Document DM9D1RGLwqDq9kB697KXR58ea

506 CHAPTER 26 1948\ Guide - Surplus or Heating Equivalent Direct. Radiation for the purposes of this code .shall be construed.to mean difference between, the. total EDR at a specified inlet temperature, and. the.EDR required to heat the air'from that temperature to 70 F. If no direct heating surface*(radiation) is installed to take Care of the normal heat transfer l6sses,-and: the unit ventilator isTo be usedfor both heating and ventilation, then the combined requirements; must be taken care of by the unit ventilator. ;. . Table 3. Typical Capacities ofUnit Ventilators for an Entering Air Temperature of Zero Cubic Feet op 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 1 Final Air Temperature F Dbg. . . 750 1000 1260 1560 500 750 1000 1250 214 320 427 534 56 95 84 95 112 95 141 95 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 (ty -- t) Hy = 0.24 WQ - to) = = +Ht 0.24 W {ty -- to) ff Hv =W d60Q (1) (2) (3) (4) From Equations 2, 3, and 4: ;. Ht = H + 0.24 d 60 0(1 -to). (5) where . 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. 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, Fahrenheit degrees. to = outside temperature, Fahrenheit degrees. ty = temperature of the air leaving the uriit, 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 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 5: Ht = 24,000 + 0.24 X 0.075 X 60 X 1000 (70 - 0) = 99,600 Btu per.hour Unit Heaters, Unit Ventilators, Unit Humidifiers 507 Heat Required for Ventilating and Recirculating . - 1 When part of the air handled by the unit is taken from the robmarid the remainder from the outside, , Ht = 0.24 Wo (ty - to) + 0.24 Wi (ty - t) (7) : Wo = do 60 Q0 (8) WV= A 60 Qi / <y = 0.24 (Wo + Wi) + * * ' (9) (10) ! .. Ht = H + 0.24do 60 Qo (l - to) .'. -'(11) where ..Wo = weight of air, pounds per hour taken from out-of-doors. Wi = weight of air, pounds per hour taken from the room. . . do = density .of air, pounds per cubic foot at temperature (o-. di .= density of air, pounds per cubic foot at temperature.!. Qo = volume of air taken in from the outside, cubic feet per minute. (?i = 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 sufficient, to warm the air needed to meet the ventilating requirements, a corresponding 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 operating 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 necessary 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 system and there is less danger of overheating, but if the electric energy fails there will be available only the heating effect of the units acting as 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 comer 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