Siphon Abrasive Delivery

Siphon Abrasive Delivery vs. Direct Pressure Abrasive Delivery

The first dry blasting machines relied on siphon abrasive delivery method which uses a gun to siphon, or pull, the abrasive to the blasting nozzle where it is then boosted in particle velocity and injected into the cabinet. Think of this like using a cue ball to hit the eight ball into the side pocket. While it gets the job done, some energy is lost when the two balls come in contact with each other. Similarly, while siphon delivery equipment is easy to make and simple to operate, it requires higher blasting pressures to achieve maximum abrasive impact velocity.

Dry blasting cabinets utilizing direct pressure abrasive delivery developed later.  These machines use a pressure vessel or pot to pneumatically push the abrasive to the nozzle.  With direct pressure, the abrasive has no delivery weight so it travels faster and faster inside the abrasive hose until it passes out the nozzle office at a much faster speed than siphon or compressed air quantity to orifice speed. To keep most abrasives from exceeding maximum impact velocity, the pressure on direct pressure machines is reduced by 50-percent compared to the siphon delivery method. Because direct pressure has a more focused patter it can create higher frictional heat than siphon systems and delivers abrasive at about twice the speed of siphon delivery methods. Cabinets using direct pressure operate on less compressed air and produce more frictional heat than siphon types.  This allows direct pressure to do things siphon is unable to do. Because direct pressure provides a more focused pattern of abrasive delivery it is better for removing heavy coatings, can push heavy steel shot and grits the siphon system can’t delivery and the direct pressure can push abrasive through drilled holes by using the part as the blasting nozzle. A siphon can’t continue abrasive delivery when the nozzle is held close to the part surface or against a drilled hole…

Why two types of abrasive cabinets? Think of Siphon and Direct Pressure machines as the tortoise and hare respectively. The Siphon, like the tortoise, is slower, but the abrasive continuously cycles through the machine. The Direct Pressure machine, like the hare, is much faster, but the pressure pot needs to re-pressurize and cannot run continuously.

So if Siphon delivery is slower, why do most automated machines use for that abrasive delivery method? Because the slower siphon is actually faster when automated because it never needs to stop and re-pressurize.  There are some Direct Pressure units that allow pressure pots to run without stopping, but they result in a brief moment where no abrasive is delivered – something that is unacceptable during automation. Click here to learn which abrasive delivery is right for you!

Here are a few takeaway tips regarding Siphon abrasive delivery:

  • If you use the same compressed air volume and the same abrasive velocity, the siphon delivery method is almost always faster; they must both use different nozzle sizes and different blasting pressure to have equal abrasive velocity and cfm usage.
  • The direct pressure nozzle is smaller and cleans faster than the siphon, why?  Pressure has no nozzle; it really has an air jet only that is called a nozzle. The siphon air jet is inside the gun where you can’t see it and it’s much smaller than the siphon nozzle you see. This is often only the first assumed error about pressure cleaning faster!
  • You can’t have an Industrial Siphon blasting cabinet without an Industrial Dust Collector that stores lots of dust, operates negative pressure and automatically cleans the dust collector filters!  The largest cabinet with less than 200 sq.ft. filter area will soon become a huge mess and should not be called Industrial or Production!
  • Double the gun size and you triple the cleaning speed.  Double again and you triple again…. that produces nine times the parts in the same time.  You must have a production dust collector or you simply have a mess!  Frictional Heat is King and each gun size has greater frictional heat at the same pressure.
  • Don’t be fooled by an Ad that says “Heavy Duty Sandblasting Cabinet”. Siphon blasters have different categories that all relate to the dust collector size included and each category type has its own market but they don’t mix well!