6.9 Pressure losses in compressed-air pipes

In practice, it is often necessary to determine the dimensions of a compressed-air pipe in dependence on the volume flow of the compressor; and the pressure losses to be expected between compressor and consuming unit of compressed air have to be calculated.

It is difficult to determine this pressure loss on the basis of formulas.

It is easier to determine the pressure loss with the help of tables.

1st Step

At first, the total length of the pipe L_tot has to be determined.

This one is composed of the straight length of the pipe L_straight

and of the equivalent length of the pipe L_equivalent.

L_equivalent stands for built in fittings, valves, bends, T-pieces, reductions, etc

Ill.: Pressure drops via valves and fittings

The equivalent length of the pipe L_equivalent is determined for each fitting built in with the help of the table in dependence on the internal diameter and it is expressed in meters. This means, the pressure loss at that fitting is identical to the pressure loss of a straight pipe with equivalent length.

2nd Step

The pressure loss depends on

  - operating overpressure (in b)
  - volume flow (in l/s)
  - total length of the pipe (in m)
  - internal diameter of the pipe

Ill. Pressure losses

Click to enlarge!

On the basis of the second diagram the pressure loss (in b) can be determined, e.g.

operating overpressure = 7 b
volume flow = 100 l/s = 6 m3 / min
total length of pipe = 50 m
internal pipe diameter = 32 mm
From the diagram
pressure loss = 0.42 b
(at an internal pipe diameter of 40 mm the pressure loss decreases to 0.15 b)

Remark:

The pressure losses of a compressed air/abrasive mixture are higher between blasting pot and jet nozzle than the pressure losses calculated only for the transport of compressed air. As a tendency, it can be recognised that the pressure loss is three up to four times higher between a 32 mm blasting hose and a 42 mm blasting hose.