Document DE7VwLeq8GeKgMkkgJQBOvKQ

622 CHAPTER 29 1951 Guide ture difference. In the. theoretical case of a dehumidifier haying 100 percent saturating efficiency, and where this air is delivered directly to the room without temperature increases due to heat gain, then the effec tual temperature difference is the difference between room temperature and apparatus dew-point temperature. If duct heat gains are considered a part of the room load, this still holds true. The apparatus dew-point, as outlined previously, is fixed by the latent and sensible loads of the space, but in many cases, it is desirable to deliver more air to the spaces than is indicated by the difference between the room temperature and the apparatus dew-point. It has been indicated that where a percentage of air is passed through the dehumidifier without being treated, the relationship is modified in direct proportion, and that if room air is passed through untreated, no effect on the heat balance results. Similarly, if room air is passed around the dehumidifier and mixed with the treated air, the heat balance is not adversely affected. Therefore, if the quantity of air passed through the dehumidifier is determined by the usual methods, room air can be passed around the dehumidifier and mixed with the dehumidified air, increasing the supply air quantity and temperature and decreasing the effectual temperature difference. Thus if the difference between the room tem perature and the apparatus dew-point indicates that 10,000 cfm at 30 deg below room temperature will be required to hold conditions, that quantity can be passed through the dehumidifier and cooled to 30 deg below the room temperature, then mixed with 10,000 cfm of room air, resulting in a supply air quantity of 20,000 cfm and an effectual temperature dif ference of 15 deg instead of 30 deg. Air supply outlets and grilles having a high induction ratio are available, and through their induction effect cause a large amount of room air to be mixed with the supply air within a short distance of the grille. A proper selection of outlets may make it possible to introduce air at low temperatures and high velocities without causing objectionable drafts or cold spots, but care must be used to see that too little air motion is not a result. Low effectual temperature dif ferences may be required for this reason. While the use of a high effectual temperature difference results in a saving in initial cost of fans and ducts, and in the operating cost of fans, this difference should be carefully con sidered. If the sensible heat load of a space is subjected to substantial variations, low effectual temperature differences should be considered, since systems employing low effectual temperature differences require less precision in controls. Reduction of air quantity by slowing down- the fans for the winter season, and increasing the temperature difference, often is feasible. A saving in fan power can thus be effected, provided the air distribution remains adequate. Extremes should be avoided in all cases. For summer air conditioning, low. supply air temperatures result in larger heat gains to the air passing through the ducts, as well as in poor control. Too high a supply air tem perature may result in excessive initial and operating costs. Suggested limits for the effectual temperature difference are- from 12 to 20 deg, the actual selection being based on the requirements of the particular case. For winter air conditioning, too high supply air temperatures result in excessive Jieat losses from the ducts and stratification within the room unless thorough mixing is assured, while too low supply air temperatures may cause drafts, high operating costs, etc. Suggested limits are from Central Systems for Air Conditioning - 623 15 to 35 deg. There can be no set rule, and each case should be judged according to its particular requirements of the installation. Reference may be-made to Chapter 30 for further discussion of the most satisfactory design difference between the entering air temperature and volume in relation to the desired room condition. INDUCTION CONVECTORS--LOW PRESSURE TYPE Induction convectors located in the room that is to be served, utilize a jet of primary conditioned air to mix with a stream of secondary room air as shown in Fig. 4. The mixture is discharged into the room through a grille at the top of the convector. Heating coils are located in the second ary air stream. The output is controlled either by manually or auto matically throttling the air jet. Heat may be supplied to the coil in summer as well as in winter. These induction convectors present several advantages. Since the secondary air stream is thoroughly mixed with the high velocity low temperature air stream before leaving the discharge outlet of the device, the resultant temperature of the mixture is satis factory even though the primary air is introduced at a temperature too low for ordinary methods of distribution. One of these devices usually is provided under each window in place of the customary direct radiator, and combines the air distribution system with the heating system. An air conditioning system without induction convectors may require in stallation of direct radiation for maintenance of minimum temperatures during air conditioning shut-down periods, but when induction convectors are used they may be selected with heating coils of sufficient capacity to maintain, by thermal circulation, a reasonable temperature when the pri mary air supply system is shut off. The use of low temperature, dehumidi fied air which has not been reheated or mixed with room air before delivery to the room, may permit a reduction in fan capacity and the.use of smaller ducts. In some cases a by-pass may be desirable in order to maintain the primary air volume and to provide additional control. This system can provide a degree .of zoning that is usually difficult with conventional design since the air delivered by each unit can be controlled individually.