Compressed Air

Compressed air and VFDs: Get the most from your system: Your questions answered

Webcast presenter Jarrett Affolter answered questions about topics such as cost benefits of a compressed air system, recommended volume for an air tank, and when to use fixed speed or variable speed.
By Jarrett Affolter, Ingersoll Rand October 12, 2018
Courtesy: CFE Media

Jarrett Affolter, product support leader for North America, Ingersoll Rand. Courtesy: Ingersoll RandThe “Compressed air and VFDs: Get the most from your system” webcast was presented live on Oct. 9, 2018, by Jarrett Affolter from Ingersoll Rand. The webcast can be found here. He supplied written answers to some of those questions that weren’t addressed from the webcast attendees:

Question: Discuss cost and benefits of using a receiver vs. using larger pipe to store air.

Answer: The cost of installing large pipe in the system usually would be much higher than additional receivers. Benefit of low pressure drop would need to be evaluated as well as the option for future growth in the system and if the larger pipe would offset increased pressure drop as a function of more air demand in the pipe.

Q: Discuss cost and benefits of using a bigger compressor vs adding receivers to store air.

A: It’s much more cost-efficient to use more storage in systems with large variation in load vs. installing a bigger compressor. Systems with a more consistent load profile would benefit from a properly sized air compressor that can run in the “sweet spot” of its efficiency curve.

Q: Discuss advantages and disadvantage to using air cooled compressors and water cooled compressors.

A: Air-cooled machines don’t require a separate cooling system that consumes power and needs maintenance. Cleaning air cooled heat exchangers is much more cost effective as well. Cooling water quality is always a concern and must be maintained within specified parameters which can turn costly. The benefit of not having seasonal temperature changes, or the effects of extreme temperatures is where water cooled becomes ideal. This allows for a more predicted control of temperatures and gives air treatment equipment (dryers and filters) a consistent inlet temperature for a more overall compressed air energy efficiency profile.

System design considerations include air distribution, control valves, and treatment equipment. Courtesy: Ingersoll RandQ: Having variable frequency drives (VFDs) and the motor ramps up and down, what is the best “dead band” we should set the pressure in percentage of the pressure?

A: Most VFDs have a “target” pressure you set to have the VFD hover around it by varying speeds. The other parameter is the unload or auto stop pressure that shuts the machine down when the system isn’t using any air. If the system has a varying load, which results in unloaded operation or short cycling, set the unload or auto stop pressure to the highest value the system will allow for. If the system has relatively consistent load and therefore unloading or shutting down is not very frequent, dial the unload or auto stop pressure to just a few PSI above the target to reap as much energy efficiency as possible. These parameters may need to be tested over time to balance energy efficiency and cycling concerns.

Q: What about if you have a system that is appropriate 16 hours to use an VFD, what for the others 8 hours that the demand is very low?

A: Size a VFD to cover the low-demand profile, and then size a fixed speed to run as base load for the 16hrs with the VFD trimming the pressure.

Q: Our vacuum starts at high speed and slows via VFD and runs a minimum run time. What is the minimum suggested run time?

A: It depends on the time between shutdowns. All motors will have a limit of how often they can start in an hour. Traditionally, this is 6 times an hour for most industrial motors. The manufacturer of the motor will have the accurate number. If the run time allows the compressor to exceed the maximum starts per hour, the run timer needs to be adjusted to compensate and keep the starts under the max start parameter.

Q: What is the recommended volume of the air tank for the VFD compressor?

A: There is no such thing as too much volume in a compressed air system. The only real constraints are price of the tank, room to locate it, and the installation costs.

Q: What are some times that you would use a fixed speed over a variable speed?

A: If the load profile is constant, a VFD compressor does not have much value. If the environment is not suitable, a fixed speed is the right choice.

Q: If the customer is facing budget issues with replacing their existing old entire compressor with a new variable speed drive (VSD) compressor, is it recommended to suggest installing VFD only on the existing motor along with control panel and discharge sensor instead of buying a new unit?

A: Most existing air compressors can benefit from a VFD “retrofit”, however some details need to be understood. Does the existing motor work with a VFD. If not, it will need to be upgraded to a motor suitable for VFD operation. Next, the mechanical limitations of the air end needs to be considered to determine the level of turn down it can operate with.