Document 0gVE2BMOv2M59bp70QndvqoJO

American Society of Heating and Ventilating Engineers Guide, 1937"" W ~ weight of air circulated, pounds per hour. 0.24 = specific heat of air at constant pressure. From Equations 1, 2 and 3: Bt = H + 0.24 d 60 Q (t -- L) (4) Example 1. The heat loss of a certain room is 24,000 Btu per hour, and the ventilation 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; A, = 0 F. Substituting in Equation 4: flt = 24,000 + 0.24 X 0.075 X 60 X 1000 (70 ----------- -0) = 99,600 Btu hr -- ___ 24,000 0-24 x 0.075 X 60 X 1000 + 70 92,2 F When part of the air handled by the unit is taken from the room and the remainder from the outside, c where Ht 0.24 Wa (ty - tj + 0.24 Wi {ty - t) W0 = weight of air, pounds per hour taken from out-of-doors, Wi - weight of air, pounds per hour taken from the room. (5) = do 60 <?0 where Wi = dt GO Qi do - density of air, pounds per.cubic foot at temperature 'todi = density of air, pounds per cubic foot at temperature t. Qo volume of air taken in from the outside, cubic feet per minute. Qi - : volume of air taken in from the room, cubic feet per minute. (6) (7) hr = H 0.24 {Wo + Wi) +t (8) Ht = 17 + 0.24 do 60 Qo (t -- fo) Equations 5, 6, 7, 8, and 9 may be used in the same manner as (i9s) illustrated above for Equations 1, 2, 3, and 4. It may be noted in Equa tion 9, representing the total heat requirements, that as the quantity Qo is diminished the heat requirements for the unit diminish very materially. . In Example 1, if the quantity of air taken in from the.outside is reduced to zero, or all of the air handled by the unit is recirculated, the total heat requirements Ht reduce from 99,600 Btu to 24,000 Btu, or to about one fourth. Such a unit handling one third of its air volume from the outside and two'thirds from the room would show a total heat requirement of 24,000 = 59t200 Btu. Units designed .and operated 252 13---Unit Heaters, Ventilators, Air' Conditioning, Cooling Units ChaJ'ter Table 3. Typical Capacities of Unit Ventilators for an Entering Air Temperature of Zero Cobio *^0* 4bpbb Minute 600 750 1000 1200 1500 Total Capacity en Squabb Feet op Equivalent Direct Heaiino Subpace (Radiation) EDR Mbh Capacity Available pob Heat ing the Room in Squabe Feet or Equivalent Direct Heating Subpace (Radiation) Final Am Tempera* tube (Dbg Fahb) EDR Mbh 285 350 455 565 705 68 84 110 136 169 95 115 150 190 235 23 28 36 46 56 105 105 105 105 105 on this principle show an average heat requirement and, therefore, a boiler capacity requirement of less than 50 per cent of that required for units taking all their air from the outside. If all of the air is recirculated, the total heat, required is the same as the heat loss of the room, or Ht = H = 0.24 W {ty - t) (10) If the heat loss of the room is to be taken care of by the direct heating surface, the unit ventilators will be required to warm the air introduced for the ventilating requirements. Therefore: Hv = 0.24 W {ty - to) (11) In this case *5, should be equal to or slightly higher than t. If the unit ventilator were of such capacity as to exactly provide for the ventilating requirements, the direct radiation would be selected on the usual basis. However, it is necessary to employ a unit which may not exactly meet the ventilating requirements, since standard units are usually rated in terms of the volume of air that will be delivered at a certain temperature ty for an initial temperature of ta. Therefore a certain amount of heat (7/h) may be available from the unit ventilator for heating purposes, as pre viously stated, and the amount of equivalent direct heating surface may, if desired, be deducted from the,amount required for heating the room. COOLING UNITS Cooling units as applied to industrial product conditioning and pro cessing are similar in construction to unit heaters except that the heat transfer surface is supplied with refrigeration instead of with steam dr hot water. They are normally installed within the space to be served, or at least closely adjacent thereto. Occasionally they are provided to receive outside air in which case this air is invariably filtered or washed to prevent any possible contamination of the product. Cooling units are provided in two major typessimilar to unit heaters, either floor mounted with housed fan, or suspended with propeller type fans. Normally, air outlet velocities are lower than for heating, due laigely to the effect of high velocities, on the product. Cooling units are 253