The 4 Myths of Compressor fed Air Knives

Compressed air knives are often described by those that sell them to be a quick and cost effective alternative to Sonic blower fed air knives, having similar performance with less upfront costs.  But is it really that simple? How do both methods stack-up I terms of “total cost of ownership”?

Myth 1:Compressor and Sonic blower fed air knives output nearly equal performance

Compressor fed air knives utilize relatively low volumes of compressed air at higher pressure compared to Sonic blower fed air knives. The volume of compressor fed air knives is often described as being 30-40 times greater than the actual compressed air flow due to the coanda effect.  The orifice gap for compressor fed air knives is ≈0.001” to 0.002” (25 – 50 microns) wide and the ambient air surrounding the air knife is drawn into the flow, “amplifying” the flow by 30 or 40 times the compressor volume.

Compressor efficiency (an oxymoron of sorts) is generally accepted to be 4 CFM per 1 HP.  This is the 1^st area you need to verify when reviewing compressor fed air knife “efficiency” claims.

Let’s say that you have a high efficiency compressed air knife designed to be driven by a compressor and it is 18.9” long and having a flow rate of 55 CFM; it would typically require 13¾HPr (10.25 kW). If you overstate the air compressors efficiency, inflating it to 5 CFM/HP, that number would drop to 11 HPr. Always correct the compressor fed air knife performance chart to reflect the performance of your compressor, as it is the only way to accurately calculate energy consumption.

Contrast that performance with a Sonic XE™ air knife having the same length with an orifice gap width of 0.055" (1397 microns) and operated at 3 PSIG. The XE air knife would require a SONIC 70 blower using 6.9 HP (5.1 kW), therefore using a 7.5 HP motor and the flow rate through the air knife would be 254 CFM.

The coanda effect does not amplify the velocity of compressor fed air knives and therefore the Sonic blower fed air knife ends-up having 3x the exit velocity of the compressor fed air knife (35,200 FPM vs 11,800 FPM).  That’s more air applied at a higher velocity with a Sonic blower fed XE air knife.

Past experience confirms Sonic can match the blow off effect of a high efficiency air knife at just 2 PSIG (55" H20) in most applications reducing the power required to 3.8 HP (2.8 kW), therefore using a 5HP motor for a resulting flow of 215 CFM and exit velocity of 29,100 FPM.

Either way, the savings are significant:

Based upon the typical 4 CFM/HP compressor efficiency, 10.75 kW - 2.8kW (compressor energy required – Sonic blower energy required) = a 7.95 kW savings x $0.083 x 40 hours per week x 52 weeks = $1372.48 savings per year!

This 75% energy reduction with Sonic blowers and XE air knives vs. compressor fed air knives is true for any size application.

Myth 2: compressor fed air knives cost less than Sonic blower fed air knives

Sticking with the example above, the compressor fed high efficiency air knife would cost approximately 40% more; approximately $494 compared to a Sonic XE air knife costing approximately $352.

The aforementioned 5 HP Sonic blower with all related plumbing – that’s everything needed to get the air from the blower to the knife, including a well-made adjustable air knife bracket to enable precise air knife alignment, costs about $5800.  However, the cost of ownership must be considered.  While capital expenditure and operating budgets are often treated separately, to the company using the equipment the real question is How much will this air knife system cost the company each year?

We’ve documented nearly $1400 per shift annually is saved in energy consumption alone based upon the current cost for energy in the Mid-Atlantic region.  The breakeven ROI ends-up being about 4 years for one shift, 2 years for two shifts and less than 18 months for 24/7 applications.  Thereafter the system continues to save you energy/money.  The savings are approximate, older/less efficient compressors and higher energy costs can significantly reduce the ROI time.

Scale the Sonic blower up to a 10Hp, 20Hp, 30Hp, 40Hp or 50Hp, and the appropriate size Sonic XE air knives compared to high efficiency compressed air knives to meet the equally large applications and the equivalent compressor power to supply those air knives would be 40Hp, 80 Hp, 120Hp, 160Hp and 200HP of compressed air to match Sonic’s air knife performance. Now that’s ROI !!!

Myth 3: Compressor fed air knives are quieter than Sonic XE air knives

If you had a high efficiency air knife you may measure the noise in a laboratory environment to be approximately 69 dbA. However in actuality once the air velocity impacts the target, the surface shear force results in noise equal or greater than a Sonic blower fed XE air knife.

Myth 4: The annual cost of compressor fed air knives is less compared to Sonic blower fed air knives.

If we base Sonic blower maintenance costs on 1 shift/day that equates to about 10,400 hours after 5 years.  During that time Sonic’s customers would expect to replace (2) drive belts at $52/each and (5) filter elements at $62/each; equating to approximately $83/yr. for maintenance during the first 5 years.

For multiple shift applications the customer would expect to replace a bearing cartridge (performed by the customer with a screwdriver) and belt tensioner (requires a wrench), costing about $1350 for the “example system” we’ve been discussing, increasing the average annual maintenance cost to about $356/yr.

What is often overlooked by proponents of compressor fed high efficiency air knives is the associated compressor maintenance and related accessories.  For example:

·         A compressed air system will require oil, filters, dryers, belts and air-end rebuilds.

·         The cost for a back-up compressor is certainly more costly than a back-up blower head.

·         A Sonic blower system can usually be located within 10-15 feet of the actual air knives because they are relatively compact and quiet compared to air compressors, which are often located in a separate room far away from the actual air knives.  This equates to additional plumbing and accessories such as air/liquid separators to remove moisture that accumulates over long plumbing runs when using air compressors.

Conclusion

At the end of the day, performance is usually what matters.  Can throughput be increased or the rejection rate decreased?  It is intuitive that drying applications benefit from warm, dry blower air compared to cool compressed air which often has condensate and trace amounts of blower lubrication entrained within the air?

Many OEM’s will implement compressor fed air knives on their systems because it’s lowers the initial cost of their product, as they aren’t supplying the compressor nor paying the ongoing utility costs.  As a result we are continuously evaluating applications and suggesting ways to upgrade the performance of existing equipment.

Because it’s all about performance, we offer complete drying and blow-off systems with Sonics' Performance Guarantee; we’re not just supplying a one component and letting you fend for yourself.  We evaluate your application and provide a turnkey solution that is cost effective.

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