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9 Heating Ventiuiting Air Conditioning Guide 1938 e. The return line should be pitched downward toward the furnace. It should be designed to minimize friction. /. The top of the shoe or boot should never be above the grate level. 8 0 Discuss the use of a booster fan. What effect has a booster fan at low operating temperatures? At high ones? A booster fan is useful in accelerating the air flow past the surface of a low temperature furnace, where only a small weight differential in the air is created, and in unbalancing a gravity system so flow is established. The first use involves the entire plant, and increases efficiency about 10 per cent with low temperature operation; the second involves only the leaders in which air flow is accelerated." At high operating tempera tures the difference in weight between warm outgoing air and cool incoming air is great enough to make a booster unnecessary with ordinary gravity systems. 9 Is it desirable to use high side wall locations for warm air registers in gravity circulating systems? High side wall locations are not recommended on account of the tendency for stratifica tion of the air in the room resulting in high temperatures at the ceiling. \ 392 Chapter 20 MECHANICAL WARM AIR FURNACE SYSTEMS Furnaces, Fans and Motors, Sound Control, Air Washers and Filters, Air Distribution Design, Automatic Controls, Design o Heating System, Selecting the Furnace, Selecting the Fan, Heavy Duty Fan Furnaces, Humidification, Cooling Methods, Cooling System Design MECHANICAL warm air or fan furnace heating systems1, which are a special type of central fan systems, are particularly adapted to residences, small office buildings, stores, banks, schools, and churches. Circulation of air is effected by motor-driven fans instead of by thedifference in weight between the heated air leaving the top of the-casing and the cooled air entering its bottom, as in gravity systems described in Chapter 19. The advantages of mechanical systems, as compared with, gravity systems are: 1. The furnace can be installed in a comer of the basement, leaving more basement' room available for other purposes. 2. Basement distribution piping can be made smaller and can be so installed as to give full head room in all parts of the average basement, or be completely concealed from view except in the furnace room. . 3. Circulation of air is positive, and in a properly designed system can be balanced in such a way as to give a greater uniformity of temperature distribution. 4. Humidity, control is more readily attained. 5. The air may be cleaned by air washers or filters, or both. 6. The fan and duct equipment may be utilized for a complete cooling and dehumidifying system for summer, using either ice, mechanical refrigeration, or low temperature water for cooling and dehumidifying, or adsorbers for dehumidifying. 7. The use of the fan increases the volume of air which can be handled; thereby increasing the rate of heat extraction from a given amount of heating surface and insuring sufficient air volume to obtain proper distribution in a large room. Much of the equipment used in central fan systems is the subject matter of other chapters. It is the purpose of this chapter to discuss the co ordinated design and to deal in detail only with problems not covered elsewhere which refer particularly to the whole problem of fan warm air furnace heating and air conditioning. , University of Illinois, Engineering Experiment Station Bulletin No. 266 by A. P. Kratz and S. Konzo tor details of tests conducted in Warm Air Research Residence. 393