Tài liệu HVAC Systems Design Handbook part 5 docx

95 Chapter 5 Design Procedures: Part 3 Air-Handling Systems 5.1 Introduction In most HVAC systems, the final energy transport medium is moist air—a mixture of dry air and water vapor. This is conveyed through filters, heat exchange equipment, ducts, and various terminal devices to the space to be air-conditioned. The power to move the air is sup- plied by fans. This chapter discusses fans and duct systems, together with related subjects such as grilles, registers, diffusers, dampers, fil- ters, and noise control. 5.2 Fans According to Air Moving and Conditioning Association (AMCA) Stan- dard 210, 1 ‘‘A fan is a device for moving air which utilizes a power- driven, rotating impeller.’’ The three fan types of primary interest in HVAC systems are centrifugal, axial, and propeller. The fan motor may be directly connected to the impeller, directly connected through a gearbox, or indirectly connected by means of a belt-drive system. 5.2.1 Fan law equations The fan law equations are used to predict the performance of a fan at some other condition than that at which it is tested and rated. The HVAC designer is particularly interested in the effects on horsepower, pressure, and volume consequent to varying the speed of the fan in a system. Source: HVAC Systems Design Handbook Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 96 Chapter Five The fan laws expressed in the following equations relate only to the effect of varying speed, assuming that fan size and air density remain constant. RPM 2 CFM ϭ CFM (5.1) 21 RPM 1 2 RPM 2 SP ϭ SP (5.2) ͩͪ 21 RPM 1 2 RPM 2 TP ϭ TP (5.3) ͩͪ 21 RPM 1 3 RPM 2 BHP ϭ BHP (5.4) ͩͪ 21 RPM 1 3 where CFM ϭ airflow rate, ft / min SP ϭ static pressure TP ϭ total pressure BHP ϭ brake horsepower, bhp Expressed in simple language, the fan laws say that when fan size and air density are unchanged, the airflow rate varies directly as the change in speed, the pressure developed by the fan varies as the square of the change in speed, and the power required to drive the fan varies as the cube of the change in speed. The complete fan laws also include terms for changes in fan size and air density. The laws are valid only when fans of different sizes (diameters) are geometrically similar. 3 CFM RPM D 222 ϭϫ (5.5) ͩͪ CFM RPM D 111 22 TP SP VP RPM Dd 22 2 222 ϭϭ ϭ (5.6) ͩͪͩͪ TP SP VP RPM Dd 11 1 111 35 BHP RPM Dd 2222 ϭ ⅐ (5.7) ͩͪͩͪ BHP RPM Dd 1111 where D ϭ fan diameter and d ϭ air density. For further variations, see ASHRAE Handbook, 2000 HVAC Systems and Equipment, Chap. 18, Table 2, p. 18.4. Design Procedures: Part 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Design Procedures: Part 3 97 Figure 5.1 Principle of operation of centrifugal fan. Figure 5.2 Cutaway view of centrifugal fan. 5.2.2 Centrifugal fans A centrifugal fan creates pressure and air movement by a combination of centrifugal (radial) velocity and rotating (tangential) velocity. As shown in Fig. 5.1, these two effects combine to create a net velocity vector. When the fan is enclosed in a scroll (housing) as shown in Fig. 5.2, some of the velocity pressure is converted to static pressure. The fan characteristics can be changed by changing the shape of the blade. Typical shapes (Fig. 5.3) are forward-curved, straight radial, back- ward-inclined (straight or curved), and airfoil. Design Procedures: Part 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 98 Chapter Five Figure 5.3 Centrifugal fan blade types. A. Forward curved. B. Radial. C. Backward- inclined. D. Airfoil. The geometry of the fan wheel, inlet cone, and scroll also has an effect on fan performance and efficiency. Figure 5.4 shows a typical cross section for a backward-inclined (BI) or airfoil (AF) fan wheel. For a given wheel or diameter, as the blade gets narrower and longer, higher pressures can be generated but flow rates are reduced. The inlet cone is shown curved (bell-mouth) to minimize air turbulence. Straight cones are also used, at the cost of some reduction in perform- ance. The clearance between the inlet cone and the wheel shroud must be minimized for efficiency, because some air is bypassed through this opening. The