Industrial Machinery Repair Part Episode 2 Part 2 pptx

Dust Collectors 259 Table 12.2 Common failure modes of cyclonic separators THE PROBLEM THE CAUSES Continuous release of dust-laden air Intermittent release of dust-laden air Cyclone plugs in inlet chamber Cyclone plugs in dust removal section Rotor-lock valve fails to turn Excessive differential pressure Differential pressure too low Rotor-lock valve leaks Fan has high vibration Clearance set wrong • Density and size distribution of dust too high • • • • Density and size distribution of dust too low • • Dust load exceeds capacity • • • • Excessive moisture in incoming air • Foreign object lodged in valve • Improper drive-train adjustments • Improper lubrication • Incoming air velocity too high • Incoming air velocity too low • • • • Internal wear or damage • Large contaminates in incoming air stream • • Prime mover (fan, blower) malfunctioning • • • • • Rotor-lock valve turning too slow • • • Seals damaged 260 Dust Collectors Troubleshooting This section identifies common problems and their causes for baghouse and cyclonic separator dust-collection systems. Baghouses Table 12.1 lists the common failure modes for baghouses. This guide may be used for all such units that use fabric filter bags as the primary dust-collection media. Cyclonic Separators Table 12.2 identifies the failure modes and their causes for cyclonic separa- tors. Since there are no moving parts within a cyclone, most of the problems associated with this type of system can be attributed to variations in process parameters, such as flow rate, dust load, dust composition (i.e., density, size, etc.), and ambient conditions (i.e., temperature, humidity, etc.). 13 Fans, Blowers, and Fluidizers Technically, fans and blowers are two separate types of devices that have a similar function. However, the terms are often used interchangeably to mean any device that delivers a quantity of air or gas at a desired pres- sure. Differences between these two devices are their rotating elements and their discharge-pressure capabilities. Fluidizers are identical to single-stage, screw-type compressors or blowers. Centrifugal Fans Centrifugal fans are one of the most common machines used in industry. They utilize a rotating element with blades, vanes, or propellers to extract or deliver a specific volume of air or gas. The rotating element is mounted on a rotating shaft that must provide the energy required to overcome inertia, friction, and other factors that restrict or resist air or gas flow in the appli- cation. They are generally low-pressure machines designed to overcome friction and either suction or discharge-system pressure. Configuration The type of rotating element or wheel that is used to move the air or gas can classify centrifugal fans. The major classifications are propeller and axial. Axial fans also can be further differentiated by the blade configurations. Propeller This type of fan consists of a propeller, or paddle wheel, mounted on a rotating shaft within a ring, panel, or cage. The most widely used pro- peller fans are found in light- or medium-duty functions, such as ventilation units where air can be moved in any direction. These fans are com- monly used in wall mountings to inject air into, or exhaust air from, a space. Figure 13.1 illustrates a belt-driven propeller fan appropriate for medium-duty applications. This type of fan has a limited ability to boost pressure. Its use should be limited to applications where the total resistance to flow is less than 262 Fans, Blowers, and Fluidizers Figure 13.1 Belt-driven propeller fan for medium duty applications one inch of water. In