Compressors and Vacuum Pumps that Meet Medical Demands

by the Applications Engineers at KNF Neuberger, Inc.

High quality and long life required from medical equipment are pushing advances in compressor and vacuum pump technology.

Compressors and vacuum pumps are the heart of many medical devices, such as diagnostic equipment, oxygen generators, aspirators, and nebulizers. Unlike many other applications, however, the special requirements of medical OEM’s demand more than simply a pump with the correct flow rate and pressure level.

For example, portable devices that allow patients the freedom to lead essentially normal lives are becoming commonplace. Thus, equipment size and weight are at a premium. A growing trend is the move away from hospital stays to outpatient care in clinics and offices, and even at-home treatment. Here, quiet pumps and motors are a must. Intense competition means OEMs are demanding longer life than ever before, abut at the same or lower cost than last year’s model. And whether supplying oxygen to a patient or drawing medical gases for analysis, the compressor or vacuum pump must run perfectly clean and contamination-free,. Finally, there are ever-present liability concerns. Because human lives are at risk, the devices must perform precisely as intended for the life of the unit.

Size constraints

Compact size has become a necessity for compressors and vacuum pumps in many medical devices. Smaller equipment takes up less space increasingly crowded clinics, laboratories, and doctors’ offices. Smaller equipment also appears less threatening to patients. Patients who are intimidated by test equipment become nervous, which can skew test results. The trend to more in-home care is also contributing to the demand for smaller medical devices which, in turn, is squeezing the envelope available for vacuum pumps and compressors.

Getting big-compressor performance in a small package can create headaches for designers. When required to produce a certain volume of air at a specified pressure, compressors are limited by laws of nature. The difficulty in making a compressor smaller while maintaining flow and pressure capabilities is that few options are available to the designer: increase piston size or stroke, or run the compressor faster. You don’t have much else to work with, and then you’re at the mercy of changes in atmospheric pressure.

KNF is putting more performance in smaller packages by using high-efficiency pumps and motors. Brushless dc motors are driving the market right now. Customers not only demand smaller compressors for portability, but they are also looking for longer life, protection from electromagnetic interference and, in some cases, speed control. Brushless dc provides all of that, and they are approaching the cost of the traditional brush-type dc motor.

Noise factors

Another consideration is the amount of noise generated by a compressor or vacuum pump. An inherently quiet model allows medical equipment to be used with little or no muffling. Mufflers might otherwise make the equipment significantly larger than necessary.

No hard and fast rules are recognized regarding the amount of noise that is considered acceptable, as it depends on where the equipment will be used. A good rule of thumb is to select a model that does not generate enough noise to be intrusive. In other words, it blends in with background noise. However, this level varies widely: background noise in a critical care unit measures about 50dBA, while a nurses’ station is about 60dBA. Vacuum pumps and compressors now on the market meet these tight specifications without mufflers or quiet boxes.

KNF applies a number of proven methods to reduce noise. These include external vibration dampeners, acoustic baffles, and mufflers, along with optimum valve design, strict control of part dimensions, and proper motor sizing. Another common practice is to seal the crankcase chamber, which reduces noise, but tends to impede cooling and shorten component life. KNF takes a unique approach to quiet compressor operation by intercepting noise before it reaches the pumping surface. They use an acoustically absorbent diaphragm that is separated from the working diaphragm. This second diaphragm is said to intercept noise before it reached the surroundings but not affect pump cooling or reliability.

Better components and assembly methods are key factors in making compressors quieter. Six or seven years ago, the typical sound level of a compressor was about 62 decibels from a 3-ft distance. It is significantly lower today.

Clean operation

Factors other than size and noise may not be as apparent to the use, but are just as important. Oilless vacuum pumps and compressors are necessary in most medical application because oil mist could affect laboratory test results or endanger a patient’s health. Oilless models require no lubrication and eliminate the need for costly, bulky oil-entrapment systems.

An added benefit to oilless operation is that such compressors and vacuum pumps eliminate the need for lubricant maintenance by medical personnel or homebound patients. Oilless designs provide added engineering flexibility so the compressor or vacuum pump can be oriented at odd angles or mounted in areas where regular maintenance might be difficult.

Another consideration is that may types of medical analysis equipment are exposed to diluted acids, including nitric and hydrochloric, or humid, moisture-laden air. These atmospheres can corrode and damage pumps and compressors. Corrosion-resistant materials are often available. Most rotary-vane pumps are made of case iron that will corrode. Similarly, diaphragm and piston pumps can be ordered with corrosion-resistant materials.

Plastics have a place in compressor components. Weight and cost consideration encourage the use of aluminum and engineered plastics that have the requisite quality.

Pump selection

Choosing a compressor or vacuum pump that best meets design requirements depends on the application. In general, four types used in medical equipment are rotary vane, diaphragm, wobble piston, and linear.

Rotary Vane Pumps

Rotary vane models are, perhaps, the type used most often in medical devices. This is because they offer relatively high flow capacities for their size, are simple and economical to install and operate, and produce relatively little noise or vibration. In addition, they provide smooth, pulse-free air without receiver tanks, and have low starting and running torque.

