Handbook of Mechanical Engineering Calculations P10 doc

SECTION 10 MATERIALS HANDLINGChoosing Conveyors and Elevators for Specific Materials Transported 10.1 Determining Equipment Design Parameters for Overhead Conveyors CHOOSING CONVEYORS AND

SECTION 10 MATERIALS HANDLING

Choosing Conveyors and Elevators for

Specific Materials Transported 10.1

Determining Equipment Design

Parameters for Overhead Conveyors

CHOOSING CONVEYORS AND ELEVATORS FOR

SPECIFIC MATERIALS TRANSPORTED

Determine the maximum allowable product weight between supports that can behandled by a belt conveyor at any one time when it conveys 100,000 lb / h (4540

kg / h) of pulverized aluminum oxide in an abrasive state at a belt speed of 50 ft /min (15.2 m / min) with a center-to-center distance of 32 ft (9.75 m) between beltsupports Compare this capacity with that at belt speeds of 150, 250, and 350 fpm(45.7, 76.2, and 106.7 m / min) Choose the type of conveyor and elevator to handlethis material under the conditions given

Calculation Procedure:

1. Find the maximum allowable product weight at the given belt speed

2. Determine the maximum allowable product weight at other belt speeds

Typical conveyor belt speeds vary from a low of 150 ft / min (45.7 m / min) to ahigh 800 ft / min (243.8 m / min), depending on belt width, type of material con-veyed, belt construction, etc

lb (161.4 kg) when the speed is 150 ft / min (45.7 m / min) Likewise for the two

3. Verify the type of conveyor and elevator to use

With such a wide variety of conveyors and elevators to choose from, it is wise toverify the choice before a final decision is made Table 1, presented by Harold V

Hawkins, Manager, Product Standards and Services, Columbus McKinnon ration, lists preferred types of conveyors and elevators for a variety of materials inboth bulk and packaged forms Entering this table at aluminum oxide in pulverizedform shows that a belt or screw conveyor is preferred, while a flight elevator isrecommended for vertical lifts of this material.

Corpo-While Table 1 gives general recommendations, the engineer should rememberthat a careful economic study is required to keep the capital investment to theminimum consistent with safe and dependable conveying of the material Alongwith capital cost, the operating and maintenance costs must also be evaluated before

a final choice of the conveyor and elevator is made

Related Calculations. Choose the belt length to accommodate the maximumexpected product capacity Belt speed should be compatible with the process equip-ment served and with the other materials-handling equipment associated with theconveyor belt

Belt conveyors are suitable for bulk materials of many types However, acteristics of the material conveyed must be considered before a final choice of the

char-belting material is made Thus, as outlined by K W Tunnell Company: (a) Material

stickiness may prevent materials handled from discharging completely from the

conveyor belt, or may interfere with the belt drive components: motors, chains, etc

dam-age to the belt materials (c) Chemical reactions between the conveyed product and

the belt material can cause damage Thus, oils, chemicals, fats, and acids can

dam-age belts (d) Excessively large lump size may require an oversize belt system to

handle the conveyed product safely

One way around these problems is use of metal-belt conveyors These are similar

in design to conventional rubber and composite-material conveyor belts except thattheir surface is made of woven or solid metal Popular materials include carbonsteel, galvanized steel, stainless steel, and other metals and alloys

With today’s emphasis on environmental and safety aspects of engineering cisions, it is wise for the design engineer to refer to the appropriate codes andspecifications governing the particular type of equipment being considered Thus,

de-in the materials handlde-ing field, ANSI B 20.1 ‘‘Conveyors, Cableways, and RelatedEquipment’’ should be consulted before any final design choices are made

Likewise, state and city codes should be checked before a firm equipment lection decision In certain instances the local code may be more restrictive thanthe national code OSHA—Occupational Safety and Health Administration—regulations are important where human safety is involved Since these regulationsvary so widely with material handled, type of equipment used, and location, nogeneralizations about them can be made other than to recommend strongly that theregulations be studied and followed

se-DETERMINING EQUIPMENT DESIGN

PARAMETERS FOR OVERHEAD CONVEYORS

Select suitable equipment for the overhead conveyor shown in Fig 1 Determinethe total chain pull and horsepower required if the conveyor is 700 ft (213 m) long,the coefficient of friction is 0.03, the total chin pull is 60 lb / ft (89.4 kg / m) com-prised of the components detailed below Use the design method presented the

K W Tunnell Company

FIGURE 1 (a) Plan of conveyor layout (b) Elevation of conveyor layout.

