Pump technology that comes from the aircraft industry outperforms conventional piston pumps in medical applications that require high reliability and clean operation. These so-called bellows-type pump compressors can generate partial vacuum or pressurize a fluid. A closer look at their construction helps understand its advantages.

Like other positive-displacement pumps, a bellows pump forces fluid from an inlet to a discharge area. The pumps are driven by a motor, the shaft of which carries an eccentric cam to generate oscillations. This, in turn, connects to a drive shaft or driver. The bellows is welded at its stationary end to a terminal that houses a reed-valve assembly. As the motor shaft turns, the driver moves up and down, which forces the bellows to compress and then expand with each rotation of the eccentric.

The bellows is manufactured from contoured diaphragms stamped from thin stainless steel. Two diaphragms are welded at the inside diameter to form what's called a convolution. The bellows capsule comprises several convolutions stacked on an arbor and welded at the outside diameter.

The latest technology

Compressor technology uses a specially shaped plate or diaphragm in the bellows that is well suited for pump applications including the delivery of hyperbaric-chamber oxygen. Further, the bellows and reed valves experience stress levels below the defined endurance limits of the material. This allows an almost infinite pump life.

In addition, all wetted surfaces are made of stainless steel, except the valve-assembly gaskets. These are typically made from Teflon or Viton, and in some cases, replaced by aluminum O-rings.

The pumps are welded and so do not leak media. The bellows also provides a hermetic seal for the gas being pumped. The standard leak rate is 1 × 10^-4 scc/sec He (standard cubic centimeters per second of helium), but pumps can be specified for leak rates as stringent as 1 × 10^-9 scc/sec He.

In addition, bellows pumps do not shed particles during use, so media stays pure. And there are no wearing or sliding seals in the pumping mechanism. This eliminates lubricants that could become a source of contamination. Better yet, with no seals to wear out, pump performance does not degrade over time. Additionally, motor and driver bearings are permanently lubricated and sealed, and do not contact the media being pumped. Thus, bellows pump-compressors can run for a minimum of 10,000 hours of continuous operation.

Factors to consider

In a bellows pump, the factors that affect flow are: effective area of the bellows, stroke, motor speed, and valve efficiency. Actual Free Flow = EA × Stroke × Motor Speed × Valve Efficiency. These parameters are optimized for the required flow and discharge pressures. Consider these design variations when selecting a bellows pump.

• They come with either small, single heads, or twin heads. Single heads are best for light-duty, low flow, and low-pressure applications. A typical plot curve for a single-head pump shows a performance of 0.4 scfm (standard cubic feet per minute) at free flow. At a design pressure rating of 10 psi, a single-head pump's flow rate drops to 0.1 scfm.

• Twin-head pumps, on the other hand, are better for high flow and high-pressure applications. Such pumps can be plumbed in series or in parallel. Plumbing in series connects the inlet of one head to the system, while the outlet of that head connects to the inlet of the second head. The outlet of the second head connects to the system. A pump plumbed in parallel connects the inlets and outlets of each head to the system.

Twin-head pumps are capable of up to 6 scfm at free flow when plumbed in parallel and 3 scfm when in series. At a design pressure rating of 40 psi, parallel-plumbed pumps deliver about 0.5 scfm, while series-plumbed pumps deliver 2 scfm. Thus, parallel-plumbed pumps give a higher flow at lower discharge pressures, while series-plumbed pumps give a higher flow at higher discharge pressures.

Other considerations include media temperature and discharge pressure. High-temperature models can handle gases at up to 450° F. The pumps use a pedestal-mount motor instead of an integral one (which is limited to a maximum temperature of 105° F). This configuration keeps motors away from the high-temperature gas.

And high-pressure models allow discharge at up to 100 psi. The bellows in these are made of a slightly thicker material than standard models and can withstand a higher differential pressure. This is the pressure difference from the inside to the outside of the bellows.

Lastly, double-containment pumps move rare, toxic, or expensive gases. The pumps have a primary bellows, just as in a standard bellows pumps, plus an additional containment bellows. A third, smaller bellows mounts externally to the pump head. A leak in either of the larger bellows breaks the vacuum, causing the small bellows to extend and trip a sensor, which warns of the failure. Gas that is being pumped is still hermetically sealed by the remaining, intact bellows.

Of course, bellows pumps can be modified for various flow requirements. For yet lower flow rates, pumps can be built using different eccentrics that shorten the stroke. A variable-speed motor can also be used to slow the cycle rates. Other options include modified port configurations such as VCR fittings welded to the caps, and a variety of motors such as dc, three-phase, explosion proof, or totally-enclosed fan-cooled.

HOW THE PUMPS COMPARE

A table lists the company's bellows pump-compressor models for various design requirements.

Model	Flow rate at 0 PSI (SCFM)	Design Pressure rating PSI
MB-21	0.2	10
MB-41	0.4	10
MB-118	1.0	25
MB-158	1.4	25
MB-111	1.0	40
MB-151	1.4	40
MB-302	3.0	40
MB-601	Series: 2.5 Parallel: 6.0	40
MB-620	Series: 3.9 Parallel: 6.0	40