Page 132 - Boiler_Operators_Handbook,_Second_Edition

What the Wise Operator Knows 117

it does in fuel. aware of what it’s efficiency is at the actual operating
Now I hope you’re beginning to see where you can conditions. When we lower steam pressure, or raise
make some difference. All that attention to the tuning it, we’ve changed operating conditions for the boilers,
of the boiler to get optimum boiler efficiency is not as economizers, boiler feed pumps and system steam traps.
productive as making certain that the energy converted An increase or decrease in pressure will alter the pres-
to steam and hot water is used efficiently. sure drop in steam mains to amplify the change at the
Plant efficiency deserves all our attention because steam utilization equipment. In some cases we’ll have
it is the sole purpose of the boiler plant to deliver heat to charts or graphs that will predict the efficiency at the
the facility. I’m careful to point out that when I say “fa- new condition. Some, like pump curves, do so with an
cility” I mean the buildings, production equipment, etc., accuracy that we can use. We may have to measure per-
served by the boiler plant. The facility itself is involved formance of other equipment to determine if the change
in the energy equation under these conditions because it is beneficial or detrimental.
can contribute to the performance of the boiler plant. It In some cases operating efficiencies are described
does so primarily by returning condensate and, in some using terms other than percent. Chillers, for example,
cases, generating some of the steam or producing some will list the kilowatts per ton values at different loads. In
of the heat. those instances the important thing to know is whether
A facility can also waste much of the heat energy the ratio should be increased or decreased to increase
produced in the boilers to increase fuel and electric- efficiency. As operators we don’t have to know the value
ity consumption. It may not be your responsibility to precisely, we only need to know whether we want to in-
reduce that waste but you should be monitoring and crease it or decrease it. In the case of kW per ton we want
documenting it for the benefit of the owner so it can be to decrease it. In the next section we’ll discuss some of
reduced. To identify your own overall performance, cal- these parameters which are much easier for boiler op-
culate the plant efficiency as defined. To get a measure of erators to use.
the facilities performance, compare fuel used to produc- At the risk of being accused of trying to gener-
tion quantities (production ratios) heating degree days, ate too many new terms I’ll stick my neck out and talk
or a formula you develop that accounts for the load about “cycling efficiency.” It isn’t addressed in any of
variations. the literature and is not given the attention it deserves.
You can also keep track of the difference in energy I’ve discovered it’s very important and have developed
returned by the facility. It can make a difference. If the an analysis method to determine it. It’s surprising how
third shift is assigned cleanup and discovered that the many boilers are out there serving a load only by cy-
hot condensate did a better job of cleaning than the heat- cling. Very few of them are in boiler plants manned by
ed domestic water you would catch them doing it. After operators but you may have to attend to one.
all, condensate is distilled water and it will dissolve a lot Whenever the load on a boiler is less than that
more than city water. boiler’s output at low fire the boiler has to cycle to serve
Which efficiency should you use? Well, I’ve al- the load. All the time it sits there it’s radiating heat,
ready said plant efficiency is the one you should moni- that radiation loss that’s only a few percent at the most
tor for overall plant performance. For comparing boilers at high fire but may be 10% or more of the input when
use what I call the boiler operating efficiency which is it’s cycling. When the pressure or temperature control
basically combustion efficiency with an accounting for switch contacts close the boiler starts, warms up, and
radiation loss. serves the load until the pressure or temperature control
Blowoff and blowdown losses as explained earlier switch contacts open. Every time it’s off the boiler loses
are functions of water treatment and operation, not boil- heat to the load and air drafting through it. When it
er efficiency. They have to be accounted for in Plant Ef- starts the boiler loses heat as the purge air cools it down.
ficiency because the heat lost to blowoff and blowdown Those heat losses, purge air cooling and off cycle cool-
isn’t delivered to the facility. Steam generated that’s ing become very significant as a percentage of the input.
used in the deaerator isn’t delivered to the facility nor is Cycling efficiency accounts for all those losses.
steam used to heat the plant. Now most engineers will tell you that it really
For all practical purposes every piece of equipment doesn’t matter much because the boiler input is very low
has an operating efficiency that is separate and distinct when it’s cycling. That’s true, but a boiler that is serving
from predicted efficiency. We seldom manage to oper- a load at 5% of capacity may be operating at a cycling ef-
ate equipment at its designed capacity so we should be ficiency of 30% or less which means it burns more than