12.2 Compressed Air Treatment, Storage, and Distribution

Moisture is the enemy

Atmospheric air carries water vapor. Compressing it concentrates that water into a much smaller volume, and cooling it downstream condenses the water out — inside the piping, inside the tools, and inside the control valves, unless the system is designed to remove it first.

The treatment train, in the order the air meets it:

Pressure dew point

The specification that matters is pressure dew point: the temperature at which water begins to condense out of the air at system pressure. The rule is simple — the pressure dew point must be below the lowest temperature the air line will ever see.

A refrigerated dryer cannot protect an outdoor air line in winter, because its dew point is above freezing. That mismatch is the cause of most frozen air line calls.

Filter placement

A coalescing filter goes upstream of the dryer, to keep oil aerosol out of the desiccant bed or refrigerated heat exchanger where it would foul the media. A particulate afterfilter goes downstream of a desiccant dryer, to catch attrition fines from the desiccant. Filters are changed on differential pressure, not on the calendar alone; a plugged filter is a permanent energy cost in the form of pressure drop.

Receivers and drains

An air receiver does four things: stores air for peak demands, damps the pulsation from a reciprocating compressor, provides residence time for condensate to fall out, and gives the control system enough volume that the compressor does not cycle constantly.

Receiver requirements a mechanic must know:

• The receiver is a pressure vessel and carries an ASME code stamp and nameplate.
• It must have a pressure relief valve sized for the compressor's full capacity, set no higher than the vessel's maximum allowable working pressure, and it must not be isolated from the vessel by any valve.
• It must have a drain at the lowest point and a pressure gauge.
• Drain it. A receiver half full of water has lost its storage volume and is corroding from the inside.

Distribution

• Lay out the main as a loop header where practical. A loop feeds every drop from two directions, halving the effective length and the pressure drop, and it lets sections be isolated for repair.
• Size the pipe generously. A common design target is no more than about 2 psi total pressure drop from the receiver to the point of use. Undersized pipe is paid for every hour the plant runs.
• Pitch the header toward drain points and fit drip legs with drains at the low points and the ends of runs.
• Take drops from the top of the header and bring them over in a gooseneck before turning down. Condensate lies along the bottom of the pipe; a bottom takeoff delivers it straight to the tool.
• Put a shutoff valve, filter, regulator, and lubricator (where lubrication is wanted) at the point of use, and a drain at the bottom of each drop.
• Reduce system pressure to what the equipment actually needs. A widely used rule of thumb is that each 2 psi of reduction in system pressure saves roughly 1 percent of compressor energy.

Leaks

Leaks are the single largest avoidable loss in a compressed air system, and in poorly maintained plants they commonly account for 20 to 30 percent of total compressor output.

Leak rates rise steeply with hole size, because flow area goes with the square of the diameter. Published free-air leakage at 100 psig gives a sense of scale:

A single 1/8-inch leak is a meaningful fraction of a small compressor's entire output, running 8,760 hours a year.

Finding leaks. During production the plant is far too noisy for the ear. An ultrasonic leak detector hears the high-frequency turbulence of a leak through background noise and is the standard tool. Soap solution confirms a suspected leak. A shutdown test — pressurizing the system with production stopped and measuring how long the compressor runs to hold pressure — quantifies total system leakage.

Safety rules for compressed air

• Never use compressed air to clean skin or clothing. Air can be driven into the bloodstream or into a body cavity with fatal results.
• OSHA 29 CFR 1910.242(b) permits compressed air for cleaning only when it is reduced to less than 30 psi, and then only with effective chip guarding and personal protective equipment.
• Depressurize and bleed a line before breaking any connection. Use safety clips or retainers at hose couplings so a parted hose cannot whip.
• Never exceed the pressure rating of a hose, fitting, or receiver, and never repair a receiver by welding without a qualified engineering assessment.