Your questions answered: Compressed air systems

In this “back to basics” webcast, the audience learned several aspects of compressed air systems. Here are the answers to many questions.

By Plant Engineering March 24, 2023
Courtesy: Plant Engineering

Compressed air insights

  • Compressed air systems are used in many applications, and learning how to assess the system is key.
  • Some installations require specialty piping or variable speed drives.

Compressed air is a necessity in many industrial applications, but it can be costly if you’re not careful. By investing some time and thought into your compressed air system, you can identify inefficiencies that, when fixed, will make compressed air an affordable solution.

After watching the webcast, Back to basics: Compressed air systems, several items were left unanswered. Read more about compressed air systems here.

Expert presenters:

  • Frank Moskowitz, Draw Professional Services, Cave Creek, Arizona
  • Ron Marshall, CET, Marshall Compressed Air Consulting, Winnipeg, Manitoba.

Do medical air applications pose other design challenges?

Frank Moskowitz: Requires oil free with a specific moisture and particulate rating. Easily accomplished with today’s air quality equipment.

Are there differences between the efficiencies of the various brands of compressors? How do we find the numbers?

Ron Marshall: Yes, there can be significant differences between makes and models. Most reputable companies selling compressors in North America are part of the Compressed Air & Gas Institute’s third-party verification program and as such must publish accurate compressor efficiency data on their websites, this is a great resource for anyone contemplating a compressor purchase.

Why is water still condensate after after-cooling and air dryers?

Frank Moskowitz: Condensate is any water that has condensed out of the gas stream. So regardless of where it is showing up, liquid is still called condensation.

My company is proposing on a project that uses turbine air compressors using a 20,000 horsepower, synchronous motor. This is for a wind tunnel. Have you ever dealt with compressors of this type and size?

Ron Marshall: No, the largest I have ever dealt with is 10,000 horsepower. These units for wind tunnels would be more classed as blowers than air compressors.

Define RFL.

Frank Moskowitz: I think you mean FRL. That stands for filter, regulator, lubricator.

For compressors and receiver tanks: How do we calculate the compressor size required with respect to the stored air in a receiver tank?

Ron Marshall: Typical rule of thumb is to have 3-5 gallons of storage times the cubic feet per minute (cfm) rating of the largest compressor. So, if your trim compressor (one that puts out part load) is 1,000 cfm rated, you should have 3,000-5,000 gallons of storage capacity at the compressor room. More is usually better; some problematic systems need more storage to enable the control to be stable.

Does the variable speed compressor require variable frequency drives (VFDs) and if yes, is the VFD same as used in motors or does it require a specialized VFD?

Ron Marshall: The drive depends on the compressor design, some have switched reluctance or direct current (DC) drives to change the rpm of the motor. When used with an inverter the motors should always be inverter duty type or early failure will be the result.

During the March 2023 webcast, the various types of compressed air systems were reviewed. Courtesy: Plant Engineering

During the March 2023 webcast, the various types of compressed air systems were reviewed. Courtesy: Plant Engineering

What exactly is the process variable for the VFD? Is it air flow or something else?

Ron Marshall: The VFD controls on pressure, kind of like cruise control on a car. A target pressure is set and the drive will speed up or slow down the motor rpm in order to maintain target pressure.

Is “blue pipe” PEX? What do you think about using PEX in some distribution?

Frank Moskowitz: PEX pipe is high-density polyethylene, which is used for potable and nonpotable plumbing applications. Its pressure rating reduces as the temperature climbs. It is not good for compressed air for that reason. Blue pipe is aluminum, and highly rated for compressed air applications

Have you seen issues with aluminum “blue pipe” being a “fragile” installation that if not installed correctly can fail dangerously?

Frank Moskowitz: Can only fail if not properly hung at the proper distances. If properly supported, it is a fantastic piping structure.

At what point would you utilize a condensate treatment system? What size of compressor is small enough to not include a treatment system?

Ron Marshall: Lubricated compressors pass a little lubricant into the system, which shows up in the condensate. Dumping oily condensate into a system pollutes the environment. Always treat and dispose of the condensate in an environmentally friendly manner. Price out the treatment system and you can determine if it is right for your system.

True or false: Inlet valve modulation is a very efficient control at part load.

Ron Marshall: Fales, modulation mode is the least efficient way to run a partly loaded compressor. Variable speed drive (VSD) or start/stop are the most efficient ways.

Do you have a resource for sizing storage tanks near a high-volume intermittent application?

Yes, check out this link to the “Improving Compressed Air System Performance: A Sourcebook for Industry” publication and check out Page 43. Use of this equation is explained in the Compressed Air Challenge’s Level 2 course. Check availability here:

For constant load, how many gallons of receiver should exist for each cfm of used air?

Frank Moskowitz: For constant load, a receiver is not required. The 3-5 gallons per cfm is for a load noload lubricant injected rotary screw compressor to enable it to blow down and reach its unloaded power before it reloads. The rule of thumb is for the trimming compressor only.

What is the effect of altitude (local barometric pressure) on power consumption?

Frank Moskowitz: At higher altitudes, the air gets thinner. This phenomenon reduces your motor’s ability to dissipate the heat it generates. Depending on elevation, ambient temperature and your motor’s temperature rise characteristics, you will probably need to de-rate your motor to help it stay within the temperature parameters of its insulation class.

My organization has old pneumatic control systems for some of our buildings’ HVAC systems that run on 20 psi supply pressure. Our reciprocating compressors are often running around 80-120 psi and then being regulated down to 20 psi for the control system. I would like to drop the supply pressure to reduce energy consumption, but I am wondering what risks I should consider in reducing the pressure substantially and thus increasing cycling frequency?

Frank Moskowitz: Reciprocating compressors store pressure in the receiver. That’s the energy that you use to feed the HVAC pneumatics. If it’s working correctly, you should see the reciprocating cycle occasionally to replenish the air used. If you lower it to deliver the 20 psig, you will not be building stored energy and the compressor will run all the time. Not good.

Could the additional tank near the big air users be connection to the loop or does it have to be connected to the branch of that big air user? Is one a better practice than the other?

Ron Marshall: Usually it is connected to the branch where the big air user is located. For this type of scheme to work, the receiver must be isolated from the plant through some sort of restriction that would reduce the flow of air to the tank while it is filling, so as to reduce or smooth out the compressed air demand to the air compressors. This stabilizes the system and help provide better air pressure.

How much outside air does a compressor need?

Frank Moskowitz: Whatever the cfm rating of the compressor is. That’s the air it will suck in.

For hot humid environments (Florida, etc.), what is the desired temperature and humidity for an air compressor room?

Frank Moskowitz: I would try to keep the compressor room at around 10°F higher than the ambient if you can. That would show a good ventilation system. As far as humidity, if your after-cooler is clean and working and the water separator and trap are working, the humidity makes no difference.

Is there any benefit of using more than one VSD compressor set?

Frank Moskowitz: Some customers do this when they have varied flow for different shifts. Day versus night or weekday versus weekend. Sometimes on size does not fit everywhere so a second VSD of different size for different shifts does work.

How do I calculate pressure drop of a compressed air piping system?

Frank Moskowitz: See this link for a calculator.

How do you handle drop legs in cleanroom applications?

Ron Marshall: These have special requirements. A cleanroom should never have any moisture present and should be designed carefully to prevent. The design of these drops should be specified by a cleanroom design expert.