Document 158Y702K81v4gZYK4LrpMb60E

. Heating Ventilating Air Conditioning Guide 1939 1. To supply a given quantity of outdoor air for ventilation or to mix indoor ,, outdoor air. (See A.S.H.V.E. Ventilation Standards, Chapter 45). an<1 2. To warm the air to approximately the room temperature if the unit is intended f ventilation only, or to a higher temperature if it is intended to take care of all or a nan of the heat transmission losses from the room. wrt 3. To control the temperature of the air delivered so as to prevent both cold draft and overheating. (See Chapter 37). 8 4. To deliver air to the room in such a manner that proper distribution is obtained without drafts. 5. To recirculate room air for the purpose of heating or promoting comfort when ventilation is unnecessary. (Ordinances should be consulted). 6. To perform all its functions without objectionable noise. 7. To clean the air properly. SPLIT AND COMBINED UNIT VENTILATOR SYSTEMS i In a split system the unit is used primarily for ventilation. Air is ;g delivered to the room at very near the room temperature, and enough separate direct heaters are placed in the room to warm it to the desired temperature, independently of the unit. Their principal advantage lies 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 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 wiil 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 blit 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, blit 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. 442 4: CHAPTER 2Z. unit heaters. Ventilators. Air Conditioning, Cooling Units Table 2. Typical Capacities of Unit Ventilators for an Entering Air Temperature of Zero Cubic Febtop jita pea Mroiw* ' goo 750 inno 1200 1500 Total Capacity in Square Feet op Equivalent Direct Hrating Surface (Radiation) Capacity Available for Heat ing the Room in Square Feet op Equivalent Direct Heating Surface (Radiation) Finai/Air Temperatube (Deo Fahr) EDR Mbh EDR Mbh 285 68 95 350 84 115 455 110 150 565 136 190 70S 169 235 23 28 36 46 56 105 105 105 105 105 VENTS8 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 witii 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 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 'A.S.H.V.E. Research Report No. 936--Investigation of Air Outlets in Class Room Ventilation, by ~ Larson. DAV. Nelson, and R. W. Kubasta (A.S.H.V.E. Transactions, Vol. 3S, 1932, p. 463). Research Report No. 1017--Air Supply to Classrooms in Relation to Vent Flue.Openings, op r. L. Houghten. Carl Gutberlet, and M. F. Lichtenfels (A.S.H.V.E. Transactions, Vol, 41.1936. p. 279). 443