Page 36 - Boiler Operator’s Handbook

Operating Wisely 21

Understanding saturation is the key to under- because it’s typically assumed that the fluids flow from
standing steam explosions. When water is heated to higher pressures (higher in the diagram) to lower pres-
saturation conditions higher than 212°F, as in a boiler, sures. I included arrows to guide the novice. Valves,
it cannot exist as water at that temperature if the vessel both isolating and control are not shown, nor are steam
containing it fails. Under those circumstances the satu- traps always shown. They are understood to exist in
rated condition becomes one atmosphere and 212°F as components that use the steam and where required to
the water leaks out. A portion of the water is converted isolate systems for maintenance.
to steam to absorb the heat required to reduce the tem- The simplest steam and water cycle exists in a
perature of the remaining water to 212°F. How much low-pressure heating system. Water is pumped into the
steam is generated is determined by the original boiler boiler, is heated to saturation temperature, then con-
water temperature but every pound of water converted verted to steam. Almost all of the energy added to the
to steam expands to 26.8 cubic feet. The rapid expansion water and steam in the boiler is latent heat. We refer to
of the steam is the steam explosion. that latent heat as the latent heat of evaporation; the en-
Let’s do the math for a heating boiler operating at ergy used to convert the water to steam. The steam then
10 psig. The 240°F water has to cool to 212°F releasing 28 leaves the boiler and flows through piping and control
Btu per pound. It can only do so by generating steam at valves to radiators, convectors, kitchen equipment, etc.
212°F which contains 1,150 Btu per pound. One pound where the steam is condensed. Those users of the heat
of steam can cool 41 pounds of water (1,150 ÷ 28). The primarily use the latent heat of condensation. There may
volume of 42 pounds of 240°F water at 0.01692 cubic feet be some heat transferred to the user by cooling the con-
per pound (0.71 cubic feet) becomes 41 pounds of water densate. The condensate is then returned to the boiler
at 212°F (0.01672 × 41 = 0.685 cubic feet) and one pound completing the cycle. Figure 1-7 is an example of a steam
of steam (26.8 cubic feet) so the original volume of water cycle diagram for a conventional heating plant, perhaps
expanded 38.71 times (0.685 + 26.8 = 27.48 ÷ 0.71) and it a school, a restaurant, or an apartment building.
happens almost instantly. I can hear it now! “Hey, Ken, what’s so simple about
Other situations involving steam at saturation are this diagram?” I couldn’t resist throwing in some things
described in the discussion of equipment where it must to make you think about your facility. When steam is
be understood. used to heat things at temperatures close to, at, or above
212°F the temperature of the condensate from that heat-
ing equipment is much higher and any drop in pressure
THE STEAM AND WATER CYCLE will result in some of the condensate flashing into steam.

For most of you who are operating commercial, in-
dustrial, or institutional boiler plants the concept of the
steam and water cycle is a foreign concept. Understand-
ing a cycle becomes important when you’re generating
power with the steam. By generating power I mean
generating electricity or powering a mechanical device
normally with a steam turbine. In order for this edition
of the book to be more meaningful for plant operators
I’ll attempt to present some descriptions of steam and
water cycles.
For almost any plant there is a steam and water cy-
cle; the exception being hot water heating boilers where
steam is not generated. In hot water heating plants the
cycle is simply heat in and heat out. It is added to the
water in the plant boilers, the water is transferred by
circulating pumps (or natural circulation) to radiators,
convectors, kitchen equipment, etc. where the water
is cooled by the users of the heat, then the water is re-
turned to the boiler to be heated again.
Flow arrows are seldom shown on a cycle diagram 1-7. Low pressure steam cycle