Document 3NMGn7qpmX1VbzR5gKm3vn2V3

American Society of Heating and Ventilating Engineers Guide, 1936 Table 3. Typical Capacities of Unit Ventilators for an Entering Air Temperature of Zero Cubic Feet or Ajb per Minute 600. 750 1000 1200 1500 Total Capacttt in Square Feet or Equivalent Direct Heating Surface (Radiation) Capacitt Available for Heat ing the Room in Square Feet or Equivalent Direct Hbatino Surface (Radiation) Final Air Tempera* tube (Deo Faith) EDR Mbh EDR Mbh 285 68 95 350 84 115 455 110 150 565 136 190 705 169 235 23 28 36 46 56 105 105 105 105 105 Ht = 0.24Wo (V - to) 0.24 Wi y - t) (5) where W0 = weight of air, pounds per hour taken from out-of-doors. Wi -- weight of air, pounds per hour taken from the room. W0 = do 60 Qo where Wi = di 60 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. H 0.24 (Wo + Wi) + t (6) (7) (8) Ht = H + 0.24 do 60 Qa (t - to) (9) Equations 5, 6, 7, 8, and 9 may be used in the same manner as is 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 aunit 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 + 24,000 _ gg goo Btu. Units designed and operated o 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 234 Chapter 12--Unit Heaters, Ventilators, Coolers, and Air Conditioners 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 - <o) (11) In this case ty 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 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. UNIT COOLERS Unit coolers as applied to industrial product conditioning and pro cessing are very similar in construction to unit heaters except that the heat transfer medium is supplied with refrigeration instead of with steam or 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 almost always filtered or washed to prevent any possible contamination of the product. Unit coolers are provided in two major types similar 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 largely to the effect of high velocities on the product. Consequently unit coolers are occasionally supplemented with duct distribution. Product cooling, originally was accomplished by means of stationary pipe coils. This was later supplemented with the forced fan bunker systems in which air was passed over banks of coils. The present trend in this field is toward a more accurate control of both temperature and humidity, thus placing these units in the classification of complete air conditioning units as discussed in the next section. However, in the majority of these cases dry-bulb temperature is controlled separately from the control of humidity, thus classifying these units as unit coolers. The principal field for unit coolers is in cold storage plants, fur storage, fruit packing houses, provision stores, brewery fermentation and stock. rooms, candy plants, and other industrial process, work. In replacing bunker and wall coils in meat-storage plants, unit coolers give distinct advantages in compactness, lower first'cost and maintenance expense, ease of defrosting, freedom from drip and the maintenance of sanitary . conditions, as well as uniform temperature and humidity under variable load conditions. Unit coolers by means of their positive air circulation prevent dead-air spots, frequently objectionable in this industry. Typical unit coolers are shown in Figs. 7 and 8. The former indicates a 235