there is a disadvantage of higher tendency to powder build-up. Optimizing a two stage process as to proper outlet temperature and powder moisture is not exactly easy and requires trial and error. This means that the extent of two stage drying process has to be chosen with respect to the product in question, mainly to its moisture/stickiness relationship.

5. Type of product plays a dominating role and here is no alternative. In some cases, however, it is possible to influence product sticking behaviour. For instance, pre-crystallization of lactose helps enormously when drying whey products, agglomeration generally improves the dryer performance, and dosing of free-flowing agent through the chamber ceiling helps to overcome deposit problems with high fat products.

6. Drafts of cold air along the chamber and other dryer components, especially cyclones, may cause deposits. Pressure relief doors or panels, which according to the safety directives lead to the outside free area and have heavy frames, may create cold bridges that generate deposits. Thus any draft of cold air in direct contact with the crucial parts of the installation has to be avoided. Good results have been obtained by shielding the pressure relief doors or

panels from the outside atmosphere with plastic foil or polystyrene panels, and heating the space next to the doors to about 60 - 70°C.

7. If the surrounding temperature around the dryer is too low, deposits may appear on the supporting structure of the chamber. Deposits do not arise only by accumulation of wet, sticky material on the walls. Also dry powder can create powder-drifts on the leeward side of previous deposits. It is not unusual that, in order to provide a pleasant atmosphere for the personnel, the whole dryer area is air-conditioned. Apart from such an installation requiring

relatively high both initial and operating costs, it is often a source of problems when drafts of cool air flow across vital parts of the installation. Besides, with the well instrumented and operated plant, there is no reason for operators to stay long in the dryer area.

8. The air disperser is one of the critical points where a dangerous deposit can occur around the hot edge. The hot edge is normally cooled by blowing relatively cold air around it or by forcing cold air to pass a space between the hot air disperser parts and the chamber ceiling. If this air is too cold, condensation may cause an annular deposit around the air inlet. The inner edge of such deposit is often burnt. It can be solved by heating the air to not less than 40°C.

9. A very important factor is also the temperature profile in the hot air duct prior to the air disperser. Temperature stratification can be caused by malfunction of the steam air heater, i.e. condensation pots or leaking finned tubes, or by an uneven air velocity profile into the heater.

10. The reason for deposits can be poor atomization due to worn-off nozzles. It is advisable to frequently check the state of nozzle inserts under the microscope. For rotary atomizers poor atomization can take place after an overhaul, if the wheel is rotating in the wrong direction. However this has a minor effect with a straight vane wheel, but can be a serious problem with a curved vane wheel.

It has been mentioned above that deposits can also cause corrosion of stainless steel. This can happen with powders containing chloride ions, e.g. hydrochloric acid whey. Apparently innocent powders can also initiate corrosion. Corrosion has been shown to occur when drying coffee whitener having glucose syrup, manufactured by the acid conversion process instead of the enzymatic. Such pit-corrosion does not appear during normal operation, but during the down-time period if the dryer is allowed to stand cold, with powder deposits, for longer periods of time.

The best way to avoid start-up and shut-down deposits is logically to avoid these procedures. The dryer can operate continuously for long periods with many products, and the only reason for interrupting the operation is the necessity of CIP, mainly of the evaporator and the feed system including the atomizing device. The problem of the evaporator can be solved by installing a second evaporator which will feed the dryer while the first is under CIP. As to the atomizing device, nozzles have an advantage being interchangeable. Another possibility is installing a second feed system involving feed pump, feed line and nozzles and to replace nozzles of system I by nozzles of system II during operation. A number of plants equipped for continuous operation are now in operation.

Very helpful means for approaching a deposit-free chamber is Computational Fluid Dynamics (CFD) program. The use of CFD design concepts minimizes deposit formation and removes potential ignition sources. This is a great benefit when designing new plants. An example of a CFD-diagram illustrating the air flow in a Multi-Stage-Dryer is shown in Fig. 12.6.

Şek.12.6. Example of CFD diagrams. A. Air velocity profile. B. Evaporation rate. C. Temperature profile.

12.4.

Fire precaution

Fires in milk powder plants have probably been experienced from the beginning of using dryers, but the first publications on this subject appeared around 1970’s by Písecký [40] and Sapryngin & Kiselejev [41]. The main reason for these accidents was that the potential danger had not been recognized at that time. Two main factors were responsible for a series of fires during that period: starting production of high-fat milk powders and introducing a new process element, i.e. a fluid bed, into the system. An explosion may also occur under certain circumstances.

