An air preheater takes energy from a process stream to heat incoming air to a system. The availability of a preheater heat source is specific to the individual process. Heat may be available from condensate streams in a steam generating process. In that case, a water to air preheater is required. Gases downstream of a combustion process may require further cooling after the main heat recovery has been accomplished. If so, a gas to air preheater is called for.
Liquid to air preheaters
For liquid to air preheaters, extended surfaces are used to enhance the heat transfer coefficient of the air. Fin and tube pitching depends on the potential for fouling of the surfaces. Tube materials are based on an understanding of the corrosion potential from the tube side fluid. As the liquid can readily be supplied in small bore pipework, the overall layout of equipment is generally not an issue.
Gas to air preheaters
For gas to air preheaters, we potentially have to examine more factors. Using products of combustion to preheat air is a common requirement. However, the quality and temperature of the gas stream may vary significantly.
We consider significant solids carry over in the gas stream when determining the pitch of tubes and use an appropriate fouling factor. In the extreme mechanical methods for solids removal may need to be studied.
Tube expansion on start-up needs to be taken into account as an integral part of the design and may dictate significant aspects of the equipment layout.
Bringing together the gas and air streams can also be a significant factor as large diameter ducting affects the available layout options.