Industrial Utility Efficiency    

Double-acting Reciprocating Air Compressors for P.E.T. Bottle Blowing

P.E.T. Stretch Blow Molding

In the 1970’s, the beverage industry began using P.E.T. (Polyethylene Terephthalate) 2 liter plastic bottles primarily for carbonated soda. The most prevalent bottle-blowing machine in use today is the stretch blow-molding machine. A preform plug is inserted into the blow-molding machine, heated, and then compressed air is injected, “blowing” into the preform to create the bottle. These machines can produce from 1100 bottles per hour to 13,000 bottles per hour, depending on the volume the user requires.


Double-acting, Oil-free, Water-cooled, Reciprocating Air Compressors

Most P.E.T. bottle-blowing machines require anywhere from 550 psig (38 barg) to 580 psig (40 barg) and an air flow of 247 cfm FAD (420 m3/hr) to 3700 cfm FAD (6290 m3/hr). The air compressor technology used most prevalently for this application is the double-acting, oil-free, water-cooled, reciprocating air compressor. With the higher pressures and air flows that are required, the P.E.T. bottle blowing market is a strong niche market for the double-acting oil-free reciprocating (piston) compressors. These compressors are heavy duty, long lasting and highly efficient. A double-acting piston air compressor compresses air on both the up and down stroke of the piston and they are available in multi-stages to provide the higher pressures and air flows required for P.E.T. bottle blowing.

These air compressors are available as oil-free. This means that no oil is injected or enters the air compressor cylinder from the crankcase of the compressor. The oil is kept in the crankcase and the cylinder is separated from the crankcase by a distance piece with packing to prevent oil from travelling up the piston rod. The air compressor does not add any oil to the air, which reduces the amount of oil that is passed downstream of the compressor. These oil-free compressors do not add any oil or hydrocarbons to the air that is being compressed - extending the life of coalescing air filters used downstream of the air compressor.

Another new development is the availability of water-cooled motors. The water-cooled motor is more efficient than air-cooled motors and reduces the radiated heat from the motor to the atmosphere. This keeps the compressor room temperature down. The water-cooled motor provides more usable horsepower with less energy. The increase in efficiency is about 2.5% depending on motor size and shaft power requirements. Since the compressor is water-cooled, the small additional requirement for motor cooling is negligible. The reduction in energy cost to produce usable horsepower more than offsets the additional costs for the cooling water. With the elimination of cooling fan noise, the sound level for the water-cooled motor is typically 70 db(A) - a significant reduction when compared to air-cooled motors. To-date the water-cooled motor is used more in Europe but is beginning to be considered and implemented here in the U.S.


Gardner Denver Bellis & Morcom Compressor for P.E.T. Bottle Blowing Applications

Gardner Denver’s Bellis & Morcom Air Compressors

Gardner Denver’s Belliss & Morcom double-acting, oil-free, high pressure, air compressors are a natural fit for P.E.T. bottle blowing applications. Mike Bakalyar, Manager Enhanced Services at Gardner Denver, said, “Belliss has a prominent market position with a market sweet spot of 200 to 400 HP (150 to 299 kw)”. Gardner Denver sells direct to the P.E.T. bottle blower and Mike continued, “This is a perfect vertical market application for Belliss & Morcom products.”

The Belliss & Morcom compressors are manufactured in the United Kingdom and they come in 50 HZ or 60 HZ electrics. Mike Bakalyar clarified, “For U.S. customers, the compressor is brought in and either sent to the customer directly, or is packaged to meet specific customer requirements”. Packaging can include unitizing ancillary equipment such as control receivers, dryers, filters, and control panels - or any other additions to the base compressor.

The air compressors are typically direct-driven using a flange mounted motor with the rotor attached to the crankshaft. They may also be supplied as v-belt driven if this is a customer requirement. Mike Bakalyar said, “Gardner Denver utilizes a systemic approach when working with a P.E.T. customer and user and shows the customer the total life cycle cost and based on that a decision is made on which Beliss & Morcom model should be used.” Because the Beliss & Morcom compressors are in a “V” design and are balanced, there is no extra mounting cost required. The plant floor just needs to be able to handle the total static weight of the compressor system.

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The Systemic Approach to Designing a System

The large P.E.T. bottle manufacturers have been consolidating over the past few years, and users of PET containers are placing bottle blowing lines in their filling or processing plants. These moves are creating new compressed air system designs. Mike Bakalyar commented, “We are working with many customers who have been users of P.E.T. containers for years but are new to the blow molding processes. Gardner Denver has the capabilities to guide these customers towards best practices in compressor selection, control strategy, and system interface - all of which influence total cost of ownership”.