In order to be able to evaluate the risk of fires and explosions it is important to know all characteristics of the products produced. Today many plants are multi-product plants operated with frequent changes of products. Plants are often pushed to their capacity limit, increasing the risk of fires and explosions. It is important to realise that only the product produced that can ignite, not the plant.

Fires and explosions can be ignited in many ways and in EU Directive ATEX 1999/92 and EN 1127 a list of 13 potential ignition sources that have to be listed. Some of these may not be relevant for spray drying or for the products produced in the milk industry though.

Every product should have a MSDS (Material Safety Data Sheet) with the following data:

LEL = Lower explosion limit

MIT = Minimum ignition temperature of dust cloud or gas

LIT = Minimum layer temperature (5 mm dust layer)

MIE = Minimum ignition energy

MAIT = Minimum auto-ignition temperature (Grewer)

LOC = Limiting oxygen concentration

Kst = Maximum rate of pressure increase (1 m3 vessel with dust)

Kg = Maximum rate of pressure increase (gas)

Pmax = Maximum explosion pressure

BZ = Burning behaviour (Brennzahl)

Experience has shown that the main reasons for fires are the following:

1. Deposits in the system leading to heat development and spontaneous combustion in the powder deposit layer due to the exothermic Maillard reaction between carbohydrates and protein.

2. Deposits on areas exposed directly to the incoming hot air and becoming overheated, burnt, and eventually glowing.

3. Particles in the supply air getting hot in the air heater.

4. Friction of metal parts, causing local areas of heating.

5. Incidents caused by inappropriate dryer operation.

Self-ignition within deposits is the most important cause of fires in milk powder plants. The exothermic reaction develops heat due to reaction between carbohydrates, usually lactose, and proteins. The rate of this reaction depends upon the thickness and porosity of the layer, surrounding air temperature and composition of the product. The time necessary for developing a smouldering or glowing core within a deposit varies between one and many hours (see Table 12.1).

**Tablo. 12.1.** Critical conditions for fire development. (For whole milk powder)
Thickness of powder layer in mm	Temperature °C	Time hours.min
10	204	00.08
20	173	00.40
30	156	01.00
40	145	02.10
50	133	04.00
60	128	06.30
70	122	10.15
80	115	14.00

Therefore, the most hazardous powders are theoretically those consisting mainly of lactose and proteins, like skim milk powder. However, the heat created by the exothermic reaction partly dissipates out of the layer due to ventilation and heat conduction. The rate of heat dissipation depends on the layer porosity. Skim milk powder deposits are usually porous so that the generated heat can easily dissipate. On the other hand, fat containing powders, especially high-fat products can create compact non-porous deposits in which the fat seals the pores, preventing heat release. The particle size of powder deposits plays an important role too. Fines of particle size less than 50 μm, having high specific surface area and little interstitial air have a higher fire risk than a powder with particle size above 100 μm. This explains many of the incidents in bag filters. The critical self-ignition temperature depends on the product

composition. It is lower with fat containing powders and especially when using fat, containing a high amount of unsaturated fatty acids. Higher moisture content will accelerate the selfignition process as well.

Apart from the hot spots around the air inlet, the temperature in the system is seldom higher than 100°C. The exothermic reaction under these conditions cannot take place in layers thinner than 50 mm. However, heat development can reach critical levels within a few hours with layer thicknesses above 150 mm.

Agglomeration helps to increase layer porosity and thus reduce the temperature rise. If all the conditions for positive heat and temperature development are available, a glowing core will form inside the layer.

The exothermic reaction leading to spontaneous combustion can take place also in powder deposits in other components than the chamber itself, i.e. cyclones, bag filters etc. The reaction rate depends also on the specific surface area of the product and therefore may proceed faster in deposits consisting of fines.

Deposits on hot spots exposed directly to the hot drying air become burnt and glowing on their surface. Such spots can occur around the air disperser and the atomizing device. The most frequent cause in pressure nozzle dryers is concentrate leakage at the nozzle assembly.

