Document wreDQg5pNvaa6jkrQov49dExE

208 CHAPTER 13 1960 Guide Table 27 .... Rates of Heat Gain from Occupants of Conditioned Spaces* Dagraa of Activity Typical Application Total Hoot Adalft, Total Hoot AdMela Bta/Hr jodad* Btu/Hr SotuAlo Hoot Btu/Hr latent Hoot Btu/Hr Seated at rest................................................... Theater--Matinee............. Theater--Evening............. 390 390 330 180 350 195 150 155 Seated, very light work................................ Offices, hotels, apartments Moderately active office work.................... Offices, hotels, apartments 450 475 400 195 450 200 205 250 Standing, light work; or walking slowly.. Department store, retail store, dime store........... 550 450 200 250 Walking; seated............................................... Drugstore, Bonk................ Standing; walking slowly............................. 550 500 200 300 Sedentary work................................................ Restaurant*........................ Light bench work............................................ Factory................................. 490 800 550 220 750 220 330 530 Moderate dancing........................................... Dance hall........................... 900 850 245 605 Walking 3 mph; moderately heavy work.. Factory............ .................... Bowling4............................................................ Bowling alley...................... 1000 1500 1000 1450 300 465 700 985 * tfeU: Tabulated value*are baaed OB 80 K room dry-bulb temperature. For 78 F room dry-bulb, the total heat remains the earn*, bat the aenaibte heat valuea should be increased by approximately 10 percent, and the latest beat values decreased accordingly. b AAjmtad total koat fata is baaed on normal percentage of men, women, and children for the application listed, with the postulate that the gala from an adult fe male ta 85 percent of that for an adult male, and that the gam from a child is 75 percent of that for an adult mala. c Adjusted total heat value for sedentary teorfc, rctfovraat, includes 60 Btu per hour for food per individual (30 Btu sensible and 30 Btu latent). 4 For hewitny figure one person per alley actually bowling, and all others as aitting (400 Btu per hour) or standing (550 Btu per hour)- up ft lower limit to the heat gain to a room from the appli ance when in operation. Experienced judgment must be used in the application of data given in Table 28. Consideration must be given to the heat contributed by appliances 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 appli ances are hooded to allow 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 property hooded with a positive fan-exhaust system through the hood, 50 percent of tiie heat will be carried away and 50 percent dissipated in the space to be conditioned. In the case of gas-burning appliances where the heat of combustion adds considerably to the total heat output, it is believed that a well-designed hood will remove more than 50 percent of tho heat generated at the appliance. The same effectiveness of the hood should be figured for both latent and sensible heat. For each meal served the beat transferred to the dining space is approximately 50 Btu per hr, of which 75 percent is sensible and 25 percent latent. Heat gains from cooking appliances, motors, lights, and people are not considered in cooling load calculations if the appliances are located in a separate kitchen that is not part of the conditioned area. LOAD FROM MOISTURE TRANSFER THROUGH PERMEABLE BUILDING MATERIALS The diffusion of moisture through all common building materials is a natural phenomenon which is always present to a greater or lesser degree. The permeability and permeance values for various build ing materials are given in Table 2 of Chapter 10, together with an explanation of moisture transmission through these materials In the usual comfort air-conditioning application, it is common practice to neglect moisture transfer through walls, because the actual rate is quite small and the corresponding latent-heat load is hardly significant. Vapor barrier* are frequently employed in modern construction for the purpose of keeping moisture transfer to a minimum, and reducing the deteriorating and insulation-destroying effects of moisture. Industrial jobs, on the other hand, frequently call for a low moisture content to be maintained in a conditioned space. Here the matter of moisture transfer cannot be neg lected; indeed, it is quite possible to have the latent-heat load accompanying this transfer be of greater magnitude than any other latent-heat load. The equation for computing this load is: (5)-4X (d^ZZ) X ~ where M -- permeance of the specimen in perms, or grains per (square foot) (hour) (inch of mercury vapor-pressure difference). 7000 " grains per pound. The factor 1076 is defined in list of symbols at Equation 6. (Sensible cooling of the water vapor is included in the factor 1076.) The only means of preventing moisture transfer is to use a vapor proof waff, or to apply a special lining, which is Cooling Load vapor proof. All openings in moisture -proof construction must be equipped with special gaskets to prevent entrance of moisture. When moisture transfer contributes an appreciable part of the latent-heat load, it is recommended that estimates nhrmld be made intentionally liberal in order to avoid later difficulties with insufficient dehumidifying capacity. Storage spaces, for example, would require sufficient dehumidifying capacity to handle the moisture brought in with goods to be stored, in addition to moisture leaking in subsequently. MiSCRLANEOUS HEAT LOADS This designation is intended to cover the various small heat gains from exposed piping, ducts, work done by circu lating fan, and unforeseen contingencies. 