Page 267 - Boiler_Operators_Handbook,_Second_Edition

252 Boiler Operator’s Handbook

The dissolved oxygen forms bubbles of gas in the water. provisions are made to heat the water and can include
Complete deaeration is not achieved until those bubbles any type of heat.
are removed. It’s getting the bubbles out that makes the Vacuum deaerators are not normally used in boiler
difference in deaerators. plants because the water is heated to higher tempera-
Boiler feed tanks come with two kinds of heaters. tures anyway. I thought I would explain vacuum de-
The water in the tank can be heated by a submerged aerators because someone in the plant may be having
heating coil or a sparge line. A sparge line simply in- trouble with one and might say “gee, the boiler operator
jects steam directly into the tank. The steam heats the should know about this thing.”
water, condensing and becoming part of the feedwater, By heating the water to a saturation temperature
while agitating the water. Agitation is important in that higher than 212°F the pressure in the deaerator will be
it helps remove the bubbles of oxygen from the water. above atmospheric and that higher pressure will push
Sparge lines are noisy and that should be considered the air and noncondensables out to atmosphere. That’s
when adopting a method of heating the water although typical of all boiler plant deaerators. The variations in
I prefer the noise and lower oxygen content to a quiet the four types depends on how difficult it is to get the
steam trap that needs maintenance. bubbles of air and noncondensables out. Noncondens-
For all practical purposes boiler feed tanks simply ables are gases other than air that can be released by
provide a place for storage of boiler feedwater and to bringing the water to boiling. They include chlorine gas,
return condensate with some capability of oxygen re- ammonia, and others that aren’t normally found in air
moval provided occasionally. They’re normally fitted but can be found, in very small quantities, in water.
with a float controlled makeup valve to admit makeup Flash type deaerators use this concept to produce a
water to maintain a constant level in the tank. The cold pressure just slightly higher than atmosphere to remove
makeup water, being more dense than the condensate, the gases. The makeup water is heated in an external
tends to simply drop to the bottom of the tank, mixing heat exchanger to a temperature higher than 212°F then
with the condensate as it enters the feed pump suction passed through a spray valve into an open tank where
piping. Dripping or, better yet, spraying the makeup some of the water flashes into steam. Since all the water
into the top of the tank will help reduce oxygen from it. is above the saturation temperature it cannot hold any
Heaters and sparge lines seldom manage to effectively oxygen so it should be removed with the flash steam
deaerate that water. Deaerators, on the other hand, are which may, or may not, be recovered. There are a num-
designed to remove air and the key is their operating ber of these devices in the field but (and I know I’m go-
pressure. Boiler plant deaerators are always operated so ing to get some heat from manufacturers for this one) I
pressure will force any removed air out of them. don’t think they’re capable of doing a decent job and I
Deaerators are provided in five types, vacuum, don’t recommend them.
flash, spray, scrubber, and tray. A vacuum deaerator The best choices for deaerators for boiler plants are
is typically a vessel filled with packing and operated spray, scrubber or tray types and which one depends
under a vacuum. The packing is not like pump or valve upon the normal temperature difference between the
packing, it’s like fill, loose pieces of ceramic or plastic makeup water and the boiler feedwater. They are all
materials stacked randomly that act sort of like splash called DC heaters (for direct contact) because the water
blocks so a lot of the water surface is exposed as it tum- is heated by mixing steam with the water; the steam is
bles down through the packing. Producing a sufficient condensed and becomes part of the feedwater in the
vacuum in a vacuum deaerator will bring the water to a process. Heating the water to saturation only removes
saturated condition. For example, pulling a vacuum of the oxygen and gases from solution, it doesn’t get the
29”Hg (inches of mercury) produces a condition where little bubbles of air and gases out of the water. To do that
79°F water will boil. As long as the water is warmer than you need some agitation and how you get the agitation
the saturation temperature that matches the pressure is determined by the temperature difference. All these
inside the deaerator it will be at boiling and a little is deaerators have spray nozzles that serve to break up
actually vaporized. The air and noncondensable gases the water as it enters the deaerator. The purpose of the
are removed from the deaerator by the vacuum pump or water spray nozzles is to break the water up into small
steam jet ejector, whichever is used. A steam jet ejector droplets so they can be heated rapidly by the rising
will normally discharge to a condenser that uses the re- steam.
maining energy in the steam to preheat the water before These deaerators also always have a vent condens-
it enters the deaerator. When a vacuum pump is used er. A vent condenser can be an external heat exchanger