Air heating in Spray drying / Spray drying

Newcospray 01/18/2024
Efficiency in the transformation of liquids to powder in the spray drying process is a constantly evolving field of study, and among the various variables to be considered, the air heating system stands out as a critical component. In this analysis, we will explore in detail the functionality and importance of this system in the context of process engineering.

Hot Air Generation
The process starts with the generation of hot air, driven by specific energy sources such as gas, diesel, electricity or steam (cogeneration). The choice of heat source is determined by considerations of energy efficiency and source availability.

In case of heating:

Indirect:
steam/fuel/gas/thermal fluid

Direct:

Gas/Electricity

Indirect heating
Steam: It is a simple radiator where, divided by sections, the condensate part meets the cold air and the final heating part meets the high pressure steam. Under normal conditions it is possible to obtain an air temperature 10ºC lower than the corresponding vapor saturation enthalpy. The boiler usually needs 2-3 bars more than the working pressure of the radiator to compensate for the losses of the pipe and the regulating valve. Efficiency is usually around 98-99%.

Fuel or gas: They have separate flows of combustion air and process air. Both gases pass through a heat exchanger made of heat-resistant steel (e.g. X15CrNiSi25-21). The end cover of the heater must be removable to clean the ducts. Efficiency is around 85% at a temperature of approximately 250ºC.

Thermal fluid: Usually used when the required temperature is so high that steam pressure would be inappropriately high. The oil or thermal fluid is heated with gas or fuel oil and fed into a pressure exchanger. The required pressure is zero. Keep in mind that if oil is used, there may be leaks and the probability of fire increases.

Air heater mounting a vertical direct gas burner.

Direct heating
Only used if contact of combustion gas with product is allowed. The presence of NOx in the final product means that this heating system is not used in the food, feed or pharmaceutical industries. Despite this, a direct gas heater is very inexpensive, very efficient and can reach process air temperatures of up to 2000ºC.

It must be taken into account that when combustion is induced, vapours appear in the process air, pre-humidifying the drying air (44 mg/kg dry air /ºC). This means that the inlet temperature must be somewhat higher than if it were completely dry air.

Direct electric heaters are inexpensive and ideal for pilot plants or small food manufacturing plants. The electric calorie/€ is more expensive than gas calorie/€, which is why electric heating is not used in industrial plants.