2. Determine the path of the conveyor on a scaled plant layout

Draw a plan and elevation of the conveyor, Fig 1, on a scaled layout of the plant.Show all obstructions the conveyor will encounter, such as columns, walls, ma-

FIGURE 2 Clearance design for overhead conveyor turns and rises.

chinery, and work aisles Indicate the loading and unloading zones, probable drivelocation, and passage through walls

3. Develop a vertical elevation to determine incline and decline dimensions

Show the inclines and declines, and their dimensions, Fig 1b A three-dimensionalview of the installation can be prepared at this point to help people better visualizethe final installation and the various routes of the conveyor

4. Determine the material movement rate, unit load size, spacing, and carrier design for the conveyor

Information for these variables can be obtained from the flow chart and the sonnel in charge of the process being served by the conveyor It is important thatthe conveyor be designed for the maximum anticipated load and material size

per-5. Modify turn radii to provide adequate clearances

Prepare drawings showing needed load spacing on turns, Fig 2 Without adequateclearnaces, the conveyor may not provide the desired transportation capabilityneeded to serve properly the process for which the conveyor is being designed

6. Design the load spacing for clearances on inclines and declines

As inclines and declines get steeper, Fig 2 load spacing has to be increased toprovide a constant clearance or separation between loads Table 2 gives selectedclearances on inclined track for overhead conveyors for a given separation at variousincline angles

7. Redraw the conveyor path and vertical elevation views using newly mined radii and incline information

deter-Show the new radii and incline information as determined by the redesign of thesystem layout, Figs 1 and 2

8. Compute the chain pull in the conveyor

The chain pull is the total weight of the chain, trolleys, Fig 3, and other nents, plus the weight of the carriers and load Thus, for the given system, the

28.6 38.7 43.2 57.5

26.4 35.2 39.7 53.0

23.5 31.1 35.2 46.7

19.7 26.4 28.9 39.4

15.2 20.3 22.9 30.5

The given chain load of 60 lb / ft (89.4 kg / m) is comprised of 10.0 lb / ft (14.9

kg / m) for the chain and trolleys, 12.5 lb / ft (18.6 kg / m) for the carriers, and 37.5

(572 kg)

For this initial calculation, inclines and declines are assumed to be level sections

if the number of declines balances out the number of inclines However, for eachadditional incline, the total line load rise has to be added to determine the totalchain pull If, for example, a vertical incline in this installation raises the line load

9. Select the tenative conveyor size based on the trolley load and chain pull

Use the manufacturer’s data to choose the tenative conveyor size In making yourchoice, try to comform to standard conveyor sizes and layouts because this willreduce the capital cost of the installation Further, the installation will probably bemade faster because there will be less customizing required

10. Select vertical curve radii

Again, work with the standard radii available from the manufacturer, if possible.This will reduce installation costs and time

11. Determine the conveyor power requirements and drive locations

Make point-to-point calculations of the chain pull around the complete path of theconveyor, Fig 1 Use the following equations to compute point-to-point chain pull:

FIGURE 3 Power- and free-trolley overhead conveyors.

conveyor is traveling up the curve; minus when conveyor is traveling down the

lb)(maximum speed, ft / min) / 0.6(33,000) Thus, if the drive capacity required is

12. Design the conveyor supports and superstructures

Refer to the manufacturer’s data for suitable supports and superstructures It is best,

if possible, to use standard supports and superstructures This will save money andtime for the firm owning the plant being fitted with the conveyor

13. Design guards required by laws and codes

Federal, state, and applicable codes require guards of various types under hightrolley runs, particularly over aisles and work areas Guard panels are normally

over-be most helpful to the engineer in achieving an economical and safe design for theinstallation being considered The steps, illustrations, and table in this procedureare the work of the K W Tunnell Company SI values were added by the handbookeditor.

BULK MATERIAL ELEVATOR AND

CONVEYOR SELECTION

Choose a bucket elevator to handle 150 tons / h (136.1 t / h) of abrasive material

a speed of 100 ft / min (30.5 m / min) What hp input is required to drive the elevator?The bucket elevator discharges onto a horizontal conveyor which must transport thematerial 1400 ft (426.7 m) Choose the type of conveyor to use, and determine therequired power input needed to drive it

Calculation Procedure:

1. Select the type of elevator to use

Table 3 summarizes the various characteristics of bucket elevators used to transportbulk materials vertically This table shows that a continuous bucket elevator would

be a good choice, because it is a recommended type for abrasive materials Thesecond choice would be a pivoted bucket elevator However, the continuous buckettype is popular and will be chosen for this application

2. Compute the elevator height

To allow for satisfactory loading of the bulk material, the elevator length is usuallyincreased by about 5 ft (1.5 m) more than the vertical lift Hence, the elevator

3. Compute the required power input to the elevator

kW)

The power input relation given above is valid for continuous-bucket, discharge, perfect-discharge, and super-capacity elevators A 25-hp (18.7-kW) mo-tor would probably be chosen for this elevator

centrifugal-TABLE 3 Bucket Elevators

4. Select the type of conveyor to use

Since the elevator discharges onto the conveyor, the capacity of the conveyor should

be the same, per unit time, as the elevator Table 4 lists the characteristics of varioustypes of conveyors Study of the tabulation shows that a belt conveyor would prob-ably be best for this application, based on the speed, capacity, and type of material

it can handle Hence, it will be chosen for this installation

5. Compute the required power input to the conveyor

The power input to a conveyor is composed of two portions: the power required tomove the empty belt conveyor and the power required to move the load horizontally.Determine from Fig 4 the power required to move the empty belt conveyor,after choosing the required belt width Determine the belt width from Table 5.Thus, for this conveyor, Table 5 shows that a belt width of 42 in (106.7 cm) isrequired to transport up to 150 tons / h (136.1 t / h) at a belt speed of 100 ft / min