A wheel of a rotary atomizer can generate heat due to friction with the atomizer skirt or liquid distributor if incorrectly assembled or with the deposits if the wheel is not properly cleaned. Generally, a flooding sensor to detect concentrate leakage is standard component of both nozzle and rotary atomizers and the operation must be stopped whenever flooding occurs, in order to avoid concentrate entering into the air disperser.

The appearance of glowing material inside the dryer does not always leads to a fire. There have been cases where a glowing particle or lump has left the system without causing any harm. If, on the other hand, such a particle or glowing lump falls into a fluid bed and mixes with fine fluidized or elutriated powder particles, then a dust explosion with subsequent fire can take place.

Another cause of ignition can be impurities in the air supply system both prior to and after the air heater and inside the air disperser. Reasons of such contamination of the heating element or hot surfaces may be:

- absence or not properly working air filters,

- splashing of concentrate into the air disperser,

- CIP-device spraying into an air disperser, which is not properly shielded,

- natural draft immediately after plant shut-down, causing fine particles to flow back into the air disperser,

- fines return system causing fines to blow into the air disperser.

A light source in front of a chamber porthole (inspection port) should never be installed without a timer switch as permanent operation can cause overheating the deposits on the glass up to ignition temperature.

The return of fines to the atomizer cloud for agglomeration requires special attention as this operation takes place close to the hottest parts of the dryer. The reason for any deposits appearing round these hottest parts must be found and removed.

The outlet air passing the cyclones is relatively cold so that there is no danger of direct overheating. However when wall deposits and cyclone cone blockages occur exothermic reactions can develop heat and cause smouldering even at that low temperature after sufficient time. Therefore cyclones are components to which extra attention should be paid frequently during operation. Measuring the cyclone tip surface temperature is a useful and inexpensive method to detect a blocked cyclone. The surface temperature of a blocked cyclone is considerably lower than under normal running conditions.

There are known cases where fire has been caused by welding works close to an operating dryer. The consequences of flame sterilization of a spoon for microbiology samples at the fluid bed sampling porthole have been experienced as well. Such happenings may sound an extreme. However, many cases of fires, the cause of which has never been found, may very well belong to this category. A person involved in such cases and surviving the accident with shock

is not always willing to disclose the facts. However, this emphasizes the necessity of training and education of the entire staff in order to avoid such accidents.

The simplest procedure to detect the beginning of heat discolouring of the powder is the scorched particle test (see section 11.5) conducted frequently in at least hourly intervals. Many, if not most, recorded accidents could have been avoided if this test had been conducted and consequent action taken, i.e. the installation stopped immediately. No statistics are available on the number of fires appearing after a positive scorched particle test, when operation was allowed to continue just to empty the evaporator. However, the number would probably not be negligible.

Nowadays temperature surveillance of nozzles and areas around them can be done by means of infrared cameras (GEA Niro SPRAYEYE™).

A dust explosion occurs when air-borne, finely dispersed combustible solids are exposed to an ignition source, and requires the following conditions:

- sufficient concentration of an exposable air-borne dust,

- source of ignition of sufficient strength,

- presence of oxygen in the surrounding atmosphere.

Powdered milk products, in general, are not considered as particularly hazardous powders. The minimum explosion level of dust concentration for milk powders is considered to be 50 g/m3. The average concentration of the milk powder in spray dryers is also around this figure. However, not all of this can be considered as exposable dust and not all regions of an installation have this critical concentration. Particle size or specific powder surface area also plays an important role. Therefore coarse agglomerated powders are considerably less hazardous than non-agglomerated products with small mean particle size.

To minimize the consequences of a dust explosion and to protect both personnel and equipment, the initial explosion must be contained, suppressed or vented. The containment method means construction of a dryer as a pressure vessel, which is strong enough to withstand an explosion without rupturing. This method is suitable only for small laboratory scale units due to fabrication costs.

Explosion suppression requires detection of an explosion in its very early stage, activating an instantaneous injection of a chemical suppressant to extinguish the flame before an overpressure develops. An explosion is detected in milliseconds by a pressure or infrared sensor. The most applied method is using explosion vents in a form of hinged doors or bursting panels that are ducted to the outside of the building. The vent duct should be as short as possible, preferably < 3 m. An example of a sanitary explosion vent module is the GEA Niro DRIVENTTM.

