ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 44-2012 CENTRIFUGAL PUMPS.pdf

1、44.1CHAPTER 44CENTRIFUGAL PUMPSCentrifugal Pumping 44.1Construction Features 44.1Pump Types 44.2Pump Performance Curves 44.4Hydronic System Curves 44.5Pump and Hydronic System Curves. 44.6Pump Power . 44.7Pump Efficiency . 44.7Affinity Laws 44.7Radial Thrust 44.9Net Positive Suction Characteristics

2、44.10Selection of Pumps. 44.10Arrangement of Pumps. 44.11Motive Power . 44.14Energy Conservation in Pumping 44.15Installation, Operation, and Commissioning . 44.15Troubleshooting 44.16ENTRIFUGAL pumps provide the primary force to distributeCand recirculate hot and chilled water in a variety of space

3、-conditioning systems. The pump provides a predetermined flow ofwater to the space load terminal units or to a thermal storage cham-ber for release at peak loads. The effect of centrifugal pump perfor-mance on the application, control, and operation of various terminalunits is discussed in Chapter 1

4、3. Other hydronic systems that usepumps include (1) condensing water circuits to cooling towers(Chapters 14 and 40), (2) water-source heat pumps (Chapter 9),(3) boiler feeds, and (4) condensate returns (Chapter 11). Boilerfeed and condensate return pumps for steam boilers should beselected based on

5、boiler manufacturers requirements.CENTRIFUGAL PUMPINGIn a centrifugal pump, an electric motor or other power sourcerotates the impeller at the motors rated speed. Impeller rotationadds energy to the fluid after it is directed into the center or eye ofthe rotating impeller. The fluid is then acted on

6、 by centrifugalforce and rotational or tip-speed force, as shown in the vector dia-gram in Figure 1. These two forces result in increased fluid veloc-ity. The pump casing is designed for maximum conversion ofvelocity energy of the fluid into pressure energy, either by the uni-formly increasing area

7、of the volute or by diffuser guide vanes(when provided).CONSTRUCTION FEATURESThe construction features of a typical centrifugal pump areshown in Figure 2. These features vary according to the manufac-turer and the type of pump.Materials. Centrifugal pumps are generally available in bronze-fitted or

8、iron-fitted construction. The choice of material depends onthose parts in contact with the liquid being pumped. In bronze-fittedpumps, the impeller and wear rings (if used) are bronze, the shaftsleeve is stainless steel or bronze, and the casing is cast iron. All-bronze construction is often used in

9、 domestic water applications.Seals. The stuffing box is that part of the pump where the rotat-ing shaft enters the pump casing. To seal leaks at this point, amechanical seal or packing is used. Mechanical seals are used pre-dominantly in clean hydronic applications, either as unbalanced orbalanced (

10、for higher pressures) seals. Balanced seals are used forhigh-pressure applications, particulate-laden liquids, or for ex-tended seal life at lower pressures. Inside seals operate inside thestuffing box, while outside seals have the rotating element outsidethe box. Pressure and temperature limitation

11、s vary with the liquidpumped and the style of seal. Packing is used where abrasive sub-stances (that are not detrimental to operation) could damage me-chanical seals. Some leakage at the packing gland is needed tolubricate and cool the area between the packing material and shaft.Some designs use a l

12、arge seal cavity instead of a stuffing box.Shaft sleeves protect the motor or pump shaft.Wear rings prevent wear to the impeller and/or casing and areeasily replaced when worn.Ball bearings are most frequently used, except in low-pressurecirculators, where motor and pump bearings are the sleeve type

13、.The preparation of this chapter is assigned to TC 6.1, Hydronic and SteamEquipment and Systems.Fig. 1 Centrifugal Pump44.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)A balance ring placed on the back of a single-inlet enclosedimpeller reduces the axial load, thereby decreasing the size of t

14、hethrust bearing and shaft. Double-inlet impellers are inherently axi-ally balanced.Normal operating speeds of motors may be selected from 600 to3600 rpm. The pump manufacturer can help determine the optimumpump speed for a specific application by considering pump effi-ciency, the available pressure

15、 at the inlet to prevent cavitation, main-tenance requirements, and operating cost.PUMP TYPESMost centrifugal pumps used in hydronic systems are single-stagepumps with a single- or double-inlet impeller. Double-inlet pumpsare generally used for high-flow applications, but either type is avail-able w

16、ith similar performance characteristics and efficiencies.A centrifugal pump has either a volute or diffuser casing. Pumpswith volute casings collect water from the impeller and discharge itperpendicular to the pump shaft. Casings with diffusers dischargewater parallel to the pump shaft. All pumps de

