Continuing my discussion of spray dryers, in this issue, I'll explore the effects of airflow in a spray dryer as well as maintenance issues.
As a review, spray dryers or towers, as they are sometimes called, atomize or spray the feed material into the drying chamber in fine droplets. They are continuous processing machines that come in a range of configurations.
Evaporation of moisture from the droplets and formation of dried particles proceed under controlled temperature and airflow conditions within the tower, and the dried product is discharged from the tower cone. The dryers have relatively short residence times. Varying the gas velocity within the dryer can alter the residence time inside the dryer.
Spray dryers typically employ induced-draft fans to extract the moisture-laden air from the system. In some instances, special fans are installed to achieve dryer-specific operations such as air sweeps.
Due to the fine product that is produced on spray dryers, they inherently require dedicated dust-collection systems such as cyclones, bag houses, scrubbers and electrostatic precipitators.
Spray dryers are controlled by programmable logic controllers (PLCs) or solid-state controllers. In spray drying systems, the exhaust air temperature or humidity provides an input signal that, by way of a setpoint, will modulate the energy supplied to the process. Mechanically, these dryers are relatively low maintenance units. They can be fabricated from materials ranging from basic carbon steel to sophisticated duplex stainless steels. These dryers must be fully insulated to allow energy-efficient operation. Tall-from dryers have a pump and exhaust fan that require differing amounts of maintenance depending on the service, environment and abrasion characteristics of the product. Likewise, nozzles may wear -- specifically, the orifice plates -- and may require frequent replacement due to the wear adversely affecting the spray pattern. Dryers using rotary atomizers can become somewhat of a maintenance challenge, having to lift relatively large motors and gearboxes to the top of the tower for replacement. Facilities to assist in the maintenance and replacement of rotary atomizers can be designed into the system.
Spray dryers have a unique position in the arena of thermal drying. There is no other high volume method of producing a free-flowing powder from a liquid in one step. They offer unique, unmatched versatility in the production of powders and can control the powder characteristics to a specified requirement.
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