by Eric Pepe, KNF Neuberger, Inc.
High quality and long life required from medical equipment are pushing advances in compressor and vacuum pump technology.
A number of breakthroughs in pump technology have occurred over the past several years, including better materials, advanced controls and more efficient construction, giving engineers more design flexibility than ever before. While a pump is generally the heart of a system, clear guidelines regarding pump selection are often lacking.
Too often, standard products are purchased too late in the design process to meet the complex, dynamic needs of a system. Instead, a modified design should be investigated that will more closely meet specific system needs. Most designers tend to think of pumps as commodities, rather than dynamic subsystems, because most pump suppliers provide standard products that cannot be modified to meet specific requirements.
Today, more and more suppliers are willing to modify their products in order to optimize their product to meet the needs of the design. Frequently, the resulting pump is less costly than standard product.
Performance SpecificationsAmong the types of pumps appropriate for modest fluid flow rates, diaphragm, rotary vane, peristaltic and linear pumps are used most often. Performance can vary dramatically depending on the equipment and the environment in which they are placed. In addition, system performance varies as conditions within the equipment change. For example, failing to consider temperature or electrical power variations beyond defined tolerance limits could cause the pump to malfunction and shut down the entire system.
Critical performance requirements:- The type of medium being pumped,
- The temperature of the medium,
- UL, CE or other approvals required,
- Physical dimensions of the pump,
- Production quantity required,
- Actual conditions at the pump inlet and outlet ports,
- Electrical motor requirements,
- Ambient operating temperature,
- The pump's duty cycle and expected lifetime.
Most pumps must be at a noload condition on inlet and outlet to restart properly. However, some pumps can be modified to start against vacuum or pressure. If this condition exists within the pumping system, the designer must communicate this to the pump supplier.
Otherwise, a standard pump, not capable of starting under load, may be integrated into the device, resulting in the need for a complex venting mechanism.
The Importance of Tolerance
It is critical that designers specify the systems tolerance to various performance requirements, including electrical power, temperature, flow rate, vacuum and pressure.For example, specifying a pumps power requirement at 115 VAC is not enough. Rather, if a pump operates in a system that varies up to 10 V, specify 115V with a tolerance of +/- 10V. Will the pump operate on 115V/60Hz exclusively, or will it operate in Europe as well, requiring that it operate with both 50 and 60 Hz current?
Rather than defining one static vacuum and flow rate, designers must specify the tolerance of flow rate for a range of vacuum or pressure.
For example, if a pump is capable of creating a vacuum greater than that required by a device, and the device contains elements that can be adversely affected, resulting failure can cause equipment shutdown; likewise if too high a pressure is developed.
Diaphragm Pump Advancements
Diaphragm pumps now offer a multitude of solutions not previously available to the designer. Typically, pump heads were constructed of aluminum or stainless steel, and coated with PVDF. Newer models now include parts intricately molded of RytonŽ engineered plastics for excellent rigidity, longterm stability and lower cost. Similarly, solid PTFE head parts are now available for pumping problem corrosives.
One recent design improvement is in the valve system. Traditional designs use one large valve port sealed by an elastomer flapper plate. This design has flow rate limitations, and proved to be unreliable in the presence of liquid condensate.
The newest multi-port designs use a series of smaller holes arranged in a circle. These holes are covered with an elastomeric disk. Because aggregate port volume is larger, flow is increased. An added benefit is that it is now tolerant to any liquid condensate encountered in the gas stream.
Molded, Structured Diaphragm
The pump's diaphragm has also seen many changes over the years. Traditionally, the diaphragm consists of a flat elastomeric sheet clamped in place around the edge. It is fastened to the oscillating connecting rod at the center using a screw and metal disk. Material is machined from the head to provide room for this disk, resulting in unproductive dead space.
An initial improvement involved molding a threaded stud into the diaphragm. The resulting smooth diaphragm surface fitted tightly against the pump's head, eliminating dead space, and improving end vacuum capability. Today, a computer-designed structure, molded into the undersurface of the diaphragm, provides control over diaphragm flexibility for the first time.
The resulting improvements in efficiency can be observed in the progressive reductions in size of today's diaphragm pumps. Not only are these pumps smaller and lighter than their predecessors, they benefit by increased performance in flow, end vacuum and reliability. Pump failure is usually not the fault of the pumping system, but rather miscommunication of the complete system requirements to the supplier. Working closely together, designers and suppliers can select the right pump to better match a system's requirements.
