PET Bottle Blowing Efficiency
Compressed Air Best Practices interviewed Dean Smith of iZ Systems. Mr. Smith has over 20 years experience as a compressed air energy efficiency and productivity consultant to the PET bottle blowing industry.
Good morning Dean. With stretch blow molders using PET, what are the typical energy costs associated with compressed air?
Good morning. The PET industry is in a state of flux right now. A number of new bottle blowing facilities are being brought on-line. They are in the “discovery” phase right now as they realize how challenging the required compressed air systems are to manage – from an energy efficiency standpoint. The average high-volume stretch blow molder (SBM) working with PET usually has 2,000 to 4,000 horsepower of installed air compressors with the related energy costs running between $1 to $4 million per year. This typically represents 35-40% of the facilities’ total energy bill.
What are the challenges to compressed air energy efficiency?
Rotary reheat stretch blow molding (RSBM) machines, from all the leading manufacturers like KRONES and SIDEL, provide challenges to the efficiency of compressed air systems. These challenges include:
• Significant pressure drops in the RSBM machine
• Large instantaneous air demand swings (2000-3000 scfm)
• Very large horsepower sizes for individual compressors (400-1200 HP)
• High historical pressure requirements (600+psig)
• Multiple pressure requirements in one facility
Our experience is that 15% energy savings are possible on the supply side with another 15% achievable by focusing in on the pressure drops within the blow molding machinery.
Please describe the pressure drops in the blow molding machines.
Stabilizing air pressure is our primary objective. We find that pressure is fluctuating 50-60 psi in most stretch machines and is significantly lower than expected. Correcting this can lead to increases in productivity and reduced air consumption.
We first recommend that blow molders understand the air pressure requirements of their blow molding machines. For example, the blow process consumes as much as 60% of the air. The remaining pneumatic applications, using 40% of the air for control components and packaging or decorating, will typically lower pressures. We recommend that the blow molder install a dedicated piping system to the blow air circuits.
The second step is to modify the pneumatic circuits on the blow molding machinery which is typically sized, by the manufacturer, based on average air demand rather than peak air demand. The pneumatic circuit on the molding machines consists of solenoids, regulators, and tubing which when undersized, creates pressure drops during the blow cycle. The pressure drop is really a lag in the flow of compressed air, which slows inflation and subsequent cooling of the container. Pressure drops in these pneumatic circuits can be as high as 50 to 75 psig! If sized appropriately to match the peak air demand by examining the Cv (critical velocity) of the components, we can minimize the pressure drop, increase productivity and reduce plant air pressure - which also saves energy.
The key component, in the pneumatic circuit, is the regulator. A carefully selected pressure flow control valve will always stay partially open and simply modify flow and pressure as demanded. This creates the pressure required to maximize efficiency. Regulators are designed for continuous flow and simply cannot keep up with the rapidly changing, pulsing demand requirements of RSBM.
We recently went through this process at one of the nations’ largest blow molding facilities. The blow molding machines were actually able to increase output, at the lower pressure, because the pressure inside the mold was stabilized.
How do you manage multiple pressures in one plant?
There is no one “right answer” to this question. Each facility has to be evaluated individually. You can have two to four pressures in one plant. The way PET is going, virtually all RSBM can actually run below 500 psi (34 bar) in he air header in the blow. In fact, the carbonated soft drink (CSD) containers (cold fill ) are now running below 400 psi (27 bar) and represent a very large portion of the indutry’s air demand. Meanwhile, the pre-forms need pressures from 150 to 250 psi but can laos run at significantly lower pressures if modified.
Ideally, for every pressure you need to run, you’d generate compressed air at that pressure. An air compressor designed to run at 600 psi is not as efficient as another one designed to manufacture air at 300 psi. After ten years of pretty dramatic changes in pressure requirements, there’s a real need for an air compressor manufacturer to optimize a 425 psi machine for CSD applications. Today, the industry is forced to use a 600 psi design machine that is then regulated down.
In theory, you’d have optimally designed equipment and distribution for each pressure in the system. The pitfalls are the increased capital requirements on the supply side and the second limiting factor is you would end up with a lot of piping for the different machines. In the real world, we have to take the compressors we have and work with them to optimize the system’s efficiency.
Pressure drops in the pneumatic circuits of blow molding machines can be as high as 50 to 75 psig.Some manufacturers, like Sidel, design circuits to effectively manage this issue.
What are your thoughts on supply-side equipment?
Compressor and dryer selection is the first step in the efficiency challenge but the proper application of systemic principals is more important in than in most compressed air systems because of the unique requirements of PET listed above. The basic offerings are:
• Large reciprocating compressors
• Large centrifugal compressors with unique control requirements
• New VSD offerings which have to be applied appropriately to gain the value of the VSD
• Drying technology
While the full load efficiencies of these compressor technologies are similar, it is not reasonable to think that any single technology is appropriate for all high-pressure air systems considering how different the compressors are and how widely varying the system’s needs will be. For example, the proper application of the new high-pressure VSD offerings is different depending upon the size of the facility. A smaller facility can operate well with the low-pressure compressor supplying the high pressure booster directly - but in a larger facility the need for a common low pressure system supporting the plant needs as well as the booster requirements becomes critical to maximum efficiency. Improperly applying this in one major facility, which we worked on, was wasting more than $300,000 per year in energy.
High-pressure centrifugal compressor offerings can be utilized to lower long-term system operating costs - but only if properly sized and managed based on the system’s need for turndown relative to its normal variations in sustained production loads. Using automation to coordinate the use of the high-pressure centrifugals, we have been able to extend maintenance intervals and lower those costs by more than 30% in some facilities.
Reciprocating compressors can provide excellent trim capability due to their ability to start and generate air quickly - but only if the compressors are properly sized for the instantaneous variation in air demand. Otherwise, you will end up with multiple compressors running with very high unloaded hours and wasted energy. A system we are currently working on is running more than 40% unloaded time on a set of five reciprocating compressors - obviously wasting a lot of energy AND increasing maintenance costs by that amount.
What is the most often over-looked compressed air system component?
The systemic principles which are most overlooked that sacrifice efficiency are automation and controls, storage, and monitoring pressure drop especially at the blow machines. Additionally, in larger facilities, we recommend significant data acquisition to monitor compressor and system performance because the annual energy costs for a typical individual high-pressure compressor will exceed $250,000 - so it is critical to know when that machine is not performing to its full capability.
In all larger, multiple compressor air systems, coordinating the operation of the compressors relative to the variations in air demand requires some form of automation to maintain a reasonable level of efficiency. This is particularly the case, in PET systems, because of the variations in compressor technology and the variations in air demand so typical in these systems. Maximizing the value of these controls requires a proper system design including storage, storage pressure differential, and pressure flow control valves. Normally, we start with an audit so that we can record and fully appreciate the needs of the individual system. There are a lot of common principles applicable in this industry - but in order to minimize the capital costs of modifications and/or upgrades to the system, an audit can be very helpful. For example, at these elevated pressures, unnecessarily over-sizing the air headers or a single storage tank pays for an audit many times over.
When it comes to high-pressure storage, the general principles we outlined in the original Compressed Air Challenge® technical materials can be applied, but the elevated costs of the tanks due to pressure ratings makes it important to size these vessels with as much care as possible. Our approach is to maximize the storage differential with the minimal amount of compressor power possible. This allows us to minimize the tank size (and capital requirements) and yet still achieve the required total stored air to support air demand swings and properly manage air compressor cycling.
Thank you for your insights.
To read more Plastics Industry articles, please click here.