The air is drawn into the system by an inlet fan figure 2. On some smaller spray drying systems the inlet fan is dispensed with relying on the exhaust fan to draw all the air through the system. This would be too noisy on larger plant when an inlet and exhaust plant is employed.
Spray drying requires large volumes of air with enough pressure to overcome all the resistance in the system.
For effective evaporation the incoming air must be heated. There are two main methods used to heat the air, direct and indirect heating.
Direct heating is where fuel such as oil or gas is burned in the inlet air duct. Oil can be used as a fuel when drying most chemicals or minerals but is not permitted for food materials. Natural gas is acceptable for most products including foods.
There are three types of Indirect heating, the steam boiler and the thermal oil heater both of which require a separate heat exchanger in the dryer’s inlet air stream. Air can also be heated by indirect air heater which passes hot air through tubes and heats the drying air which passes over the outsides of the hot tubes. This type of heating would normally uses gas or oil as fuel but in some areas it may be more economical to use alternative fuels.
Figure 3 in the diagram shows a direct gas burner, direct gas burners are suitable for most applications and are in most cases, the most economical method of heating. Direct gas heating has the advantage of responding quickly to changes of temperature parameters and can be designed to give very low emissions.
On small spray drying plant and where installations are located in areas where electrical generation is inexpensive, direct electrical heating can be an option.