17、scribed in this chapterhave volute casings except the vertical turbine pump, which has adiffuser casing.Pumps may be classified as close-coupled or flexible-coupled tothe electric motor. The close-coupled pump has the impellermounted on a motor shaft extension, and the flexible-coupled pumphas an im

18、peller shaft supported by a frame or bracket that is con-nected to the electric motor through a flexible coupling.Pumps may also be classified by their mechanical features andinstallation arrangement. One-kilowatt and larger pumps are avail-able as close-coupled or base-mounted. Close-coupled pumps

19、havean end-suction inlet for horizontal mounting or a vertical in-lineinlet for direct installation in the piping. Base-mounted pumps are(1) end-suction, frame-mounted or (2) double-suction, horizontalor vertical split-case units. Double-suction pumps can also bearranged in a vertical position on a

20、support frame with the motorvertically mounted on a bracket above the pump. Pumps are usuallylabeled by their mounting position as either horizontal or vertical.Circulator PumpCirculator is a generic term for a pipe-mounted, low-pressure,low-capacity pump (Figure 3). This pump may have a wet rotor o

21、rmay be driven by a close-coupled or flexible-coupled motor. Circu-lator pumps are commonly used in residential and small commercialbuildings to circulate source water and to recirculate the flow of ter-minal coils to enhance heat transfer and improve the control of largesystems.Close-Coupled, Singl

22、e-Stage, End-Suction PumpThe close-coupled pump is mounted on a horizontal motor sup-ported by the motor foot mountings (Figure 4). Mounting usuallyrequires a solid concrete pad. The motor is close-coupled to thepump shaft. This compact pump has a single horizontal inlet andvertical discharge. It ma

23、y have one or two impellers.Frame-Mounted, End-Suction Pump on Base PlateTypically, the motor and pump are mounted on a common, rigidbase plate for horizontal mounting (Figure 5). Mounting requires aFig. 2 Cross Section of Typical Overhung-Impeller End-Suction PumpFig. 3 Wet-Rotor Circulator PumpCen

24、trifugal Pumps 44.3solid concrete pad. The motor is flexible-coupled to the pump shaftand should have an OSHA-approved guard. For horizontal mount-ing, the piping is horizontal on the suction side and vertical on thedischarge side. This pump has a single suction.Base-Mounted, Horizontal (Axial) or V

25、ertical, Split-Case, Single-Stage, Double-Suction PumpThe motor and pump are mounted on a common, rigid base platefor horizontal mounting (Figures 6 and 7). Sometimes axial pumpsare vertically mounted with a vertical pump casing and motormounting bracket. Mounting requires a solid concrete pad. Them

26、otor is flexible-coupled to the pump shaft, and the couplingshould have an OSHA-approved guard. A split case permits com-plete access to the impeller for maintenance. This pump may haveone or two double suction impellers.Base-Mounted, Horizontal, Split-Case, Multistage PumpThe motor and pump are mou

27、nted on a common, rigid base platefor horizontal mounting (Figure 8). Mounting requires a solid con-crete pad. The motor is flexible-coupled to the pump shaft, and thecoupling should have an OSHA-approved guard. Piping is horizon-tal on both the suction and discharge sides. The split case permitscom

28、plete access to the impellers for maintenance. This pump has asingle suction and may have one or more impellers for multistageoperation.Vertical In-Line PumpThis close-coupled pump and motor are mounted on the pump cas-ing (Figure 9). The unit is compact and depends on the connected pip-ing for supp

29、ort. Mounting requires adequately spaced pipe hangersand, sometimes, a vertical casing support. The suction and dischargepiping is horizontal. The pump has a single or double suction impeller.Vertical Turbine, Single- or Multistage, Sump-Mounted PumpVertical turbine pumps have a motor mounted vertic

30、ally on thepump discharge head for either wet-sump mounting or can-typemounting (Figure 10). This single-suction pump may have one ormultiple impellers for multistage operation. Mounting requires asolid concrete pad or steel sole plate above the wet pit withaccessibility to the screens or trash rack

31、 on the suction side formaintenance. Can-type mounting requires a suction strainer. Pip-ing is horizontal on the discharge side and on the suction side.The sump should be designed according to Hydraulic Institute(2008-2010) recommendations.Fig. 4 Close-Coupled Single-Stage End-Suction PumpFig. 5 Fra

32、me-Mounted End-Suction Pump on Base PlateFig. 6 Base-Mounted, Horizontal (Axial), Split-Case, Single-Stage, Double-Suction PumpFig. 7 Base-Mounted, Vertical, Split-Case, Single-Stage, Double-Suction Pump44.4 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)PUMP PERFORMANCE CURVESPerformance of a c

33、entrifugal pump is commonly shown by a man-ufacturers performance curve (Figure 11). The figure displays thepump power required for a liquid with a density of 1000 kg/m3(water) over a particular range of impeller diameters and flows. Thecurves are generated from a set of standard tests developed by

