Page 416 - Boiler_Operators_Handbook,_Second_Edition

Controls 401

operator knows his combustion air comes from the out-
doors and makes sure the sources of that air flow are
not blocked by leaves, snow, and other forms of debris.
There are some offsetting conditions because a fan will
deliver more pounds of cold air than hot air (see the sec-
tion on centrifugal fans) so the air to the burner actually
increases as the boiler room gets colder. It tends to offset
the additional friction as the operators start closing ev-
erything to keep warm; but it can’t do it all.
I have a problem with the typical approach of tun-
ing up the boilers in the summer when there aren’t a lot
of no heat calls so the technicians have plenty of time.
I would rather pay the technician overtime to tune my
boiler in the winter when the doors are shut and the air
is cold. That’s when the boiler is burning the most fuel
and I want the most efficient operation. Any jackshaft
controlled boiler should be tuned in cold weather with
all doors, windows, etc., adjusted to winter positions.
Some of that increased flow of colder air is re-
quired later in the winter when the gas or oil gets colder.
Figure 11-30. Linkage control valve with covered ad-
There isn’t a significant difference in the volume of oil
justments
as it cools and change in flow is not as measurable as it
is with gas. Colder gas is more dense and the boiler will
finding a technician that’s even capable of understand- burn more gas at each setting of the control valve. The
ing linearity, let alone adjusting the actuating motor and colder air doesn’t necessarily compensate for it.
fan damper linkage to produce it. Some technicians will There are also variations in fuel and air flow as-
tell you it’s impossible to establish linearity on a small sociated with changes in atmospheric pressure because
boiler but that’s because they don’t know how to do it. the pressure of fuel after a pressure regulator is equal to
In a few pages I’ll tell you how. the sum of spring force and atmospheric pressure in the
A jackshaft system provides simple, highly reli- pressure reducing valve. The fuel gas pressure can vary
able control but its performance is affected by external a fair amount depending on where the regulator vent is.
conditions and devices. Wise boiler operators need to be Pressure is higher when the vent is on the side the wind
aware of how they can alter the air to fuel ratio indepen- is hitting. A pressure below normal atmospheric is often
dent of the jackshaft controls and maintain their plant produced on the downwind side of a building.
accordingly. Wind forces can also affect the difference between
The flow that is the most susceptible to external in- the air inlets to the building and the stack to alter com-
fluences is combustion air flow. I’ve often walked up to bustion air flow. Air density also varies slightly with
the door of a boiler plant and banged my nose because atmospheric pressure. All these variations in tempera-
the door didn’t budge when I pulled on the handle (I ture, wind, atmospheric pressure, and human gener-
pulled myself into the door or wall) once I put enough ated interferences require all burner adjustments have
pull on it the door opened and I found myself blown a cushion of excess air to absorb those variations. We’ll
into the plant by the air flow. It’s no wonder the op- accept a little loss in efficiency to ensure we don’t oper-
erators were having problems with the boiler smoking, ate fuel rich so we generate carbon monoxide and other
they had closed all the windows, doors and operable hazardous and poisonous gases.
louvers (many also boarded up the fixed louvers) so the A typical jackshaft system is adjusted for about
air had only cracks and seams to get through on its way 15% excess air at high loads, producing a flue gas with
to the inlet of the forced draft fan. 3% oxygen remaining, to ensure the boiler will always
Such conditions also aggravate the situation be- operate without going fuel rich. In testing it can prob-
cause the soot formed on the fire sides of the boiler from ably fire at 1/2 to 1% excess air without combustibles.
the smoke act to restrict the flow of flue gases through Almost any boiler will require some increase in excess
the boiler to block off the air flow even more. A wise air below 50% firing rate because the drop in velocity