addition, it should not be used in corrosive environments or where explosive gases are present. Axial Axial fans are essentially propeller fans that are enclosed within a cylindri- cal housing or shroud. They can be mounted inside ductwork or a vessel housing to inject or exhaust air or gas. These fans have an internal motor mounted on spokes or struts to centralize the unit within the housing. Electrical connections and grease fittings are mounted externally on the housing. Arrow indicators on the housing show the direction of airflow and rotation of the shaft, which enables the unit to be correctly installed in the ductwork. Figure 13.2 illustrates an inlet end of a direct-connected, tube-axial fan. This type of fan should not be used in corrosive or explosive environments, since the motor and bearings cannot be protected. Applications where concentrations of airborne abrasives are present should also be avoided. Axial fans use three primary types of blades or vanes: backward-curved, forward-curved, and radial. Each type has specific advantages and disadvan- tages. Fans, Blowers, and Fluidizers 263 Figure 13.2 Inlet end of a direct-connected tube-axial fan Backward-Curved Blades The backward-curved blade provides the highest efficiency and lowest sound level of all axial-type, centrifugal fan blades. Advantages include: ● Moderate to high volumes ● Static pressure range up to approximately 30 inches of water (gauge) ● Highest efficiency of any type of fan ● Lowest noise level of any fan for the same pressure and volumetric requirements ● Self-limiting brake horsepower (BHP) characteristics (Motors can be selected to prevent overload at any volume, and the BHP curve rises to a peak and then declines as volume increases.) The limitations of backward-curved blades are: ● Weighs more and occupies considerably more space than other designs of equal volume and pressure ● Large wheel width ● Not to be used in dusty environments or where sticky or stringy materials are used because residues adhering to the blade surface cause imbalance and eventual bearing failure 264 Fans, Blowers, and Fluidizers Forward-Curved Blades This design is commonly referred to as a squirrel-cage fan. The unit has a wheel with a large number of wide, shallow blades; a very large intake area relative to the wheel diameter; and a relatively slow operational speed. The advantages of forward-curved blades include: ● Excellent for any volume at low to moderate static pressure using clean air ● Occupies approximately same space as backward-curved blade fan ● More efficient and much quieter during operation than propeller fans for static pressures above approximately one inch of water (gauge) The limitations of forward-curved blades include: ● Not as efficient as backward-curved blade fans ● Should not be used in dusty environments or handle sticky or stringy materials that could adhere to the blade surface ● BHP increases as this fan approaches maximum volume, as opposed to backward-curved blade centrifugal fans, which experience a decrease in BHP as they approach maximum volume Radial Blades Industrial exhaust fans fall into this category. The design is rugged and may be belt-driven or directly driven by a motor. The blade shape varies consid- erably from flat surfaces to various bent configurations to increase efficiency slightly or to suit particular applications. The advantages of radial-blade fans include: ● Best suited for severe duty, especially when fitted with flat radial blades ● Simple construction that lends itself to easy field maintenance ● Highly versatile industrial fan that can be used in extremely dusty environments as well as