34、theHydraulic Institute (2008-2010). The tests are performed by the man-ufacturer for a given pump volute or casing and several impellerdiameters, normally from the maximum to the minimum allowable inthat volute. The tests are conducted at a constant impeller speed forvarious flows. Pump curves repre

35、sent the average results from testing severalpumps of identical design under the same conditions. The dischargepressure decreases as the flow increases (Figure 12). Motors areoften selected to be non-overloading at a specified impeller size andmaximum flow to ensure safe motor operation at all flow

36、require-ments.The pump characteristic curve may be further described as flator steep (Figure 13). Sometimes these curves are described as a nor-mal rising curve (flat), a drooping curve (steep), or a steeply risingcurve. The pump curve is considered flat if the pressure at shutoffis about 1.10 to 1.

37、20 times the pressure at the best efficiency point.Flat characteristic pumps are usually installed in closed systemswith modulating two-way control valves. Steep characteristicpumps are usually installed in open systems, such as cooling towers(see Chapter 14), where higher pressure and constant flow

38、 are usu-ally desired.Pump manufacturers may compile performance curves for a par-ticular set of pump volutes in a series (Figure 14). The individualcurves are shown in the form of an envelope consisting of the max-imum and minimum impeller diameters and the ends of their curves.This set of curves i

39、s known as a family of curves. A family of curvesis useful in determining the approximate size and model required,but the particular pump curve (Figure 11) must then be used to con-firm an accurate selection.Many pump manufacturers and HVAC software suppliers offerelectronic versions for pump select

40、ion. Pump selection softwaretypically allows the investigation of different types of pumps andFig. 8 Base-Mounted, Horizontal, Split-Case, Multistage PumpFig. 9 Vertical In-Line PumpFig. 10 Vertical Turbine PumpFig. 11 Typical Pump Performance CurveCentrifugal Pumps 44.5operating parameters. Correct

41、ions for fluid density, temperature,and motor speeds are easily performed.HYDRONIC SYSTEM CURVESPressure drop caused by the friction of a fluid flowing in a pipemay be described by the Darcy-Weisbach equation:p = f (1)Equation (1) shows that pressure drop in a hydronic system(pipe, fittings, and equ

42、ipment) is proportional to the square of theflow (V2or Q2where Q is the flow). Experiments show that pres-sure drop is more nearly proportional to between V1.85and V1.9, ora nearly parabolic curve as shown in Figures 15 and 18. The designof the system (including the number of terminals and flows, th

43、efittings and valves, and the length of pipe mains and branches)affects the shape of this curve.Equation (1) may also be expressed in specific energy form:h = (2)whereh = loss through friction, m (of fluid flowing)p = pressure drop, Pa = fluid density, kg/m3f = friction factor, dimensionlessL = pipe

44、 length, mD = inside diameter of pipe, mV = fluid average velocity, m/sg = gravitational acceleration, 9.8 m/s2The system curve (Figure 15) defines the system pressure requiredto produce a given flow rate for a liquid and its characteristics in apiping system design. To produce a given flow, the sys

45、tem pressuremust overcome pipe friction, inside pipe surface roughness, actualfitting losses, actual valve losses, resistance to flow due to fluidviscosity, and possible system effect losses. The general shape ofthis curve is parabolic since, according to the Darcy-Weisbachequation Equation (2), the

46、 pressure loss is proportional to thesquare of the flow.If static pressure is present due to the height of the liquid in thesystem or the pressure in a compression tank, this pressure issometimes referred to as independent pressure and is added to thesystem curve (Figure 16).Fig. 12 Typical Pump Cur

47、veFig. 13 Flat Versus Steep Performance CurvesLD-V22-hpg- f LD-V22g-=Fig. 14 Typical Pump Manufacturers Performance Curve SeriesFig. 15 Typical System Curve44.6 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)PUMP AND HYDRONIC SYSTEM CURVESThe pump curve and the system curve can be plotted on the

48、same graph. The intersection of the two curves (Figure 17) is thesystem operating point, where the pumps developed pressurematches the systems pressure loss.In a typical hydronic system, a thermostat or controller varies theflow in a load terminal by positioning a two-way control valve tomatch the l

49、oad. At full load the two-way valves are wide open, andthe system follows curve A in Figure 18. As the load drops, the ter-minal valves begin closing to match the load (part load). Thisincreases the friction and reduces the flow in the terminals. The sys-tem curve gradually changes to curve B.The operating point of a pump should be considered when thesystem includes two-way control valves. Point 1 in Figure 19 showsthe pump operating at the design flow at the calculated design pres-sure loss of the system. But typically, the actual system curve is