Page 34 - Boiler Operator’s Handbook

Operating Wisely 19

the same amount of heat as one with steam in it even it to go somewhere. The normal means for the expansion
though the water is at the same temperature. of the water in a hot water system is an expansion tank,
The rate of heat transfer from a hot metal to steam a closed vessel containing air or nitrogen gas in part of
and vice versa is always greater than heat transfer from it. Modern versions of expansion tanks have a rubber
a hot metal to water. It’s because of the change in vol- bladder in them to separate the air and water. The blad-
ume more than anything else. Take a simple steam heat- der prevents absorption of the air into the water. The air
ing system operating at 10 psi (240°F). Check the steam or nitrogen compresses as the water expands, making
tables and you’ll find a pound of water occupies 0.01692 room for the water with a little increase in overall sys-
cubic feet and a pound of steam occupies 16.6 cubic feet. tem pressure. Tanks without bladders normally have a
As the steam is created it takes up almost 1,000 times gage glass that shows the level of the water in the tank
as much space as the liquid did. That rapid change in so you can tell what their condition is.
volume creates turbulence so the heating surface always A hot water system will also have a means to add
has water and steam rushing along it. It’s about the water, usually directly from a city water supply. Most
same effect as you experience when skiing or riding in have a water pressure regulator that adds water as need-
a convertible, you’re cooler because the air is sweeping ed to keep the pressure above the setting of the regula-
over your skin. When the steam is condensing it col- tor. A relief valve (not the boiler’s safety valve) is also
lapses into a space one one-thousandth of it’s original provided to drain off excess water. Older systems can
volume and more steam rushes in to fill the void. That’s be modified and added to the extent that the expansion
the mechanism that improves heat transfer with steam, tank is no longer large enough to handle the full range of
not the fact that steam has more heat on a per pound expansion of a system. In some newer installations I’ve
basis. found tanks that were not designed to handle the full
Steam may have more heat per pound but those expansion of the system. Those systems require auto-
pounds take up a lot more space. One cubic foot of water matic pressure regulators to keep pressure in the system
at 240°F contains 12,234 Btu but one cubic foot of steam as the water shrinks when it cools and the relief valve
only contains 69.88 Btu. Say, that provokes a question. to dump water as it expands while the system heats up.
Why don’t we only use hot water systems because wa- The tank should be large enough, however, to prevent
ter can hold more heat? The best answer is because we the constant addition and draining of water during
would have to move all those pounds of water around normal operation. A good tight system with a properly
to deliver the heat. To deliver the heat provided by one sized expansion tank should retain its initial charge of
pound of steam would require about 200 pounds of water and water treatment chemicals to simplify system
water. Steam, as a gas, naturally flows from locations maintenance.
of higher pressure to those of lower pressure, we don’t All hot water systems larger than a residential unit
have to pump it. The rate of water flow is restricted to should have a meter in the makeup water line so you
about 10 feet per second to keep down noise and ero- can determine if water was added to the system and
sion. Steam can flow at ten times that speed. Nominal how much. Lacking that meter a hot water system can
design for a steam system is a flowing velocity of about operate with a small leak for a long period of time dur-
6,000 feet per minute. If you found that confusing, check ing which scale and sludge formation will occur until
the units, there are 60 seconds in a minute. you finally notice the stack temperature getting higher
Hot water is a little easier to control when we have or some other indication of permanent damage to the
many low temperature users. A hot water system has a boiler or system.
minimal change in the volume of the water at all operat- Steam compresses so there is seldom a problem of
ing temperatures. For that reason we will pay the cost of expansion with steam boilers unless you flood the sys-
pumping water around a hot water system in exchange tem. However, since steam temperature and pressure
for avoiding the dramatic volume changes in steam is related when using steam at low temperatures we
systems. Never forget that there is a change in volume frequently get a vacuum and air from the atmosphere
in a hot water system; to forget is to invite a disaster. leaks in. We will say a vacuum “pulls” air in but it really
Water changes volume with changes in temperature at doesn’t have hands and arms that can reach out to grab
a greater rate than anything else, almost ten times as the air. The atmospheric air is at a higher pressure so it
much as the steel most of our boiler systems are made will flow into the vacuum. In those cases where we have a
of; see the tables in the appendix. Unlike steam it doesn’t tight system the vacuum formed as steam condenses will
compress as the pressure rises so the system must allow approach absolute zero so the weight of the air outside