8.5 Vacuum Systems, Pumps & Venturi Generators

Task E-22 covers pneumatic and vacuum systems, 9 of the 135 exam questions. Millwrights service vacuum on packaging lines, plywood and veneer presses, dust collection, degassing systems, vacuum lifters and pick-and-place tooling.

Measuring Vacuum

Vacuum is not a separate quantity — it is pressure below atmospheric. The confusion comes from the three scales in common use:

Worked conversion. A gauge reads 20 in Hg of vacuum at sea level. As a percentage that is 20 / 29.92 = 66.8%, and the absolute pressure remaining is (1 - 0.668) x 101.3 = 33.6 kPa absolute.

The hard ceiling: because vacuum works by removing pressure rather than adding it, the maximum differential available at sea level is only about 101 kPa (14.7 psi). There is no way to get more, and altitude reduces it further. Every vacuum-lifting calculation starts from that limit.

Vacuum Pump Types

Gas ballast on an oil-sealed rotary vane pump admits a controlled bleed of atmospheric air into the compression stage so that condensable vapour — usually water — stays as vapour and is exhausted rather than condensing into the oil. Running a pump on a wet process without gas ballast turns the oil milky, destroys its sealing ability and ruins the pump. If oil looks milky, run the pump with ballast open and the intake blanked until it clears, then change the oil.

Venturi Vacuum Generators

         VENTURI VACUUM GENERATOR (EJECTOR)

   compressed air  ==>  [ nozzle ]>>>>[ diffuser ] ==> exhaust (silencer)
                              ||
                              || entrained air
                              ||
                        vacuum port  <-- to cup / tooling

Compressed air accelerates through a nozzle, and the resulting low-pressure region draws air in through the vacuum port. Advantages: no moving parts, instant vacuum and instant release, tolerant of duty cycling, and mountable right at the tooling to minimize evacuation time.

The drawback is air consumption. A generator running continuously on an idle machine wastes large volumes of compressed air. Best practice is an air-saving circuit: a vacuum switch senses that the set vacuum level is reached, a non-return (check) valve holds the vacuum in the sealed volume, and the generator shuts off until the level decays. A blow-off pulse of positive air then releases the part instantly.

Vacuum Lifting

Vacuum cup capacity follows the same force-equals-pressure-times-area relationship as hydraulics, with the differential capped at atmospheric pressure.

$F = \Delta P \times A \times n$

Worked example. Four cups of 150 mm diameter operate at 70 kPa of vacuum.

$A = 0.7854 \times 150^2 = 17{,}671\text{ mm}^2 = 0.01767\text{ m}^2$ $F = 70{,}000\text{ Pa} \times 0.01767 \times 4 = 4{,}948\text{ N} \approx 504\text{ kg}$

That is the theoretical holding force. A vacuum lifter must then apply the manufacturer's and the regulation's safety factor — commonly at least 2:1 for horizontal lifting and higher for vertical or tilting handling — plus allowances for surface porosity, contamination, temperature and acceleration forces. Any vacuum lifting device used overhead requires a vacuum reservoir or a fail-safe backup and an audible low-vacuum alarm, so a loss of power or supply air does not drop the load.

Filtration, Moisture and Maintenance

• Inlet filtration is mandatory: any dust, fibre or liquid drawn into a vacuum pump will damage vanes, seals or rotors. Check and change inlet filters on a schedule and after any process upset.
• Exhaust mist filters on oil-sealed pumps capture oil aerosol and return it to the sump; a plugged exhaust filter causes high back-pressure, oil carryover and pump overheating.
• Moisture separators and knockout pots protect the pump from condensate.
• Oil condition on an oil-sealed pump: change on schedule and immediately whenever it turns dark, milky or carries an odour.
• Vacuum hose and tubing must be rated for vacuum — ordinary flexible tubing collapses. Reinforced hose or rigid tube is required.
• Leak-check with a vacuum decay test: isolate the system, record the vacuum, and time the rate of rise. A rapid rise means leakage; a slow rise means outgassing.

Troubleshooting

Diagnostic method: blank the pump inlet and measure the vacuum the pump alone can achieve. If it reaches specification at the inlet, the pump is good and the fault is in the system — then isolate and blank sections progressively to find the leak. If the blanked pump cannot reach specification, the pump itself needs service.

Safety: never place a hand or any body part over an open vacuum port or cup — a large cup at full vacuum can hold hard enough to cause serious injury. Vacuum vessels can implode: they must be rated for external pressure, and an atmospheric tank is never used as a vacuum receiver.