Collecting data on operational performance helps to peel the onion and expose waste in both supply and demand segments of the system. There are many cases when applied conventional wisdom can have very costly long term consequences. Bakalyar said, “We work with the customer to ensure there is a clear understanding of how system design choices will influence total life-cycle costs. It is our objective to help the customer avoid unintentional cost leakage in the support of their manufacturing processes.”

A systemic approach to designing a system, can include taking a look at what the air demand is and whether it is a constant demand or cyclical one and how many compressors the customer currently has. Depending on how much air demand is going to be added or perhaps subtracted, a determination will be made if it will be better to add compressors or to go to one large compressor. A total operating cost analysis, with all the proposed compressors and ancilliary equipment, will then be performed. Routine maintenance costs are included based upon the recommended maintenance intervals. Even though a large horsepower double-acting compressor can run effectively and efficiently at part load, it might be better to have more than one compressor so that you have built in a back-up compressor for maintenance periods and in the event a compressor is down for repair. Therefore, when you have more than one compressor with controls that will bring on each compressor as needed by the air demand, this is sometimes is more cost effective than having one large compressor that runs with a part load requirement. The systemic approach works with the user to determine which scenario will provide the user with the most effective and cost efficient system.


Bellis Graph

Example of a Compressor System Design for a P.E.T. Application

Compressor Controls for Energy Efficiency

As the price of electricity and water increase, P.E.T. bottle blowing manufacturers are becoming increasingly of the energy costs of their compressed air systems. Bellis & Morcom air compressors have compressor controls designed to operate the compressor in the most efficient mode possible. There are three main control systems: 1) Step Loading, 2) Variable speed and 3) Air Recovery.


Step Loading Control

The Step Loading Control will hold the suction unloading valves (SUV) open so that no air is brought into the compressor and no compression or pressurization of the air is taking place. The compressor is still running but no air pressure is developed. In this mode the compressor is operating at 8-10% of Full Load Brake Horsepower. So, if you have a compressor that requires 210 brake horsepower at full load, when you are in this mode your brake horsepower will drop to 16.8 to 21 brake horsepower. Mike Bakalyar said, “Because unloading is nearly instant, the power curve is essentially straight between full load and the unload point. Since this basic standard control is very efficient the improvement provided by the following enhanced controls needs to be carefully considered and properly applied to gain meaningful operating savings.”


Variable Speed Control

With the variable speed controller, you have to be careful not to slow the compressor down below the minimum speed that will affect the lubrication in the bottom end. The variable speed controller can lower your speed to 50-60% of full speed, keeping power consumption linear to the demand being supported. Variable speed control also lowers operating unloaded brake horsepower to 4-5% of full load brake horsepower. For our 210 full load brake horsepower example, this reduces your unloaded power to 8.4 to 10.5 brake horsepower. This again is a dramatic drop in electrical costs. Variable speed control is most effective when applied in a single compressor application or assigned to a designated trim compressor in a multiple compressor system.


Recovery Mode Control

The Recovery mode controller allows users to use some of the “blow-by” air. Since the air used to blow the bottle material into the mold at high pressure is vented at the end of the cycle, it is possible with some mold designs to capture a portion of this vented air. The air recovery mode control will capture this air and feed it back to the compressor. In this mode the recovered air is introduced at the second stage of the compressor eliminating the power associated with compression in the first stage. The compressor in this control mode is performing the role of a booster since the first stage work is recovered from the process. The benefit here is that you have spent energy and money to get the air to 550 psig and by the time it comes back to the 2nd stage it is at around 150 psig, it will take less work (and money) to increase the pressure back to the desired 550 psig.

Another option for some P.E.T. bottle blowing users is to capture the “blow-by” air from the process and feed it into their plant air system which will lessen and sometimes eliminate the need for their plant air compressor to operate - except in peak plant air demand periods.


Control Automation

Control automation and system management options can ensure that multiple compressor installations are operated at best practice efficiency. System design configuration and associated control strategy is designed to utilize the best combination of the above control options to deliver the lowest possible energy consumption at all levels of demand and to ensure component reliability. Mike Bakalyar indicated, “We approach applications from a systemic view and design to consider issues such as load cycles, operating base load assets at demand target pressures, utilizing design capacitance, and other factors that work together to influence total system costs.”



A double-acting piston air compressor compresses air on both the up and down stroke of the piston and is available in multi-stages to provide the higher pressures and air flows required for P.E.T. bottle blowing.



Double-acting, water-cooled, oil-free reciprocating air compressor systems provide the P.E.T. bottle blowing industry with the pressure, air flow and cost effective controls required to operate the energy-efficient and reliable systems. Despite the changes in the manufacturing landscape, one thing that remains clear is that with all the bottled water and carbonated sodas being consumed worldwide, the P.E.T. bottle blowing market will be around for some time to come, and with it the double-acting, water-cooled, oil-free reciprocating, air compressors.


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