In 1979, I received a call from a business friend who had . purchased his first single-stage cup blowing machine with a base. He was surprise to find that he actually needed to use more than . 100 pounds per square inch of air in the machine to blow the bottles. As a result, I began developing a compressor system for polyethylene terephthalate (PET). Since then, I have designed and supplied over 350 systems – from Tobago to Tibet and many other locations.
In the more than three decades I have designed and supplied air compressor systems for PET,. pressures have ranged from 250 psi for blowing bubbles to 650 psi for complex, . high-speed bottle designs. The trend today is to reduce blowing pressures as much as possible to save energy.
Types of Air Compressor Systems for PET
PET air systems began with three-stage air-cooled lubricated piston units for smaller . systems and water-cooled oil-free three-stage or two-stage intensifiers for larger systems. Over the past decade, four-stage centrifugal. compressors have become popular base load air compressors, . while smaller companies use air-cooled, lubricated, single-stage intensifiers. The king of the hill remains the three-stage, water-cooled, oil-free reciprocating machine.
Here is a brief overview of the types of air compressors available today.
Three-stage air-cooled piston
– 15/40 CFM; requires lubrication only; . compresses from atmospheric pressure to 580 psi; each has Largest flow rate of 40/80 CFM, . so must be use in a variety of situations; provides back-up; . makes good use of space; moderate efficiency; 10 to 12-year life; very cheap for small systems.
Single-stage booster – 10/500 CFM; lubricated and non-lubricated configurations available; . uses existing or required 125 to 175 psi inlet air, depending on pressure;. moderate use of space; efficient; 10 to 15 year service life; . very cheap solution for providing PET compressed air
Single-stage intensifier – 600/2,000 CFM; oil-free and water-cooled; uses 125 psi inlet air; . low speed; requires a lot of space; efficient; moderate cost; . 20 to 30 year service life; used where large amounts of low pressure air are available;. realistic cost for large systems
Four-stage high-pressure centrifuges – 1,800/5,000 CFM;. oil-free; space-saving; low energy costs at non-full load; . long-term low maintenance, high overhaul costs; . moderate cost per CFM; 10 to 15-year service life; realistic choice for base-load. applications in large systems
Three-stage,
water-cooled piston – 90/1400 CFM; . oil-free; water-cooled; . cheap footprint depending on configuration; most efficient option;. can incur high maintenance costs if not inspected after 4,000 to 6,000 hours; . highest investment cost per CFM; 20 to 30 year service life; . essential element of most compressor systems in large PET production plants
Three-stage oil-free, water-cooled piston air compressors
oil-free, water-cooled reciprocating air compressor . from AF Compressors with high-pressure refrigeration dryer for PET systems.
high-pressure piston air compressor for PET – Sauer Compressors USA, WP271L.
Single-stage booster compressor for small PE bottle filling
The long service life of the single-stage . booster makes it a cost effective solution for small PET bottling facilities.
Determining Blow Molding Air Requirements
Now that we know what equipment is available, we can check number of,. other aspects to determine which compressed air system makes the most sense for . different applications in the plant. Based on over 30 years of best practices, . here are 11 key points to consider . when determining the blowing air requirements for your application.
1. type of plant.
Is this a established plant or an addition to an existing facility? What is the availability of services and parts? Always try to keep it simple . when selecting a compressor for start-up and remote applications.
2. Water cylinder type
. Simple design water bottles tend to have lower pressure requirements. than complex design food and pharmaceutical bottles. The market will dictate the design and so the complexity and type of compressor.
3. Air Quality
. Many small and young blowing companies use three-stage . lubricated pistons or lubricated intensifiers with three-stage filtration. The final stage of filtration is a carbon filter that removes tastes and odors. Regular filter changes are essential for the proper use of this . type of system (1000 hours for carbon filters and 2000 hours for particulate. and coalescing filters). Air quality sampling can be performe by many companies, . including Trace Analytics, to ensure that air quality is maintaine. and that the process meets ISO requirements.
Oil-free compressors ensure that no lubricants are use in the cylinders, . but they can also remove particulates and anything else that is in the intake air. I recommend the use of particulate filters and a carbon filter to ensure good air quality.
Purge system air dryers are . refrigerated type with a pressure dew point of 38° F. Some large companies . use desiccant dryers because there have been problems with . freeze dryer failures in drainage systems in the past. In most environments, a maintained freeze dryer will prevent moisture . buildup in the bottles and mold.
