Document KGvODwqxXvm8BNM8dE4qG38Ko

14 CHAPTER 2 1962 Guide And Data Book Table 1 .... Factors for Warm Duct Design Satie of Syttea Caid Ak <o Syftea Toted Air-a/O Hath of Warm Duct Area to Cold Duel Area SjrdtO wftfc Supplementary Heating SfOnm at Feriaetnr of flofldtng 100% Air Sytten 1.0 to 0.9 0.89 to 0.85 0.84 to 0.80 0.79 to 0.75 0.74 or smaller 0.70 0.70 0.75 0.75 0.80 From Air Ceadihmrinf, Beating end Ventilating. 0.80 0.80 0.85 0.85 0.90 1958, D- 81. operation. The second term of Equation 8 represents heat by passed in the warm duct and serves to control zones momen tarily below their individual peakB. The quantity of cold air as derived from Equations 7 and 8 should be used in selecting cooling coils and determining cold main sees. The quantities of cold and warm air needed for winter peak can be determined from Equations 7 and 9. The second term of Equation 9 represents the net coolingeffect of bypassed cold air in the cold duct. When calculating air quantities from Equations 7 and 9, care should be exercised to use the air tem peratures of cold, warm, and room ah which are to be main tained during winter operation. The warm sir quantity thus determined can be used in rising the beating coil. The warm duct axes are seldom derived from consideration of warm air requirement under winter peak. The maximum flow in the warm duct will ordinarily occur during light summer loads or during intermediate season operation while warm duct tem peratures are still kept at a low level For this reason the warm air duct sixes are fixed ordinarily as a certian percentage of the cold air duct area as shown in Table 1. Example 1 illustrates the use of Equations 7, 8, and 9 to determine air quantities. Example 1: Determine the cold and warm air quantities for peak summer and winter operation of a system for which the following data are known: Total supply air quantity Q 60,000 cfm. Summer U - 78 F 1. - 50F t. - 86F Ha 1,400,000 Btuh Winter tr - 75 F U - 55 F 1. - 125 F * 2,100,000 Btuh Solution far Summer Operation: From' Equations 7 and 8, 0c + Q* - 60,000. 1,400,000 + 1.080.(86 - 78) = 1.080.(78 - 50). Solving these equations:. Q. -- 49,500 cfm (summer). Qm * 10,500 cfm (summer). Solution for Winter Operation: From Equations 7 and 9, 0. + Q. - 60,000 2,100,000 + 1.08 0.(75 - 55) * 1.08 0- (125 - 75) Solving these equations Q. - 45,000 cfm (winter)i 0 " 15,000 cfm (winter). Sizing of Cold and.Warm Ducts After the general scheme of the ductwork is established, and the air to be supplied to each section of the system is cal lable 2____ Recommended Maximum Velocities tn Cold and Warm Ducts CFM Carried by Hm Ovcf Maxiera Vdotiliw FPM 60,000 to 40,000 40,066 to 25,000 25,000 to 15,000 15,000 to 10,000 10,000 to 6,000 6,000 to 3,000 3,000 to 1,000 6,000 5,000 4,500 4,000 3,500 3,000 2,500 Fnm Xv Cenditiening, Heating end Ventilating, April 19SS, p. 81. culated, the actual rising of the ductwork does not differ greatly from that used for other systems, and discussed in Chapter 12 of the 1961 Guide And Data Book. In dual-duct systems where volume regulation is accom plished at terminal points, the duct suing technique becomes even less critical than for other systems, and extreme precision in duct siring becomes superfluous. The cold ducts may be sized by equal friction, static regain, or velocity methods, or a combination of all three, if deared. Volume regulators in mix ing units will absorb all pressure unbalance caused by the initial design, partial load operation, or future unavoidable changes in load distribution and will produce a mechanically stable system under all normal operating conditions, irrespec tive of the method or procedure used in duct rising, providing the power needed in the longest duct is estimated correctly. As pointed out previously, the maximum flow through the warm ducts is not created by winter peaks, but rather by in termediate operation while low temperatures are maintained in the warm chamber. For this reason the warm duct sizes are usually determined as a ratio of area to cold duct rises. The set of factors from Table 1 can be used for this purpose. On multi-zone installations there will be an infinite set of operating conditions which will create rates of air flow in the cold and warm ducts entirely different from those used in the original design. For this reason, the possibility of achieving uniformity in duct pressures, for the purpose of volumetric control by some program of duct sizing, is very limited.' Every effort should be made in designing the ductwork to reduce the total power needed to produce the required rate of flow in the supply system. This will assure a quieter system, reduce duct leakages and will approach in the majority of cases the condition of maximum owning and operating economy. The chief justification for high duct velocity is space limita tion created by past and present architectural and structural practices. By necessity, in a majority of multi-zone installa tions, relatively high velocities and high friction rates must be used in some parts of the distribution system. On the other hand, where space is not at a premium, the use of high frictibn rates is not economically justifiable. In most installations there will always be a point or points of greatest space re striction where the duct velocity must be high. As soon as these points are passed and space becomes less critical, the friction rates should be gradually reduced toward the k>w end of the duct system. A substantial