Rotary vane Positive Displacement (PD) pumps may be the most commonly used pumps in re-circulating chillers today. Performance and low cost have made them the favored pumps for a variety of applications. Turbine (TU) pumps are also used in re-circulating chillers, however, due to their higher initial cost often times PD pumps are selected over TU pumps where flow and pressure requirements can be met by a PD pump.
With the requirement for increased productivity across the business landscape, recirculating chillers are now relied on to operate more hours per day. The reliability and life of re-circulating chillers are critically important to supporting the needs of today’s complex applications and operational demands. Pump selection is now more important than ever to ensure that re-circulating chillers are able to meet the demanding needs of today’s operating environment
Considerations include but may not be limited to:
Due to the stringent requirements of today’s most complex applications, the Thermo Scientific ThermoFlex re-circulating chiller platform is available with a wide variety of pumps to meet the most demanding application needs
MTBF vs. Life
For example, a battery may have a useful life of 1 hour and an installed population of 1,000,000 batteries.
If 10 batteries fail to reach their 1 hour life out of the installed population, the batteries would have an MTBF of 100,000 hours.
1,000,000 hours/10 failures = 100,000 hours MTBF.
1,000,000 hours/10 failures = 100,000 hours MTBF. What this teaches us is that component life is what matters the most when selecting equipment, i.e., we want to know how long we can expect the battery to last. MTBF speaks more to the quality of the battery, that is, it tells us the likelihood that the battery will last the full life of 1 hour.
So how does this relate to Positive Displacement pumps (PD) and Turbine pumps (TU). They both share a similar long MTBF; however, the Turbine pump has a life that is three times that of the Positive Displacement pump. This means that while they share a similar reliability during their stated life, the TU pump will run longer before wearing out. Based on life or product uptime, the TU pump seems the obvious choice. While there are other considerations for users who do not run their re-circulating chillers 24 hours a day, 7 days a week, the PD pump may have an acceptable life at a lower cost.
Materials of construction
PD pumps are contact pumps that have pump vanes and an inner shell that the vanes contact which both are made of carbon. Carbon is desirable in several ways; it is a low cost, low friction material that is easy to manufacture. Carbon also provides a positive seal capable of creating the high pressures required by many applications. Alternatively, carbon also has some undesirable characteristics. During the break-in of a new pump head the carbon on carbon contact causes fine carbon particles to be released into the recirculating flow, this is known as carbon shedding. Carbon shedding may result in a grey discoloration of any filters in use. This is completely normal and is not an indication of corrosion, algae or other contaminates. The somewhat brittle nature of the carbon means that it cannot tolerate debris. Subsequently, to maximize PD pump life, the pumps should be protected by a full flow filter of 100 microns or finer. Full flow filters are standard on all ThermoFlex re-circulating chillers.
Corrosion caused by dissimilar wetted metals, pump cavitation and non-compatible fluids can all cause damage or increased wear to the carbon. If the carbon becomes damaged, it is possible for the larger released pieces of carbon to damage the pump vanes leading to rapid failure of the pump head.
Turbine pump construction does not contain carbon. They are a close tolerance pump that requires filtration, but other than the pump shaft riding in the pump seal, there is no direct contact between moving parts which is why the life of the TU pump exceeds that of the PD pump.
Applications that require high duty cycles of the re-circulating chiller or are run 24/7 should consider using a TU pump because of its longer life. While the TU pump can be more expensive than a PD pump, the additional life and increased uptime can easily offset the additional cost for many applications. For applications with mixed metals such as aluminum and copper, where electrolysis between the metals may release abrasive aluminum oxide into the re-circulating flow, the TU pump is a better choice because the abrasion to the carbon components in the PD pump may cause the fluid to turn black and lead to premature pump failure. For applications that are sensitive to hydraulic pulsations within the re-circulating flow the TU pump may be a better choice than the PD pump. This is because the PD pump with 4 vanes delivers 4 separate volumes of fluid with each rotation. As each pump vane delivers its volume of fluid a pressure wave is sent through the fluid that may still be measured at the application. Conversely, with the TU pump for a given pump speed and application pressure loss, a continuous flow is delivered
There is more to pump selection than just matching up the flow and pressure requirements of the application; pump life, MTBF, wetted materials, duty cycle, sensitivity to down time, cost of ownership and fluid pulsations should all be considered when selecting a pump. While the PD pump continues to be suitable for many applications, longer life as well as other performance related characteristics of the TU pump make it the obvious choice for today’s complex and challenging applications where productivity and uptime are paramount.
- Equipment usage - daily operating hours
- Preventative maintenance programs
- Wetted materials in the re-circulating chiller and application
- Water types such as deionized, filtered, distilled and reverse osmosis
- Additives such as freeze point suppressants, biocides, algaecides and inhibitors
- Pressure variations (pump pulsations) for applications that are sensitive to vibrationLife = the expected time that a product can be used as intended, measured in hours.MTBF (Mean Time Between Failure) = how many combined hours of run time the installed population of a product will operate until there is a failure that falls within the stated life of the product.