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What are spray dryers?
Spray drying is a method of rapidly drying a liquid mixture or slurry into a dry powder using a hot gas. Spray dryers are often used in the food, pharmaceutical and chemical industries. A spray dryer operates by injecting a mixture or slurry into a hot air stream through a spray nozzle. The liquid in the mixture is vaporized by the hot air, and the solids then fall out in the form of a powder. In this article, we will only be discussing dryers that process mixtures that are water-based. However, spray drying can also be done on mixtures where the liquid is flammable.
Spray dryer variety
Spray dryers come in many designs, shapes, and sizes. Every dryer is somewhat unique, but fundamental parts will remain unchanged. Drying chambers are usually cylindrical or cone shaped; but some designs use a box shaped drying chamber. The dryers are usually followed by cyclone collectors or baghouse collectors for separation of the dry powder. Sometimes there might be external fluid beds as well. For box type dryers, there can often be fluid beds or baghouses that are integral to the drying chamber itself. Spray dryers are usually direct gas fired, but they can sometimes be indirect fired and might also be equipped with heat recovery systems.
Large spray dryers are common in the food industry, especially for dairy products such as milk, whey, and cheese. They are often used for products that might be thermally sensitive because the drying temperature can be more consistently controlled.
Even when the input mixture is water-based, fires and explosions can and do occur in spray dryers. This is mainly due to the combustibility and explosibility of the solids in the mixture. For example, milk, whey, and cheese powder are all combustible and can be explosive in powdered form.
Fires are generally more common than explosions in spray dryers, but a fire that is not promptly controlled can lead to an explosion. Fires and explosions are both generally caused by solids that build up on the interior of the dryer, especially at the spray nozzles. These accumulations can then start to overheat and burn, causing a fire or explosion. A fire or explosion can occur directly in the drying chamber, or hot embers could be carried downstream causing a fire or explosion in a fluid bed or bag house.
Protecting your spray dryer
Fire and explosion protection for a spray dryer would typically incorporate the following elements:
- A Deluge water spray system inside the drying chamber: activated by heat detection.
- A form of explosion relief for the dryer, cyclones, fluid beds and baghouses depending on size and arrangement. Explosion venting should be ducted to the exterior of the building. If explosion venting is not possible, explosion suppression may be a viable alternative.
- Sprinkler protection for bag houses.
- Sprinkler protection for the general building area.
- Dust tight or Class II Division 2 electrical equipment, as appropriate.
The following items should be considered for loss control management:
- Internal cleaning of the dryer and nozzles should be completed on an appropriate frequency to help prevent accumulations that could overheat and ignite.
- Spray nozzles should be inspected frequently for accumulations. Installation of cameras at nozzles can help facilitate frequent inspection.
- Building areas should be placed on an appropriate housekeeping schedule to remove fugitive dust accumulations from all surfaces including floors, equipment, and building structures.
- Combustion controls for gas fired burners should be tested annually, at minimum.
- CO detection in the dryer exhaust stream can help provide early warning of accumulations in the dryer that may be starting to overheat and combust.
- Establish a robust Management of Change program. Changing the material being dried has often proven to be the impetus for a fire or explosion in a dryer. Whenever a product change occurs, a thorough engineering review should be completed. Changes in fat content or other characteristics of the material being dried can have an effect on buildup inside the drying chamber.
Spray dryers are a critical piece to many manufacturing processes. Being aware of the hazards and putting protections in place reduces risk associated with property loss. Contact Risk Logic with for additional guidance on property loss prevention for spray dryers and similar types of equipment.