Many publications on this subject are available. The most important are issued by VDI 3673

[42] (Verein Deutscher Ingenieure, 1979), ABPMM (Association of British Preserved Milk

Manufacturers, 1987) [43] and IDF (International Dairy Federation, Bulletin No 219/1987) [44].

Generally it has been accepted to use the venting area as recommended by VDI 3673, i.e.:

[12,1]

…

where: V is the vessel volume in m3

K is a constant defining critical volume ratio.

The recommendation not to use the whole volume of the dryer, but only a part, expressed by the constant K, is based on the consideration that the critical dust concentrations of 50 g/m3 in dryers commonly used in dairy industry are only present in the conical part of the chamber or even just a part of the cone. This means that for the calculation [12,1] only 25% of the chamber volume, or even less, is used.

Explosion vents can prevent severe injury and damage to both personnel and equipment. However, a fire, even a short one, can cause irreversible damage to the stainless steel. Therefore it is important to cool the walls as quickly as possible. A fire extinguishing system must be able to deliver quickly 20-60 m3 of water and to ensure effective wetting of the walls. For this purpose, a number of spray nozzles are installed in all parts of the installation (chamber, ducts, fluid beds, cyclones, bag filters). The extinguishing system is controlled by a separate control panel with separate sensors placed throughout the plant. Control is usually based on the outlet temperature with a two level alarm system: Level 1 is a visual and audible warning, and Level 2 automatically activates the fire extinguishing system.

Preventing the possibility of fire or explosion in the dryer requires:

1. To follow strictly the operating instructions for the dryer and operating parameters for the product in question.

2. To follow strictly the instructions for dry cleaning and washing of the installation and checking all parts for deposits after each operation and cleaning. It is very important to completely dry out the plant after wet cleaning.

3. To follow strictly the operating parameters during the operation and check the relation between the amount of feed and the amount of final powder to ensure that the product leaves the plant continuously.

4. To frequently check for presence of scorched particles in the product, using the scorched particle test or by visual control by sifting about 1 kg of powder.

5. To check smell. Exhaust air exhibiting a typical acrolein smell is also a quite useful way of detection, especially as it appears in the very early stages of the combustion. Once dried milk smoulders, it produces also significant amounts of carbon monoxide. A modern method of tracing early stages of smouldering by the detection of CO was suggested by Steenbergen et al. [53]. Based on this principle, a fire detection system was developed, consisting of an air sampling and air sample preparation unit together with a sensitive CO analyzer.

CO emission sources in the neighbourhood of the drying plant may cause interference, but the system can compensate for this. CO detection systems are installed in many new dryers nowadays.

6. Last but not least: education and training of the personnel enabling them to be able to make a fast and qualified decision to stop the plant immediately when an indication of danger occurs.

Should, in spite of all precautions, a fire occur the following basic procedures are recommended:

1. Stop all fans.

2. Stop all heat supplies.

3. If fire extinguishing equipment has not started automatically, activate it manually.

4. Change from feed to water with highest possible rate.

5. Call the fire brigade.

Experience has shown that only a few percent of the registered fires are accompanied by explosions and that the present standard of explosion vents and fire extinguishing systems is safe enough to avoid damage to both personnel and equipment. Apart from the proper functioning of the pressure relief venting and fire extinguishing equipment, which must be checked regularly, the most important aspects are qualified training of the personnel and the following of the elementary rules. Many of the fires registered could have been avoided if these two conditions had been met.

12.5.

Principles of good manufacturing practice

The vast amount of products produced by the milk powder industry is used for human consumption and a significant part for baby nutrition. Therefore milk powder factories have to be designed and operated to ensure both safe and wholesome manufacture, and avoid risks associated especially with pathogenic microorganisms.

Several General Codes of Hygienic Practice are available (IDF Document 123-1980 [45] and 178-1984 [46], FAO/WHO Codex Alimentarius Commission’s Document CAC/RCP 31 1983 [47], ABPMM publication 1987 [48]) and IDF Recommendations for the Hygienic Manufacture of Spray Dried Milk Powders (IDF Bulletin No.267/1991 [49]). All these documents give advisory guidelines requiring that each factory elaborates its own Code of Practice adapting the

general rules to local conditions.

Processing equipment, from a hygiene point of view, is safe due to the design being based upon many years of experience by recognized manufacturers. Designing a building and assembling the individual processing units into a production line often requires compromises due to local conditions and requires close cooperation of the architect with the specialists from the equipment manufacturer and dairy company in order to avoid costly errors. The most

important principles are briefly discussed below.

