Steam has some design and operating advantages over hot water heating systems. For instance, one pound of steam at 212°F when condensed (latent heat of condensation) into one pound of hot water gives up approximately 1000 Btu per pound of steam.

On the other hand, a hot water heating system with supply water temperatures at 200°F and return water temperatures at 180°F only gives up 20 Btu per pound of water (1 Btu/lb/°F). Another advantage is that steam, based on its operating pressure, flows throughout the system on its own while a pump and motor is needed to circulate hot water.

In an open vessel, at standard atmospheric pressure (sea level), water vaporizes or boils into steam at a temperature of 212°F. But the boiling temperature of water, or any liquid, is not constant. The boiling temperature can be changed by changing the pressure on the liquid.

If the pressure is to be changed, the liquid must be in a closed vessel. In the case of water in a heating system, the vessel is the boiler. Once the water is in the boiler it can be boiled at a temperature of 100°F or 250°F or 300°F as easily as at 212°F.

The only requirement is that the pressure in the boiler be changed to the one corresponding to the desired boiling point. For instance, if the pressure in the boiler is 0.95 pounds per square inch absolute (psia), the boiling temperature of the water will be 100°F. If the pressure is raised to 14.7 psia, the boiling temperature is raised to 212°F.

If the pressure is raised again to 67 psia, the temperature is correspondingly raised to 300°F. A common low pressure HVAC steam heating system will operate at 15 pounds 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 required to convert one pound of water at 32°F to one pound of 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).

Refer to the figure below:

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 “superheated 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).

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1 comment:

James Feder said...

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