Industrial Utility Efficiency    

Blow-Off Air in Plastic Injection Molding

Over the last several decades, Air Power USA has reviewed many various types of plastic injection molding operations throughout the U.S.. This article identifies the #1 most common compressed air energy savings opportunity we have identified over all these years – BLOW OFF AIR. NOTE: Please look at the footnote at the end of this article for clarification on an item you will see in each economic analysis.

The #1 Opportunity in all Plants – Compressed Air Blow Offs:

Regardless of application, there are several guidelines that should always be applied to compressed air used for open blow off:

  • Use high pressure only as a last resort
  • All blow off air should be regulated
  • All blow off air should be regulated to the lowest effective pressure—higher pressure means higher flow, which may not be needed
  • Use Venturi air amplifier nozzles whenever and wherever possible—this will usually reduce blow off air at least 50%, freeing up more air flow for other applications
  • All blow-off air should be shut off (automatically) when not needed for production
  • Consider a separate low pressure air source for blow off air – i.e., blowers.

Plants with many 1/8” and 1/4” inch lines running as blow off on units will use approximately 10 and 25 cfm each, respectively, at 60 psig.

One savings approach is to use an air amplifier which requires less compressed air. Air amplifiers use venturi action to pull in significant amounts of ambient air and mixing it directly into the airstream, which amplifies the amount of air available at the point-of-use. Air amplifiers have amplification ratios up to 25:1. Using 10 cfm of compressed air can supply up to 250 cfm of blow off air to the process and generate a 15 cfm of savings per 1/4” blow off. Savings may be available using 1/8” lines, but the cost effectiveness will not be as great.

Another method for blow off to be investigated is the use of blower generated low pressure air. This air is much less costly to produce on a dollar per cfm basis. It is the volume of air (cfm) that creates the mass or weight of the air that performs the blow off. The pressure influences the “thrust” out to the end of the nozzles where it quickly dissipates. Often a higher volume or weight of air at lower thrust (pressure) improves productivity and quality of the blow off over the higher pressure version.

Replace Blow Off Air with a Mechanical Device and Eliminate the Compressed Air Use

At a plastic injection molding facility on the West Coast, compressed air was used in a sub-assembly area on parts sorters. Figure 1 shows one of (100), 1/8” tube air blows used in the tumblers/sorters throughout the plant. These blow offs are set to use 45 psig inlet regulated compressed air and use a measured 3 cfm. Reviewing production records indicates an “on time” for this process of about 70% of the total 8760 production hours per year. This results in an average use of 2 cfm each for a total usage of 200 cfm for the (100), 1/8” blow offs spread over 64 stations.

Figure 2 shows the mechanical device installed to perform the same task in the parts sorter to replace the 1/8” tube blow offs. This worked as well or better and used no air. This type of modification worked on all parts sorters of many different types of formed parts used in the sub-assembly area.

 

Economic Benefits of this Project:
Total amount of compressed air saved 200 cfm
Annual recoverable energy cost per year (calculated) $70.39 cfm/yr
Recoverable electrical energy cost value of the 200 cfm of compressed air saved $14,078/year
Cost of 200 mechanical blocks installed at $5.00 each $1,000

Try Venturi Amplifiers or Flow Inducers Whenever Possible

Many plants today have realized that an open tube blow off is inherently inefficient and that proper nozzles not only create a potentially more effective flow pattern but also induced ambient air flow allowing for potential lower volume of air use with enhance performance.

The fact is that all flowing air will bring along or induce ambient air to flow. However, there are general purpose dispersion nozzles often applied when a select designed venturi amplifier with amplification up to 25:1 may do even better using significantly less compressed air.

At a Northern Indiana plastics molding plant, the 1/4” tube open blows (at 80 psig and 32 cfm compressed air flow in and about 80 cfm out to process) had been replaced with a very good dispersion control nozzle and reduced the compressed air from 32 cfm to 26 cfm and still delivered 95 cfm to the process; somewhat higher than the original 80 cfm of combined compressed and ambient air. The productivity and quality remained about the same.