Powder plants should be located in areas free of any air-borne pollution such as industrial exhaust and flue gas, agricultural odours and dust from heavy traffic roads. Service roads and yards must be hard surfaced with wear-resistant material such as asphalt or concrete, and with good drainage and maintained in good condition free of cracks. The drainage system must be designed to handle peak volumes of rainwater.

Buildings must be of a type which will prevent contamination of the whole internal environment. Pitched roofs with an angle greater than 20° are preferred over flat roofs. Walls and floors must be waterproof, non-absorbent with smooth surface to allow easy cleaning. Ceilings must be executed to avoid condensation.

The maintenance of a positive air pressure in the buildings and an air flow from the processing and packaging areas to the raw material area by means of an air conditioning system is recommendable. However, the temperatures must not be kept below normal room temperature levels and direct flow of any cool air onto any critical parts of the dryer must be avoided.

Potable water must be available for hygiene and processing operations and must meet the requirements of WHO-standard for drinking water. The main drying air must be taken both from the outside and top of the building. Drying air

of temperatures higher than 120°C requires only coarse filtration corresponding to EU1-EU4 class. All air coming in contact with the product with temperatures less than 120°C must be filtered to at least Eurovent EU7 class. The use of more effective filtration, up to EU13, may be required, depending on the product. Air filters must be cleaned when necessary (as indicated by pressure drop).

All seals, gaskets and sleeves, especially on units operating under partial vacuum, require special attention and maintenance to avoid that the surrounding air is sucked into the dryer.

Cooling water for process air conditioning must be recirculated under partial vacuum to avoid penetration into air ducts in case of leaks. Air de-humidifying units must allow for cleaning and maintenance of all heat exchange surfaces.

The effluent waste water system must be designed to handle peak load conditions and must be maintained to the highest operational standards at all times.

The changing areas, showers and toilets with hand washing and sanitizing facilities must be provided with good ventilation and without direct open access to the processing areas. There must not be any other access to the operation rooms than through the changing area and the protective working clothing must be taken on or off whenever passing this area. In processing areas, hand washing, sanitation and hand drying facilities must be available. Paper towels are preferred. Visitors and contractors must observe the same rules as production personnel.

All material fed to the spray dryer processing area must receive a heat treatment corresponding to pasteurization. The pasteurizer must be provided by flow-diversion valve and pasteurization temperature must be measured and recorded. The thermometer must be checked at least once a week.

For concentrated milk handling, parallel systems of balance tanks, which are switched and cleaned every four hours, are recommended to reduce potential bacterial growth. Concentrate reheating facilities prior to spray drying is recommendable to avoid the bacterial growth in the feed line.

Spray dryer insulation must be constructed to avoid invasion by water and should preferably be removable for inspection.

Washing facilities for the rotary atomizer or nozzle units must be constructed to avoid wetting the top of the dryer. The walls of the spray dryer must be checked at least once a year for cracks. The fire water sprinklers must be checked frequently for leakage. The water from fire sprinklers must be drained after each use. The pressure relief doors leading directly to the outside can in some climates cause excessive cooling of external chamber surfaces causing condensation on the internal surfaces. Enclosing the venting duct with a plastic membrane and possibly heating the air between the membrane and pressure relief doors is recommendable.

Not only the chamber with removable insulation panels (see Fig. 4.3), but also all the other components of the installation have to be designed to be cleaning-friendly and of hygienic construction such as fully welded fluid beds etc.

The spray dryer building interior must be maintained in dry conditions, and dry cleaning by means of vacuum cleaner has to be preferred. Wet cleaning if and when required must be conducted in such a way to avoid excessive wetting.

Any water remaining on the floors should be wiped off and dried with a cloth.

Operators of a spray drying installation are responsible for cleaning and disinfection of the equipment. The evaporator, spray dryer feed line and the atomizing unit must be wet cleaned and disinfected after each operation. The spray drying chamber, internal and external fluid beds, cyclones, ducts and powder transport lines should be dry cleaned when necessary. Wet cleaning should be applied as a consequence of product accumulation, discoloration or inferior

product quality. The wet cleaning procedure has to be done according to the instructions from the equipment manufacturer. An important step in the wet cleaning procedure is an inspection to check whether all powder residues have been completely removed. All water remaining in the dryer after wet cleaning has to be completely drained and dried by circulation of hot air through the plant. The plant must be completely dry before starting a new production run and the dryer has to be inspected with special attention to critical wet spots as manholes, sleeves, gaskets, fluid bed plenum etc.

