Page 39 - Boiler_Operators_Handbook,_Second_Edition

24 Boiler Operator’s Handbook

Steam leaving the last stage of the turbine enters heat exchangers, typically, an economizer, and feed
the condenser where the latent heat is removed and water control valves. Newer plants may operate with-
the steam converted to water. The condenser can be air out feed water control valves by using variable speed
cooled which normally consists of multiple heat exchang- control of the boiler feed pumps to change the feed wa-
ers with fan forced atmospheric air used to cool them. Air ter flow. Some plants, especially supercritical, may use
cooled condensers are only used where water comes at booster pumps to help increase feed water pressure.
a premium or is in very limited supply; most condens- Controlling flows of extraction steam are one of the
ers are water cooled. Water cooled condensers can use ways a utility plant boiler operator earns a salary. Maxi-
water from cooling towers but can be from a river, lake, mizing electricity generation by ensuring maximum
or the ocean which normally provides the highest plant flow of steam through each turbine stage produces more
efficiency because that water is the coldest. The colder Kwh output and that’s the product the plant is selling to
cooling water reduces the pressure on the steam side its customers.
of the condenser because the colder surfaces lower the At least most of the water, sometimes a liquid
saturation temperature of the steam. The pressure in the and sometimes steam, is circulated within the system
condenser is always less than atmospheric so that pres- leaving the boiler then returning to it; that constitutes a
sure is referred to as a vacuum. The vacuum, typically cycle. The wise operator should be able to describe the
expressed in inches of mercury, can be estimated from cycle in his or her plant. If there isn’t a diagram for your
the steam tables by adding ten to twenty degrees to the plant laying around or buried in a file somewhere then
cooling water temperature. In the example of Figure 1-8 try making your own. Displaying the cycle diagram in
you’ll note the temperature of the condensate is higher at the plant not only informs visitors of the scope of the
the discharge of the condensate pumps. That’s because system but serves as a reminder as to what an operator
the condensers are designed to sweep the steam leaving should monitor to make the cycle as efficient as possible.
the turbine over the droplets of condensate before turning
up to contact the tubes that condense the steam.
The steam condensate is removed from the con- COMBUSTION
denser by condensate pumps that force the water to
flow from the condenser to the deaerator. On its way Most of our fuel that we use is called “fossil fuel”
the condensate will first pass through low-pressure because its origin is animal and vegetable matter that
heat exchangers that use extraction steam and operate was trapped in layers of the earth where it became fos-
at a partial vacuum, draining their condensate into the silized, breaking down, for the most part, into hydrocar-
condenser. Condensate from auxiliary systems that heat bons. Hydrocarbons are materials made up principally
equipment at low temperatures, such as space heating, of hydrogen and carbon atoms. It’s the hydrocarbon
is typically drained into one of the low-pressure heat portion of fossil fuels that generates more than 90% of
exchangers. the energy we use today, from the propane that fires up
When the condensate reaches the deaerator it is your barbecue to the coal burned in a large utility boiler
mixed with some extracted steam to heat it to saturation to make electricity. The normal everyday boiler plant
conditions. That steam source is commonly augmented that you’re operating also burns hydrocarbons but we
with steam from a pressure reducing station supplied concentrate mainly on four forms, natural gas, light oil,
by a higher pressure source to maintain the plant’s heavy oil, and coal.
heat balance (explained later). When water is heated to The principal difference in these fuels is the hydro-
saturation any gases that are absorbed in the water can- gen/carbon ratio and the amount of other elements that
not remain absorbed because the water is about to boil. are in the fuel. Despite the fact that our typical hydro-
The gases form bubbles in the water, which is agitated carbons vary from a gas lighter than air to a solid they
to force the bubbles out of the water and out a vent at all burn the same, combining with oxygen from the air
the top the deaerator. The heated, deaereated water is to release energy in the form of heat. It’s not necessary
collected in a storage tank which is either part of the de- to know how it does it, only to understand that certain
aerator or as separate vessel normally designed to hold relationships exist and generally what happens depend-
enough feed water to keep the plants boilers operating ing on changes you make or changes that are imposed
for 20 minutes. on you by the system. If you look at a number of what
Boiler feed pumps draw water from deaerator stor- we call “ultimate analysis” of fuels you’ll discover that
age and pump it to the boilers through high-pressure the fuel gets heavier with an increase in the amount of