saving in duct friction is usually achieved by a careful study of the duct mains adjacent to tire apparatus room. The connections to the cold and warm plenums should be made as streamlined as possible to reduce the loss at the entrance to the high-velocity duct. In actual layout this is usually accomplished by using a conical shaped fitting be* , tween the high-velocity ductwork and the plenum. In many Dual-Duct, Induction Unit, and Fan-Coil Conditioner Systems 15 the space conditions at tire apparatus room are not critical and a large duct can be used in this area. The duct velocity can then be stepped up gradually when the point of space restriction is reached. Maximum velocities recommended for cold and warm ducts am given in Table 2. Precautions The use of dampers for adjustment of distribution should be avoided, because they are practically useless in a dualduct system with constantly changing pressure characteristics. In systems with relatively high velocities, they are a source of noise, leakage and unnecessary expense. Splitters of any kind should be avoided, because they not only are a source of noise but can actually cause distributional troubles. Uuvaned short radius rectangular elbows should not be used. The small ducts and take-offs from mains to mixing units should not be undersized. Offsets should be avoided if possible. When not possible, the transformations and bends should be kept easy and usually require turning vanes. Refum Air Ducts Return air ducts should be sized at low velocities as in any conventional system. Attempts to pressurize the return duct have not as yet proved to be entirely successful; clogging of high pressure throttling valves by lint and dust still presents a problem. One scheme to simplify the arrangement of the dual duct system is to use hung ceilings or corridors for return plenums and collect all return air at central points on each floor. In some localities there are code restrictions to this method- Low Pressure Ducts Low pressure ducts leaving the mixing units are sized as any other conventional ductwork. Experience has indicated that the least expensive and the safest way to assure a quiet installation is to have some length o! lined ductwork on the leaving ride of the mixing unit. The lined ductwork, especially when it contains one or two elbows, is a very effective sound attenuator. The lined ductwork will provide a necessary safety factor in acoustical design if noise regeneration should occur in the air distributing system be cause of poorly constructed ducts, fittings and tap-offs. Types of Dual-Duct Mixing Units Dual-duct miring units are designed to meet two basic con ditions of application. One type has deen designated undermndovi unit and is limited insize, capacity and architectural adaptability to the conditioned space in which it is installed. The space limitations and location also limit the acoustical treatment, especially for large air capacities. Capacities cur rently in use are generally less than 600 cfm per unit. The second type unit is designed for ceiling or remote instal lation and is usually concealed or isolated from the space it conditions. Appearances, space, and acoustical treatment are, .therefore, flexible. Capacities of this type unit are generally 100 to 4000 cfm although, in some instances, units up to 10,000 cfm may be considered. Larger capacities may justify separate air conditioning systems. warm air is usually recirculated air, without outside heat, or a mixture of recirculated and outdoor air. The central condi tioning system thus makes available to all conditioned spaces a warm and cold air' supply. The terminal or room units merely mix tire required proportions of the two air streams to satisfy each conditioned space individually. Terminal Units ' There are two types of terminal units; those used merely to control space temperature and those which maintain both the temperature and the desired air flow volume requirements. The space thermostat controls an operator which modulates the valving device to proportion the warm and cold air de livered to the space. Either type may be acoustically treated for noise attenuation. Without volume control, the air flow to the conditioned spaces varies with changes of pressure in the two ducts which, in turn, vary with the relative demands for warm or cold air. The potential volume variation increases with the size and pressure of the distribution system and may easily vary as much as two to one. Some compromise can be effected by a system of static pressure regulators in the ducts. A terminal device more commonly used today is the con stant volume type of unit with thermostatic mixing of the warm and cold air. This unit not only serves to keep the dis tribution system stable, but also simplifies the initial balanc ing of the system. Figs. 9, 10 and 11 show common methods of controlling this type of unit to satisfy both the temperature and volume requirements. The mixing is similar in each case with operator controlled valves positioned by a room thermo stat. In Fig. 9 the air flow is controlled by a spring-loaded valve which closes as the system pressure rises, the spring and movement being calibrated to maintain constant flow and the adjustment may be preset for any prescribed volume. The by passing of air from one duct to the other, while unequal CONTROL OF DUAL-DUCT SYSTEMS Dual-duct systems require a supply of warm air in one duct and a supply of cold air in the second duct. In summer the