Steam Temperatures.
—While the temperature of steam is an unimportant factor in the heating of buildings there are many uses in which it is of the greatest consequence. When steam is employed for cooking or baking it is not the quantity of heat but its intensity that is necessary for the accomplishment of its purpose.
Steam cookers must work at a temperature suitable to the articles under preparation, and the length of time required in the process. Examination of the table on page 3, will show that steam at the pressure of the air or 0, gage pressure, has a temperature of 212°F., which for boiling is sufficiently intense for ordinary cooking; but for all conditions required of steam cooking, a pressure of 25 pounds gage pressure is required. The temperature corresponding to 25 pounds is shown in the table as 267°F. Baking temperatures for oven baking as for bread requires temperatures of 400°F. or higher. To bake by steam at that temperature would require a gage pressure of 185 pounds to the square inch.
The British thermal unit is the English unit of measure of heat. It is the amount of heat required to raise the temperature of a pound of water 1°F. From the table it will be seen that steam at 10 pounds gage pressure, is only 27.4° hotter than it was at 0 pounds. In raising the pressure of a pound of steam from 0 to 10 pounds, the steam gained only 27.4 B.t.u. of heat. The amount of heat gained by raising the pressure to 10 pounds is small as compared with the heat it received on vaporizing. The extra fuel used up in raising the pressure is not well expended. It is customary, therefore, in heating plants, to use only enough pressure in the boiler to carry the steam through the system. This amount is rarely more than 10 pounds and oftener but 3 or 4 pounds pressure.
Gage Pressure—Absolute Pressure.
—In the practice of engineering among English speaking people, pressures are stated in pounds per square inch, above the atmosphere. This is termed gage pressure. It is that indicated by the gages of boilers, tanks, etc., subjected to internal pressure. Under ordinary conditions the term pressure is understood to mean gage pressure, the 0 point being that of the pressure of the atmosphere. This system requires pressures below that of the atmosphere to be expressed as a partial vacuum, a complete vacuum being 14.7 pounds below the normal atmospheric pressure.
In order to measure positively all pressures above a vacuum, the normal atmosphere is 14.7 pounds; all pressures above that point are continued on the same scale, thus:
Gage pressure 0 = 14.7 absolute
Gage pressure 10 = 10 + 14.7 = 24.7 absolute
Gage pressure 20 = 20 + 14.7 = 34.7 absolute
Absolute pressures are, therefore, those of the gage plus the additional amount due to the atmosphere. All references to pressure in this work are intended to indicate gage pressure unless specifically mentioned as absolute pressure.
Steam heating as applied to buildings may be considered under two general methods: the pressure system in which steam under pressure above the atmosphere is utilized to procure circulation; and the vacuum system in which the steam is used at a pressure below that of the atmosphere. Each of these systems is used under a great variety of conditions, and to some is applied specific names but the principle of operation is very much the same in all of a single class.