Document mmNqRvYq47D01EgVO5nDadp8b

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 Table 3. Typical Capacities of Unit Ventilators for an Entering Air Temperature of Zero Cubic Feet of Air per Minute Anemometer Rating Condensate Rating Total Capacity in Square Feet, Equiv- alent Direct Radiation Capacity Available for Heating the Room. Square Feet Equivalent Direct Radiation Final Air Temperature . Deg F 750 1000 1260 1560 500 750 , 1000 1250 214 320 427 534 56 95 84 95 112 95 . 141 95 If no direct heating surface (radiation) is installed to take care of the normal heat transfer losses, and the unit Ventilator is to be used for both heating and ventilation, then the combined requirements must be taken care of by the unit ventilator. When all of the air handled by the unit is taken from the outside, the total heat to be suppliedJs_obtained by means of Equations 1, 2 and 3. Ht = 0.24 W (ty - to) (1) W = dWQ (2) hr H 0.24 W ft (3) where 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 + Hv. Q = volume of air handled by the ventilating equipment, cubic feet per minute. t = temperature to be maintained in the room, degrees Fahrenheit. to = outside temperature; degrees Fahrenheit. ty = temperature of the air leaving the unit, degrees Fahrenheit. 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 d 60 Q (t - to). (4) Example1. 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. Substituting in Equation 4: ' , Ht = 24,000 + 0.24 X 0.075 X 60 X 1000 (70 -- 0) = 99,600 Btu per hour * = '24,000 ______ + 70 = 92 2 F ty 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, CHAPTER 22. UNIT HEATERS, UNIT VENTILATORS, UNIT HUMIDIFIERS Ht = 0.24 Wo (ty - to) + 0.24 Wi (ty -- t) where IV0 -- weight of air, pounds per hour taken from out-of-doors. Wi = weight of air, pounds per hour taken from the room. (5) . Wo = d0 60 Qo Wi = di 60 Qi where do = density of air, pounds per cubic foot at temperature to. di = 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) H ty = 0.24 ( Wo + Wi) + 1 (8) Ht = H + 0.24 do 60 Qo (t - to) (9) Equations 5, 6, 7, 8 and 9 may be used in the same manner as is illus trated previously for Equations 1, 2, 3 and 4. It may be noted in Equa tion 9, representing the total heat requirements, that as the quantity Q0 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 to 24,000 Btu per hour, 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 require ment of 24,000 + ----g--24,000 _ gg^oo Btu per hour. Units ' designed and operated 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 ty should be equal to or slightly higher than l. 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 t0. Therefore a certain amount of heat (Hh) 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. 437