Document peVV3ryjxXZdM7jLdMbxb6jy7

American Society of Heating and Ventilating Engineers Guide, 1936 LOCATION OF UNIT The location of the unit ventilator in a room is important. Wherever possible it should be placed against an outside wall. It is difficult to obtain proper air distribution if the unit is erected either on an inside wall or in a corner of the room. Standard units discharge the air stream up-, ward, but for special cases units may be installed to discharge air hori zontally., Units may be set away from the wall or partially recessed into the wall to save space without materially affecting the results. The air inlet may enter the cabinet at the back at any point from top to bottom. ' VENTS4 . The size and location of the vent outlet is important. In many cases the sizes for public buildings are regulated by law, but the location of the vents generally is left to the discretion of the engineer. Best results have been obtained with a velocity through the vent openings nearly equal to that at which the air is introduced into the room, thus maintaining a slight pressure in the room. Calculated velocities at the vent openings of from 600 to 800 fpm produce the best diffusion results from this system. The cross-sectional area of the vent flue itself may be figured on the basis of 15 sq in. of flue for each 100 cfm. Thus the vent flue area of a flue for a room equipped with one 1200 cfm unit ventilating machine would be 180 sq in. The area of vent flue opening from the room ,may be 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 outside 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 each classroom has a separate vent flue there is a minimum ,fire risk arid 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 5000 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 = 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. Capad- ^Investigation of Air Outlets in Class Room Ventilation, by G. L:-Larson, D.-W. Nelson, and R.,W. Kubasta (A.SIH.V.E. Transactions, Vol. 38, 1932)., Air Supply to Classrooms in Relation to Vent Flue Openings,.by F. C. Houghten, Carl Gutberlet, and M. F. Lichtenfels (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, June, 1935). 232 Chapter 12- Unit Heaters, Ventilators, Coolers, and Air Conditioners ties should be determined in accordance with the A.S.H.V.E. Standard Code for Testing and Rating Steam Unit Ventilators6. Typical capacities are given in Table 3. The amount of heat to be supplied by the unit ventilator will depend on the amount of air passed through the unit and the temperature range through which the air is heated. The weight of air (W) to be drculated per hour is fixed by the ventilating requirements. If no direct heating surface (radiation) is installed, the combined heating and ventilating requirements must be taken care of by the unit ventilators, and the total heat to be supplied is obtained by means of the following formulae: When all of the air handled by the unit is taken from the outside, Ht = 0.24 W(ty- ta) (i) W = d 60 Q (2) where ty = H 0.24 W + < (3) d = density of air, pounds per cubic foot. H = heat loss of room, Btu per hour. Hv = heat required to warm air for ventilation, Btu per hour. Ht = total heat requirements for both heating and ventilation, Btu per hour = H + By. Q = volume of air handled by the ventilating equipment, cubic feet per minute. t = temperature to be maintained in the room. ta = outside temperature. ty = temperature of the air leaving the unit. W = weight of air circulated, pounds per hour. 0.24 - specific heat of air at constant pressure. From Equations 1, 2 and 3: Ht = H + 0.24 dSO Q (t -- to) (4) Example 1. The heat loss of a certain room is 24,000 Btu per hour, and the ventilating requirements are 1000 cfm. If the room temperature is to be 70 F and all air is taken from the outside at zero, what will be the total heat demand on the unit if it is required to provide for both the heating and ventilating requirements (combined system)? Solution. H = 24,000; d = 0.075 Q = 1000 cfm; t = 70 F; to = 0 F. Substituting in Equation 4: Ht = 24,000 + 0.24 X 0.075 X 60 X 1000 (70-0) = 99,600 Btu ty =----- :--------2^000k 70 -- 92 2 F y 0.24 X 0.075 X 60 X 1000 + 7U - - When part of the air handled by the unit is taken from the room and the remainder from the outside,. Adopted 1932. See A.S.H.V.E. Thansactions. VoL 38. 1932. ' 233