Wiring diagram of low pressure compressor, booster, freeze dryer and filter.
Wiring diagram of low pressure compressor, booster, freeze dryer and filter.
4. The duration of the project
. It is obvious that the investment in a complete PET blowing system can be a significant expense. If it is a 5-7 year business plan, a small three-stage PET air compressor or booster is the best choice. Larger, water-cooled . PET compressors should be consider for longer life projects when the budget allows. The normal investment for a blow molding compressor system is 20% of the cost of a new blow molding machine/mold.
5. Operating time
. An obvious consideration is the number of hours per year the equipment will be in use. A single shift, over five days, equals 2,000 hours, . which is common for many small specialty bottle manufacturers. This is a logical place for a small air-cooled unit. When production hours increase to 4,000-6,000 hours per year, . consider moving to larger water-cooled units, . as they tend to have a longer life.
6. Required blowing pressure
. As mentioned above, blowing pressures started at . 250 psi in the early years and now hover around 450 to 500 psi in most applications. Cylinder design and machine speed are key factors affecting blowing pressure requirements. The higher the pressure, . the more heat is generated in the compressor, . resulting in shorter maintenance intervals and higher energy costs. Some of the plants I’ve worked with have reduced their pressures to as low as 50 psi,. which has reduced maintenance and saved 5% in electricity costs. This required some adjustments to the blowing machine and. monitoring of the cylinders during the pressure reduction process.
Reducing Compressed Air Pressure:. How It Works in the Plastics Industry – Webinar Recording
Download the slides and watch the free webcast recording to learn.
Best Practices for Blow Molding Recovery
Understanding the requirements of PET blow molding machines
Flow monitoring for high pressure applications
Reducing the blowing pressure of PET bottles
Improve the audit process
Reducing the impact of plant pressures
Common low pressure applications in the plastics industry
Turn reduced plant pressures into energy savings
7. Equipment location
: . Compressors are often installed in any available space,. and system placement is a secondary concern. The following should be consider when determining the location of the compressor:
Space: Make sure there is at least 3 feet of space around each major component.
Access: Will you need forklift access to remove major components such as engines, etc.?
Ventilation: The room should be designe so that the temperature in the . room does not exceed the outside temperature during the hottest period. Air conditioning is a must.
Suspended portal: For large water-cooled compressors. where major components must be remove for servicing, a suspended portal or. A-frame should be incorporate into the room layout.
Future Expansion: . If you think you may need space, piping, compressors,. or electrical equipment in the future, plan the layout .
8. Maintenance capabilities:
. What internal capabilities do you have for daily, weekly and monthly maintenance. ? Unfortunately, high-pressure compressors for PET. are the most expensive component to maintain when blowing bottles. Train your service team to follow the maintenance recommendations and record them daily. Maintenance at 2,000, 4,000 and 6,000 hour intervals should in a on time. If you do not have your own. capabilities, contract with an appropriate service organization. Timely maintenance is necessary to avoid downtime.
Maintenance checklist for low pressure air compressor and high pressure booster
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9. Backup System:
Facilities located in remote . locations or requiring 98% capacity may need a 100% backup system. The question is how you can get your existing system back up and running. Many companies look for a good used or refurbished system to use as a backup system,. or move their old system to backup status.
10. Energy cost:
each horsepower used for 4,000 hours at 0.08 cents per kW costs $257. If you need 100 CFM at 500 psi, you will use about 50 horsepower, which costs $12,835 . per year and represents an investment of $50,000 to $80,000. If you go to 500 CFM at 500 psi, you will spend about $64,172 per 100 hp, an investment of $20,000 to $150,000. You can see how energy costs affect the bottom line. Here are some key saving points:
The compressor should not run idle for more than 10 minutes.
The pressure differential between the. compressor and the blower should be no more than 15 psi.
Set the pressure to the smallest required to blow a good bottle.
In multi-machine systems, install a controller to ensure proper load control.
Install a flow meter to make sure there is no . increase in air consumption over time due to mold wear or leaks in the pipes/equipment.
Monthly electronic compressed air measurement bulletin
If compressed air flow, . dew point, oil, pressure and kWh measurements are not available, . how can the system be manage ? The contents provide practical examples of system. evaluation and process profiles of the tools needed.
