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Heating and Ventilating Systems 29: Figure 3-2. Steam Boiler

This document discusses steam heating systems. It explains that steam is water that has been heated to its boiling point of 212°F and converted to vapor form through the addition of latent heat. Producing 1 pound of steam from 32°F water requires a total of 1150 Btu of heat, including 180 Btu of sensible heat to raise the water's temperature and 970 Btu of latent heat to convert it to vapor. Steam heating systems are classified as low pressure (15 psig or less) or high pressure (over 15 psig).

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106 views2 pages

Heating and Ventilating Systems 29: Figure 3-2. Steam Boiler

This document discusses steam heating systems. It explains that steam is water that has been heated to its boiling point of 212°F and converted to vapor form through the addition of latent heat. Producing 1 pound of steam from 32°F water requires a total of 1150 Btu of heat, including 180 Btu of sensible heat to raise the water's temperature and 970 Btu of latent heat to convert it to vapor. Steam heating systems are classified as low pressure (15 psig or less) or high pressure (over 15 psig).

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© © All Rights Reserved
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Heating and Ventilating Systems 29

Figure 3-2. Steam Boiler

per square inch gage pressure (psig), which is a pressure of 30


psia and a temperature of 250°F.
The amount of heat required to bring the water to its boiling
temperature is its sensible heat. Additional heat is then required
for the change of state from water to steam. This addition of heat
is steam’s latent heat content or “latent heat of vaporization.” To
vaporize one pound of water at 212°F to one pound of steam at
212°F requires 970 Btu. The amount of heat required to bring
water from any temperature to steam is called “total heat.” It is
the sum of the sensible heat and latent heat. The total heat re-
quired to convert one pound of water at 32°F to one pound of
30 HVAC Fundamentals

steam at 212°F is 1150 Btu. The calculation is as follows: the heat


required to raise one pound of water at 32°F to water at 212°F is
180 Btu of sensible heat. 970 Btu of latent heat is added to one
pound of water at 212°F to convert it to one pound of 212°F
steam. Notice that the latent heat is over 5 times greater than
sensible heat (180 Btu × 5.39 = 970 Btu). The total heat is 1150 Btu
(180 + 970). See Figure 3-3.

Point 1 — One pound of ice (a solid) at 0°F.

Point 1 to Point 2 — 16 Btu of sensible heat added to raise the


temperature of the ice from 0°F to 32°F. Specific heat of ice is
0.5 Btu/lb/°F.

Point 2 to Point 3 — Ice changing to water (a liquid) at 32°F.


It takes 144 Btu of latent heat to change one pound of ice to
one pound of water.

Point 3 to Point 4 — 180 Btu of sensible heat added to raise


the temperature of the water from 32°F to 212°F. Specific heat
of water is 1.0 Btu/lb/°F.

Point 4 to Point 5 — Water changing to steam (a vapor) at


212°F. It takes 970 Btu of latent heat to change one pound of
water to one pound of steam.

Point 5 to Point X — X amount of Btu of sensible heat added


to raise the temperature of the steam from 212°F to X°F. This
is called superheating the steam and the result is “super-
heated steam.” For example, if the final temperature of the
superheated steam is 250°F then 19 Btu of sensible heat
would have to be added (250°F – 212°F = 38°F. 38°F × 0.5
Btu/lb/°F specific heat for steam × 1 lb of steam = 19 Btu).

PRESSURE CLASSES OF STEAM HEATING SYSTEMS

Steam systems may be classified as low pressure (15 psig/30


psia or less) or high pressure (greater than 15 psig/30 psia). It is

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