Regulators and Final Control Elements

7 7.1 DAMPERS AND LOUVERS

1362

Introduction 1362 Damper Designs 1363 Parallel-Blade Dampers and Louvers Rotating Disc Dampers 1365 Multiple-Disc Dampers 1365 Fan Suction Dampers 1366 Variable-Orifice Dampers 1366 Conclusions 1367 Bibliography 1367 7.2 EELECTRIC ENERGY MODULATION

1364

1368

Introduction 1368 On/Off Power Control 1369 Throttling Power Controllers 1369 Saturable Core Reactors 1369 SCR Power Controllers 1370 Zero-Voltage-Fired SCR Controllers 1371 Controlling Infrared Tungsten Heaters 1372 Common SCR Limitations 1372 Ignitron Tube 1373 Power Amplifier 1374 References 1374 Bibliography 1374 7.3 LINEAR AND ANGULAR POSITIONING OF MACHINERY

1375

Open- and Closed-Loop Positioning 1376 Positioning System Components 1376 Motors and Actuators 1377

Electric Motors 1377 Motor Drivers 1378 Controllers 1378 Sensors for Feedback 1378 Communication Protocols 1380 Conclusions 1380 References 1380 Bibliography 1381 7.4 PUMPS AS CONTROL ELEMENTS

1382

Rotodynamic or Centrifugal Pumps 1384 Radial-Flow 1384 Axial- and Mixed-Flow 1385 Peripheral or Regenerative Turbine 1385 Positive-Displacement Pumps 1386 Rotary Pumps 1386 Gear Pumps 1387 Reciprocating Pumps 1387 Air Pumps and Air Lifts 1388 Condensate Pumps 1388 Air Lifts 1388 Design of Pumping Systems 1388 Head Requirements 1388 NPSH Calculation 1389 Specific Speed 1390 Horsepower 1390 Installation Considerations for Wastewater Pumping Stations 1390 Metering Pumps 1391 Plunger Pumps 1391 Diaphragm Pumps 1392 Pneumatic Metering Pumps 1393 1359

© 2006 by Béla Lipták

1360

Regulators and Final Control Elements

Installation Considerations 1393 NPSH and Pulsation Dampening 1395 Opposed Centrifugal Pumps 1395 Bibliography 1396

7.5 REGULATORS—FLOW

Stability 1422 Safety 1423 Installation 1423 Bibliography 1423 ISA Specification Form (Table 7.7s) 1425 Instructions for ISA Form S20.51 1425

1397

7.8 REGULATORS—TEMPERATURE

Introduction 1398 Purge Flow Regulators 1398 Flow Regulators for Chromatographs 1399 Variable-Orifice Flow Regulators 1399 HVAC Balancing Flow Regulators 1399 Oil Flow Regulator 1401 Industrial Flow Regulators 1401 Thermal Mass Flowmeters 1402 Bibliography 1403

7.6 REGULATORS—LEVEL

1405

Introduction 1405 Float-Type Level Regulators 1405 The Phenomenon of Offset 1406 Span, Dead Band, and Hysteresis 1406 Power Generated by the Float 1406 Specific Gravity and Temperature Effects Stuffing Boxes 1408 Installation 1408 Conclusions 1409 Diverter Valves 1409 Altitude Valves 1410 Bibliography 1411

7.7 REGULATORS— PRESSURE

1412

Introduction 1413 Regulators vs. Control Valves 1414 Control Valves 1414 Regulator Applications 1414 Gas Industry Applications 1415 Liquid Pressure Regulation 1415 Gas Station Safety Regulators 1416 Pressure Surge Relief Valves 1416 Pressure Regulator Designs 1416 Weight-Loaded Regulators 1418 Spring-Loaded Regulators 1418 Pilot-Operated Regulators 1418 Regulator Characteristics and Sizing 1419 Seating and Sensitivity 1419 Droop or Offset 1420 Noise 1421 Cavitation 1422 Sizing and Rangeability 1422

© 2006 by Béla Lipták

1407

1426

Introduction 1427 Control Quality 1427 Types of Temperature Regulators 1428 Direct- and Pilot-Actuated 1428 Remote or Internal Element 1428 Accessories and Special Features 1429 Thermal Systems 1430 Bulbs, Wells, Fittings 1430 Vapor-Filled System 1431 Liquid-Filled System 1434 Hot Chamber System 1435 Fusion-Type System (Wax-Filled) 1435 The Regulator Valve 1436 Single- and Double-Seated Valves 1436 Three-Way Valves 1436 Valve Features 1438 Conclusions 1438 Bibliography 1438 7.9 THERMOSTATS AND HUMIDISTATS

