Your eyes are often bigger than your stomach

A report card on compressed air knowledge: Part 2

By Michael Camber

Missed part 1?

Click here for the results of a recent quiz of automotive service shops about compressed air.

The point least understood among the surveyed compressor users is how to size a system for the shop.  We presented a scenario with five techs, each using 15 cfm tools, and gave five different compressor size (hp) options. Less than 40% picked the most appropriate option, and most incorrect answers were heavily over-sized.  We see this in the field all the time.  Operators routinely overestimate the size of compressor needed.

Depending on the type and design efficiency, most commercial/industrial compressors produce between 3 and 4.5 cfm per horsepower.  The (simplistic) quiz scenario assumed all air users would need air simultaneously.  Using 4 cfm/hp, a 20 hp compressor with 100% duty cycle would supply air to all five users.  As mentioned above, 40% chose this option, but another 40% chose either the 30 or 50 hp options. 

The downsides to oversizing include:

  • spending more up-front for the equipment (compressors, dryers, etc)
  • paying more for parts and service
  • increased wear from over-cycling (starts and stops)
  • reduced energy efficiency (this applies more to rotary type machines than pistons, which mostly operate on start/stop controls)

Now factor in the reality that it is very rare for all techs to be operating air tools at the same time.  So let’s say only three of five people will be using air simultaneously (total of 45 cfm).  A 15 hp compressor would easily handle it.  If only two out of five were using air at the same time, an efficient 7.5 hp with 100% duty cycle might suffice.  Or bump it to 10 hp (40 cfm) and add 100-200 gallons of storage to be safe.  Far more cost effective than buying the 20 hp.

For energy efficiency, it would be better to get two smaller machines than one large one. This also give you back-up, but it does drive up the initial cost.

If you anticipate that you will need more air in the future,  arrange your compressor room/area to allow for additional machines, but only buy what you need in the short term.  The money you save on energy and service will help pay for the upgrade later.

Download our Compressed Air System Installation Guide E-book for more installation tips.

A Report Card On Compressed Air Knowledge

By: Michael Camber

One of our media partners recently quizzed their subscribers to assess users’ knowledge about compressed air systems.  The respondents were in the automotive service industry, but the questions were not industry or application specific and the answers are a fair reflection of compressed air knowledge of most businesses with compressors 25 hp and below.  We thought it would be useful to present the results and discuss areas where understanding of compressed air systems needs a boost.

Duty Cycle

A strong majority (84%) of respondents knew that reciprocating/piston type compressors operate at higher internal temperatures than rotary type compressors, but nearly 1/3 mistakenly thought that shop recips could safely run at 100% duty cycle.  Some recips are built to run at higher duty cycles than others, but all air-cooled units need at least some downtime to cool off.  The consequences of overrunning them include loss of lubrication, seizure, motor failure and higher oil carry-over into paint and equipment.

Check out our Piston vs. Rotary Screw Infographic for a quick comparison.

Piping Material Choices

Regarding piping, nearly 2/3 know that copper or aluminum are better choices– especially if air quality is important– but a full 20% picked PVC as top choice.  While PVC doesn’t accumulate or add contaminants, it presents one major problem:

Wait for it ….

Yes, PVC is less safe than any metal pipe and is subject to rupturing and fragmenting.

Solving Pressure Problems

When presented with five possible solutions for solving a low pressure problem, everyone recognized that buying another (or larger) air compressor to get more flow is not the first step to address the problem.  Checking for leaks, checking pipe size, adding storage and doing a pump-up test to check compressor function were all known to be better first steps.  We were pleased to see that over 80% recommended doing all of these things before buying another compressor to get more flow (cfm).

Filters Are Not Dryers

Likewise, we were pleased to see that everyone knew that a compressed air dryer is a simple solution to address the common summertime problem of increased water in compressed air lines and tools.  It’s apparent though that not everyone understands the difference between dryers and other devices that remove some moisture. Filter style moisture separators (with drain valves) are effective at removing liquid water.  Storage tanks do the same and can allow some moisture vapor to cool and condense to liquid, but the key to effective drying is reducing the compressed air pressure dew point below the ambient temperature.  Tanks and filters cannot do it. A dryer can.

The last quiz question was about compressor sizing. Since this topic is a bit more complex it deserves its own post. Stay tuned…