Page 152 - Boiler_Operators_Handbook,_Second_Edition

Special Systems 137

and brass boiler parts are more malleable and slightly In that case, the bulb was clamped to the pipe where the
stronger than ice so the effect is not as dramatic, but it cold water enters the tank.
does happen. Each time the system filled the storage tank until
Boiler damage due to thermal shock is normally the hot water flowed out of the storage tank into the pip-
result of repeated heat/cool cycles. Damage occurs when ing and into the bottom of the boiler for a short period
the metal is over-stressed because the surface is cooled until the temperature controller finally responded to
or heated at a rate that exceeds the heat flow through it. the change from cold to hot water. When the circulat-
As a result one surface is at a different temperature than ing pump started again the hot water was immediately
the one opposite it. The differences in thermal expansion displaced by cold water. The thick metal at the bottom of
result in compressive stress at the hottest surface and the boiler was repeatedly subjected to swings between
tensile stress at the coldest surface. When the difference hot and cold water entering the boiler which resulted in
in stress reaches the breaking point of the metal then tiny cracks around the bottom of the boiler shell.
micro cracks form in the colder surface. Repeated expo- As you can tell, simply heating hot water isn’t as
sure to the heating and cooling expands the cracks until simple as it sounds. There’s even an unusually different
leaks are evident. Thermal shock can also be associated attitude about scale formation among people that main-
with rapid changes in firing rate but most service water tain these devices. Why? They manage to get away with
heaters are designed to accommodate the changes associ- a considerable amount of scale because water tempera-
ated with their on/off operation. tures are so low. It’s a common practice to allow scale
You would think that a hot water heater with to build in one of these heaters (keep in mind, you can’t
normal temperature differentials of 140°F would be treat it because it has to be potable where someone could
damaged regularly by thermal shock if even smaller drink it) until you can hear the loose scale (they call it
temperature differentials are a problem. They don’t be- lime deposits) rattling in the bottom of the heater where
cause the overall temperature differential is distributed steam is forming under the material and then collapsing
along the length or height of the boiler. The boiler in Fig- as it contacts the colder water.
ure 4-10 would normally have 40°F water entering the Since water is not concentrated in a service water
bottom (at T1) and 180°F water leaving the outlet (at T2) heater you would not expect it to form scale except
with the temperature between those two levels varying under unusual conditions, but it happens regularly. It’s
almost linearly from top to bottom. The high tempera- not uncommon for scale to form on the heat transfer
ture differentials between the products of combustion surfaces to the point that the heater capacity is less than
and the water in the boiler do not produce a significant demand and you can’t make enough hot water. I can
temperature difference across the thickness of the metal recall one location where the solids content of the water
because the heat flows through the metal much faster was so high that a mere 6°F increase in water tempera-
than through the thin film of flue gas between the metal ture was all that was required for scale formation. The
and the products of combustion. The temperature dif- best solution for these applications is water softeners
ferential across the metal is normally less than 30°F. but that’s not always accepted by the powers that be so
Thermal shock occurs when a liquid in contact you should be prepared to clean a service water heater
with the metal is quickly displaced by other liquid at regularly as part of its maintenance when the calcium
a temperature significantly lower or higher than the and/or magnesium content of the water is high.
original liquid. The direct contact with the metal parts
and turbulence associated with the rapid replacement of
the liquid heats or cools the metal surface rapidly, faster WASTE HEAT SERVICE
than the heat transfer through the metal itself.
So what caused the damage to the boilers I men- As far as I’m concerned these are the best boilers;
tioned earlier? What can cause thermal shock? Well, in the cost of fuel, the single largest cost for any other kind
the case I first examined, the temperature control was of boiler plant, is zero! That one great benefit also en-
different. Instead of installing a temperature switch that courages us to put up with some unique and sometimes
penetrates the storage tank at a level above the water hazardous flows that contain the heat we extract with the
inlet (as shown in Figure 4-9) the contractor provided a boiler. I think the one most hazardous I’ve seen is a sulfur
“strap-on” aquastat. That is a temperature switch with dioxide stream from firing pure sulfur to make sulfuric
a bare thermal sensing bulb that is simply clamped to acid. Knowing what you know about problems with sul-
the outside of a tank or pipe to sense the temperature. fur in conventional fuels should make you appreciate the