Biomedical Engineering Reference
In-Depth Information
working chamber, similarly to a piston pump. In combination with automatic inlet and
exhaust valves, this change in volume produces a pumping action. To prevent excessive
stretching of the elastic diaphragm, a support is arranged below the diaphragm.
Simple diaphragm pumps can also be driven by an electromagnet or by a voice-coil
type of arrangement excited by an AC supply. They are less efficient than the cam driven
variety as they rely on the stiffness of the diaphragm material or a spring to perform the
pump stroke.
Depending on the type of drive used, diaphragm pumps can be classified as high
speed and low speed. Low-speed pumps operate up to about 300 strokes/min. The flow
rate,
Q
(cm
3
/s), of such pumps exhibits a reasonably linear relationship with the speed
of rotation of the drive motor. This makes such low-speed diaphragm pumps especially
suitable for metering in medical applications. The flow rate can be varied by changing the
stroke of the diaphragm and changing the rotation speed of the motor.
High-speed diaphragm pumps operate at between 2500 and 3000 strokes/min, about
10 times as fast as their slow-running counterparts. Their size is generally in inverse
proportion to their speed, which allows very compact pumps to be manufactured.
The important characteristics of these pumps include the following:
•
Reliability, thanks to their simple and sturdy construction.
•
Chemical resistance; all liquid-contact parts can be made of chemically resis-
tant materials such as polytetrafluoroethylene (PTFE), perfluorinated elastomer
(FFPM), or polyvinylidene fluoride (PVDF).
•
Can operate in any position.
•
Very compact.
•
Quiet.
•
Self-priming because they can also handle gases and gas-liquid mixtures.
•
Can run dry.
•
Maintenance-free.
•
Have long working lives.
A typical high-speed diaphragm pump running at 3000 strokes/min has the flow rate
versus head characteristic shown in Figure 8-75. Note that deformation of the diaphragm
FIGURE 8-75
Performance curves
for a small
high-speed
diaphragm pump.