clean air ● Appropriate for high-temperature service ● Handles corrosive or abrasive materials Fans, Blowers, and Fluidizers 265 The limitations of radial-blade fans include: ● Lowest efficiency in centrifugal-fan group ● Highest sound level in centrifugal-fan group ● BHP increases as fan approaches maximum volume Performance A fan is inherently a constant-volume machine. It operates at the same vol- umetric flow rate (i.e., cubic feet per minute) when operating in a fixed system at a constant speed, regardless of changes in air density. However, the pressure developed and the horsepower required varies directly with the air density. The following factors affect centrifugal-fan performance: brake horsepower, fan capacity, fan rating, outlet velocity, static efficiency, static pressure, tip speed, mechanical efficiency, total pressure, velocity pressure, natural frequency, and suction conditions. Some of these factors are used in the mathematical relationships that are referred to as Fan Laws. Brake Horsepower Brake horsepower (BHP) is the power input required by the fan shaft to produce the required volumetric flow rate (cfm) and pressure. Fan Capacity The fan capacity (FC) is the volume of air moved per minute by the fan (cfm). Note: the density of air is 0.075 pounds per cubic foot at atmospheric pressure and 68 ◦ F. Fan Rating The fan rating predicts the fan’s performance at one operating condition, which includes the fan size, speed, capacity, pressure, and horsepower. Outlet Velocity The outlet velocity (OV, feet per minute) is the number of cubic feet of gas moved by the fan per minute divided by the inside area of the fan outlet, or discharge area, in square feet. 266 Fans, Blowers, and Fluidizers Static Efficiency Static efficiency (SE) is not the true mechanical efficiency, but it is convenient to use in comparing fans. This is calculated by the following equation: Static Efficiency (SE) = 0.000157 ×FC × SP BHP Static Pressure Static pressure (SP) generated by the fan can exist whether the air is in motion or is trapped in a confined space. SP is always expressed in inches of water (gauge). Tip Speed The tip speed (TS) is the peripheral speed of the fan wheel in feet per minute (fpm). Tip Speed = Rotor Diameter × π × rpm Mechanical Efficiency True mechanical efficiency (ME) is equal to the total input power divided by the total output power. Total Pressure Total pressure (TP), inches of water (gauge) is the sum of the velocity pressure and static pressure. Velocity Pressure Velocity pressure (VP) is produced by the fan when the air is moving. Air having a velocity of 4,000 fpm exerts a pressure of one inch of water (gauge) on a stationary object in its flow path. Natural Frequency General-purpose fans are designed to operate below their first natural fre- quency. In most cases, the fan vendor will design the rotor-support system so that the rotating element’s first critical speed is between 10 and 15% above the rated running speed. While this practice is questionable, it is acceptable if the design speed and rotating-element mass are maintained. However, if either of these two factors changes, there is a high probability that serious damage or premature failure will result. Fans, Blowers, and Fluidizers 267 Inlet-Air Conditions As with centrifugal pumps, fans require stable inlet conditions. Ductwork should be configured to ensure an adequate volume of clean air or gas, stable inlet pressure, and laminar flow. If the supply air is extracted from the environment, it is subject to variations in moisture, dirt content, barometric pressure, and density. However, these variables should be controlled as much