Selecting the right pump for food and beverage applications can make all the difference. Pumping ground beef, orange juice and tahini will require different styles of pumps. Each style of pump has a different mechanical design that gives it particular operating characteristics. In food processing, gear pumps are a popular option for multiple reasons. To aid in proper pump selection, this article will help engineers fully understand how gear pumps work and in which situations they will be the ideal choice.
Let us start with an overview of gear pumps for food and beverage applications.
High viscosity (thick rather than runny) fluids benefit from a pump that mechanically conveys rather than using pressure alone. This is the action that gear teeth provide, trapping a volume of fluid and moving it forward (a process called positive displacement). In some other styles of pumps, viscous food can get stuck or pump unevenly. Gear pumps work best when the fluid includes fat, oil or glycerin, which helps to seal and lubricate the spaces between the moving teeth.
The continuous turning action of the gears creates a steady fluid movement. In contrast, some other pumps, like air-operated double-diaphragm (AODD) pumps (also good for high viscosity fluids), have a pulsing flow.
Material flow can be precisely managed by controlling the speed and revolutions of the gears, including movement in reverse. This means gear pumps are ideal for precise metering.
Shaft deflection, which can damage a pump, is a concern with some pump styles, but a gear pump eliminates the risk of shaft deflection. As a result, the same pump can handle a range of viscosity foods at different flow rates.
Gear pumps have a compact design, making them easy to integrate into a production environment.
Gear pumps having a sanitary design is essential to food safety and compliance with food safety standards. A sanitary gear pump will use food-grade seals and Teflon gears, which eliminate the risk of metal shavings entering the food stream. A sanitary pump will not have a dead leg or other space where material can accumulate. Trapped food could become a breeding ground for bacteria.
How Gear Pumps Work
There are two designs of gear pumps: internal gear pumps and external gear pumps. Internal gear pumps have a small gear that rotates inside a larger gear that has its teeth pointing in. The small gear drives the pump, and the outside gear turns in response. The fluid enters the pump through the inlet port. Trapped between the gear teeth, the fluid is carried around the inside of the pump housing and discharged via the outlet port. An internal gear pump handles a wide range of viscosities and is self-priming. It offers better suction than an external gear design, and it is better suited to high viscosities and shear sensitive fluids.
External gear pumps have two gears that mesh and rotate in opposite directions. One gear is the driver; the other gear just turns. The process fluid enters through the inlet port and is trapped between the gear teeth. Then, the fluid is transported around the outside of the gears and discharged via the outlet port. This design of the pump offers higher flow and higher pressure than a pump with an internal gear design.
In both designs, the gears are mounted on short shafts with friction bushings securing the gears axially on each side. The wet end of the pump is isolated by a seal.
Gear Pumps in Low Flow Processes
Low flow rates are common in food and beverage processing when ingredients are combined and mixed. A pump that meters precisely is essential. Flow rates must be predictable, and the volume of fluid must be controllable. For example, operators want to know the exact amount of habanero concentrate that will dispense reliably.
A positive displacement pump, like a gear pump, delivers a fixed volume with each movement of the gear teeth. The output is proportional to the speed of the gears and the volume of fluid in the gear cavity. Flow is easily regulated by varying speed with a fine degree of control.
Gear pumps are compatible with servo motor-driven dispensing equipment. Sensors or flow meters can be fitted to gear pumps to provide feedback and closed loop control.
Typically, a low-flow application requires a steady fluid flow, in which case self-priming is essential. Gear pumps are naturally self-priming; they create suction that draws fluid into the pump without the need for ancillary priming mechanisms.
Selecting a Gear Pump for Viscosity
There is a difference between theory and practice when it comes to pump selection. The specified flow rate of a pump may not be the flow rate seen on the line because the viscosity of the process fluid impacts the flow rate and pressure. As viscosity increases, flow and pressure may decrease. That is why it is important to specify a gear pump that can handle the expected viscosity to achieve the desired flow rate and pressure.
Gear pumps are available that can handle up to 100,000 centipoises (the measure of viscosity), about the viscosity of mincemeat.
For low viscosity fluids, such as light oils or water, a gear pump can be selected that has a relatively small clearance between the gears and between the gears and casing. A tighter seal means more fluid is pumped with the progression of the gear teeth. For high viscosity fluids such as heavy oils or molasses, select a pump that has a larger clearance between the gears and housing. This may prevent the pump from experiencing clogging that could eventually damage the pump.
The space between the gears and the pump casing may permit backflow and bypass, which affects precision in metering applications. In low-flow, metering applications, close tolerances from the pump manufacturer are important to
limit this effect.
Cleaning a Gear Pump
While pumps used in food processing are preferably cleaned-in-place (CIP) by circulating a hot cleaning solution, manual friction cleaning may be the only option for highly viscous foods. Product can collect, especially near the bushings. This requires the pump to be disassembled, cleaned and reassembled. Some pumps are designed with clean-out-of-place (COP) in mind. They are easy to disassemble, have fewer parts and are designed for one-way assembly.
Comparison With Lobe Pumps
Gear pumps are not the only style of pump used for food processing. Lobe pumps are another popular choice for viscous materials. When selecting between gear pumps and lobe pumps, engineers should consider precision, cost and cleaning.
Lobe pumps have bigger cavities compared to gear pumps. This makes them good at moving thick materials but not so good at accurately metering process fluids.
Lobe pumps cost more. A similarly sized gear pump might cost half the price of a lobe pump because a lobe pump uses costly stainless-steel lobes rather than Teflon gears.
Will the pump be used with a CIP system? The Teflon-based gears in a gear pump can swell when the cleaning solution exceeds 200 degrees. If the tolerance is tight between the gear and the chamber wall, then a lobe pump may be the only option.
Is the process fluid abrasive? Steel lobes resist wear better than Teflon gears.
Talk With an Expert
Except in cases where a lobe pump will perform better, a gear pump is often a good choice for food and beverage applications. Gear pumps excel at moving viscous materials.
They handle low-viscosity fluids as well. They offer precision and predictable flow. Talk with an expert at a pump manufacturer for selection advice based on the needs of the specific application to ensure a gear pump is the right tool for the job.