Page 324 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 309

a boiler and at low loads (to avoid steaming you have to kets” from the air side to the flue gas side and back. The
keep enough water going through it). If you have a feed- metal baskets absorb heat from the flue gas then give
water recirculating loop you would use that to maintain it up to the combustion air. The major manufacturer of
temperatures, otherwise during startups you should regenerative air heaters makes a “lungstrom” (for it’s
add water to the boiler more frequently to provide some designer) air preheater (Figure 10-42) in a plant right
consistency to cooling of the economizer and you may near where I grew up, Wellsville, New York. The re-
even have to accelerate blowdown to provide enough generative air heater occupies less space than a tubular
water flow (that’s a typical operation with HRSGs with heat exchanger and can prevent corrosive conditions by
integral economizers). That little bit of extra work is simply stopping the rotation.
worth the savings in fuel over the operating life of the To accommodate varying combustion air supply
equipment. temperatures air preheaters are frequently fitted with
steam air heaters to prevent acid condensation. There’s
Air Preheaters a loss of efficiency associated with the steam use but
Use some caution with this term. Normally we it’s recovered in added energy from the flue gas which
mean a heat trap when we use the term preheater but couldn’t be absorbed without damage to the preheater.
some manufacturers will call a steam coil installed in an Some of those heaters are adequate to permit startup
air supply an air preheater because it does do what the and low load operation without starting and stopping
name implies. Within the trade such devices are called or bypassing the air heater.
“steam air heaters” to differentiate them from our tradi- Air heaters are a little easier to operate than econo-
tional heat traps. An air preheater uses energy left in the mizers since you can leave them off line until the boiler
flue gas that leaves the boiler to preheat the combustion is carrying a load then close the bypass damper or start
air. This makes the air preheater the only true heat trap the rotor motor to put them into service. By simply not
because it does trap the heat without adding any surface running the rotor motor during boiler warm-up the flue
to the boiler. The way the air preheater increases the gas side is heated to prevent corrosion. The rotor should
efficiency of the boiler is by raising the temperature of be run while purging the boiler to ensure all the baskets
the combustion air using the stack heat instead of fuel. are purged. There are small pie piece shaped sections
There’s also a slight increase in heat flow through the that are sealed between the gas and air sides while
boiler heating surface due to higher furnace tempera- the rotor is stopped. Regenerative air heaters require
tures. additional maintenance because of the moving parts
An advantage of air heaters is higher temperature and seals to separate the flue gas and air sections but
differentials. Instead of using 200°F plus feedwater to performance is usually more consistent than tubular air
cool the flue gas you use combustion air entering at 80°F. heaters. They can be cleaned in service whereas tubular
There’s potential for lower flue gas outlet temperatures air heaters are usually bypassed for water washing or
which means higher boiler efficiency but corrosion of require a full boiler shutdown to clean them.
metal parts of the preheaters and ductwork to and in- You may never encounter them but I should men-
cluding the stack must be given consideration.
There are two basic designs of air preheater, tubu-
lar air preheaters which consist of a box and tube heat
exchanger to transfer heat from flue gas to combustion
air and regenerative air preheaters.
Tubular air preheaters are normally arranged with
the flue gas passing through the center of the tubes and
combustion air surrounding the tubes. Corrosion during
startup and low load operation is eliminated by bypass-
ing the air around the heat exchanger so the flue gases
keep it hot. Modulating the bypass damper to allow par-
tial air flow doesn’t work very well because the cold sur-
faces where the air first enters will promote condensation
anyway. The bypass should be either open or closed.
Regenerative air preheaters use a rotating element
to transfer the heat. A shaft rotates an assembly of “bas- Figure 10-42. Lungstrom air preheater