Air drying
The following are significant sources of exergy loss for air drying:
• A sizable amount of exergy is lost with exiting air, even if it is assumed to reach the wet-bulb temperature in the drying process. At higher wet-bulb temperatures, the water present in the exiting air makes a significant contribution to the total exergy loss of the exiting air.
• The exergy exiting with the product is seen to be quite small, as might be expected, since it was shown earlier that
little exergy was put into the solid products.
• The exergy loss from the walls of the dryer, due to heat rejection, is significant and needs to be taken into
consideration. For example, in spray drying this amount may reach up to 25% of the total exergy input. Of course,
this loss can be reduced by appropriately insulating the dryer. Another important aspect is the size of the dryer. For
example, the jet-type ring dryer has a much smaller loss from its walls than an equivalent-capacity spray dryer,
due to its smaller dimensions.