forward-curved (FC) wheel (Fig. 5.5) usually has a short, wide blade and a flat shroud. The inlet cone is curved or tapered and is mounted to minimize the clearance between the inlet cone and shroud. This type of fan handles large air volumes at low pressures. The illustrations show single-width, single-inlet (SWSI) fans. Double- width, double-inlet (DWDI) fans are also made. 5.2.3 Fan testing procedures Fans for HVAC applications should be tested and certified for perform- ance rating in accordance with AMCA Standard 210, 1 promulgated by the Air Moving and Conditioning Association. Also, ASHRAE Stan- dard 51 prescribes the test setup and data-gathering procedures for fan testing. For a line of several sizes of geometrically similar fans, Design Procedures: Part 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Design Procedures: Part 3 99 Figure 5.4 Cross section of BI, radial, or airfoil fan. Figure 5.5 Cross section of FC fan. only the smallest fan in the line is actually tested. Performance of all other sizes is calculated, by using formulas based on the fan laws. The testing setup and procedures are designed for ideal inlet and outlet conditions, with a minimum of turbulence. Later in this chapter we discuss the effect of the nonideal conditions usually found in HVAC system installations. The test procedure includes measuring the airflow and horsepower against varying pressures, for a constant fan speed. Pressure is mea- sured in inches of water, by using an oil- or water-filled manometer. Design Procedures: Part 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 100 Chapter Five Figure 5.6 Normalized curves for a BI fan. Figure 5.7 Normalized curves for an FC fan. Airflow is measured in cubic feet per minute. The data can then be plotted as a series of curves similar to Fig. 5.6. This figure contains ‘‘normalized’’ typical curves for a BI fan. Airfoil fan curves are similar with slightly higher efficiencies. The curves for an FC fan have a dif- ferent shape, as shown in Fig. 5.7. When the fan speed is varied, the result is a family of parallel curves, as shown in Fig. 5.8. 5.2.4 Fan performance data The HVAC system in which a fan is to be installed has a system-curve characteristic relating to the HVAC system geometry. In accordance Design Procedures: Part 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Design Procedures: Part 3 101 Figure 5.8 Fan performance at various speeds. with the laws of hydraulics, the system pressure loss varies as the square of the change in airflow rate. The system curve can be super- imposed on a fan curve, resulting in something like Fig. 5.9. For pur- poses of illustration, this shows two different system curves. These two curves are the recommended limits between which the fan can be efficiently and safely used. The manufacturer’s performance tables normally cover this area of the graph. Operation at conditions outside the recommended limits can result in inefficiency, noise, and instabil- ity (surge). The point of intersection of the fan curve and the system curve de- termines the actual operating condition—flow rate versus pressure. This assumes that the system resistance has been accurately esti- mated and that the fan is installed so that inlet and outlet conditions are comparable to those used in the laboratory test. In fact, this is seldom or never the case. AMCA Publication 201, 2 Fans and Systems, discusses system effects in detail and includes a great deal of data on multipliers to be used for various system effects which are too volu- minous to include in this book. The effect is illustrated in Fig. 5.10, which is taken from AMCA Publication 201. The theoretical fan se- lection would be at condition 1 on the calculated duct system curve. However, if the actual system curve is as shown by the dashed line, then the fan selected at condition 1 will actually perform at condition 4, with a higher pressure and lower flow than the design values. To get the design airflow rate, the fan will have to be speeded up to get to condition 2. This might not be possible with the original fan and Design Procedures: Part 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 102 Chapter