as possible. As a minimum, inlet filters should be installed to minimize the amount of dirt and moisture that enters the fan. Excessive moisture and particulates have an extremely negative impact on fan performance and cause two major problems: abrasion or tip wear and plate-out. High concentrations of particulate matter in the inlet air act as abrasives that accelerate fan-rotor wear. In most cases, however, this wear is restricted to the high-velocity areas of the rotor, such as the vane or blade tips, but can affect the entire assembly. Plate-out is a much more serious problem. The combination of particulates and moisture can form “glue” that binds to the rotor assembly. As this con- tamination builds up on the rotor, the assembly’s mass increases, which reduces its natural frequency. If enough plate-out occurs, the fan’s rota- tional speed may coincide with the rotor’s reduced natural frequency. With a strong energy source like the running speed, excitation of the rotor’s nat- ural frequency can result in catastrophic fan failure. Even if catastrophic failure does not occur, the vibration energy generated by the fan may cause bearing damage. Fan Laws The mathematical relationships referred to as fan laws can be useful when applied to fans operating in a fixed system or to geometrically similar fans. However, caution should be exercised when using these relationships. They only apply to identical fans and applications. The basic laws are: ● Volume in cubic feet per minute (cfm) varies directly with the rotating speed (rpm) ● Static pressure varies with the rotating speed squared (rpm 2 ) ● Brake horsepower (BHP) varies with the speed cubed (rpm 3 ) The fan-performance curves shown in Figures 13.3 and 13.4 show the per- formance of the same fan type designed for different volumetric-flow rates, operating in the same duct system handling air at the same density. 268 Fans, Blowers, and Fluidizers Static pressure, in. wg CFM, thousands SP BHP System resistance Brake horsepower Point of rating 440 RPM 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 2468101214161820 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Figure 13.3 Fan-performance curve 1 CFM, thousands System resistence Brake horsepower Point of rating 528 RPM SP 2.0 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 2 4 6 8 101214161820 BHP Static pressure, in. wg Figure 13.4 Fan-performance curve 2 [...]... 401 422 444 1.10 1 .29 1.50 1.74 2. 01 381 401 420 441 4 62 1.35 1.56 1.78 2. 03 2. 32 4 02 421 440 459 479 1.81 1.83 2. 08 2. 35 2. 65 421 439 458 477 496 1.85 2. 10 2. 37 2. 67 2. 98 438 456 475 494 513 2. 10 2. 37 2. 66 2. 98 3. 32 18776 17708 18640 195 72 20504 1800 1900 20 00 21 00 22 00 467 489 5 12 535 558 2. 31 2. 65 3. 02 3.43 3.87 483 504 526 548 570 2. 63 2. 98 3.36 3.77 4 .23 499 520 541 5 62 584 2. 97 3.33 3. 72 4.45... 5. 02 5 32 551 571 590 610 3.68 4.07 4.49 4.95 5.43 21 436 22 368 23 300 24 2 32 25164 23 00 24 00 25 00 26 00 27 00 5 82 605 628 6 52 576 4.36 4.89 5.46 6.09 6.75 593 616 639 6 62 685 4. 72 5 .26 5.85 6.48 7.15 605 627 650 6 72 695 5. 12 5.67 6 .26 6.90 7.58 618 640 661 683 705 5.54 6.10 6.70 7.34 8.04 631 6 52 673 694 715 5.95 6.54 7.16 7.81 8. 52 26096 28 00 700 7.47 708 7.88 718 8. 32 727 8.78 738 9 .27 27 028 29 00 723 8 .24 ... 8 .24 7 32 8.66 741 9.11 750 9.58 760 10.08 Continued Fans, Blowers, and Fluidizers 27 1 Table 13.1 continued 7/8" SP 1" SP 11/4" SP 11 /2" SP 13/4" SP RPM BHP RPM BHP RPM BHP RPM BHP RPM BHP 3 82 393 406 421 438 1 .27 1.44 1.63 1.84 2. 08 403 413 425 439 454 1.46 1.63 1.83 2. 06 2. 31 444 451 461 473 486 1.85 2. 05 2. 27 2. 51 2. 79 483 488 493 305 517 2. 28 2. 49 2. 