1440

Introduction 1441 Accuracy of Thermostats and Humidistats 1441 Conventional 1441 Advanced 1441 Thermostats 1441 ElectroMechanical Designs 1442 Electrical/Electronic Design Features 1442 Microprocessor-Based Units 1443 Control by Phone 1443 The Proportional-Only Controller 1443 Thermostat Action and Spring Range 1445 Thermostat Design Variations 1445 Special-Purpose Thermostats 1446 Humidistats 1450 Relative Humidity Sensors 1450 Humidistat Design Features 1452 Bibliography 1453 7.10 VARIABLE-SPEED DRIVES

1454

Introduction 1455 Transportation Efficiency 1455 Characteristics of Variable-Speed Drives

1455

Contents of Chapter 7

Electromechanical Drives 1456 Mechanical Variable-Speed Drives 1456 Hydraulic Variable-Speed Drives 1458 Magnetic Variable-Speed Drives 1459 Variable Voltage 1461 Pole-Changing AC Motors 1461 Solid-State Variable-Speed Drives 1461 Underlying Semiconductor Technology 1461 Drive Circuit Topologies 1462 Variable-Speed DC Motors 1462

© 2006 by Béla Lipták

Thyristor DC Drives 1464 Variable-Frequency Drives 1465 Comparison of Solid-State Drives 1468 Efficiency of Solid-State Drives 1469 Recent VFD Developments 1469 Efficiency of Variable-Speed Drives and Pumps 1469 Evaluation of VSD Efficiencies 1470 Conclusions 1471 Bibliography 1471

1361

7.1

Dampers and Louvers A. BRODGESELL

(1970)

B. G. LIPTÁK

(1985, 1995, 2005)

Flow sheet symbol Types of Designs:

A. Multiple blade or louver B. Rotating disc, including multiple disc C. Radial vanes D. Iris-type variable orifice (Note: Some butterfly valves and guillotine-type slide gate valves are similar in their functions to dampers. They are discussed in Chapter 6.)

Design Pressure:

Type A designs can usually handle up to 10 in. H2O (2.5 kPa) shut-off differentials; type D units can handle up to 15 psid (103 kPa)

Materials of Construction:

Steel, galvanized steel, aluminum, and fiberglass; stainless steel is used in special cases

Sizes:

Type A dampers are available up to 6 ft × 8 ft (1.8 m × 2.4 m) for HVAC applications and in even larger sizes for boilers and other industrial applications

Flow Characteristics:

See Figure 7.1b 2

Leakage through Each ft 2 (0.092 m ) of Damper Area:

At 3 in. H2O (0.75 kPa) shut-off pressure differential: the leakage of standard type 2 2 A units is 50 SCFM (250 l/s/m ); special low-leakage type A, 5 SCFM (25 l/s/m ); 2 positive-steel type B, 0.5 SCFM (2.5 l/s/m )

Costs:

Type A costs range from $100 to $250 per square foot of area ($1000 to $2500 per square meter) as a function of size, features, and accessories

Partial List of Suppliers:

Air Clean Damper Co. (www.aircleandamper.com) Arrow United Industries (www.arrowunited.com) Babcock & Wilcox Co. (www.babcock.com) Bachmann Industries (www.bachmannusa.com) Belimo Air Controls (www.belimo.org) Damper Design Inc.(www.damperdesign.com) Flextech Industries (www.flextech-ind.com) FMC Corp. (www.fmc.com) Honeywell Sensing and Controls (www.honeywell.com/sensing) Johnson Controls (www.jci.com) Louvers & Dampers Inc. (www.louvers-dampers.com) Miracle Vent Inc. (www.miraclevent.com) Polymil Products Inc. (www.polymil.com) Ruskin Air & Sound Control (www.ruskin.com) Safe-Air/Dowco (www.safeair-dowco.com) Vent Products Co. (www.ventprod.com) Young Regulator Co. (www.youngregulator.com)