Five Figure 5.9 Recommended performance of a typical centrifugal fan. (Reprinted from AMCA Publication 201-90, Fans and Systems, with written peermission from Air Move- ment and Control Association, International, Inc.) horsepower selection, and a different size fan will be needed. If the problem is discovered after installation, it could be very costly to fix. The most common design and installation errors relate to fan inlet and outlet conditions. The ideal in both cases is a gradual transition with no turns close to the fan. Turning vanes must be provided in inlet duct elbows. An inlet condition that creates a swirling motion in the direction of rotation will reduce the pressure-volume curve by an amount depending on the intensity of the vortex; this is the principle used by inlet vane dampers. A condition that causes a swirl opposite to the direction of rotation will cause a substantial increase in horse- power. Installation in an intake plenum (as in most packaged HVAC sys- tems) or discharging directly into a plenum (as in most multizone and dual-duct systems) will affect fan performance adversely. The performance curves indicate fan classes. Classes I, II, III, and IV relate to structural considerations required to accommodate higher speeds and pressures. These include stronger frames and wheels and larger shafts and bearings. The fact that a fan has a class rating means that it can be operated at any or all possible points within that class. However, if a selection approaches the upper pressure limit of a class, it would be prudent to specify a fan design in the next higher class. Design Procedures: Part 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Design Procedures: Part 3 103 Figure 5.10 Deficient duct system performance, system effects ignored. (Reprinted from AMCA Publication 201-90, Fans and Systems with written permission from Air Move- ment and Control Association, International, Inc.) 5.2.5 Inlet vane dampers for fan volume control A common method of fan volume control employs the inlet vane damper. This consists of a ring of pie-shaped elements which open and close in parallel. Control may be manual or automatic. When properly installed to provide an inlet swirl in the direction of fan rotation, the damper alters the fan performance curve as shown in Fig. 5.11. The fan horsepower is also reduced, although not as much as would be predicted by the fan laws, because the damper increases the system pressure loss. Use of discharge dampers is not recommended for vol- ume control, only for fan isolation. A variation of the inlet vane damper concept is a cone at the fan inlet which can be moved in or out, thereby varying the size of the fan inlet. 5.2.6 Mechanical and structural considerations The mounting and driving mechanism for the fan wheel entails many mechanical and structural considerations. The bearings which support Design Procedures: Part 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 104 Chapter Five Figure 5.11 Typical normalized pressure-volume curve-inlet vane control for a centrif- ugal fan. (Reprinted from AMCA Publication 201-90, Fans and Systems with written permission from Air Movement and Control Association, International, Inc.) the shaft come in many kinds, depending on the speed of rotation, weight of the fan wheel, belt tension, power transmitted, and whether the fan wheel is overhung (Fig. 5.12) or supported between bearings (Fig. 5.13). Sometimes three bearings are used; then alignment must be precise. Bearing supports must be strong enough to support the bearings without flexing. The drive shaft must be strong, true, and rigid enough to support the fan wheel between the bearings and to transmit the required power without undue flexing over a specified rotational speed. All rotating shafts have a critical speed at which excessive vibration, noise, and possible failure will occur. Many shafts have two or more critical speeds. Sometimes the lower critical speed is less than the normal speed range of the fan. This is satisfactory when the fan is accelerated quickly through the critical speed. It will not be satisfactory if the fan is to be used in a speed-controlled VAV application. Design Procedures: Part 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. [...]