Document eJe7vm2wo4v7vxZOz5739a6g

American Society of Heating and Ventilating Engineers Guide heated. -The cost, of installation of a combined system is usually lgc that of a split system and there is less danger ofoverheating,- but'f* eiectric energy fails there will be practically no heating. - .. LOCATION OF UNIT . ' The location of the unit ventilator in a room is important. Whp possible it should be placed against an outside wall. It is difSc,,? obtain proper air distribution if the unit is erected either on an insidetor in ai corner of the'room.. Standard;units discharge the air stream' ward, but for special cases units may be installed to discharge air h" zontally. Units may be set away from the wall or partially recessed' -the wall-to save-space without-materially affecting-the-results. Thi*1 inlet may enter the cabinet at the back at any point from top to bott VENTS -The size and location of the vent outlet-is important. In many the sizes for public buildings are regulated by law, but the location of vents generally-is left to the discretion of the engineer. Best results have been obtained with a velocity through the ven openings nearly equal to that at which the air is introduced into the roo' thus maintaining a slight pressure in the room. Calculated velocities the vent openings of from 600 to 800 fpm produce the best diffusion resuljj from this system. ----- The cross-sectional area of the vent flue itself may be figured on i basis of 15 sq in. of flue for each 100 cfm. Thus the vent flue area .of* flue for a room equipped with one 1200 cfm unit ventilating machin woujd be 180 sq in. The area of vent flue opening from the room maybe figured on the basis of 25 sq in. per 100 cfm. In school buildings provided with wardrobes or cloakrooms the vents may be so located that the air shall pass through these spaces, heating and ventilating them with air which otherwise would be passed to.the outsidJ without being used to the.best advantage. Many state codes for venti lation of public buildings make this arrangement mandatory.. There has been, much controversy over the use of corridor ventilation in school building practice, one group holding the view that when eadi classroom has a separate vent flue there is a minimum fire, risk and less likelihood of cross-contamination, while others emphasize the economy features of the corridor discharge and minimize the fire, contamination, and other hazards. CAPACITIES Unit ventilators are available in air capacities ranging from 450 cfm to 6000 cfm and with corresponding heat capacities (above that required for ventilation purposes based upon an outside temperature of zero and an inside.temperature of 70 F) ranging from 30 Mbh to 144 Mbh (1 Mbh r 1000 Btu. per hour). Some manufacturers furnish a unit with several heating capacities for each air capacity, thus enabling the engineer to select the unit best adapted to the heating and ventilating load. Capaci ties should be determined in accordance with the A.S.H.V.E. Standard 230 ^^B5F|^^er43--Unit Heaters, Ventilators, .and Coolers Testing and Rating Steam Unit Ventilators1. Typical capacities Jj^pwuihnTtaoblfehe3.at to be supplied by the unit ventilator will depend on f|l|S|lggrwouhnict ohf tahier apairsissedhetahtreodu.ghThtheewuenigiht taonfdatihre(Wte)mtpoebreatcuirrecuralantvfde Ijgjnjggf is fixed by the ventilating requirements. mated ^I^K>d^^ect heating surface (radiation) is installed, the combined H||and ventilating requirements must be taken care of by the mfit BSftOafo1'5' and the total heat to be supplied is obtained by means of the ffSfigTng formulae: |Kn all of the air handled by the unit is taken from the outside, mm" =6.2tW(ly -to) - -- -- (1) W = dQ (2) H h = 0.241F + t (3) density of air, pounds per. cubic foot. > heat loss of room, Btu per hour. ... i'heat required to warm air for ventilation, Btu per hour. . -- total heat requirements for both heating and ventilation, Btu per hour = H -f- Hy. -- volume of air handled by the ventilating equipment, cubic feet per hour. < temperature to be maintained in the room. = outside temperature. . , = temperature of the air leaving the unit. = weight of air circulated, pounds per hour. = specific heat of air at constant pressure. EStii Equations 1, 2 and 3: Br Hi = H + 0.24 dQ (t - to) (4) Betohpte 1. The heat loss of a certain room is 24,000 Btu per hour, and the ventilating KtqSirenients are 1000 cfm. If the room temperature is to be 70 F and all air is taken [raiftljie outside at zero, what will be the total heat demand on the unit if it is required itoiprovide for both the heating and ventilating requirements (combined system)? H = 24,000; d = 0.075 (? = 1000 x 60 = 60,000 cfh;l = 70 Fife = OF. Kuhgituting in Equation 4: MK'- Ht = 24,000 + 0.24 x 0.075 x 60,000 (70 -0) = 99,600 Btu tv -- 24,000 0.24 x 0.075 x 60,000 + 70 = 92.2 F Kwieh part of the air handled by the unit is taken from the room and the ^^Shder from the outside, Hi = 0.241F0 (ty - t,,) + 0.24 W\ (ty - t) (5) 11932. See A.S.H.V.E. Transactions. Vol. 3S. 1932.............................. - - 231 If A