Vacuum is a very widely used utility in the process industries – from pharmaceutical and chemical to food and biotechnology. While some applications require fairly deep vacuum – so-called “fine” vacuum – many other operations rely on vacuum in the “rough” range. These rough vacuum applications include suction operations to drive liquid movement or accelerate filtration, and evaporative applications, in which vacuum is used to dry materials or separate liquids by controlling their boiling points. Using common scales, rough vacuum is used to describe vacuum between atmospheric pressure and 1 torr (29.88 inches of mercury gauge pressure at sea level, represented as “in. Hg”). Fine vacuum refers to the range between 1 torr and 0.001 torr (29.919999 in. Hg).
Traditional approaches
Longstanding practice has been to use oil-sealed (rotary vane) vacuum pumps for many industrial vacuum applications. The technology is fairly robust, pumps can readily be re-built when needed, pumping speeds are fairly high, and the technology is suited for continuous duty. Further, rotary vane pumps can deliver pressures in the fine vacuum range. These pumps have some important drawbacks, however; they are not particularly resistant to corrosive conditions, and so need frequent service. The service demands lead to process downtime, or the need to keep back-up capacity at the ready in order to protect against unplanned maintenance. Further, while rotary vane pumps are capable of vacuum in the fine vacuum range, many process operations are best managed in the rough vacuum range, where rotary vane pumps are less efficient, and where they may actually complicate process management with excess vacuum. In addition, tightly controlling vacuum with rotary vane pumps has historically been something of a challenge.
Addressing the shortcomings
Diaphragm vacuum pumps can overcome these drawbacks and provide additional advantages in the many situations in which rough vacuum levels are a better fit for the process being considered.
- Fit to the need: When pumps designed for deeper “fine” vacuum are used for operations that do not require the deeper vacuum, the mismatch can cause problems. Operations in which the vacuum is intended to induce a pressure differential for liquid transfer, for example, can result in evaporation of process liquids when a fine vacuum rotary vane pump is used. Besides the loss of liquids, the evaporated liquids end up in the pump, contaminating the pump oil and shortening the intervals between oil changes. In a drying process, excess vacuum can cause “boiling retardation,” or “bumping” and foaming that can lead to sample losses. With dry pumps, it is easier to select a model that delivers the more appropriate vacuum, one that is a better fit to the process and that does not risk product loss.
- Oil-free operation: Diaphragm pumps are dry; they have no oil or water in the wetted path. The reciprocating drive arm moves a diaphragm that pushes gas or vapors through the pump. That means there is no oil to get contaminated with process vapors, or to break down and lose its ability to lubricate. The lack of oil changes provides an immediate savings, but also ensures that there is no contact between pump oil and process fluids or vapors. Further, by eliminating oil from the pump, there is no longer a need for the costly disposal of contaminated waste oil.
- Long service cycles: High quality diaphragm pumps can deliver long service intervals. Pumps from some manufacturers have recommended maintenance intervals of as much as 15,000 hours in normal fixed speed operation. Even in continuous duty, that means nearly two years without service interruption.
- Corrosion resistance: When wetted components are built of chemical-resistant materials such as fluoropolymers, the pumps are exceedingly resistant to corrosion. Combined with the long maintenance intervals available with some dry diaphragm pump designs, the corrosion-resistant construction further reduces service demands.
- Control: Vacuum control involves a careful balancing of pumping speed, system leakage, and vapor generation. Since vapor generation is strongly influenced by temperature, the vapor pressures of the components of the evaporating mixture, and surface area, any change in any of these variables will impact the vacuum level needed to optimize conditions. Too much vacuum, and the mixture may “bump” and cause product loss, or time lost for product recovery. With too little vacuum, the process will proceed more slowly, possibly affecting productivity or production rates. Diaphragm pumps can achieve superior levels of control that result in greater precision while reducing both power consumption and wear and tear on pump, by employing variable speed motors or variable frequency drives.
Conclusion
Confronted with all of these advantages, an obvious question is, “Why aren’t more industrial processes supported with diaphragm pump technology?” The answer is two-fold. First, as mentioned previously, certain operations require deeper vacuum than the diaphragm pump can provide. For these applications, reliance on alternative technologies – such as rotary vane, scroll, claw or liquid ring – may be needed. Second, the physics of the technology limits the size of diaphragm pumps, so they are not a practical way to produce vacuum for large processes needing very high pumping speeds. Maximum speed for a commercial, single-stage diaphragm pump is in the range of 12 cfm. When quadruplexed into a pumping station, speeds of up to 50 cfm can be achieved, with the full control capabilities, chemical resistance and service advantages previously described. This may be an appropriate capacity for a pilot plant, or for a specialty chemical or biotechnology facility, but may be insufficient to serve the needs of a full scale, commercial facility for pharmaceutical or chemical production.