Document g2X9z4KGQnvgn40K1R5npro8V

American Society of Heating and.Ventilating Engineers Guide, 1929 and fan assembly and forced up through the machine, using either polyphase, alternating current or direct current motor. All of the. air may be driven through the radiator to be heated and thence to the room, orall of the air may be driven through the cold air or by-pass chamber and thence to the room, or part of the air may be driven through the radiator and part through the cold air chamber in any desired proportion, depending upon the.position of the by-pass damper.. The closing of the fresh air damper simultaneously opens the recirculat ing grille at the floor line, so that there is a free path for the air to circulate by gravity through the radiator. Thus, when the motor is not operating and the fresh air damper is closed, the radiator of the unit becomes an enclosed direct radiator, functioning in the same manner, as any other enclosed direct radiator. By starting the motor during heating-up period in the morning, the air may be drawn from the room at the floor line,' heated, discharged, recirculated, reheated and redischarged, this process Continuing until the room has reached the desired temperature, thus effecting a tremendous saving in time and fuel in preparation of the room for occupancy. With this system of ventilation, the air outlets for the room serve strictly in the capacity of vents to permit of displacement .and they should be small, with as little exhausting effect as possible. Under such conditions their location is unimportant further than that they be placed at or very near the floor. In the typical layout they are usually placed in the wall opposite the fan unit. In school work where it is desired to circulate ajr through adjoining wardrobes, room outlets should be in low panels of doors or near floor in partitions. Wardrobe outlets may be either at 'floor or ceiling. The latter arrangement is usually preferable from a purely ventilation standpoint but the former provides a better heating effect. By such a plan direct radiation may usually be omitted from such rooms. When half doors or no doors are used between classrooms and wardrobes, outlets from the latter must be at the floor. With the mechanical unit ventilation system exhaust fans or aspirating coils in vent flues are neither necessary nor desirable, it being the idea to force the air out of the rooms under back pressure. .Where the outlets are properly proportioned this has an inflation effect that retards infiltra tion and assists in diffusion. One vent for each machine is.sufficient and both the grille and flue should have a net free area of about 18 sq. in. for each 100 cu. ft. of air delivered per minute by the ventilator. In order to carry out the principle of diffusion and get proper results from the- unit system of ventilation, consideration must be given to the number, size, location and general application of the mechanical venti lators. With a correctly designed system and proper equipment, good diffusion will be effected if the frequency of: air change in the room: ventilated is equivalent to five or more volumes per hour but the extent to which the desirable effect of air motion is present will be governed both by the frequency of air change, and the ceiling height. , ; On the same- principle that underlies the necessary .'distribution of direct radiation there is a limit to the amount of air which can be dis tributed from a single point in ventilation work if good results are to be 390 Chapter XXIV--Systems of Ventilation expected. Practical experiments seem to indicate that this limit is about 1500 cu. ft. per minute with this type of apparatus. In a practical way this determines the number of unit machines to be used in a given case. Where the very best results are desired it is recommended that the capacity of any single machine be limited to 100 cu. ft. per minute per foot of ceiling height. Mechanical unit ventilators should be located centrally on the outside wall of the room which they serve. Corner locations are liable to result in inefficient and unbalanced distribution. In fact, under adverse con ditions, drafts may result from such a location. Unit ventilators may be recessed but they should never be enclosed or concealed. Not only are enclosures liable to affect the perfection of diffusion, but from a practical standpoint they interfere with proper care and attention by rendering the machine inaccessible. Moreover, there is a certain psychological value to an exposed machine. The occupants quickly learn its purpose and operation with the result that they appre ciate its value and see that it is operated and properly cared for. No single mechanical unit ventilator should be made to serve more than one room by the extension of ducts from the outlet, since this is contrary to all the basic principles of the system. Adjacent rooms, if not sufficient in size and importance to be equipped with individual ventilating systems probably do not require any ventilation. Where the total heat required is in excess of the rated capacity of the unit, the unit must be supplemented by direct radiation. Otherwise, the unit can be used for both heating and ventilating without direct radiation. No special treatment of vent flues is required by this system, the vitiated air being discharged from the building in the same manner, as all other mechanical systems. It has been found that the best results have been obtained where the cross sectional area of the vent fjue does not exceed 20 sq. in. per 100 cu. ft. of air per minute. Industrial Service Unit heaters for industrial work consist of a heating element over which air is forced or drawn by means of a power driven fan which also distributes this heated air to the space to be heated. The area that can be served by one unit varies according to the type of unit and the location of the unit within the building. Industrial units are available in capacities from that of the smallest disc or propeller, type fan up to that of gangs of centrifugal fans, and in ranges, expressed in heating output, from perhaps one hundred square feet of radiation up to several thousand square feet of radiation. They may be placed on the floor, or may be suspended from overhead construction, and may be heated by steam or water-or other fluid, or may be direct fired, with coal, oil or gas. There has been a remarkable recent expansion in the demand and in the production of all sorts of industrial heaters, and their use no longer is confined, by any means, to industrial buildings alone. The tendency is apparent to return after some digression, toward a location for the units as near the floor as is practicable, especially with 391