Compressed air for PET blow-moulding production

Compressed air is critical to blow moulding production and the opportunities to improve supply side efficiency are highly prevalent.

Measurement and recording of actual pressure flow inside the production equipment can assess the actual performance of the compressed air system. This identifies any areas where compressed air problems are causing limitations on productivity and quality of production, the improvement of which in turn can lead to lower energy use costs and increased production rates.

All blow moulding processes require stable compressed air pressure delivered to the moulding machine to control quality and maintain productivity. In most blow moulding processes, compressed air is used to inflate the parison, a tube-like piece of plastic with a hole in one end through which compressed air can pass. The compressed air also cools the part after inflation to final form, but prior to ejection from the mould.

In PET bottle blowing, high-speed machines use compressed air to produce bottles at rates of over 20,000 bottles per hour. The rate of pressure rise becomes dependent upon the pressure differential driving the flow from the air inlet of the machine to the cavity. The higher the inlet pressure the faster the rate of pressure rise.

Increasing the system pressure is a common way to maximise productivity and still produce good product. Unfortunately, higher pressure leads to wasteful artificial demand, elevated compressor energy and maintenance costs, and inefficiency in managing the system.

The real costs of higher system pressure

Blowing the part as quickly as possible leads to very high rates of flow in supply components creating high pressure drop. A blow machine running 24,000, 500 mL bottles per hour can consume 90 m³/min depending upon setup creating significant pressure drop in the headers and filters delivering the air. In order to make acceptable bottles with this level of pressure drop the system has to operate at dramatically higher than necessary pressure.

This higher than necessary pressure means each bottle requires a greater volume of air, and because the header pressure is elevated to increase the inflation pressure differential, the blow pressure continues to rise to higher than required pressure after the bottle is fully moulded. For every bar of pressure increase above the required blow pressure, the volumetric flow required increases by the volume of the bottle. For example, one bar in excess pressure for a 500 mL bottle times the production rate equates to 1.5 m³/min in artificial demand.

Higher maintenance and energy costs on the compressors

The most common compressor for achieving these pressures is a 3-stage reciprocating machine which uses valves to control the flow of air through the stages. At these higher pressures the temperatures are higher, increasing the stress and wear on critical components. Where it was possible to substantially decrease the discharge pressures, maintenance cycles are extended by as much as 25–30%. Power is also reduced at the lower discharge pressures by a ratio of 1% energy reduction for every 5% pressure reduction. A reduction of 700 kPa or 16% will mean about a 3% energy reduction at the compressors.

Capturing the efficiency opportunities

The first step in capturing efficiency opportunities is to minimise the pressure drop within the moulding machines, which normally requires removing and/or replacing pneumatic components with those of higher flow capability. The regulators and filters are critical items and must be examined closely by measuring the pressure drop while the machine is blowing bottles. Localised storage receivers can minimise pressure drop by supporting the very high rates of flow during each blow cycle with stored air. This storage must be located as close to the point of consumption as possible; for example, it must be tied into the pneumatic circuit after the filter and regulator to be of any value.

System management

Managing this level of pressure change requires significant modifications in the approach to system management. While compressed air storage tanks can be expensive, maximising the storage is essential with most compressed air systems as the lack of appropriate storage is even more costly if additional compressors are required to run part loaded to deal with the rates of pressure change.

An appropriate automation system which calculates the rate of pressure change and makes intelligent decisions regarding the appropriate supply-side response can make a significant difference in energy costs and reduce compressor cycling, wear and motor starts. Avoiding unnecessary compressor starts due to the rate of change can mean saving many thousands of dollars in energy costs per year.

Kaishan Compressors offers an assessment and advisory service for upgrading, replacement, design and installation of new systems to match production demands.