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tion unsatisfactory. In addition to excessive humidity, especially in the winter months, being objectionable for physical comfort, it takes a great deal of heat to evaporate a pound of water into a pound of water vapor, even without raising the temperature of the vapor above that of the liquid. As an illustration : If the amount of heat necessary to evaporate one pint of water were converted into work, it would lift a man weighing 150 lbs. from sea level to the top of a mountain 5,000 ft. high.
The peculiar thing about dry air is that it must be heated to a much higher temperature to feel the same degree of comfort. Air at 58° F. is comfortable, provided the relative humidity is at least 65%; while if the air is extremely dry, say 30% humidity, it is necessary to carry a temperature of 75° for the same degree of comfort. The best conditions in a theatre would be a temperature of 70° and about 50% relative humidity. You will remember the fine days in early June when you halted just outside the front door, took a long deep breath and started for the day's work with a smile on your face, a wonderful vigor in your stride, a keen, brilliant light in your eyes, and your mind clear. That's not due to Springtime — it's because the temperature is about 70° and the relative humidity 50%.
During the summer months, of course, the heaters are not in use, but the supply fan continues to deliver its full volume of air into the building, and the air washer continues to remove all traces of dust, soot, etc., from the air and often cools it from 10° to 15° by evaporation. The amount of cooling, of course, depends upon the outside relative humidity. If the air is already nearly saturated, little, if any, cooling is obtained, but if the outside air is comparatively dry, as is the case in many parts of the country, there is a very marked cooling even if the water in the air washer is recirculated, as most of this cooling is obtained by evaporation, rather than from the air coming in contact with water at a lower temperature than air. This is generally referred to as evaporative cooling. As an illustration: Moisten one of your fingers and blow upon it — your finger feels cool. Now blow upon one of your dry fingers and there is a warm feeling. When you blew upon your moist finger your breath immediately began to evaporate the moisture. You will remember I referred above to what an astonishing amount of heat is required to evaporate a pint of water. In the summer when no heat is being applied to the spray of water, the air must give up some of its heat in order to evaporate the water. In other words, dry air, greedy for water vapor, is forced to pay for the water vapor with heat; hence the cooling effect when air is passed through a spray chamber. Needless to say the hotter it is outside, the greater the cooling obtainable inside by means of this evaporation. An installation was recently made in Denver, Colo., where the cooling, due to evaporation, was from 25 to 30 degrees below the outside temperature. This would naturally tend to make a theatre a very popular place during the warm months, and a theatre so equipped is due to
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