Plastic Molder Reduces $3.2 Million Energy Spend with Pneumatic Cylinder Retrofits
A plastics molding plant had engaged us to conduct an ‘on-site’ Energy Assessment of their facility. The annual ‘spend’ for electricity, natural gas, and water was about $3.2 million for this modern 275,000 square foot, fully air-conditioned facility. During the Review, several opportunities were identified and delineated in lighting, HVAC, process ventilation, the water systems and energy supply contracts. However, the most significant savings were in their compressed air system.
The Supply Side of the Compressed Air System
Production operations (7/24/360 ) required two x 700 horsepower, water-cooled rotary screw air compressors. One air compressor was fully loaded and the other was a Variable Speed Drive air compressor controlling the wet receiver pressure at 120 to 125 psig.
The plant air distribution piping system was robust. The system had 8” and 6” mains, a 4” loop on each of two plant production levels and 2” size pipe drops to each of about fifty (50) large plastic molding machines. System pressure drop from compressor discharge to the mold machines was only about 4 to 6 psig.
Air pressure below 110 psig to the inlet regulator on the individual molding machines resulted in production problems. Due to an intermittent 10 psig system pressure variation (caused by manual air-hose usage), the utilities team was operating the air compressors at a 122 psig set-point. We documented air pressure to the individual molding machines between 110 and 118 psig over several days. Production was good, everyone seemed pleased.
The air compressors were relatively new. In fact the original 600 horsepower motors had been recently replaced with 700 horsepower units to assure adequate pressure. Four, originally installed 350 horsepower air-cooled rotary screw air compressors had been replaced due to on-going operational problems, frequent rentals needed and production quality issues. Yet, I noted that these new 3,200 SCFM compressors were 115 psig nominally rated.
The Demand Side of the Compressed Air System
A plant expansion was being contemplated and the engineering staff had planned to include another $350,000 compressor in the project to handle the new load. After some lengthy discussion about the compressors and energy costs, it was decided that we should conduct a more thorough analysis of the equipment and mold machines to try and identify opportunities for improvement.
The mold machines were of european design. They were robust, high-tech, compact and very productive. They operated amazingly well at high-cycle frequency rates for their eight years of age.
It was difficult for me to understand the need for 110 psig pressure to make the machines operate reliably. I had visions of a huge ‘artificial demand’ and long-term compressor reliability concerns. Close analysis identified the “Achilles Heal” in the demand side of the system. Actually, it identified two of them! They were located in two different types of mold machines.
Type #1 Mold Machine
One type of molding machine utilized a vertical 40 mm diameter x 16” long air cylinder that lowered and raised the heavy mold press assembly. This air cylinder was located in the center of the 20 foot square, highly automated process. Cylinder cycle time was 10 seconds. Compressed air supply, from the 2” size pipe, passed through a ½” diameter pipe to an undersized ½” Regulator/Filter/Stop Valve Station. Then, 105 psig compressed air passed on through a 6 mm inside diameter plastic hose to the solenoid station operating the cylinder.
Trials had been conducted at a lower pressure using a small surge tank. Lower supply pressure increased cycle time and reduced machine productivity. The air cylinder had difficulty lifting the heavy mold assembly back to the START position. Further discussion with the maintenance crew identified air cylinder re-builds as a regular repair issue. We also learned that a replacement air cylinder option was now being offered by the european mold machine supplier in a larger 50 mm size. Wonder why?
Our recommendation, on Mold Machine Type #1 was to install new 50 mm size cylinders and use the old 40 mm cylinder body as a surge vessel for the Solenoid valve return stroke. We also replaced the ½” air supply pipe with 1” pipe, replaced the ½” size Regulator station with ¾” size and changed out the 10 feet of 6 mm tubing to 12 mm size. The cost estimate was $3,000 per machine on each of 20 identical machines.
Type #2 Mold Machine
The second type of mold machine was totally different. It had sixteen very small 1.5” diameter x 1.0” stroke air cylinders (in a common mold body plate) fed by two 3/8” rubber air hoses. All of the cylinders actuated simultaneously. Again, the plastic air lines to the solenoid valve appeared to be undersized. Extensive trials by the maintenance team, using a one-gallon surge tank, did not permit the lowering of the plant air pressure. Lower air pressure produced defective molded parts. They had determined that 102 psig was needed to consistently assure the production of quality parts. To change the stainless mold plate body was obviously ‘out of the question’.
Our recommendation, on Mold Machine Type #2, was to install an air amplifier and surge tank unit on each machine. This had the effect of raising the compressed air supply pressure from 95 psig to 115 psig for the air line that feeds from the solenoid valve to the mold body plate. The cost estimate was $4,000 per machine on each of 30 identical machines.
Summary and Comment
The changes to the two mold machines were estimated to cost $180,000. They included installing the new 50 mm size air cylinders on the twenty Type #1 Molding Machines and the air amplifier and surge tank units on the thirty Type #2 Molding Machines. These changes would allow the air compressors to be lowered from 122 psig to a 102 psig nominal control setting. Compressor reliability, plant-wide air leaks reduction, motor power savings and electric demand reduction were estimated to total a savings of $300,000 per year.
If your air compressors operate above 95-100 psig for normal industrial process systems, understand why and have a sound technical reason for that condition. Not only energy cost savings, but also long term reliability of the air compressors can significantly improve when operating at 95 -100 psig versus units operating continually at 115 -125 psig. Most process equipment should operate reliably with 85 psig air supply to the machine manifold (with some specific exceptions such as large air cylinders, Pulse-Jet bag houses and sandblast units). Tackle those issues locally and closely review the compressed air specifications for equipment provided by vendors of automated production systems.
About the Author
Gary W. Wamsley is President of JoGar Energy & Utility Services, Inc. a small Atlanta-based Consulting Firm that specializes in “On-Site” Energy Assessments & Utility System Reviews for commercial and industrial plants. He also conducts technical training for engineers and utility personnel. He is Mechanical Engineer with 40 years of management, technical staff and plant operational experience in large and small facilities for the tire & rubber, aerospace and pulp & paper industries.
• A Registered Professional Engineer in four states
• A Certified Energy Manager with AEE
• A Certified Plant Engineering Manager
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