... 22 46 23 48 24 50 25 32 14 32 14 34 15 36 16 36 16 38 17 38 17 41 18 43 19 43 19 45 20 47 21 47 21 50 22 50 22 54 24 33 13 35 14 35 14 38 15 38 15 40 16 43 17 43 17 45 18 45 18 48 19 50 20 50 20 53 21 53 21 58 23 36 13 36 13 39 14 39 14 41 15 41 15 44 16 44 16 47 17 50 18 50 18 52 19 52 19 55 20 55 20 61 22 36 12 39 13 39 13 42 14 42 14 45 15 45 15 48 16 48 16 51 17 54 18 54 18 57 19 57 19 60 20 63... ASHRAE Handbook, 1997 Fundamentals, Chap 32, Table 2 38 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 39 11 42 12 42 12 46 13 46 13 49 14 49 14 53 15 53 15 56 16 56 16 60 17 60 17 63 18 63 18 67 19 44 11 44 11 48 12 48 12 52 13 52 13 56 14 56 14 60 15 60 15 60 15 64 16 64 16 68 17 68 17 72 18 50 10 50 10 55 11 55 11 55 11 60 12 60 12 65 13 65 13 70 14 70 14 75 15 75 15 75 15 80 16 85 17 54 9 54 9 60... Design Procedures: Part 3 Design Procedures: Part 3 TABLE 5. 3 123 Equivalent Spiral Flat Oval Duct Dimensions Major axis (a), in Duct diameter, in 5 5 .5 6 6 .5 7 7 .5 8 8 .5 9 9 .5 10 10 .5 11 11 .5 12 12 .5 13 13 .5 14 14 .5 15 16 17 18 Minor axis (b), in 3 8 9 11 12 15 19 22 4 7 9 10 12 13 15 18 20 21 5 8 10 — 11 13 14 18 19 21 6 7 8 9 10 11 12 14 16 8 9 — 11 12 14 15 17 19 20 23 25 28 30 33 36 39 45 52 59 ... 29 13 2. 25 15 6 17 7 18 7 20 8 20 8 23 9 23 9 25 10 28 11 28 11 30 12 30 12 2 .50 17 6 17 6 19 7 19 7 22 8 25 9 25 9 28 10 28 10 30 11 30 11 33 12 2. 75 Aspect ratio 18 6 21 7 21 7 24 8 24 8 27 9 27 9 30 10 30 10 33 11 33 11 3.00 21 6 21 6 25 7 25 7 28 8 28 8 32 9 32 9 35 10 35 10 39 11 3 .50 24 6 24 6 28 7 28 7 32 8 32 8 36 9 36 9 40 10 40 10 4.00 30 6 30 6 35 7 35 7 35 7 40 8 40 8 45 9 45 9 5. 00 36 6... 22 22 23 23 24 24 25 25 26 26 27 27 27 27 28 28 29 29 30 30 31 31 32 32 33 33 35 35 23 18 24 19 25 20 25 20 26 21 28 22 29 23 30 24 31 25 31 25 33 26 34 27 35 28 36 29 36 29 39 31 26 17 26 17 27 18 29 19 30 20 30 20 32 21 33 22 35 23 35 23 36 24 38 25 39 26 39 26 41 27 44 29 26 15 28 16 30 17 30 17 32 18 33 19 35 20 35 20 37 21 39 22 39 22 40 23 42 24 42 24 44 25 47 27 28 14 30 15 32 16 32 16 34 17... 14 11 14 11 15 12 16 13 18 14 19 15 20 16 20 16 21 17 1. 25 9 6 11 7 12 8 12 8 14 9 15 10 17 11 17 11 18 12 20 13 21 14 21 14 23 15 24 16 1 .50 Equivalent Rectangular Duct Dimensions 11 6 11 6 12 7 14 8 14 8 16 9 18 10 18 10 19 11 21 12 23 13 23 13 25 14 26 15 1. 75 12 6 14 7 14 7 16 8 18 9 18 9 20 10 20 10 22 11 24 12 24 12 26 13 28 14 2.00 14 6 14 6 16 7 16 7 18 8 20 9 20 9 23 10 25 11 25 11 27 12 27... website Design Procedures: Part 3 Design Procedures: Part 3 121 tions, and many equipment elements may contribute a large amount of leakage Any air leak means a loss of energy—not only the thermal energy required to heat or cool the air but also the fan work required to move the air 5. 3.8 Duct design velocities A very simple example of duct layout and sizing is shown in Figs 5. 24 and 5. 25 and Table 5. 4.. .Design Procedures: Part 3 Design Procedures: Part 3 1 05 Figure 5. 12 SISW centrifugal fan with overhung wheel 5. 2.7 Axial fans Axial-flow fans impart energy to the airstream by giving it a swirling motion Straightening vanes must be provided in the tubular housing to improve flow and efficiency for use with duct systems, as shown in Fig 5. 14 Belt or direct drive may be... Use as given at the website TABLE 5. 4 Calculations for Fig 5. 24 Design Procedures: Part 3 124 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Design Procedures: Part 3 Design Procedures: Part 3 1 25 As can be seen, even a simple system... 11 3 .50 24 6 24 6 28 7 28 7 32 8 32 8 36 9 36 9 40 10 40 10 4.00 30 6 30 6 35 7 35 7 35 7 40 8 40 8 45 9 45 9 5. 00 36 6 36 6 42 7 42 7 42 7 48 8 48 8 54 9 6.00 42 6 42 6 49 7 49 7 49 7 56 8 56 8 7.00 48 6 48 6 56 7 56 7 56 7 64 8 8.00 Design Procedures: Part 3 116 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All . 95 Chapter 5 Design Procedures: Part 3 Air-Handling Systems 5. 1 Introduction In most HVAC systems, the final energy transport. website. Design Procedures: Part 3 1 05 Figure 5. 12 SISW centrifugal fan with overhung wheel. Figure 5. 13 DIDW centrifugal fan with wheel between bearings. 5. 2.7