Document 0J2q56Jox3Bz64bg0V3prLNOJ

288 CHAPTER 12 1950 Guide of outdoor air at the rate of 1000 cfm and the exhaust of room air at the corresponding rate, calculate the total, sensible and latent heat gains. Solution: Substituting in Equation 13: = 1000 X 1.08 (95 - 80) -= 18,200 Btu per hr. From psychrometric data IF, = 0.01413, Wi -- 0.01122. Substituting in Equations 14 and 15: 5. = 1000 X 4840 (0.01413 - 0.01122) = 14,100 Btu per hr. <?t = ?. + 5. = 30,300 Btu. Many cooling coil manufacturers publish tables giving psychrometric data based on the average conditions of the leaving air for various coil temperatures, air velocities, and entering dry-bulb and wet-bulb condi tions. When these tables are used, it is necessary to calculate the mixed air condition entering the coil, and determine from the tables what coil and air velocity will produce the desired leaving air conditions as required for Table 24. Rates or Heat Gain fbom Occupants op Conditioned Spaces* Decree op Activity - Typical Application Total Hbat Adults, Male . Btu/Hr Seated at Rest___ ____________ Theater-Matinee Theater-Evenings . .. Seated, Very light Work Offices, Hotels, Apartments..... ............ Moderately Active Office Work Offices, Hotels, Apartments.......... ........ Standing. Light Work; or Department Store, Walking Slowly............ ............... Retail Store Dime Store.... ............. Walking; Seated; ....................... Drug Store 390 . 390 - 450 475 550 Sedentary Work .................... Restaurant __ _______ light Bench Work..... ............... Factory......................... 490 800 Walking 3 mph; 1000 Heavy Work__ .....____ ___ _____ Factory .. 150C Total Hbat Adjusted* Btu/Hr Sensible Hbat Btu/Hr 330 180 350 195 400 195 4SO 200 450 200 550 750 850 1000 1450 220 220 245 300 465 Latent Heat Btu/Hr 150 155 205 250 250 330 530 605 700 985 * Note: Tabulated values are based on 80 P room dry-bulb temperature. For 78 F room dry-bulb, the total beat remains the same, but the sensible (mat values should oe increased by approximately 10 per cent, and the latent beat values decreased accordingly. b Adjusted total heat pain is based on normal percentage of men, women, and children for the application listed, with the postulate that the gain from an adult female is 85 per cent of that for an adult male, and that the gain from a child is 75 per cent of that for an adult male. 4 Adjusted total heat value for sedentary work, restaurant, includes 60 Btu per hour for food per individual (80 Btu sensible and 30 Btu latent). d For bowling figure one person per alley actually bowling and all others as sitting (400 Btu per hour) or standing (660 Btu per hour). the space to be conditioned. When cooling coils are listed as 80 to 95 per cent efficient, the manufacturer indicates that 20 to 5 per cent of the air passes through the coil without being cooled. If data of this nature are used, the uncooled portion of the air must be added to the space load before determining the effective air quantity. HEAT SOURCES WITHIN THE CONDITIONED SPACE People. The rates at which heat and moisture are given off by human beings under different states of activity are given in Table 24. In many applications these sensible and latent heat gains become a large fraction of the total load. Appreciable variations in heat-emission rates must be recognized according to lie age and sex of the individual, state of activity, environmental influences, and duration of occupancy (since for short 1 Cooling Toad 289 occupancy the extra heat and moisture brought in by people may be a significant factor).- While Chapter 6 should be referred to for detailed information, Table 24 in this chapter summarizes practical data representing conditions commonly encountered. Lighting. In general, the instantaneous rate of heat gain from electric lighting17 may be calculated from the following relation: ,, (totel light ( use (special allow- * (wattage (factor (ance factor Bta hour (16) The total light wattage is obtained from the ratings of all fixtures installed, both for general illumination and for display use. The usefactor is the ratio of the wattage in use, for the conditions under which the load estimate is being made, to the total installed wattage. For commercial appli cations such as stores, the use factor would be unity. The special allowance factor is introduced to care for fluorescent fixtures, and for fixtures which are either ventilated or installed so that only part of their heat goes to the conditioned space. For fluorescent fixtures, the special allowance factor is recommended to be taken as 1.20 in order to allow for power consumed in the ballast. For ventilated fixtures, recessed fixtures, and the like, manufacturers' or other data17 must be sought to establish the fraction of the total wattage which may be expected to enter the conditioned space. Power. When equipment of any sort is operated within the conditioned space by. electric motors, the heat equivalent of this operation must be considered in the cooling load. The general equation for calculating this load is: - fHorSeP?Wer X ( L0ad ) X 2544, Btu per hour. *" \ Motor Efficiency / \Factor/ '^ (17) It is assumed that both the motor and the driven equipment are within the conditioned space. If the motor is without the space, then do not divide by the motor efficiency in Equation 17. The load factor is merely the fraction of the rated load which is being delivered under the conditions of the cooling-load estimate. Motor efficiencies may be approximated as follows: about 50 to 60 per cent at J hp rating, increasing to 80 per cent at 1 hp, and to 88 per cent at 10 hp and above. Appliances. Care must be taken in a cooling-load estimate to properly account for the heat gain from all appliances, electrical, gas, or steam. Table 25 presents recommended data.18 Note that the mainlaining rate in Table 25 is the heat input required to maintain the appliance at the nor mal operating temperature even though it is not being used; i.e., no coffee is being made, no toast is being made, no food is being cooked in the fry kettle, etc. The maintaining rate is useful in setting up a lower limit to the heat gain to a room from the appliance when in operation. Experienced judgment must be used in the application of data given in Table 25. Consideration must be given to the heat contributed by appli ances which are in use at the time of peak load. The quantity of heat will depend upon whether products of combustion are vented to a flue, whether they escape into the space to be conditioned, or whether appliances are hooded allowing part of the heat to escape through a stack. There are no generally accepted data available on the effects.of venting and shielding^ heating appliances but it is believed that, when they are properly hooded !i i :i :6 i !il