INTRODUCTION Dampers and louvers are used to control the flow of gases and vapors. These streams usually flow in large ducts at relatively low static pressures. There are both “commercial” and “process control” quality dampers on the market. Commercialquality units are used for the less-demanding applications, 1362 © 2006 by Béla Lipták

such as heating, ventilation, and air conditioning (HVAC), while the process control-quality units can handle higher pressures, higher temperatures, and corrosive vapors. The process control-quality units also can provide superior leakage and control characteristics. Dampers are also used to control the flow of solids or to throttle the capacity of fans and compressors. There is no

7.1 Dampers and Louvers

Top

1363

1.0 0.8 0.6

Joint unsupported by vertical edge stop

0.5 0.4

Air flow Joint supported by vertical edge stop

0.3

Vertical edge stop

0.2

FIG. 7.1a The designs of the parallel-blade and opposed-blade dampers, which are also referred to as uni-rotational and counter-rotational louver designs.

io rat are a

0 0.6 0.7 0 0 0.7 5 0.8 0

0.06 0.05

0.5

Bottom Opposed blade proportioning damper (counter-rotational louvre)

0.1 0.09 0.08 0.07

Fre e

Parallel damper (uni-rotational louvre)

0.15

Damper pressure drop (in. w.g.)

Joint unsupported by vertical edge stop

Top and bottom leaf edges deflect toward horizontal edge stop under pressure

0.04 0.03 0.02

0.015 1.0

0.01

Fraction of maximum flow

90° louvre (parallel)

0.008 0.007 0.006 0.005

0.8

0.6

0.004 500 700 1000 (150) (210) (300) 800 (240)

Butterfly

0.4

0.2

1500 (450)

3000 (900) 2000 (600)

5000 (1500) 4000 (1200)

Approach velocity ft/m (m/m) 0

0

0.4

0.2

0.6

1.0

0.8

Fraction of maximum rotation

100

A-1 B-2 C-5 D-8 E - 22 F - 50

Linear

Percent maximum flow

80

60 A

B

C

D

E F

G

40

Percent of total system drop through the damper in the wide open position

G - Inherent characteristic of an opposed blade damper with equal percentage linkage at a constant pressure drop

20

0

0

20

40

60

80

FIG. 7.1c Pressure drop through wide-open dampers. The free-area ratio of an open damper is the total open area between the blades, divided by the nominal area.

clear distinction between butterfly valves and butterfly dampers or between slide-gate valves and guillotine dampers. The design features of these dampers are quite similar to their control valve counterparts, which are discussed in Chapter 6. Dampers in general are large in size, and their operating and shut-off pressures are limited to lower values. The diameters of the largest dampers can exceed 20 ft (6 m).

100

Percent linkage movement

FIG. 7.1b The flow characteristics of a parallel-blade damper are similar to those of a conventional butterfly valve (see top part of this figure). The flow characteristics of an opposed-blade damper approach equalpercentage characteristics when the total system pressure drop is through it and it shifts its characteristics toward quick opening, as the damper receives less and less of the total pressure differential.

© 2006 by Béla Lipták

DAMPER DESIGNS Dampers and louvers can be grouped according to their shapes into parallel-blade, disc and multiple-disc, radial vane, and variable-orifice designs. Within each design category, there can be subdivisions according to leakage rates, materials of construction, actuator designs, or accessories provided.

1364

Regulators and Final Control Elements

3−1/2 in. 90 mm W

H

FIG. 7.1d 2 2 Standard commercial damper frame sizes and areas, ft /m . (Courtesy of Ruskin, previously Johnson Controls.)

Parallel-Blade Dampers and Louvers Multiblade dampers consist of two or more rectangular vanes mounted on shafts that are one above the other, which are interconnected so they rotate together (Figure 7.1a). The vanes are operated by an external lever, which can be

© 2006 by Béla Lipták

positioned manually, pneumatically, or electrically. In the uni-rotational louver (parallel damper) design, the vanes remain parallel at all rotational positions. In a counterrotational louver (opposed blade), alternate vanes rotate in opposite directions. Both designs are illustrated in Figure 7.1a.