Rotary vane pumps produce vacuum or pressure with a series of sliding, flat vanes rotating in a cylindrical case. Individual vanes slide in and out as the rotor turns, trapping a quantity of air and moving it from the inlet side of the compressor to the outlet. Oilless versions feature self-sealing graphite vanes that automatically adjust as they wear, so the pump performs with like-new efficiency throughout its service life. The small amount of inert graphite dust generated by this wearing process can be a problem in certain situations.

Rotary vane applications include oxygen concentrators and segmented circulators used to stimulate blood circulation in patients’ limbs.

Diaphragm Pumps

Diaphragm models are often used where more vacuum or pressure is needed than can be generated by rotary vane units. Though rotary vane compressors are hard pressed to generate more than 30 psi of pressure, even relatively small diaphragm compressors can easily develop 60 or more psi.

The basic compressing mechanism in diaphragm pumps does not require a sliding seal between moving parts. Instead, a diaphragm flexing back and forth in a closed chamber compresses the gas. Flexing is generated by the motion of a connecting rod under the diaphragm. Only a short stroke is required to generate pressures similar to those produced by a reciprocating piston in a cylinder. Intake and discharge valves control pumping action. Common medical applications for diaphragm pumps and compressors include compact nebulizers and portable aspirators.

KNF claims diaphragm pumps offer advantages over other types, particularly if the application is contamination sensitive. A true diaphragm pump is really contamination free, as no dynamic seals are required, and the diaphragm and housing can be made from a variety of materials. The diaphragm is typically neoprene, Viton, or Teflon, and the head can be aluminum, stainless steel, or plastic such as polypropylene or polycarbonate. KNF frequently recommends Teflon coatings and diaphragms because it introduces no contamination to the system, does not absorb gases, and is easy to keep clean.

KNF’s experience with diaphragm pumps also indicates that they are long-lived devices. The limiting component on a conventional brush-type dc pump is the motor itself—it wears out long before the diaphragm, although brushless dc motors are extending diaphragm-compressor life.

Piston Pumps

Piston pumps can develop much higher pressures than diaphragm pumps, but that medical devices rarely require high pressures. The typical diaphragm pump that operates to 80 psi well meets the flow and pressure demands of the medical market. Swing-piston pumps are the design engineer’s third option when it comes to medical devices. They essentially combine the best characteristics of piston and diaphragm compressors and vacuum pumps into a compact unit. Like a diaphragm unit, swing-piston models are quiet, compact, and oilless. Perhaps more importantly, swing-piston pumps also provide pressures to 100 psi, as much as a typical articulated piston-model.

A swing-piston compressor or vacuum pump essentially mounts a piston rigidly on top of an eccentrically mounted connecting rod. The piston is surrounded by a flexible cup. The cup functions as a seal, equivalent to the rings on a piston compressor, and as a guide member for the rod. It expands as the piston travels upward, thus maintaining contact with the cylinder walls and compensating for the rocking motion.

Swing-piston units can be very compact, and are often used for in-office testing, oxygen concentrators, and analysis equipment. The ability to supply vacuum and pressure simultaneously is also useful. A swing-piston pump can provide vacuum from one head and pressure from the other. This solution is quieter, cheaper, lighter, and cooler than separate vacuum and pressure pumps.

Linear pumps

Linear pumps typically use mechanical, magnetic, or pneumatic displacement to move a diaphragm in a linear fashion—rather than flexing the diaphragm with rotary elements, as with other types of pumps. A narrow performance window, producing pressures of around 5 psi and vacuum of 6 to 7 in.Hg, limits linear pumps applications. Linear pumps are extremely niche oriented, and are well suited for applications such as inflatable hospital beds, where alternating cushions beneath the patient constantly inflate and deflate to keep the patient massaged. This application is best fit with a linear pump, because it is virtually silent and draws very little input power.

Emphasis on Quality

Virtually all medical equipment manufacturers stress product quality, and that strategy holds today as never before. Part of the push is a result of ever increasing regulatory requirements. A more direct driver, specifically with medical analytical devices, is the need to generate test results using fewer and smaller samples. This is advantageous because using less reagent reduces operating expenses and means less waste is produced. In this age of the EPA looking over the shoulder of anyone who produces medical waste, the less you generate from an analyzer, the more attractive it is to the marketplace.

However, smaller sample size means the devices must be highly accurate. Instead of analyzing a 1-cc sample, you may be working with a nanoliter. So everything in the system has to have higher resolution, repeatability, and quality. And, producing a quality product is not enough. The ability to demonstrate to an independent third party that a product is made in a repeatable fashion is now a must. For instance, all major customers have performed quality audits on KNF’s processes and products. Because of these factors, in-house quality programs are becoming essential for suppliers to medical OEMs.

In the past, FDA Good Manufacturing Protocol and military specifications were the predominant quality guidelines for the U.S. medical industry; today, ISO 9000 standards are becoming more important. Viable suppliers should have an in-house Total Quality Management program to upgrade production, procurement, engineering, and sales in line with ISO 9000.

While such programs can be expensive, experience indicates that quality also pays. Quality forces costs up, and KNF monitors the effective costs of all quality programs. We look at the effect on the bottom line, not only to get compensated for our investment, but also to make intelligent decisions as to whether a particular aspect of a quality program is appropriate for us. It’s a question of return on investment, and we clearly see where our investment in quality will give us a return we can measure in added sales.