Document 7RnXaLaLZqxVR8Z4ONrpVXBXg

of and 1937American Society Heating Ventilating Engineers Guide, ^ temperature, independently of the unit. Their principal advantage 1;^ in offsetting the cooling effect of window and wall surfaces long before these can be heated to room temperature and in retaining heat for this purpose after the ventilation is shut down. Where the unit ventilator selected has a capacity more than sufficient to warm the air needed to meet the ventilating requirements, a cor m responding reduction may be made in the amount of direct heating surface installed. The greater the amount of excess capacity of the unit, the more efficient will be the temperature regulation of the room. The split system permits the heating of the room during failure of electric current 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 iassurance 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 iheated. 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 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. 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 Investigation of Air Outlets in Class Room Ventilation, by G. L. Larson, D. W. Nelson, and R. W. Kubasta (A.S.H.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. Licbtenfels (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, June. 1935). 250 -Unit Heaters, Ventilators, Air Conditioning, Cooling Units Id 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. Tni sscchhooooll buildings provided withn wwaaridurioUbUetOs oUrl cVliowaukiuro_o__m_s t_he_ vents v be so ilo--ca.toeAd that the air shall opass through these spaces, heating and fitilating them with air which otherwise would be passed to the outside V>hout being used to the best advantage. Many state codes for venti- laattjioonn ooff ppuubblliicc bDuUilldldiinnggss mmaaikvec tuhiiios auri riaunngjje--m__e__n__t__m__andatory. There -has tb-e--e--n---mmunchtmcwonretrvovoevresyr tohveerustheeofusceororfidcoorrridor 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 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 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. Typical capacities are given in Table 37. 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 (IF) to be circulated 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 - to) W = d 60 Q U) (2) H h = 0.24 W + t (3) where d = density of air, pounds per cubic foot. H = heat loss of room, Btu per hour. Hy = heat required to warm air for ventilation, Btu per hour. Ht = total heat requirements for both heating and ventilation, Btu per hour = H + HV. 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 UnitVentilators (A.S.H.V.E. Transactio Vol. 38, 1932). V/