7.1 Dampers and Louvers

Low-Leakage Designs The parallel-blade damper cannot provide tight shut-off because of the long length of unsealed seating surfaces. The leakage characteristics of unsealed standard dampers are given in the lower portion of Figure 7.1e. In low-leakage damper designs, blade seals are installed along the seating surfaces of the blades, resulting in the reduced leakage characteristic shown in the upper portion of Figure 7.1e. There are a number of variations in the blade-edge seal designs. Some of these designs are illustrated in Figure 7.1f. Corrosion Resistant Designs For corrosive services, both the parallel- and the opposed-blade designs are available in sizes from 12 by 24 in. up to 60 by 120 in. These units are made of fiberglass-reinforced polymer with 304 stainless steel hardware. Some design variations are shown in Figure 7.1g. Actuators and Accessories Damper actuators can be manual, electric, hydraulic, or pneumatic. Standard pneumatic actu2 ators vary their effective diaphragm areas from 2 to 24 in. (13 2 to 155 cm ), while their stroke lengths range from 2 to 6 in. (51 to 152 mm). The amount of force they produce ranges from about 10 to 300 lbf (4.5 to 136 kgf). The standard spring ranges for dampers include the spans of 3–7, 5–10, and 8–13 PSIG (0.2–0.48, 0.34–0.68, and 0.54–0.88 bars). Electronic actuators can be operated by 4 to 20 mA DC analog or by digital signals. For more accurate throttling, the actuators can also be provided with positioners. If remote indication of damper status is desired, limit switches can be installed to detect the blade angle. These can be pneumatic sensors of nozzle backpressure or mechanically actuated position sensors. The damper position switch can be furnished with an adjustable mounting flange, which allows the unit to be mounted through a duct wall with the trip lever positioned so that it is actuated by the damper blade itself.

© 2006 by Béla Lipták

Sealed, low-leakage damper

Pressure differential, inches of H2O (kPa)

4 (1.00)

3 (0.75)

2 (0.50)

1 (0.25)

0

0

1 (0.3)

2 (0.6)

3 (0.9)

4 (1.2)

5 (1.5)

6 (1.8)

7 (2.1)

8 (2.4)

80 (24)

90 (27)

Leakage, cfm/ft2 [(m3/min)/m2] Static across damper, inches of H2O (kPa)

Flow guides are sometimes installed between adjacent vanes in order to improve the effectiveness of throttling. In the top of Figure 7.1b, the blade-angle vs. flow characteristics of a parallel-blade damper and a butterfly valve are shown. The sensitivity of this design is very high at mid-flow while the last 30° of rotation is relatively ineffective. The flow characteristics of butterfly valves are similar though somewhat superior to those of louvers. The lower portion of Figure 7.1b shows an opposed-blade damper with equal percentage linkage. As less and less of the total system pressure drop is assigned to the damper, the characteristics of this damper shift toward quick opening. Figure 7.1c gives the pressure drop across wide-open dampers. Ideally the wide-open pressure drop should be between 4 and 8% of the pressure difference across the closed damper. If the damper is sized so that closed pressure difference is between 12 and 25 times the pressure drop, when the damper is open, its apparent characteristic will be nearly linear (see curves C and D in Figure 7.1b). Figure 7.1d provides some dimensional data for standard commercial damper frames, including their areas.

1365

Standard unsealed damper

4 (1.00) 3 (0.75) 2 (0.50) 1 (0.25) 0

0

10 (3)

20 (6)

30 (9)

40 (12)

50 (15)

60 (18)

70 (21)

Leakage, cfm/ft2 [(m3/min)/m2]

FIG. 7.1e The leakage rates through sealed, low-leakage dampers are shown in the top, while the leakage rates of unsealed dampers are given in the bottom portion of the figure. (Courtesy of Honeywell and Ruskin, formerly Johnson Controls.)

Rotating Disc Dampers The rotating disc damper designs are very similar to the butterfly valve designs, which were discussed in detail in Section 6.17. These dampers are usually installed in circular ducts and can be operated both manually or automatically. A corrosion-resistant version of this design is made of fiberglassreinforced plastics materials and is illustrated in Figure 7.1h. Multiple-Disc Dampers A unique variation of the butterfly design is the multiple rotating disc damper. In this design several disc elements are distributed over an area (Figure 7.1i). One advantage of this configuration is improved flow control characteristics,

1366

Regulators and Final Control Elements

Model ABK6 “K” style

Model ABF6 “F” style

Model ABFA “FA” style

45° Adjustable

90° Adjustable

Combination stationary & adjustable

Enlarged view of blade seal Pressure effects seal

High pressure side

Low pressure side

FIG. 7.1f Low-leakage damper designs tend to increase the efficiency of HVAC systems. In this figure, two blade-edge seal designs are illustrated. (Courtesy of Honeywell and Ruskin, formerly Johnson Controls.)

because each disc can have its own unique spring range and failure position. Another major advantage is the substantial reduction in leakage compared to the parallel-blade design. At a static pressure of 3 in. H2O (0.75 kPa) the leakage can be estimated as 0.01% of full damper capacity, which corresponds to about 0.5 2 2 2 SCFM (2.5 l/s/m ) leakage per ft (0.092 m ) of damper area.

