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 If you ask a dozen people what the proper operating pressure for a steam system is, you'll probably get a dozen different answers. Most folks just follow "what they were taught" without giving much thought to the results. You see most steam systems run at ridiculously high pressures.As early as 1900, residential boiler manufacturers decided that no house heating steam system should operate at a pressure higher than two psi. They could make this statement because it's latent heat, not steam pressure that does the actual heating work in a residential system. Latent heat is the energy we put into water to get it to change state from a liquid to a gas. In the early 1800s an Englishman named Thomas Tredgold coined the term British Thermal Unit. He defined the BTU as the quantity of heat needed to raise the temperature of one pound of water (about one pint) one degree Fahrenheit. For instance, suppose we had one pint of 32 degree water (water can exist as a solid or a liquid at 32 degrees. Did you know that?). If we wanted to raise that pint of water to 212 degrees we would have to add about 180 BTUs of heat. That would give us one pint of water, not steam, at 212 degrees (You see water can also exist as a liquid or a gas at 212 degrees).
Think of it. It only took 180 BTUs to get that pint of water to rise from 32 degrees to 212 degrees. But it took more than five times the heat (970.3 BTUs) to get it to move from 212 water to 212 steam. There was no change in temperature, but there sure was a change in the energy content. This energy is latent heat; it's what heats the house. We get nearly all of it back when the steam condenses in the radiators. Steam has the ability to heat when it's at zero psi pressure. You see you don't need a lot of pressure to heat the building. All you need is latent heat. To prove this is true, consider this: If you add only 10 more BTUs of latent heat per pound of steam to zero psi steam, you'll wind up with steam at 10 psi. That 10 additional BTUs is insignificant when it comes to heating the building, but it can cause us many problems with the system. As you'll see.
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But how do we get that pint of water to change state and become steam? We do it by adding a great deal of latent heat. You know the old saying, "A watched pot never boils?" Well, it's certainly true because to make that pint of water turn into steam we have to add 970.3 BTUs!| Dan Holohan - [Intro] | [Email] | [Website] |
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