An important element in maintaining high hygienic standards in a milk powder factory is regular education and training. The responsibility for arranging training of all personnel remains with the management. For this purpose, management should elaborate their own, detailed Code of Hygienic Practice based on publications mentioned in the introduction to this section.

The presence and the extent of bacterial contamination of dried milk products originates from:

a) microorganisms surviving the process,

b) microorganisms surviving and growing during the processing,

c) microorganisms contaminating the product during or after processing.

To the first two groups belong primarily the spore-forming bacteria which are heat-resistant and may survive relatively high heat treatment conditions. The presence of spore-forming bacteria may be an indication of poor raw milk quality. Some non-spore-forming bacteria may be also relatively heat resistant.

12.6.

The use of computer for quality control and trouble-shooting

Every milk powder factory should have in the organization a function to deal with troubleshooting. This function represents a link between production and quality control. The person who carries out this function - a technologist - should have a thorough knowledge of milk powder technology, and also have the daily responsibility to generate a list of the set points of technological parameters for the next operation. To be able to carry this responsibility the technologist must collect information of the operating conditions and product quality data from each production, analyze the data and draw conclusions, based on statistical evaluation of the information collected, and decide which technological parameters (or combination of parameters) are optimum for achieving the desired quality properties of the final product.

Achieving first class powder quality is the prime goal. As earlier emphasized, efforts for improving or just maintaining this quality can be considered as a kind of troubleshooting. It would not be quite correct to think that with the present state of the art of equipment and technology, the optimum operating conditions are fixed. The performance of each spray dryer is influenced by a number of factors, and the optimum operating conditions to achieve top product quality have to be found individually for each plant and for each product specification. They must be checked daily and revised at relatively short intervals. Factors that cause variations in final product quality are, for example: raw milk quality, composition and seasonal fluctuations, plant location, building execution inclusive of air flow and temperatures around the plant, type and layout of the installation and its individual components, climate and air condition variations, especially humidity, during operation, etc.

Computer control is now universally applied for spray drying installations. The whole operation, including start-up and shut-down, is controlled by a computer, and during production the operational parameters can be printed out at regular intervals as a table or a screen-dump showing the numerical values on a flow-sheet of the installation. The advantage of this system is reliability of both time interval and accuracy of information. The disadvantage is that the

computer produces information on many pages of paper.

The computer stores the information as so-called historical trends, which remain in the computer’s memory for a certain time depending on storage capacity, and can be visually studied as graphical trends of individual or combinations of parameters on the screen and printed out as screen-dumps. Again, the information collected in this way may be spread over a number of pages. Needless to say, for the technologist it is much easier to collect the information needed from a one page log sheet. Besides, computer print-outs should never be a replacement for manually written log sheets. There is a simple reason for this, namely that operators will lose the feeling for and contact with the technological parameters if they are not forced to read and write down the data every hour, and consequently become totally familiarized with them.

Control computer systems record the data at pre-programmed intervals and store them in memory for a period of time depending on the data storage capacity. The system described here makes it possible to collect the operating parameters at the end of each operation directly from the process control computer and to save them on other data storage media. Such a file contains usually much more information than those which can be found on the manually

written log sheets. From there they can be further processed and converted, depending on their format, either directly or after using the translation facility of the spreadsheet system used into a spreadsheet file. The use of a computer with spreadsheet program for this purpose was described by Písecký [50].

The analytical results from the laboratory, both in-process and final control, can be entered into these log sheets, which are then saved for future use. The information can also be used to generate a survey of production quantity for each day, or a given period of time, energy consumption, production economy, etc. The possibilities for utilizing this information are in fact almost unlimited.

The system is supposed to be a tool for the technologist and production supervisor, enabling them to follow, control and optimize the quality of the product and the production rate.

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Troubleshooting operations

Lack of capacity

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Fire precaution

Principles of good manufacturing practice

The use of computer for quality control and trouble-shooting

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Lack of capacity

Product quality

Deposits in the system

Fire precaution

Principles of good manufacturing practice

The use of computer for quality control and trouble-shooting

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