Pressure Booster selection considerations

[BLogsdon]

Our lab scientists are looking to replace the N2 bottles they use for their HPLC equipment with a N2 generator. Our manufacturing air pressure is maintained at 100 psi which is closer to 90 psi by the time it makes it to the lab. They need, at most, about 2 CFM at 110 psi of N2.

I am debating between a small dedicated compressor and a pressure booster (Festo DPA-40 or similar); however, I am not familiar with pressure boosters.

Does anyone have experience with the reliability of pressure boosters? One of the local compressor salesmen told me they last no more than a year with regular use, but I would imagine his opinion is pretty biased.

Also, are there any design considerations with regard to a booster that would be different than an air compressor?

Thanks!
Ben

 

[zdas04]

Not being familiar with Festo, I did a search and found Festo DPA and the note that says

Pressure boosters are intended for the occasional relieving of compressed air. They are not suitable as a replacement for compressors, as wear on seals and drive pistons increases significantly during continuous operation without breaks.

seems to say that this technology (basically a diaphragm pump for gas) is unlikely to be the most efficient delivery system. At your delivery volume (I'll assume that the "CFM" you are talking about is "SCFM", precision helps here since 2 aCFM at 100 psig at sea level is is 15.8 SCFM) efficiency may not be a huge issue.

With nitrogen generators you also have to be really concerned about purity. The output purity of units this small range from 95-98% nitrogen with no less than about 1% argon (which seems to be a tough problem to separate from nitrogen).

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[BLogsdon]

The literature for these lab devices is pretty vague. The spec for the largest user requiring "A nitrogen generator with a minimum capacity of 5560 L per day with 100 psi at the side panel. Maximum consumption of nitrogen gas is 21 L/min." I'm assuming they are referring to volume at room temperature and 100 psi as measured through one of those rising ball-type flow meters.

The volume of air is very small for the majority of use (closer to 14 l/min)--so even though the pressure booster will be operating continuously, it should be very slow operation...unless I'm misunderstanding the concept of operation.
I'm getting some more info from the Festo distributor to find out how it will perform at low flow rates.

 

[georgeverghese]

I'd hesitate to use inert gas from an N2 generator (which gets its feed from a crappy oilflooded air compressor) for something as delicate as a GC.

 

[israelkk]

Try

http://www.haskel.com/

they have excellent pneumatic driven boosters.

 

[BLogsdon]

I'm not worried about air quality. We're a pharmaceutical company and only have oil-free compressors, desiccant dryers, and (depending on the application) filter down to 0.02 um.

The problem is our entire process and compressed air system has been validated at the 100 psi pressure so I am unable to bump pressure up for this lab application.

Those Haskel units are really slick!

 

[georgeverghese]

If you've got good quality air, then I'd give these Haskel units a try - installing a buffer vessel between the Haskel unit and the GCs' could help to reduce run time for these boosters (assume the Haskel unit is downstream of the N2 generator) to extend operating life.

For example, the Haskel booster could kick in at 110psig and stops at 150psig to feed the buffer vessel. If you want, add a forward sensing pressure regulator on the N2 supply buffer vessel set at 100psig to feed the GCs'.

Check with Haskel if there is a risk of drive air getting into N2, or if there could be N2 leakage loss into the LP side of the drive mechanism.

 

[BLogsdon]

George, running the pressure to full compression of the booster, storing it in a receiver, and regulating it down to the desired N2 pressure is EXACTLY what the rep from Haskel suggested. Then I'll come off the regulator to a membrane N2 generator.

I'm leaning toward the Haskel unit because of their robustness.

Thanks for everyone's suggestions! I'll post an update after the project is complete.

 

[georgeverghese]

Guess you must be leaning in the direction of the N2 gen unit on the LP side ( rather than immediately d/s of the Haskel booster) due to design pressure limitations on the N2 gen unit - okay.
If not, a membrane type unit may work better on the HP side and you'd use less compressed air per unit volume of N2 generated ( ie reject-waste gas flow is less when feed air pressure is higher).
Another option may be to use an impregnated carbon bed type pressure swing adsorption unit for N2 generation.

 

[BLogsdon]

Final design:

House air supplied by oil-free compressors at about 90 PSI
Boost pressure to about 175 PSI with a 2:1 pressure booster
Store air in receiver and reduce to approximately 115 PSI
Pass air through membrane-type N2 generator (outlet approx 110 PSI)
Distribute in poly tubing to end users with point-of-use regulator​

The pressure booster I chose was from Midwest Pressure Systems. Fantastic support, high quality product, and made in the USA. I calculated, that at the flow rate I am expecting, it should last at least 3 years before requiring a rebuild.

I'll post another update once equipment is installed and tested.

 

[georgeverghese]

With this arrangement, there is a risk of off spec inert gas quality if total instantaneous consumption at lab users is more than the design case flow. How will you then insulate or buffer the membrane unit from erratic flow profile at the user end? An intermediate buffer vessel to store this inert gas will help. Ideally a high set FIC loop with FCV on the exit ( or a cruder version of this may be a backpressure PCV) on the outlet of the inert gas buffer vessel.

 

[BLogsdon]

I considered that but feel the risk is low since the N2 demand will be steady and well within the capacity for the N2 generator. In fact, this system is more regulated than is commonly done for these lab gas systems--typical lab units have a N2 membrane generator immediately downstream of a small scroll compressor with no significant storage.