Air Heaters

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Reasons for their use

• These are fitted for three main reasons
  • To increase efficiency by recovery of heat from flue gases ( except where bled steam heaters are in use, these increase overall plant efficiency but by a different method)
  • Accelerate rate of combustion
  • Avoid effect of cold air impinging on boiler surface
  • As a by product air heaters also form a convenient way to warm up a standby boiler before initial firing.
  • However, the effects of dew point corrosion and fouling in smoke tube air heaters should be taken into account when designing the heat absorption limit.I.E. the amount of heat to be removed from the flue gas should have a limit.
  • For water tube boilers gas air heaters are only considered where the temperature at inlet to economiser is greater than 200^oC. Due to greater heat transfer efficiency between gas/water economisers are preferred to gas/air exchangers.

At low loads all gas/air heaters should be bypassed to keep uptake temperatures as high as possible.

Types of air heaters

Lungstrom gas/air heater (regenerative)

The drum contained within the cylindrical casing is formed into segments into which are placed removable ( for cleaning) baskets, consisting of vertical plates (to give minimum resistance to flow) The drum slowly rotates, about 4rev/min, driven via a flexible coupling,gear train, clutch and thrust bearing by one of two electric motors; one mounted on top the other below.
As the drum rotates a segment will enter the gas side, here it removes heat from the gas, it continues to rotate until entering the air side where it will give up its heat to the air. The heat transfer is very efficient, however, soot and corrosive deposits quickly build up in the mesh and hence an effective soot blowing method is essential. This normally takes the form of an arm , pivoted at the circumference of the drum with a single nozzle at the other end. This sweeps acros the drum rather like a record arm. One of these arms are fitted top and bottom.
Gas leakage to the air side is prevented by the air being at a higher pressure and by fine radial clearance vanes fitted in the drum segments.
By passes for both air and gas sides are fitted to prevent fouling with the reduced gas flow and temperature, also during manoeuvring when the possibility of different gas/air flow rates occurring leading to high metal temperatures and possible fires.
Failure by uptake fires is not uncommon with this as in most gas/air heater designs.

Tubular gas/air heater

Shown above is the horizontal tube type air heater which was less susceptible to choking with soot than the vertical types sometimes found with older scotch boilers.
To aid cleaning water washing was sometimes carried out to aid the sootblowers

Bled steam air heater

The use of individual banks and 'U' tubes allow for ease of isolation when these become perforated without large loss of heating surface. The tubes were expanded into the headers and made