Page 144 - Boiler_Operators_Handbook,_Second_Edition

Special Systems 129

the inlet to produce a blend of return and circulated water generator (Figure 4-6) is a once through boiler.
to raise the inlet temperature well above the return water Okay, I still call them boilers; because they are
temperature. These should be started before the boiler boilers. They’re just unique boilers and that’s why we
and normally have proof of operation switches that pre- call them generators. I’ve often wondered if they were
vent boiler operation if the pump doesn’t operate. called “generators” in an effort to exclude them from the
If you live long enough and move around you requirements of the boiler construction codes but I have
might just run into one of these on a water tube boiler, never researched it. They don’t have drums and the
they are rare. Some boilers were manufactured with the headers are usually small enough that someone could
requirement that the circulation of water in the boiler argue that the code doesn’t apply. Those generators re-
had to be assisted by a boiler water circulating pump. quire water flow through them to operate because they
The pump typically takes suction from a drum or header don’t store any hot water, their water volumes are very
at the bottom of the boiler and pumps into a header a lit- low. The controls will include low water flow switches
tle higher. It’s difficult to tell if the pump was provided that prevent burner operation and will shut the burner
as an element of the design of the boiler or added once down if the water flow in the boiler is too low.
it was revealed that natural circulation couldn’t provide The controls require Btu calculation with measure-
sufficient flow through the risers. I’m only aware of one ment of the return water in order to ensure the outlet
situation where the downcomers were a restriction. On temperature is close to steady. Flow through the boiler
that boiler, a new and unique design, the manufacturer consists of several parallel circuits and the tubes are fre-
didn’t provide any downcomers. quently orificed at the headers to ensure proper distribu-
Because boiler water circulating pumps are designed tion of water. It stands to reason that a tube designed for
to pump hot boiler water you may learn by reading the in- water flow on a once through basis will have a real prob-
struction manual that the pump should not be started until lem if steam is generated in it because the larger volume
the boiler has been warmed up to a certain temperature of steam will fill the tube. Once steaming starts in one of
or pressure. That’s because the pump motor was sized for those boilers failure due to overheating rapidly follows.
handling the hot boiler water with assistance in producing Each HTHW generator is commonly fitted with its
a differential from natural circulation so operating it with own circulating pump (standby circulating pumps are
colder water would result in overloading the pump motor. normally shared) to ensure adequate water flow. Nor-
You should also find the pump will have cooling for the mally, there are separate pumps used to circulate the
lubricating oil or direct cooling of the bearings using an high temperature hot water through the system.
external source of cooling water because of high operat- The circulators (in an HTHW plant I’ve always
ing temperatures. There should also be cooling provided heard them called circulating pumps) have to pump wa-
for the pump shaft seals and, of course, pump operation ter much hotter than the standard pump. Even though
should require proof of cooling water flow before the they are installed to pump the water into the boiler like
pump motor can be started. You should find that there is a hydronic circulators they are exposed to temperatures
differential pressure switch used to prove the pump in op- that are so high the oil or grease in the pump bearings
eration incorporated into the burner management system
to safely shut down the boiler in the event of the failure of
the boiler water circulating pump.

HTHW BOILER PLANTS

High temperature hot water (HTHW) plants have
all the characteristics, features and problems associ-
ated with hydronic systems. The defined difference is an
HTHW plant operates with water temperatures higher
than 250°F. The typical HTHW boiler plant has design
conditions of 400 psig and 400°F. HTHW plants also have
some other unique characteristics that are not found in
the typical hydronic system. In most HTHW plants the
boilers are called HTHW generators. They differ consid-
erably in construction and operation. The typical HTHW Figure 4-6. HTHW generator