Document ZJYKbXoB8kGg3zmK3KwGJZGjJ

EXHAUST SYSTEMS, GENERAL 6- COMFORT VENTILATION ture from the body is slow, and a wanner feeling is experienced than would be ex pected from the temperature indicated by the thermometer In general, the most de sirable relative humidity range seems to be between 30 and 50 per cent. Relative humidity may be determined either by a hygrometer which reads direct ly m percentage of relative humidity, or by dry- and wet-bulb thermometers from which the readings are taken simultaneously in a current of air and the humidity calculated on a psychrometnc chart (Fig. 28-20). A wet-bulb thermometer is one on which a little sock has been slipped This is moist ened and as it moves about in the air, water evaporates if the air is not 100 per cent saturated The evaporation produces a cooling effect which m turn registers as a lower temperature on the wet-bulb ther mometer Common methods of ventilation Natural ventilation Wind, aided by air currents created by beating devices and higher indoor temperatures, provides a cer tain amount of ventilation even though doors and windows are closed Air enters and leaves through the pores of building materials as well as through cracks and crevices However, spontaneous ventilation should never be relied upon to give satis factory results under all conditions Heating devices, such as ovens, cause the air to expand and nse, and if suitable open ings are provided m the roof or near the ceiling, the rising air currents will pass to the outside Then, if there are suitable openings near the floor, outside air will come in to replace what is lost Skybghts and louvers satisfactorily con trol the escape of aur in some cases, but they must be adjusted when changes of weather and wind occur, and it is some times difficult to prevent downdrafts Vertical stacks through roofs usually have tops to exclude ram and snow The amount of air discharged by a vertical stack and roof ventilator depends on wind veloa- -Ftg. 28-20 Psychrometnc ehort shows characteristics of water vapor and air as temperature varies ty, temperature differences, and the type of ventilator used The stationary types are usually least effective and the wind-actuat ed rotary ones most effective, but a great deal depends upon the design of the indi vidual unit Manufacturers can supply ta bles showing the effectiveness of each de sign Stack or chimney action is the force that produces air flow due to a temperature dif ferential This action is identical to the induced draft of a furnace Air will flow out of a building if the temperature of the indoor air is higher than that of the out door air, and the converse is likewise true At one point, the effect could be neutral ized Design guides Some of the more impor tant guides to realizing the maximum bene fits of natural ventilation are the following 1 Inlet openings in the building should be well distributed on the windward side near the bottom, outlet openings should be located on the leeward side near the top Outside air will then be supplied to the zone to be ventilated. 2 Inlet openings should not be obstructed by buildings, trees, signboards, etc, out side nor by partitions inside 3 Greatest flow per square foot of total opening is obtained when inlet and out let openings have nearly equal areas 4 In order that the force of temperature difference may operate to maximum ad vantage, the vertical distance between in let and outlet openings should be as great as possible 5 Windows on the windward side of momtors should usually be kept closed, since, f they are open, the inflow tendency of the wind counteracts the outflow ten dency of temperature difference Open ings on the leeward side of the monitor result in cooperation of wind and tem perature difference 0 Furnaces that give off heat and fumes should be located in the end of the build ing exposed to the prevailing wind The strong suction effect of the wind at the roof near the windward end will then cooperate with the temperature differ ence, to provide the most active and sat isfactory removal of the heat and the gas-laden air 7 The vacuum produced by the wind is 2S-S5