72 2.99 3 .29 520 523 529 537 547 2. 73 2. 96 3 .21 3.50... OV 1/4" SP 3/8" SP 1 /2" SP 5/8" SP 3/4" SP RPM BHP RPM BHP RPM BHP RPM BHP RPM BHP 7458 8388 9 320 1 025 2 11184 800 900 1000 1100 120 0 26 2 28 1 199 319 338 0.45 0.55 0.66 0.79 0.93 28 9 308 325 343 3 62 0.60 0. 72 0.85 1.00 1.17 314 330 347 365 383 0.75 0.89 1.04 1 .21 1.40 337 351 368 385 4 02 0. 92 1.06 1 .23 1. 42 1.63 360 3 72 387 403 420 1.09 1 .25 1.43 1.63 1.85 121 18 13048 13980 149 12 15844 1300 1400 1500... 0.410 0.390 0.3 72 0.354 0.340 0.714 0.676 0. 620 0.573 0.533 0.498 0.467 0.440 0.416 0.394 0.375 0.3 52 0.341 0. 326 0.688 0.651 0.598 0.5 52 0.514 0.480 0.450 0. 424 0.401 0.380 0.361 0.344 0. 328 0.315 0.564 0.534 0.490 0.453 0. 421 0.393 0.369 0.347 0. 328 0.311 0 .29 6 0 .28 2 0 .26 9 0 .25 8 0.460 0.435 0.400 0.369 0.344 0. 321 0.301 0 .28 3 0 .26 8 0 .25 4 0 .24 2 0 .23 0 0 .21 9 0 .21 0 and temperature) directly affecting the... humidity, Table 13 .2 Air-density ratios Altitude, ft above sea level 0 Air temp., 1,000 2, 000 3,000 4,000 5,000 Barometric pressure, in mercury ◦F 29 . 92 28.86 27 . 82 26. 82 25.84 24 .90 70 100 150 20 0 25 0 300 350 400 450 500 550 600 650 700 1.000 0.946 0.869 0.803 0.747 0.697 0.654 0.616 0.5 82 0.5 52 0. 525 0.500 0.477 0.457 0.964 0.9 12 0.838 0.774 0. 720 0.6 72 0.631 0.594 0.561 0.5 32 0.506 0.4 82 0.460 0.441... 10,000 15,000 20 ,000 Barometric pressure, in mercury ◦F 23 .98 23 .09 22 .22 21 .39 20 .58 16.89 13.75 70 100 150 20 0 25 0 300 350 400 450 500 550 600 650 700 0.801 0.758 0.696 0.643 0.598 0.558 0. 524 0.493 0.466 0.4 42 0. 421 0.400 0.3 82 0.366 0.7 72 0.730 0.671 0. 620 0.576 0.538 0.505 0.476 0.449 0. 426 0.405 0.386 0.368 0.353 0.743 0.703 0.646 0.596 0.555 0.518 0.486 0.458 0.433 0.410 0.390 0.3 72 0.354 0.340... 528 2. 34 2. 63 2. 94 3 .28 3.64 471 488 506 524 5 42 2.55 2. 90 3 .23 3.56 3.97 501 517 534 5 52 570 3.09 3.43 3.75 4.19 4.61 531 545 581 578 395 3. 62 3.98 4.37 4.79 5 .25 359 5 72 587 603 619 4.16 4.54 4.96 5.41 5.80 547 556 585 604 624 4.03 4.45 4.89 5.36 5.87 561 580 599 618 637 4.38 4.81 5 .28 5.78 6.30 588 606 625 644 663 5.06 5.54 6.05 6.59 7.16 613 631 649 668 686 5.74 6 .25 6.81 7.40 8.01 637 654 6 72. .. 0.747 0.694 0.648 0.608 0.573 0.5 42 0.513 0.488 0.465 0.444 0. 425 0.896 0.848 0.770 0. 720 0.669 0. 624 0.586 0.5 52 0. 522 0.495 0.470 0.448 0. 427 0.410 0.864 0.818 0.751 0.694 0.645 0.604 0.565 0.5 32 0.503 0.477 0.454 0.4 32 0.4 12 0.395 0.8 32 0.787 0. 723 0.668 0. 622 0.580 0.544 0.513 0.484 0.459 0.437 0.416 0.397 0.380 Continued Fans, Blowers, and Fluidizers 27 3 Table 13 .2 continued Altitude, ft above sea... 7.40 8.01 637 654 6 72 690 708 6. 42 6.98 7.57 8.19 8.85 644 664 685 706 727 6.41 6.99 7.63 8.30 9.01 657 677 687 717 738 6.86 7.46 8.10 8.77 9.53 6 82 701 721 740 760 7.77 6.41 9.09 9.80 10.58 705 724 743 7 62 7 82 8.65 9.35 10.07 10.83 11.63 727 746 765 784 803 9.54 10 .27 11.04 11.84 12. 69 748 770 9.78 10.60 759 780 10.30 11.13 780 800 11.35 12. 20 801 821 12. 46 13.35 822 841 13.57 14.49 most of the fabricated . 2. 01 4 62 2. 32 479 2. 65 496 2. 98 513 3. 32 18776 1800 467 2. 31 483 2. 63 499 2. 97 516 3.33 5 32 3.68 17708 1900 489 2. 65 504 2. 98 520 3.33 536 3.70 551 4.07 18640 20 00 5 12 3. 02 526 3.36 541 3. 72 556. 438 2. 10 13048 1400 379 1 .29 401 1.56 421 1.83 439 2. 10 456 2. 37 13980 1500 401 1.50 420 1.78 440 2. 08 458 2. 37 475 2. 66 149 12 1600 422 1.74 441 2. 03 459 2. 35 477 2. 67 494 2. 98 15844 1700 444 2. 01. 4.49 195 72 2100 535 3.43 548 3.77 5 62 4.45 576 4.53 590 4.95 20 504 22 00 558 3.87 570 4 .23 584 4.61 597 5. 02 610 5.43 21 436 23 00 5 82 4.36 593 4. 72 605 5. 12 618 5.54 631 5.95 22 368 24 00 605 4.89 616 5 .26