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
 (Verein Deutscher Ingenieure, 1979), ABPMM (Association of British Preserved Milk
Manufacturers, 1987)  and IDF (International Dairy Federation, Bulletin No 219/1987) .
Generally it has been accepted to use the venting area as recommended by VDI 3673, i.e.: