small piston air compressor

How To Size a Small Piston Air Compressor for Blast Cabinets

We often get asked which small piston air compressor is needed for a particular abrasive blast cabinet. Typically this happens when a buyer finds the blasting cabinet first then goes to look for the required compressed air.

One of the most important facts to remember is the compressed air is what creates the blasting process. Blasting cabinets are like sanding belts without the belt, the cfm volumethe gun uses also determines the frictional heat generated at any setblasting pressure. Frictional heat determines just as much about cleaning speed as the blasting pressure itself. If you need a quick refresher on why friction heat is critical for abrasive blasting, check out this post.

Real World Example

Let’s say three blast cabinets are all using 80 psi to blast, but one has a 16 cfm gun size, another has a 35 cfm, and the third has 70 to 80 cfm. The frictional heat created with each set up can easily vary from 175 to 400+ degrees F among the cabinets – even more with direct pressure abrasive delivery.

5 Tips for Finding the Right Size Small Piston Air Compressor

  1. Look for cast iron compressor pumps first, they are more likely to be 100% duty cycle
  2. Horizontal tanks take up more space but normally have larger pumps. Large vertical tanks are used to make you think you have lots of air, but in reality they help pass vibration safety tests and normally they have smaller pumps. Once the tank pressure drops to100 psi all the air remaining in the tank is not high enough to maintain fast part processing.
  3. Two air cfm volumes are printed on most piston compressors. Never use displacement cfm numbers, always look for the air volume at 90 to 100 psi. Both will be listed, but you blast at high pressure and not displacement that’s mathematical cfm using piston volume and the pump rpm speed.
  4. Two stage pumps push higher pressures into the tank, which means more air volume, more atmospheres, inside the tank. Two-Stage Compressorsare a better value than smaller single stage
  5. Always double the cfm required to operate the gun size. This allows the compressor to rest and cool, and it will last much longer.

Pro Tip: As the pressure increases inside the tank less air cfmis placed into the tank, so the cfm volume gets smaller as tank pressure increases. Many compressor manufactures play a game of hide and seek. They show you a big tank, display displacement numbers with zero in the tank, then speed up the pump so it can’t operate at 100% duty cycle. Duty cycle on an air compressor is rated in 10-minute time periods. If you have a 50% duty cycle that means 5 minutes running and 5 minutes to cool.

One of our FAQ’s is, “How do I size an air compressor for 7 cfm usage?”

Our advice is to avoid buying a siphon machine that indicates it operates using only 7 cfm unless you are only blasting small items like pipe nipples or nuts and bolts. With larger items the blasted surface will likely be very blotchy, like painting you car with an aerosol spray paint can. The cabinet may be listed at a fraction of the cost compared to other machines, but you will probably be disappointed with the cleaning ability because not only is the cfm small, but the frictional heat is very low. That doesn’t mean the machine is a piece of junk, but it does mean that’s all you get with only a 7 cfm gun.

Now, there are some applications where 7 cfm can be used without issues. Many direct pressure machines, including our entire line of sandcarving models, use a small piston air compressor. The reason they work is the low volume of air used allows unmatched operator control for the detailed work used in sandcarving. The frictional heat is much lower with the low cfm volume and doesn’t blast through most protective films, which allows deeper carving but understand direct pressure machines deliver abrasive at about double the speed of siphon delivery using a much higher particle per square inch.

For a 7 cfm compressor the cfm usage of the blasting cabinet must be small enough for the pressure you want to use during blasting. With such low cfm number, you’re limiting yourself to a very small gun size. Conversely, a larger cfm usage generates higher frictional heat, and takes less time to process a part.

Pro Tip: With any direct pressure delivery as the nozzle’s internal diameter (ID) wears out larger, the machine requires more cfm to maintain the same blasting pressure. This is unlike a siphon delivery that simply wears the nozzle larger losing some velocity impact but using the same amount of compressor cfm.

Real World Example: Because we are talking about direct pressure delivery (and not siphon) using a small piston air compressor, we would advise doubling the 7 cfm volume to 14 cfm. This allows the nozzle (actually an airjet) to wear larger … The additional cfm in reserve will allow you to maintain the blasting pressure you started with for a longer period of time. If you are using 7 cfm and only have 7 cfm, as the nozzle wears the selected blasting pressure will simply drop lower and lower.

If you insist on using a small siphon cabinet and believe the information about only using 7 cfm, you have the magic cfm number that requires a 220-volt single-phase model. Unfortunately, 120-volt won’t deliver 7 cfm at 80 to 90 cfm for use with a siphon delivery cabinet. There are always exceptions but the cost is high. Remember, once the tank pressure drops below blasting pressure the air inside the tank can’t be used and most siphon units blast at 80 to 100 psi.

To Sum It Up

Media Blast & Abrasive has been manufacturing sandblasting cabinets since the 1970s. We make more than 180 standard models and sizes of abrasive blast cabinets. In addition to a comprehensive list of included features, we offer more than 80 optional features for increased productivity, durability, and operator safety. If you’re not sure whether the cabinet or small piston air compressor you are purchasing is the right fit for your needs, give us a call or visit our Buying Guide.