Page 128 - Boiler_Operators_Handbook,_Second_Edition

What the Wise Operator Knows 113

You shouldn’t confuse matching loads and react- EFFICIENCY
ing to changing loads however. I was in one plant that
started up a boiler every morning to handle the warm- There are so many definitions of efficiency and
up as the night set-back thermostats switched back up. many operators (and most engineers) are confused as to
An hour or two later the boiler was shut down until the which is which or simply assume they are all the same.
next morning. First of all, that’s rough on the boiler and I shall attempt to define the many different labels of ef-
it’s really shortening its life. It’s also wasting a certain ficiency and to clarify what they actually represent. I’ve
amount of energy because what it took to heat the boiler even created a couple of definitions because I’m certain
up is lost up the stack before the next morning. If an there’s a need for them.
operator is doing his job of checking all the operating The first point of confusion involves the defini-
limits when a boiler is started then that daily start-up tion of boiler efficiency. It can be officially defined as
would be rough on the operator; most don’t seem to one hundred times the heat absorbed in the steam and
bother. water divided by the heat energy added by the fuel and
Short-term operation for an intermittent peak load other sources of energy. That’s the definition established
shouldn’t be considered unless there are problems with by the ABMA and the one most of us accept as the true
the steam pressure or supply water temperature drop definition. Those other sources of energy include electric
associated with that load. In other words, it’s okay for power supplied to the fans, and pumps that are integral
the steam pressure or water temperature to drop a little to the boiler. If all of those values we engineers call “in-
when everything starts heating up in the morning. The puts” are accounted for then we get a correct value of
drop will limit the heat flow to the load because there’s efficiency. However, it’s the one that is seldom used.
a smaller temperature difference and everything will The energy added to the water and steam is the
eventually recover. Don’t hesitate to try it. Let the pres- “output” of the boiler. There can be multiple outputs
sure or temperature drop. A slight dip in conditions on that have to be considered. If the boiler has a reheater
an operating boiler is much less damaging than running the energy added to the steam that flows through the
a boiler up from cold. reheater is an output in addition to the water that is
If the pressure or temperature dips can’t be toler- evaporated to produce steam and the energy added in
ated you’ll learn quickly what average night-time tem- the superheater.
perature signals that limit so you can have more boiler Note that the official definition of boiler efficiency
capacity in operation when it’s necessary. considers output to include all the heat absorbed by the
I also want to mention those plants where no- water and steam. That includes the heat added to the
body seems to notice or care what the boiler to load water that’s lost in blowoff and blowdown but not the
relationship is. It’s not at all uncommon for me to find heat lost in steam for sootblowing. Since the boiler’s
a two boiler hot water plant where both boilers are output that we get to use doesn’t come from blowoff
always operating. In most of those plants the boilers or blowdown water or sootblowing steam how can it
were each sized to carry the full load and the operators be counted as output? It’s counted because the boiler
discovered they could shut one down and never worry manufacturer has no control over the quality of water
about having enough boiler capacity. The cost of fuel used to make steam and no control over the fuel fired
to simply keep a boiler hot can be considerable so they and how cleanly it’s fired. The boiler manufacturer is
also found that they saved the owner a lot of money. concerned with the heat that’s transferred through the
Of course you have to shut at least one valve when you tubes.
shut down that hot water boiler. Otherwise the hot wa- Soot blower operation to maintain boiler condi-
ter flowing through it will heat up a lot of air that’s lost tions is one of the reasons that a boiler efficiency test
up the stack. in accordance with ASME PTC-4.1 (Steam Generating
Don’t think you have to run a proportion of boil- Units Power Test Code) is supposed to be run for a mini-
ers to match the load. I’ve been in many plants with mum of twelve hours. The Test Code does account for
four boilers, any one of which could carry the full load the sootblower steam because it’s required to keep the
of the facility served. They’ll run one or two boilers in heat transfer surfaces clean.
the summer and three in the winter whether they need Several years ago the ABMA (American Boiler
them or not. They’re also usually the plants where the Manufacturer’s Association) agreed to guanantee ef-
boilers are regularly switched so they will all wear out ficiency at only one firing rate and, unless otherwise
at the same time. specified by the customer, set it at full load. If you have