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When the Pressure's On ... Articulating Piston Compressors Come Through

Glen Hatten, Product Manager, Thomas Products Division, Gardner Denver, Inc

They've been used by OEMs and their customers since the dawn of electric-motor-driven compressors. And despite the emergence of alternative technologies -- wobble piston and diaphragm among them -- articulating piston compressors remain a preferred source of air pressure and vacuum for many applications.

Robust and environmentally tolerant, articulating piston compressors tend to be the choice for the most demanding situations. Their straight up/straight down piston motion is tailor-made for delivering high-pressure, high-volume airflow -- and for stopping and restarting against back pressure. Plus, today's oilless designs eliminate contamination concerns, so that articulating piston compressors can be used even in critically clean medical, laboratory, and beverage systems. (Shown here, Thomas TA Series compressors, which use direct current motors; ac-powered models are also available.)

A Bit of History

Originally, there were only oil-lubricated compressors. Though efficient and long-lasting, oil-lubed compressors -- even when fitted with output filters -- inevitably introduce some lubricant into the air stream. For certain purposes, that's no problem. Heavy-duty air tools, for example, easily tolerate a little oil in the stream. But other applications, including many in the OEM arena, require ultraclean air so that precision parts or finished products are not contaminated.

In 1954, engineers at Bell & Gossett attacked the problem with a new articulating piston compressor design using carbon rings, instead of an oil bath, to reduce friction. The age of oilless articulating piston compressors had begun. In the mid-1960s, Bell & Gossett improved this design by changing from a black carbon ring to a polytetrafluoroethylene (PTFE) ring. This extended the compressor's service-free life considerably, though it initially limited its potential size. Further refinement solved this problem. In 1970, ITT moved the Bell & Gossett compressor division to Monroe, LA, under the new name ITT Pneumotive. Much development in articulating piston design followed. In 1980, ITT Pneumotive introduced articulating piston compressors up to 10 hp. The articulating piston compressors sold today by Thomas, which acquired the Monroe plant in 1989, are evolutions of these early models.

How an Articulating Piston Compressor Works

The key to articulating piston technology lies in the "joint" provided by a wrist pin, which creates independent motion of the piston. This independence of motion allows the piston to travel in a perfectly linear plane (straight up/straight down) relative to the cylinder wall. A high degree of compression as well as a high rate of flow at high pressure can be achieved with this design. In addition, clearance between the piston and cylinder wall can be tightly controlled, leading to maximum performance and efficiency.

An articulating piston compressor uses the same operating principle as a car engine, but in this case converts rotational energy from an electric motor into a reciprocating motion used to compress air in a cylinder. The rotational motion is transferred through an eccentric and bearing assembly (colored yellow in the diagram at right) to the connecting rod, on through the wrist pin (colored blue) to the piston itself.

As the piston moves down inside the cylinder bore, air is drawn in through a one-way intake valve. On the upward stroke air is compressed and exhausted through another one-way valve. Two piston rings, each backed with a spring to keep them in contact with the cylinder wall, provide an airtight seal. A piston skirt is also used to guide the piston in the cylinder. All components are designed for long, service-free life.

Some Key Applications

Beverage Dispensing: Compressed nitrogen systems improve beer and wine dispensing. For beer, nitrogen (often blended with carbon dioxide) provides a better head; for wine, nitrogen reduces or eliminates oxidation. Membrane-based air separation systems, using pressurized air, allow nitrogen to be generated rather than purchased in cylinders. Because they are designed for continuous operating at high pressure (175 psig for this example), articulating piston compressors are perfect for this application.

Telecommunications: Cable pressurization protects dielectric transmission lines from corrosion caused by water ingress. A compressor, coupled with a drying device, pumps pressurized air into the wire jacket to reduce moisture ingress and help maintain line integrity. Though pressures are low -- typically from 2-10 psig -- high airflow and long compressor life are essential, especially in remote locations. Articulating piston compressors, designed to run continuously for thousands of hours between service intervals, are commonly applied. (Thomas GH Series compressor, shown here, is equipped with high-torque capacitor start ac motor.)

Dental Tools: Multifunction dental carts -- incorporating drills, suction devices, and aspirators -- work with compressed air and vacuum on board, or drawn from stationary units, typically installed in the basement. Dental carts need high volumes of compressed air at high pressure. The compressor must be able to restart against 80-100 psig back pressure and also be able to withstand considerable on/off cycling. Articulating piston compressors, equipped with high-torque motors, provide the "oomph" to handle this very demanding application. (High-performance HP Series twin-cylinder compressor from Thomas is often used with dental carts.)

Tire Inflation: Central tire inflation systems (CTIS) are used in over-the-road semi-trucks, as well as in off-road vehicles. A CTIS enables the driver to adjust tire pressure depending on performance requirements or road conditions. CTIS is just one example in the fastest growing market segment for articulating piston compressors: mobile applications using low-voltage dc motors. Mobile applications, which also include air brakes and air suspensions, involve dirty, harsh conditions. With specially sealed and treated components, plus characteristically high duty cycles, articulating piston compressors are the obvious choice.

Specifically designed for mobile applications: Mobile Application Specialist: Featuring enclosed designs in both the motor and compressor sections, the new E Series articulating piston units from Thomas are designed for the wet and dirty conditions found in mobile vehicle applications.

The E Series compressor section is an oilless design using dual cast iron cylinders, high strength stainless steel valves, and PTFE (polytetrafluoroethylene) piston rings and skirts. Motor and compressor bearings are permanently lubricated and sealed. The permanent magnet dc motor is totally enclosed, fan-cooled, and industrially rated. Both 12 Vdc and 24 Vdc versions are available.

Because of its efficient design, the E Series compressor achieves high performance while operating at 1,800 rpm. This ensures significantly extended service life compared with units operating at 3,600 rpm, and also contributes to quieter operation. E Series compressors provide up to 150 psi (10.3 bar) of pressure. Maximum flow is rated at 3.35 cfm (93.8 lpm).

Fire Protection: Dry pipe sprinkler systems provide automatic protection in low-temperature spaces such as warehouses, loading docks, and walk-in freezers. These systems are constantly filled with pressurized air or nitrogen gas, which keeps the pipes clear until water is called for to extinguish a fire. Articulating piston compressors, equipped with high torque motors and designed to start against the system's full rated pressure without fail, are ideal for the strict requirements of dry sprinkler systems.

Getting the Most From Your Compressor

Nothing lasts forever, including an articulating piston compressor. But certain conditions can contribute to extended compressor life and top performance, including:

  • Operating at lower than rated pressure
  • Intermittent operation with infrequent cycling
  • Good ventilation
  • Temperatures at or below 80ºF (27ºC) and above freezing
  • Clean environment
  • Proper ac input voltage, or clean dc power (depending on type of motor).

In an ideal world, all applications would align with these conditions. Still, as we've said, articulating piston compressors are workhorses that go where other types of compressors fear to tread -- including in dirty, hot, and cold environments, and in applications having high pressure and high cycle requirements.

The main thing is to work closely with your compressor supplier and its application engineers. That way, you'll be sure to have an articulating piston compressor that meets your application requirements and provides reliable pressure or vacuum performance over thousands of hours of service.

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