During an audit site visit, a 1/4”, 25:1 amplification nozzle was tested using 10 cfm of compressed air and delivering 250 cfm of air to the process. This was a savings of 16 cfm per nozzle of which there were a total of 16 throughout the plant. The 250 cfm total flow to the process allowed an increase in line speed. The action item was to install quantity (16), 1/4” amplifier nozzles in place of standard dispersion control nozzle for this application.

Economic Benefits of this Project:
Current volume of compressed air used with (16) dispersion nozzles at 26 cfm at 80 psig 416 cfm
Total compressed air used by (16) 1/4” venturi amplifier nozzles at 80 psig 260 cfm
Total compressed air volume saved 156 cfm
Recoverable electrical energy value per cfm/year $86.34 cfm/yr
Net annual recoverable electrical energy cost savings $13,469/yr
Cost of (16) nozzles installed with regulator and filter $2,100

Comments

When a plant is confronted with a question regarding blow-off air, personnel often opt for a quick “which is the best for all operations?” solution. There is no answer for all operations. It behooves energy-oriented operations personnel to be familiar with all types of solutions. The most important questions are:

  • Which solution has the most positive impact on productivity and quality?
  • Will other methods that use less air do the same, better or worse?

In the case of the Northern Indiana plastics molding plant, the better performance of the higher flow volume actually allowed an increase in line speed of almost 2%. Alternatively, it may have also allowed the use of lower pressure to the nozzle (flow reduction). The important point is when evaluating open blowing operations, investigate carefully. All the options – blower air – will it work? What is capital cost? What is maintenance cost? What is energy cost? The same can be asked for other options like dispersion nozzles and venturi amplifier nozzles, etc.

We have found during audits and evaluations over the last twenty years that 70% of the time, or more, the correctly applied venturi nozzle produces the best results and obviously enjoys a very low capital investment.

   Win-Win  
  "70% of the time, or more, the correctly applied venturi nozzle produces the best results and obviously enjoys a low capital investment." - Hank Van Ormer, Air Power USA    

Electric Eye

On production line compressed air users, such as blow off and others, use an electric eye controller to shut off the blow air (or other uses) when there is nothing there.

 

A plastics plant in Vermont had this situation. The normal runs on the line would have open spaces of various magnitudes on a continuing basis. The plant was running one large blow air line with a dispersion nozzle using a constant 42 cfm at 85 psig.

A simple, pre-packaged electric eye unit was mounted and installed in less than 15 minutes. During a run of several days’ normal production, the average compressed air use fell to 20.45 cfm measured.

Economic Benefits of this Project:
Average uncontrolled compressed air usage 42 cfm
Average controlled usage 20.4 cfm
Net average compressed air saved 21.6 cfm
Recoverable electrical energy cost savings $cfm/year $103.12 cfm/yr
Total recoverable electrical energy savings $2,227/yr
Electric eye installed cost $1,085

 

For more information please contact Hank Van Ormer, Air Power USA, www.airpowerusainc.com

 

Footnote on Calculations in this Article:
$ cfm/yr recoverable
Average cfm reduction to actual recoverable energy cost reduction

This is not a fixed value but is calculated for each audit site and reflects a number of measures:

Actual input power measured in kW at a specific pressure

The power rate in either a blended (12 months total energy bill divided by the total kWh) or calculated base rate with all demand charges included

The specific power of each operating unit and the overall combined specific power (cfm/kW) of the combined multiple units operating at each set of conditions (i.e. production vs. non-production, etc.)

The number of operating hours per year per operating conditions.

When the system review or audit is finalized, any energy cost savings not related to cfm demand reduction are deducted from the total (such as dryer savings in direct electric kW, compressor improvements in efficiency or specific power, compressor discharge reduction; etc.) to develop an accurate estimate of actual input energy reduction per cfm of air saved.

Most importantly, this model considers each specific compressor and its part load operating efficiency depending on compressor type, capacity control and installation conditions and configurations. Differences in these factors can have a very dramatic effect on the actual recoverable input energy per cfm per year.