Air Heating System

The drying air can be heated either by Indirect heating (steam, oil and gas or hot oil) or Direct heating (gas or electricity).

INDIRECT HEATING

A steam heater is a simple radiator. The steam pressure determines the temperature obtained in ths radiator. Under normal conditions it is possible to reach air temperatures 10șC lower than the corresponding saturation enthalpy of the steam.

Modern steam heaters are divided in sections, so that the cold air first meets the condensate section, then a section with low steam pressure (which is usually the biggest one in order to utilize as much low-pressure steam as possible), and then the air finally enters the high-pressure steam section. The air heater consists of rows of finned tubes housed in an insulated metal case. The heat load is calculated from the quantity and specific heat of the air. The heater size depends upon the heat transfer properties of the tubes and fins and is usually about 50 Kcal/șC x h x m³ for an air velocity of 5 m/sec. Steam-heated air heaters will usually have an efficiency of 98-99%. As the steam boiler is usually placed at some distance from the air heater, 2-3 bar g extra pressure on the boiler should be anticipated due to pressure loss in the steam pipe and over the regulating valve. To avoid corrosion of the tubes in the air heater it is recommended to use stainless steel.

In indirect oil and gas heaters drying air and combustion gases have separate flow passage. The combustion gases pass through galvanized tubes that act as heat transfer surface for the drying air. The combustion chamber is made of heat-resistant steel. The end cover of the heater should be removable for cleaning of the tubes. Heaters of this type will in the range of 175-250șC have an efficiency of about 85%. See Fig. 41.

Hot oil liquid phase air heaters are used either alone, or when high inlet drying air temperatures are required, and the steam pressure is not high enough. The heater system consists of a heater, which can be gas- or oil-fired, and an air heat exchanger. Between these two components a special food-grade oil or heat transfer fluid, which does not crack at high temperatures, is circulated at high speed. The main advantage of hot oil liquid phase is the open pressure-less system.

DIRECT HEATING

Direct gas heaters are only used when the combustion gas can be allowed to come into contact with the product. They are therefore not common in the food and dairy industries. The direct gas heater is cheap, it has a high efficiency, and the obtainable temperature can be as high as 2000șC. When a plant is designed with an air heater with direct combustion, it is necessary to calculate the amount of vapour resulting from the combustion (44 mg/kg dry air/șC), as this will increase the humidity in the drying air. The outlet temperature has therefore to be increased in order to compensate for this increase in the humidity and to maintain the relative humidity.

Combustion of natural gas (methane) takes place according to the following stoichiometric reaction formula:

CH₄ + 2 O₂ => 2 H₂O + CO₂ + Heat

The oxygen for the combustion originates from the atmospheric air with 21% O₂ and 79% N₂.

All combustion yields small quantities of oxides of nitrogen as a result of the reaction of nitrogen and oxygen at elevated temperatures. Subsequent nitrogen oxide NO and nitrogen dioxide NO₂ formation occurs and is referred to as the sum (NOx) of the two.

It should be noted that high combustion temperatures, high heat transfer rates, high excess air, and low residence time in the combustion chamber are all factors increasing the formation of NOx.

For comparison the following approximate NOx concentration prevails:

Normal water supply:	4000 ppm
Cigarette smoke:	2000 ppm
Exhaust gas from a car:	900 ppm
Heavy traffic intersection:	75 ppm
Natural gas boiler stack:	45 ppm
WHO food limit for infants:	2-5 ppm
Spray drying chamber:	<1 ppm
Normal fresh milk:	0.1 ppm

The level of NOx in the process air after the direct fired natural gas air heater will depend on many variable factors, however, with a well adjusted air heater it should be limited to the above. Only about 2% of the NOx formed will be absorbed in the milk powder.

The level of NOx in milk powder depends not only on the method used for heating the process air, but also on the type of food used for the cows, as well as on the type of fertilizer and soil used for producing the food.

The NOx level in milk powder is:

Indirect heating: Traces - 2 ppm
Direct heating: 1 - 3.5 ppm

and the level of nitrates (NO₃) is in the order of 5-10 times the level of nitrites (NO₂).

Electric air heaters are common on laboratory and pilot plant spray dryers. The heater has low investment costs, but is expensive in operation and therefore not used in industrial size plants.