Document pmqm5g0oO80nXprLYK16roN3k

556 CHAPTER 24 1951 Guide 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. Heat Required for Ventilating Only When all of the air handled by the unit is taken from the outside, the total heat to be supplied is obtained by means of Equations 1, 2, 3, and 4. H = 0.24 W (t, -- i) H, = 0.24 W (1 - (,) Ht = 0.24 W (t, -- t,,) = H + H, W = d 60 Q (l) (2) (3) (4) From Equations 2, 3, and 4: where Ht = H +. 0.24 d 60 Q (< - !,,) (5) d = density of air, pounds per cubic foot (0.075 lb per cu ft for Standard Air by definition). H = heat loss of room-, Btu per hour. UT = heat required to warm air for ventilation, Btu per hour. Ht = total heat requirements for both heating and ventilation, Btu per hour. Q = volume of air handled by the ventilating equipment, cubic feet per minute. t = temperature to be maintained in the room, Fahrenheit degrees, to = outside temperature, Fahrenheit degrees. tj -- temperature of the air leaving the unit, Fahrenheit degrees. W -- weight of air circulated, pounds per hour. 0.24 = specific heat of air at constant pressure (approximate value). Example 1. The heat loss of a certain room is 24,000 Btu per hour, and the venti lating 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 5: Ht ~ 24,000 + 0.24 X 0.076 X 60 X 1000 (70 - 0) = 99,600 Btu per hour 24,000 tr + 70 0.24 X 0.075 X 60 X 1000 92.2 F. Heat Required for Ventilating and Recirculating When part of the air handled by the unit is taken from the room and the remainder from the outside, Ht = 0.24W. (i, - (,,) + 0.24 W\ (t, - i) Wa -- do 60 Q0 Wi -- di 60 Qi (6) (7) (8) H tr ~ 0.24 (Wo + IF,) 1 1 (9) H, = H + 0.24 do 60 <?,, (t - f,,) (10) where W, = weight of air, pounds per hour taken from out-of-doors. Unit Heaters and Unit Ventilators 557 W-, = weight of air, pounds per hour taken from the room. d,, = density of air, pounds per cubic foot at temperature 10. d-t = density of air, pounds per cubic foot at temperature (. Q0 = 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. Applications of Unit Ventilators Items to be considered in the application of unit ventilators are: (1) combination with other means of heating, (2) location of units, and (3) method of venting or exhausting. In a split system the unit is used primarily for ventilation. Air is delivered to the room at or slightly above room temperature, and enough radiation is installed in the room to take care of the normal heat transfer losses. Where the unit ventilator selected has a capacity more than suffi cient to warm the air needed to meet the ventilating requirements, a corre sponding reduction may be made in the amount of direct radiation 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 oper ating the ventilating equipment. The combined system employs a unit ventilator with sufficient capacity for both ventilation and the normal heat transfer losses. In such a case no direct radiation is required. The necessaiy operation of the fan when the room is being heated also gives assurance that some ventilation is being provided, especially if automatic dampers are used in the air intake and in the recirculating intake. These dampers are arranged to provide a certain quantity of outside air, depending upon the weather conditions. The cost of installation of a combined system is usually less than that of a split sys tem, and there is less danger of overheating, but if the electric energy fails, the only heat available is that of the units acting as gravity convectors. Location of Unit Ventilator The location of the unit ventilator in a room is important. Wherever possible it should be placed against an outside wall and on the center line of the room. It is difficult to obtain proper air distribution if the unit is installed either on an inside wall or in a corner of the room. Standard units discharge the air stream upward, but for special cases units may be installed, to discharge air horizontally. 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. Air Exhaust Vents and Flues The size and location of the air exhaust vent* outlet are important and in many cases, are regulated by law for public buildings. Where no codes govern, the location and size of vents are left to the discretion of the engineer. Best results have been obtained with a velocity through the vent open ings 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