There are a number of heater configurations: vertical cylinder; vertical tube cabin type; horizontal tube cabin type; etc. The type chosen depends to some extent on the service.
One aspect sometimes overlooked is 'box loss'. This refers to heat lost through the external metal skin. It is common to find older heaters with external skin temperature approaching 60 °C. Some idea of the loss is that 10 m2 of exposed skin at this temperature in a 15 kph wind will lose ~90000kcalh-1. It is therefore worthwhile specifying that internal refractory should not produce a skin temperature in excess of a named value - depending on ambient conditions for the location. Also, where appropriate, a cylinder design has less exposed skin area than a cabin type of the same capacity.
Older heaters often have firebrick or castable refractory. There is a wide variety of alternatives available which are far superior. Instead of costly re - -ining, it is often possible to apply a coating to the existing refractory. Some of these coatings improve emissivity and thus improve heat pickup, especially in the heater's radiant zone.
Combustion air preheat is invariably attractive to improve efficiency. There are a number of systems but the best and most reliable is simple heat exchange between flue gas and combustion air. Care is required when retrofitting such a system since the reduction in fuel tends to move the heat load from the radiant section to the convection section, possibly limiting heater capacity. Note also that it is possible to take energy from a large heater stack and share the preheated air across a number of heaters. This avoids the cost of multiple individual systems.
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