Document B81ZveO5zNgOYY7q6ZbYbDQao

796 CHAPTER 43 1948 Guide AIR QUANTITY AND EFFECTIVE TEMPERATURE DIFFERENCE ' ; The difference between the room air temperature and the .supply air temperature at the outlet to the room is known as the effective tempera ture difference. In the theoretical case of a dehumidifier having 100 per cent saturating efficiency and where this air is delivered directly to the room without temperature increases due to heat gain, then the effective temperature difference is the difference between room tempera ture 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. effective temperature difference. Thus if the difference between the room tem perature and the apparatus dew-point indicates that. i0,000 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 effective tempera ture difference of 15 deg instead of 30 deg. Air supply outlets and grilles that have a high induction ratio are available and cause a large amount of room air to be mixed with the air leaving the outlet within a short distance of the outlet through the induction effect of the air stream. A proper .selection of outlets may make it possible to introduce air at low tempera tures 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 effective temperature differences may be required for this reason. While the use of a high effective 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 considered: If the sensible heat load of a space is subjected to substantial variations, low effective temperature differences should be considered, since systems employing low effective 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 effective 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 Central Systems for Comfort Air Conditioning 797 excessive heat: 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 15 to 35 deg. There can be no set rule and each case should be judged according to its particular requirements. Reference may be made to Chapter 40 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 conditioned air (or primary air) to induce a flow of room or secondary air which mixes with the primary air Fig. 4. The mixture is discharged Fig. 4. Induction Unit (Low Pressure Type) into the room, through a grille at the.top of the convector. Heating coils are located in the secondary air stream. The output is. controlled either by . manually or automatically 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 tempera ture of the mixture is satisfactory even though the primary air is intro duced at a temperature too. low for ordinary methods of distribution. One 6f 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. With a conventional system it may be necessary to provide supplementary heating in the form of direct radiation. . Induc tion convectors may. be selected with heating coils haying sufficient capacity under gravity conditions (that is, with the fan system shut off and no primary air entering the device) to maintain the room at a reasonable temperature in winter. The use of low temperature, -dehumi dified air.which has not been reheated or mixed with room, air before