(30.5 m / min) [Note that the next larger capacity, 162 tons / h (146.9 t / h), is used

when the exact capacity required is not tabulated.] Find the horsepower required todrive the empty belt by entering Fig 4 at the belt distance between centers, 1400

ft (426.7 m), and projecting vertically upward to the belt width, 42 in (106.7 cm)

At the left, read the required power input as 7.2 hp (5.4 kW)

6.83 hp (5.1 kW) Hence, the total horsepower to drive this horizontal conveyor is

The total horsepower input to this conveyor installation is the sum of the elevator

TABLE 5 Capacities of Troughed Rest [tons / h (t / h) with Belt Speed of

100 ft / min (30.5 m / min)]

TABLE 6 Minimum Belt Width for Lumps

FIGURE 4 Horsepower (kilowatts) required to move an empty conveyor belt at 100 ft / min (30.5 m / min).

MATERIALS HANDLING 10.13

TABLE 7 Maximum Belt Speeds for Various Materials

Related Calculations: The procedure given here is valid for conveyors usingrubber belts reinforced with cotton duck, open-mesh fabric, cords, or steel wires

It is also valid for stitched-canvas belts, balata belts, and flat-steel belts The quired horsepower input includes any power adsorbed by idler pulleys

re-Table 5 shows the minimum recommended belt widths for lumpy materials ofvarious sizes Maximum recommended belt speeds for various materials are shown

(1524-SCREW CONVEYOR POWER INPUT

AND CAPACITY

What is the required power input for a 100-ft (30.5-m) long screw conveyor

conveyor capacity is 30 tons / h (27.2 t / h)?

Calculation Procedure:

1. Select the conveyor diameter and speed

Refer to a manufacturer’s engineering data or Table 8 for a listing of recommendedscrew conveyor diameters and speeds for various types of materials Dry coal ashesare commonly rated as group 3 materials, Table 9, i.e., materials with small mixedlumps with fines

To determine a suitable screw diameter, assume two typical values and obtainthe recommended rpm from the sources listed above or Table 8 Thus, the maximumrpm recommended for a 6-in (152.4-mm) screw when handling group 3 material is

TABLE 8 Screw Conveyor Capacities and Speeds

TABLE 9 Material Factors for Screw Conveyors

90, as shown in Table 8; for a 20-in (508.0-mm) screw, 60 r / min Assume a 6-in(152.4-mm) screw as a trial diameter

2. Determine the material factor for the conveyor

A material factor is used in the screw conveyor power input computation to allowfor the character of the substance handled Table 9 lists the material factor for dry

3. Determine the conveyor size factor

A size factor that is a function of the conveyor diameter is also used in the powerinput computation Table 10 shows that for a 6-in (152.4-mm) diameter conveyor

4. Compute the required power input to the conveyor

MATERIALS HANDLING 10.15

TABLE 10 Screw Conveyor Size Factors

kW) With a 90 percent motor efficiency, the required motor rating would be

this conveyor Since this is not an excessive power input, the 6-in (152.4-mm)conveyor is suitable for this application

If the calculation indicates that an excessively large power input, say 50 hp (37.3kW) or more, is required, then the larger-diameter conveyor should be analyzed Ingeneral, a higher initial investment in conveyor size that reduces the power inputwill be more than recovered by the savings in power costs

Related Calculations. Use the procedure given here for screw or spiral veyors and feeders handling any material that will flow The usual screw or spiralconveyor is suitable for conveying materials for distances up to about 200 ft (60.9m), although special designs can be built for greater distances Conveyors of this

capacity of the conveyor decreases as the angle of inclination is increased Thus

h) of various classes of materials at the maximum recommended shaft rpm As thesize of the lumps in the material conveyed increases, the recommended shaft rpmdecreases The capacity of a screw or spiral conveyor at a lower speed is found

Table 8 shows typical screw conveyor capacities at usual operating speeds

Various types of screws are used for modern conveyors These include pitch, variable-pitch, cut flights, ribbon, and paddle screws The procedure givenabove also applies to these screws

short-DESIGN AND LAYOUT OF PNEUMATIC

CONVEYING SYSTEMS

A pneumatic conveying system for handling solids in an industrial exhaust lation contains two grinding-wheel booths and one lead each for a planer, sander,and circular saw Determine the required duct sizes, resistance, and fan capacity forthis pneumatic conveying system

instal-Calculation Procedure:

1. Sketch the proposed exhaust system

Make a freehand sketch, Fig 5 of the proposed system Show the main and branchducts and the booths and hoods Indicate all major structural interferences, such as