Document Raj1bL10DO8QnVnnJvZNVV5ba

American Society of Heating and Ventilating Engineers Guide, 1928 System four is especially adapted to industrial plants and shops to convey heat to the points desired and to create an effective air motion and uniform temperature with minimum heat loss. When the fan is required to do both heating and ventilating it is necessary to keep the fans in operation. Great success has been reported in school work with systems of this kind, particularly because of its flexibility, easy temperature and humidity control, and economy of operation.1 The Fan System for Heating and Ventilating consists of a combination of a fan operating in conjunction with a blast heater, with or without a system of air distributing ducts. An air washer or humidifier may be added when required without otherwise changing the type of system. For heating purposes only, the fan system may or may not be used, depending on circumstances and the requirements to be met. The fan system may be used to supply both heat and fresh air for ventilation, or it may be used in conjunction with some form of direct radiation which is to care for the heat losses. When used for ventilating purposes, the fan will be required to supply whatever amount of air is specified to meet the ventilation requirements. The system may be arranged so that the fan may blow the air through the heaters, or may draw the air through the heaters. Each arrangement possesses its own peculiar advantages, but the selection depends largely upon the individual requirements of the installation. The draw-through apparatus is usually employed in factory buildings on account of its compactness as well as on account of the advantage gained by connecting directly to the piping system. In this case the temperature of the air delivered will be the same for all parts of the building. The blow-through apparatus is used in public buildings, or wherever different temperatures and independent temperature regulation are required for different rooms of the building. The use of a by-pass around the heating coils permits the mixture of hot and cold air in any desired proportions, by the use of a mixing damper at the point where the two ducts, one from the heater, and one from the by-pass, joint to form one duct leading to the room. In the case of public buildings, the fan frequently blows the warm air into a space termed a plenum chamber, from which the air ducts radiate to the various rooms of the building; this arrangement is sometimes called the plenum system of heating and ventilating. Air supply systems may be distinguished as upward and downward systems; the former being used frequently* in such buildings as theatersand auditoriums where people are associated closely. Air is supplied near the floor and exhausted through grilles in or near the ceiling. The downward plan is used in schoolrooms, hospitals, and other public buildings, air being introduced about 8 ft. above the floor arid drawn out near the floor. The selection of either system depends upon conditions confronting the engineer. The amount of heat to be supplied is governed by the losses from transmission plus those from infiltration, with proper allowances for heat 1 American Society Heating and Ventilating Engineers Transactions, Vol. 25, 1919. Com parative Study of. Natural and Mechanical Ventilation for School Rooms, Legg & Walker; Vol. 28. 1922, Intermediate and Junior High Schools in Detroit, H. W. Anderson; Vol. 29. 1923, Heating and Ventilating Chicago Schools, John Howatt. 292 XIX--Chapter Systems of Ventilation supplied by persons or processes. Temperatures usually Specified for various types of buildings are to be found in Table 1. ' The amount of air to be supplied depends largely upon the type of service required, the amount of heat needed, the perfection of ventilation demanded, etc. The total quantity of air to be circulated in an indirect heating system, either mechanical or gravity type, is demonstrated in. the seven cases set forth in the Fig. 1, Cases 1 to 6, page 294 as follows: H = Heat loss of room or building as determined by formulae and data given under Chapter I (B.t.u. per hour) M = weight of air passing into room per hour in pounds from the heating system Mr -- weight of air recirculated per hour, pounds Mo = weight of air drawn into the system from the outside for the ventilation require ments per hour lb. and passed through the indirect radiation system Mb -- weight of tempered air by-passed around the reheater per hour, pounds Mh = weight of air passed through heater or reheater per hour, pounds t = mean air temperature of the room of building to = mean outside air temperature t, = mean temperature of the air entering the heater to = mean temperature of the air leaving the indirect radiator tx = temperature loss assumed in the air duct system ty = temperature of the air entering the room or building 0.24 = specific heat air of constant pressure. (B.t.u. required to raise 1 lb. of air 1 deg. fahr.) The mean temperature to of the air leaving the indirect radiator (blast heater, tem pering coil, reheater, unit heater or gravity indirect radiator) should be learned from the makers, tables for the heater or indirect radiator or heater it is proposed to use. Case 1--When all of the air passing through indirect heater is recirculated air: Mo -- 0; Mh = Mr = M; Temperature of air entering heater, 1, = t; M -- 0.24 H (ty - t)~ (1) Case S--When all of the air passing through the indirect heater is drawn from the outside: Mr -- 0; Mh = M = Mo Temperature of air entering heater, 1, = /0 M = 0.24- \H-ty--------t-), --(2) Case S--When, a portion of the air passing through the indirect heater is outside air and the remainder recirculated air: H Mk = M = Mr + Mo = 0.24 (ty - t) .................. :.......................................-...... (3) In this case M0 is known from the ventilating requirements and the amount of air to be TT recirculated is ascertained by the following formula: Mr -- M -- M0 or Mr = 0.2 4 (-t-y-------/-) - Mo...................................................................................... .............................................................. (4) The mean temperature of the air entering the indirect heater is ascertained by the following formula: , _ Mo (to + 460) + Mr (t + 460) '--------------------- Mo + Mr--------------------- 460------------------- ---- -.....(5) Case 4--When all of the air circulated is drawn from the outside and passed through a tempering coil, air washer or humidifier and reheater. The temperature of the air ^ entering the reheater will have the same dewpoint tem perature as the air in the room dr building towhich the air is delivered, having tempera ture t and relative humidity as specified. If the.relative humidity is not specified it shall be assumed as 35 per cent. If the room temperature is not specified it shall be assumed as 70 deg. fahr. 293