Page 322 - Boiler_Operators_Handbook,_Second_Edition

Plants and Equipment 307

HEAT TRAPS loads and you have an opportunity for generating steam
in the economizer. Generating steam in the economizer
There’s a general use of the term heat trap to refer
will promote scaling of the water sides of the economiz-
to anything that is added to a boiler to absorb heat re-
er and potential damage from water hammer as flows
maining in the flue gas. They normally return that heat
change.
directly to the boiler. Conventional heat traps are econo-
When the feedwater control valve is between the
mizers and air preheaters. Condensing heat exchangers
economizer and boiler the probability of steaming is re-
can be used as either an economizer or air preheater but
duced because the economizer operates at a higher pres-
are commonly used to heat water for other purposes.
sure but the control valve will take a beating as some
Economizers of the water flashes to steam as it goes through it. The
An economizer traps heat by transferring energy feedwater piping in the boiler drum will also be exposed
from the flue gas to the boiler feedwater so that heat to water hammering and erosion from the flashing
doesn’t leave the boiler. Economizers are only found in steam. There are such things as steaming economizers
high pressure steam plants. They don’t work on most but they’re designed to do it; a normal economizer is not
low pressure or any of the HTHW plants, and some designed to generate steam at any load.
high pressure plants can’t benefit from the addition of If you have wide variations in load the economizer
an economizer. An economizer can work in a low pres- of each boiler should be fitted with a return line to dump
sure steam plant that has no condensate returns because some of the feedwater back to the deaerator. By adjust-
the feedwater temperature would be much lower than ing a globe valve in that line you can control the outlet
steam temperature. If you have a low pressure plant temperature at low loads.
with little condensate return such that the feedwater I always provide bypass and isolating valves be-
temperature (before heating in a feed tank) would be cause there’s no reason to limit boiler operation to include
around 100°F lower than the steam temperature then the economizer. If the economizer has problems draining
an economizer could be used to trap some of the energy it and bypassing it will not damage it because the flue gas
lost up the stack but we would probably call it a CHX for temperatures will not be hot enough to hurt it.
reasons that will become evident. An economizer is typically constructed of tubes
When the boiler feedwater is colder than the steam just like boiler tubes with those tubes rolled or welded
and water in the boiler, it can extract more heat from into headers. The tubes can be bare but are usually fit-
the flue gas. Fluids colder than what’s in the boiler can ted with fins to increase the heat transfer surface (Figure
also be used in an economizer to recover the heat. An 10-40) There are two standard arrangements of construc-
economizer on a high pressure boiler plant makes it as tion, square, where the tubes are straight and connected
efficient as low pressure boilers because the feedwater to each other by bends, and circular where the tubes
supplied to the economizer inlet is about the same tem- form a coil between the two headers. The circular econo-
perature as steam and water in a low pressure boiler. It’s
important to be certain the feedwater flows through the
economizer in the opposite direction of the flue gas so it
sees hotter flue gas as it heats up and the coldest water
is exposed to the gas just before it leaves the economizer.
Economizers can heat feedwater to a higher temperature
than the flue gas leaving the economizer because of the
counterflow arrangement.
At low loads there are some concerns with econo-
mizer operation which can restrict the turndown capa-
bility of the boiler. When the economizer is mounted in
the stack or on top of the boiler the water has to flow
down through the economizer. The natural tendency of
heated water is to rise up through colder water because
it’s lighter (the thermal-siphoning effect) so water flow
through the economizer can become unstable at low
loads. Combine that with the fact that the heating sur-
face doesn’t change so heat transfer improves at lower Figure 10-40. Finned tube economizer