'Where sufficient data are available, these various heat gains may be estimated individually. In the majority of cases, however, common practice is to lump these factors together and combine them with a safety factor according to the experience and judgment of the estimator. On this basts, a small safety factor is added to the calculated cooling load to compensate for miscellan eous effects. No rules can be given for this procedure, as experience in air conditioning is indispensable for applica tion of suitable safety factors. APPARATUS DEW POINT AND REQUIRED AIR QUANTITY THROUGH CONDITIONING EQUIPMENT In ordinary practice, with commercial apparatus, complete saturation of the air is seldom obtained. Four-row finned cooling coils contact approximately 80 percent of the air, whereas six-row finned coils contact approximately 95 percent of the air. In spray-type dehumidifiers of good design the air leaves the dehumidifier at 1 to 2 deg higher wet-bulb temperature than the spray water leaving the dehumidifier, and the difference between the dry-bulb and wet-bulb tem peratures leaving the dehumidifier may be as low as 1 deg. A spray-type dehumidifier having sufficient length of spray chamber and density of spray, together with proper arrange ment of nozzles, may approach saturation very closely. As explained in Chapter 3, and shown in Fig. 5, the slope of the line on the psychrometrie chart connecting the room condition with the apparatus dew point on the saturation line, determines the ratio of.sensible heat absorbing capacity to the moisture absorbing capacity of the supply air. There fore the room condition can be maintained as long as the supply-air temperature lies on this line, but a greater volume of supply air must be used to satisfy the room load if the cooling coil does not contact 100 percent of the air. For a given room load, the same apparatus dew point will be re quired whether the cooling appliance contacts all the air or only part of the air. From the point of view of satisfying the given cooling load requirements, the air pacing through the apparatus without being cooled below the dew-point temperature produces'two effects: 1. The air quantity which must be passed through the dehumidifier must be increased. Thus, if 20 percent of the air passing is not contacted, then (20 + 80) X 100 = 25 percent more air must be used than would be necessary if all of it were contacted. 2. Passing untreated air may change the room cooling load, which in turn may change the enthalpy-humidity difference ratio, (s'oinetimei called the sensible-heat factor). If return air only is passed through the dehumidifier or if room air only is bypassed, the room load will not change, but if some outdoor 209 air is passed through, the room sensible-heat gain and room latent heat gain will be changed due to the addition of un treated outdoor air which changes the enthalpy-humidity difference ratio. When a load calculation is made, it is neces sary to know the percentage of air treated in the dehumidifier, and calculation must be made accordingly. If the ventilation air is drawn through the dahi-miHlfW before it goes into the room, only that portion of the air not saturated must be included in the room load for the purpose of determining the apparatus dew point and supply-air quan tity. It should be noted when evaluating the load added by untreated outdoor air that the temperature difference be tween room air and outdoor air, and the moisture content difference between room air and outdoor air, should be used, rather than the difference between outdoor air and apparatus dew point, since the rise from the apparatus dew point to room condition is charged against the dehumidifier as the cooling and dehumidifying load. Fig. 5.... Apparatus Dew-Point Shown on ASHRAE PSYCHROMETRIC CHART The procedure for determining the required air quantity is based upon the thermodynamic principles of Chapter 3 and the use of the ASHRAE Psychbometric Chart. Readers are advised to review these principles, paying particular attention to the illustrative examples. Calculation of the cooling load for a conditioned space is equivalent to making, for the space, a heat halanna in which all heat, moisture, and infiltration are treated as directly entering the space. As explained in the section. Load from Outdoor Air, Ventilation, and Infiltration, the outdoor-air load normally does not become a part of the space load, be cause heat and moisture are removed in the air conditioner before this air gets into the conditioned space. The desired conditions are maintained by considering a certain quantity of air to be withdrawn from the space, passed through the x conditioning equipment, and returned to the space with such a temperature and humidity ratio that its net effect will be to counterbalance or remove the given entering amounts of heat and water vapor. This quantity of indoor air, which is considered to be circulated in this manner, is nailed the required otr quantity and its determination is normally part of every cooling-load estimate. The procedure is as follows: 1. Determine the total sensible and latent heat loads in Btu per hour for the space. 2. Compute the quantity called the enthalpy-humidity difference ratio (also referred to as heat-moisture ratio) of