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Heating Ventilating Air Conditioning Guide 1938 since the direct radiators will furnish heat, but it permits a careless operator to avoid operating the ventilating equipment. A combined system employs the unit ventilator alone, its capacity being sufficient both for ventilation and for supplying the heat loss. Direct heating surface is omitted altogether. It becomes necessary then that the fan be running whenever the room is to be heated but this also gives assurance of ventilation, especially if automatic dampers are used in the air intake from out-of-doors and in the recirculating intake arranged so as to give a certain quantity of air from the outside (commensurate with weather conditions) whenever the unit is operating and after the room is heated. The cost of installation of a combined system is usually less than that of a split system and there is less danger of overheating, but if the electric energy fails there will be practically no heating. 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 comer 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. VENTS6 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 ^Investigation of Air Outlets in Class Room Ventilation, by G. L. Larson, D. W. Nelson, and R. W. . Rubasta (A.S.H.V.E. Transactions. Vol. 38, 1932, p. 463). 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. Transactions, Vol. 41. 1935, p. 279). 446 Chapter 23. Unit Heaters, Ventilators. Air Conditioning. Cooling Units 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 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 15 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. Typical capacities are given in Table 27. Table 2. Typical Capacities op Unit Ventilators for an Entering Air Temperature of Zero Cubic Feet of Am feb Minute 600 750 1000 1200 1500 Total Capacitt in Square Feet of Equivalent Direct Heating Surface (Radiation) EDR Mbh Capacitt Avahablb fob Heat ing the Rook-in Square Feet of Equivalent Direct Heating Surface (Radiation) Final Am Tempera ture (Deg Fahr) EDR Mbh 285 68 95 23 350 84 115 ' 28 455 110 . 150 36 565 136 190 46 705 169 235 56 105 105 105 105 105 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 -- to) (1) W = d 60 Q (2) where H * 0.24B,+< (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. Hi - total heat requirements for both heating and ventilation, Btu per hour = H + Hy. Q = volume of air handled by the ventilating equipment, cubic feet per minute. t = temperature to be maintained in the room. to = outside temperature. ty = temperature of the air leaving the unit. ,, .'A S-H-V-E. Standard Code for Testing and Rating Steam Unit Ventilators (a'.S.H.V.E. Transactions. Vol. oo, 19o2, p. 25). 447