Page 268 - Boiler_Operators_Handbook,_Second_Edition

Water Treatment 253

or, as shown in the following figures, simply a length of them up further before they reach the outside and drop
tubing inside the water box above the water spray noz- into the storage section that removes the bubbles of air
zles. The purpose of the vent condenser is to condense and gases.
most of the steam that is carried out with the air and Many people get confused with the term “spray”
gases. The idea is to have only air and gases leaving the because all these deaerators have water spray nozzles.
deaerator. Of course we always adjust the vent valve on Even I will use the terms “spray-scrubber” and “spray-
a deaerator to produce a “wisp” of steam, just enough so tray” to describe scrubber and tray type deaerators to
we know all the air and gases are pushed out because a avoid that confusion. A spray scrubber uses a steam
little steam is coming out with them. Throttling the vent spray to provide the agitation to remove the bubbles of
valve too much will recover all the steam as condensate air and gas so there’s no real reason to prefix the titles of
but can also trap air and gases in the top of the deaera- the other two with the word spray.
tor to prevent steam contacting the makeup as it enters Except for power generation plants where the
through the sprays and prevent proper deaeration. makeup is primarily colder water from a condenser
Opening the vent valve too much is just wasting steam. few plants can use a spray type scrubber. The combined
Operation of that vent valve is the key function of condensate return and makeup water temperature is so
a boiler operator. The trouble is most operators solve high that steam requirements aren’t enough to perform
any control problem by simply leaving the valve so far the agitation. When the temperature difference of the
open that steam is blowing out dramatically. That’s a condensate and feedwater can be consistently more than
considerable waste of energy and water. The wise opera- about 50°F then there’s enough difference for a scrubber
tor keeps that vent adjusted so there’s only that wisp of type of deaerator (Figure 8-4) to work well. The flow of
steam coming out. steam along with the water up through the baffles of
I always dealt with spray type deaerators (Figure the scrubber provides enough energy to separate the
8-3) aboard ship because the water from the condenser bubbles. Some of the energy is achieved using the differ-
was relatively cold and only heated slightly in the air ence in density of the water and steam.
ejector condenser and turbine bleed heat exchangers so When the temperature difference between blended
there’s a considerable difference between make-up and makeup and feedwater is less than 50°F, always insist on
feedwater temperature. If you’re operating in a plant a tray type deaerator. The trays (Figure 8-5) don’t look
that generates a lot of power by condensing turbines like what you put your lunch on at the cafeteria, they’re
(a utility) then a spray type deaerator may be all that’s made up to produce hundreds or even thousands of lit-
needed. The large difference in temperature requires a tle waterfalls. Distributing the water over the trays and
lot of auxiliary steam to heat the water and the steam producing thin little falls produces hundreds of square
can be directed into the spray section where it creates feet of exposed water surface for the bubbles to escape
a violent mixing with the droplets of heated makeup from. Some scrubbing of the falling water is achieved
water before it flows up to mix with and heat the water by the steam flowing up through the trays to the water
entering at the spray nozzles. It’s the effect of all that sprays but most of the energy that’s used to force the
steam rattling those water droplets around and breaking bubbles out of the water is provided by gravity. A tray

Figure 8-3. Spray type deaerator Figure 8-4. Scrubber type deaerator