--------+-------+----+----+---------+-------------+------------------ | Temp. |Wet |Dry | Per | Evap. In | Date |outside|bulb|bulb| cent. | 24 hours | Cold-air intake | | | |saturated|quarts pints | --------+-------+----+----+---------+-------------+------------------ Dec. 13 | -13 | 54°| 63°| 53 | | Closed 8 a.m. Dec. 14 | -18 | 55 | 66 | 47 | | Open Dec. 15 | -20 | 57 | 68 | 49 | 21 | Closed 7 a.m. Dec. 16 | -18 | 57 | 67 | 51 | 20 1 | Closed 7 a.m. Dec. 17 | -22 | 58 | 69 | 48 | 18 1 | Closed 7 a.m. Dec. 18 | -16 | 55 | 65 | 51 | 17 1½ | Closed 6:30 a.m. Dec. 19 | -10 | 57 | 68 | 47 | 20 1 | Closed 8 a.m. Dec. 20 | 0 | 59 | 70 | 49 | 13 ¾ | Not open at night Jan. 8 | -12 | 58 | 71 | 43 | 18 | Closed Jan. 9 | -17 | 57 | 71 | 39 | 25 | Open 24 hours Jan. 10 | -16 | 58 | 69 | 45 | 27 1 | Open 10 hours Jan. 11 | -21 | 60 | 75 | 40 | 30 | Closed Jan. 12 | -15 | 60 | 73 | 46 | 30 | Closed --------+-------+----+----+---------+-------------+------------------ The amounts of water evaporated may seem large to those who are unaccustomed to quantitatively consider problems in ventilation but the small amount of water in the air at -21° must produce a very dry atmosphere when it is raised to 70° in temperature. The amount of moisture in air at 20°F. and at 80 per cent. humidity is only 1.58 grains to the cubic foot. If this air is now raised to 70° the moisture will still be 1.58 grains where there should be 4 grains of water to make 50 per cent. humidity. It therefore will require the addition of practically 2.42 grains of water for each cubic foot of entering air in order to bring it up to 50 per cent. humidity. In a case with the above conditions of atmosphere, suppose it is desired to know the amount of water that would be taken up in humidifying the air for a school-room of size to accommodate 40 pupils. The prescribed quantity of air for this purpose is 30 cubic feet per minute for each pupil. The air is to be maintained at a humidity 50 per cent. saturated. The problem will be one of simple arithmetic. If each pupil is to receive 30 cubic feet of air per minute or 1800 cubic feet per hour, the 40 pupils receiving 1800 cubic feet per hour will require 40 × 1800 = 72,000 cubic feet of air per hour. To each cubic foot of the air is to be added 2.74 grains of water, 72,000 × 2.42 = 164,240 grains of water. Reducing this to pounds, 164,240 ÷ 7000 = 23.46 pounds or 2.77 gallons of water per hour.