Sizing a Pneumatic System

The capacity and the horsepower requirements of pneumatic conveyors depend on eight factors:

1. The horizontal distance that the material is moved.

2. The diameter of the pipe.

3. The vertical distance that the material is moved.

4. The number of bends in the pipe.

5. The elevation above sea level.

6. The temperature of the outside air.

7. The type of material being conveyed

8. The moisture content of the material.

To size a pneumatic system the effect of each of these factors must be accounted for.

Table 20.3 is an example of the type of information that is available for sizing pneumatic systems. The data show that as the horizontal length (ft) of the pipe

Table 20.3. Pressure system conveyor capacities for dry corn (bu/hr).

Equivalent Length (ft)

Table 20.3. Pressure system conveyor capacities for dry corn (bu/hr).

Equivalent Length (ft)

Pipe size (in)

Hp

50

100

200

250

300

350

3

10

450

400

350

320

295

265

4

15

650

575

500

460

420

380

5

25

1100

1000

900

830

755

685

6

40

2100

1950

1800

1650

1510

1370

8

75

4300

3900

3500

3220

2940

2660

10

100

5800

5100

4500

4240

3980

3720

Modifications to Capacity:

1. Vertical pipe x 1.20 = Equivalent horizontal length.

3. Capacity in soybeans = 80% of Corn.

4. Add 20 ft of equivalent horizontal pipe for each 90° bend in pipe.

5. Reduce capacity by 4.0% for each 1000 ft above sea level.

6. Reduce capacity by 2.0% for each 10°F above 70°F.

Modifications to Capacity:

1. Vertical pipe x 1.20 = Equivalent horizontal length.

3. Capacity in soybeans = 80% of Corn.

4. Add 20 ft of equivalent horizontal pipe for each 90° bend in pipe.

5. Reduce capacity by 4.0% for each 1000 ft above sea level.

6. Reduce capacity by 2.0% for each 10°F above 70°F.

Source: Beard Industries, Frankfort, Indiana 46041.

increases the capacity of the system (bu/hr) decreases and as the pipe diameter increases (in) the capacity increases.

To provide an easy way to estimate the amount of reduction caused by factors 3 and 4, the equivalent feet of pipe is determined. Equivalent feet of pipe is a procedure that uses constants to determine the amount of horizontal pipe that will cause the same reduction in capacity as the factor in question. For example, the fourth modification to capacity listed in Table 20.3 shows that each foot of vertical pipe causes the same reduction in capacity as 1.2 ft of horizontal pipe. Thus the equivalent feet for a vertical section of pipe is determined by multiplying the vertical length (ft) by 1.2.

Adjustments for factors 5 through 8 are handled differently. The capacity of the system is adjusted for the effect of factors 5, 6, and 7 by reducing the capacity and appropriate percentage. Factor 8 is accounted for by either assuming a standard moisture content, or developing a separate table for a range of moistures. Expressed as an equation:

where: TEF = Total equivalent feet, FH = Horizontal distance (ft), FB = Equivalent feet for bends, FV = Equivalent feet for vertical sections.

Problem: What is the capacity of the system in Figure 20.4 if the material to be moved is dry soybeans?

Solution: The first step is to determine the total equivalent feet for the pipe. In this example, factors 1, 2, 3, 4, and 7 are used. Note: the last section of pipe does not have a length dimension because the assumption is used that the force of gravity exceeds the resistance of the pipe. Therefore, vertical down sections of pipe are

FIGURE 20.4. Pressurized conveyor for sample problem.

not included.

= ^(250 ft + 10 ft) + bends x 20^) + (20 ft x L2)) = 344 ft

Although the actual length of pipe in Figure 20.4 is 280 ft, because of the bends and the vertical up sections the equivalent distance is 344 ft.

The next step is to use Table 20.3 to determine the capacity of an 8.0 inch duct with an equivalent distance of 344 ft. The closest value is 2660 bu/hr. Note that Table 20.3 gives values for corn. Because the capacity of a pressurized system for soybeans is 80% of the capacity for corn (modification 3), the system capacity in Figure 20.4 is:

bu bu bu

Information to determine the capacities of auger, pneumatic, and bucket conveyor systems for other crops and situations are available from manufacturers and agricultural extension personnel.

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