FIG. 7.1g Corrosion-resistant fiberglass-reinforced polymer damper designs for both adjustable and stationary applications. (Courtesy of Polymil Products Inc.)

Variable-Orifice Dampers Variable-orifice dampers use the same principle as the iris diaphragm of a camera. In order to achieve control action, the closure element moves within an annular ring in the damper body and produces a circular flow orifice of variable

Fan Suction Dampers On blowers and fans, when throughputs must be controlled, radial vane dampers can be utilized. The damper consists of a number of radial vanes arranged to rotate about their radial axis (Figure 7.1j). The radial vane dampers do not provide high-quality control, and their closed position leakage rates are also high. Their control applications include furnace draft control. Usually a positioner is furnished with these units, which provides a linear relationship between the control signal and the blade pitch angle. In certain packages the positioner is factory set in the reverse acting mode, meaning that an increasing control air signal will reduce the air flow by decreasing the blade pitch. In such packages, one has to install a reversing relay between the positioner and the actuator, if direct action is desired.

© 2006 by Béla Lipták

FIG. 7.1h Corrosion-resistant single-disc damper (Courtesy of Polymil Products Inc.)

7.1 Dampers and Louvers

1367

A

D-2

D-1 D-1

D-1

D-1

D-2

Actuator D-3

D-3

D-3

Wafer valve body

D-4

D-4

Adjustable orifice

Section A-A A

FIG. 7.1i Multiple-disc dampers provide better sealing and control characteristics.

diameter (Figure 7.1k). The flow characteristics are similar to those of a linear valve. However, tight shut-off is not possible, and leakage rates are comparable to or greater than those of a butterfly valve of equal size. Maximum pressure differential is limited to approximately 15 psid (104 kPa). Dual valve units can be provided with a common discharge port for applications involving the blending of two streams. For solids service, the variable-orifice valve can be used for throttling, if the valve is installed in a vertical line. In horizontal lines, the shutter mechanism of the valve forms a dam, which makes the valve unsuitable for solids service. Standard sizes range from 4 to 12 in. (100 to 300 mm).

FIG. 7.1k Variable-orifice or iris damper. The sleeve can be made of nylon or of other materials and is provided with a built-in retaining ring at each end. When the upper end is fixed and the lower end is rotated, this gradually reduces the orifice opening. At 180° of rotation the orifice is completely closed. If the sleeve is first turned back on itself, partly “inside out,” the effect of rotation is exactly the same, but operates in a much more compact form, as a pleated duplex diaphragm.

CONCLUSIONS Dampers are suitable for control of large flows at low pressures where high control accuracy is not a requirement. Typical applications of these units include air conditioning systems and furnace draft control. Variable-orifice or iris dampers are smaller than other dampers, offer better control quality, and can also be used to control vertical solid flows. Bibliography

FIG. 7.1j Radial vane dampers are used to throttle the flow on the suction side of air fans and blowers.

© 2006 by Béla Lipták

Ball, K. E., “Final Elements, Final Frontier,” InTech, November 1986. Brown, E. J., “Air Diffusing Equipment,” in ASHRAE Handbook, Equipment Volume, Chapter 2, 1979. “Catalog of U.S. & Canadian Valves & Actuators,” Washington, D.C.: Valve Manufacturers Association of America, 2003. Dickey, P. S., “A Study of Damper Characteristics,” Reprint No. A-8, Bailey Meter Co. Daryanani, S., et al., “Variable Air Volume Systems,” Air Conditioning, Heating and Ventilating, March 1966. “Evaluation Finding for Oiles Multiple Viscous Shear Dampers,” Cerf. Report: Hitec 99-04, American Society of Civil Engineers, 1999. Lipták, B. G., “Reducing the Operating Costs of Buildings by Use of Computers,” ASHRAE Transactions, Vol. 83, Part 1, 1977. Lipták, B. G., “Optimization of Semiconductor Manufacturing Plants,” Instruments and Controls Systems, October 1982.