Document NEZR8g5GZY1Rn71m79J89jD3V

320 CHAPTER 13 -1956 Guide factor according to the experience and judgment of the estimator. On this basis, a small safety factor is added to the calculated cooling load to com pensate for miscellaneous effects. No rules can be given/for this proce dure, as experience in air conditioning is indispensable for application 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 ap proximately 80 percent of the air, whereas six-row finned coils contact approximately 95 percent of the air. In spray type dehumidifiers of good Fig. 5. Appaeatus Dew-Point Shown on A.S.H.A.E. Psychbomethic Chabt design the air leaves the dehumidifier at 1 to 2 deg higher wet-bulb tem perature than the spray water leaving the dehumidifier, and the difference between the dry-bulb and wet-bulb temperatures leaving the dehumidifier may be as low as 1 deg. A spray type dehumidifier haying sufficient length of spray chamber and density of spray, together with proper ar rangement of nozzles, may approach saturation very closely. As explained in Chapter 3, and shown in Fig. 5, the slope of the line on the psychrometric 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. Therefore 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 per cent of the air. For a given room load, the same apparatus dew point will be required 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 require ments, the air passing 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 in-. Cooling Load 321 creased. Thus, if 20 percent of the air passing is contacted, then (20 4- 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 maychange the enthalpy-humidity difference ratio (sometimes 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 outside air is passed through, the room sensible heat gain and room latent heat gain will be changed due to the addition of untreated outside air, which changes the enthalpy-humidity difference ratio. ' When a load calculation is made, it is necessary to know the percentage of air affected in the dehumidifier, and calculation must be made accordingly. If the ventilation air is drawn through the dehumidifier 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 quantity. It should be noted when evaluating the load added by untreated outside air that the temperature difference between room air and outside air, and the moisture content difference between room air and outside air, should be used, rather than the difference between outside 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. The procedure for determining the required air quantity is based upon the thermodynamic principles of Chapter 3 and the use of the A.S.H.A.E. psychrometric 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 balance in which all heat, moisture, and infil tration are treated as directly entering the space. As explained in the section, Load from Outside Air, Ventilation, and Infiltration, the outside air load normally does not become a part of the space load, because 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 conditioning equipment, and returned to the space with such a tem perature and humidity ratio that its net effect will be to counterbalance or remove the given entering amounts of heat and water vapor. This quan tity of indoor air, which is considered to be circulated in this manner, is called the required air 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-humiiddit^y difference ratio (also referred to as heat-moisture ratio) of the room load, Wi - WV Use the equation where hi -- ft, _ (Space sensible load + space latent load) Wi -- W, Space latent load/1076 h, = enthalpy of moist air supplied to the space, Btu per pound of dry air. hi = enthalpy of moist air at room design conditions, Btu per pound of dry air-. iF, = humidity ratio of moist air supplied to the space, pounds of vapor per pound of dry air.) IFi = humidity ratio of moist air at room design conditions, pounds of vapor per pound of dry air.