Page 406 - Boiler_Operators_Handbook,_Second_Edition

Controls 391

rushing in at twice the boiler capacity can, for a short pe- flow of water is required to prevent generating steam
riod of time, consume so much of the heat to simply heat in the economizer. The feed pump on-off operation
up the feedwater that some of the steam in the boiler is produces a significant change in output of a boiler,
condensed so the water level drops suddenly every time especially at low loads, that can cause bumps in the
the pump runs (see shrink and swell discussed later). whole steam system. Anything larger than a small
Sometimes it’s enough to trip the low water cutoff. Con- commercial boiler operation should have a better
siderable differences in boiler level is required for them method of water level control.
to operate without false trips. Many of the new flexitube There are two unique self contained control systems
boilers are equipped with two level controls, one set for that you should be aware of. They were used only on
controlling level when the boiler is off and another for boilers, and can still be found in many locations. One
when the boiler is firing, set at a higher level. is a thermo-mechanical system; the other is thermo-hy-
If the boiler has an economizer the continuous draulic. The key to these controls is that prefix, “thermal”
which indicates that we use temperature to detect level
and power the control valve. The thermo-mechanical sys-
tems (Figure 11-23) are manufactured by Copes-Vulcan.
The thermo-hydraulic systems (Figure 11-24) are manu-
factured by Bailey and Swartout among others. Both
systems use the difference in heat transfer rates between
steam condensing and simple water heating.
They incorporate a tube connecting ends to the
water space and steam space in the boiler. The water
level in the boiler is repeated in the tubing so the tubing
above the water level is exposed to steam and the tubing
below the water level is exposed to boiler water (actu-
ally it’s mostly condensate from the steam condensing
in the tube). Since steam condensing transfers heat
much faster than hot water the portion of the tube that
is exposed to steam is hotter. Both systems arrange con-
necting piping so the tube is at an angle, the slope of the
Figure 11-23. Thermo-mechanical boiler level control thermo-mechanical tube being much shallower than the
thermo-hydraulic, to provide
additional tube length and (as
a result) heat exchange surface
for better control.
Since the heat transfer is
much higher for steam con-
densing the lower the level of
the boiler water the hotter the
tube. The heat transfer from
the finned water jacket of the
thermo-hydraulic controller or
from the tube of the thermo-
mechanical controller to the
surrounding air is increased
slightly because of the hotter
water jacket or tube. The ex-
pansion of the tube, or the wa-
ter in the jacket, is converted to
movement of the valve; open-
ing it as the tube or jacket gets
Figure 11-24. Thermo-hydraulic boiler level control hotter.