SIZING PNEUMATIC SYSTEMS. Introduction. Pneumatic systems are sized to meet output power requirements. The air distribution system is sized to carry the required air flow with minimum friction losses through sections of pipe and various fittings.
Pressure is defined as the force exerted by a fluid per unit area.
Units in SI are Pa=N/m2. The pressure unit Pascal is too small for pressure encountered in practice.
Therefore, kPa and MPa are commonly used.
Units in British are : psf = lbf/ft2, psi = lbf/in2
You have to convert from psi to psf ( 144 in2 = 1 ft2)
Absolute pressure, is measured relative to absolute vacuum (i.e., absolute zero pressure.)
Gauge pressure, is measured relative to atmospheric pressure
PNEUMATIC DISTRIBUTION SYSTEMS
Air systems are plumbed to minimize losses between the receiver located near the compressor and the point of use.
A loss of 10% pressure is allowable under normal operating conditions with less than half the loss being attributed to the main transmission line.
Loop systems provide air through more than one path to the air drops, thereby reducing run length restrictions.
Where loops are particularly long, installing more than one compressor at convenient locations reduces the run length and resulting pressure drop.
Provision also is made to pitch air lines slightly from 0.1- .25 in./ft, to collect moisture accumulation in lines at drain points where water can be removed periodically.
VALUES FOR d 5.31 FOR SCHEDULE
40 PIPE SIZES
The friction loss through fittings is determined for each size and type of fitting experimentally, and expressed as an equivalent length of straight pipe of the same size.
Air compressors are sized to supply present equipment, with a 25%-50% capacity built in for future expansion.
First the pressure range is selected , usually 80 to 140 lbf/in ", Then the free air demands of all tools and equipment using the air are totaled-both the continuous demand and the average air demand.
The continuous demand is the amount of air required if all the air tools and equipment were operated continuously; whereas the average demand is the free air consumption rate multiplied by the percentage of time the equipment will be in use.
The compressor cannot be sized below the average value of the air consumption because, even with a large receiver supplying air during peak demands, the compressor could not recover.
Piston displacement does provide information about the size of the unit, but because volumetric efficiency decreases both with pressure and speed, it does not give an accurate estimate of the available delivery from the compressor.
For normal use , the compressor can be sized from the average demand, which is 30% to 60% of the continuous demand, depending upon the range of operating pressure.
If the compressor is running and delivering an input to the receiver (Qc), the formula becomes
Increasing the size of the reservoir by 25% for unexpected overload and 25% for future expansion of production capacity is common.