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Tools & Technical Reference

 
 

Positive Displacement Pumps: Advantages and Operating Principles

Designed to move liquid by pressurizing it, positive displacement (PD) pumps come in many configurations, such as rotary vane, gear, screw, progressive cavity, Archimedes' screw, piston, and plunger. Historically, some of the very first pumps, invented thousands of years ago for primitive irrigation purposes, were positive displacement designs. Numerous Lytron refrigeration systems use the rotary vane positive displacement configuration. Other applications for this type of pump include post-mix beverage dispensers, espresso coffee machines, reverse osmosis water filtration systems, glycol cooling loops for beer, and welding torch coolers.

Positive Displacement PumpFigure 1. In, chamber #1 is just beginning to fill,
chamber #2 is almost completely filled,
chamber #3 is just beginning to discharge,
and chamber #4 is almost completely discharged.

The working parts of Lytron's positive displacement pumps are made from brass or stainless steel and carbon graphite materials, all machined to the high precision required for reliable operation. These pumps operate by filling and discharging variable volume chambers formed by graphite vanes, which slide in and out along machined radial slots in the rotor. The rotor and vane assembly fits into a graphite jacket. The rotor and the jacket are offset to create an eccentric geometry that is essential to pump operation. (See Figure 1.)

When the rotor rotates (typically at 1725 revolutions per second), the eccentric geometry causes the vanes to slide inward along their grooves, thereby shrinking the volume of each chamber as it moves from the inlet to the outlet. Because the fluid being pumped is not easily compressed, it is squeezed out of the chamber when it reaches the pump outlet.

Note that as the rotor turns and chamber volume changes, the vanes "float" in their slot. A combination of centrifugal force and hydraulic pressure forces the vanes outward so that they remain in contact with housing's inner surface and, hence, provide an effective seal. A very thin layer of fluid between the vane and the jacket keeps friction to a minimum.

These positive displacement pumps are self-priming. Unlike centrifugal or turbine pumps, the flow rate is steady regardless of the system pressure drop. To avoid system over pressurization, the pump incorporates a relief valve set to bypass flow at 60 psi.

Positive displacement pumps are very cost-effective. For our Kodiak chillers, Lytron offers these pumps with flow rates from 1.3 up to 10 gallons per minute. The smaller pumps are available with either a brass or stainless steel housing. Brass is good for most applications; stainless steel is recommended for deionized or high purity water. If you have questions about which pump will work best in your application, contact one of our applications engineers at 781-933-7300 or send an email to info@Lytron.com.