Useful information on Micro Annular Gear Pumps
What is a micro annular gear pump?
A micro annular gear pump is a type of positive displacement (PD) pump. Gear pumps use the actions of rotating cogs or gears to transfer liquids. There are two basic designs: external and internal (Figure 1). An external gear pump consists of two identical, interlocking gears supported by separate shafts. An internal gear pump operates on the same principle but the two interlocking gears are of different sizes with one rotating inside the other. The larger gear (the rotor) is an internal gear i.e. it has the teeth projecting on the inside. Within this is a smaller external gear (the idler – only the rotor is driven) mounted off-centre and held in place by a crescent-shaped partition or spacer.
Micro annular gear pumps (also called gerotors or trochoid pumps) are a type of internal gear pump. The inner and outer gears are assembled eccentrically within the pump casing without a spacer. Generally, the number of teeth on the driven inner gear is one less than that of the idling outer gear. Since the number of teeth and the centre for each gear are different, the volume of the gap between the inner and the outer rotors changes throughout a cycle. The pump inlet is positioned where the gap begins to widen and this generates suction. The pump outlet is positioned on the opposite side of the casing where the gap between the gears closes.
How does a micro annular gear pump work?
The inner gear is off-centre with a trochoidal profile and the outer gear is machined with matching, intersecting circular arcs. The gears mesh precisely at only one point in their rotation but the teeth of the inner gear are always in contact with the outer gear. This partitions the internal void between them into dynamically-changing volumes. Close tolerances between the gears, bearings and case allow the pump to develop suction at the inlet and prevent liquid from leaking back from the discharge side.
There are four stages in a micro annular gear pump’s working cycle: filling, transfer, compression and delivery. The progress of fluid as it passes through this cycle is shown in Figure 2.
- As the gears come out of mesh on the inlet side of the pump, an expanding volume is created in the void between the two gears.
- Liquid flows into the void from the inlet port and is trapped by the gear teeth as the gears continue to rotate.
- The trapped liquid is moved around the casing and transferred from the inlet side of the pump …
- … across to the outlet side.
- As the gears become interlocked on the discharge side of the pump, the fluid is compressed …
- … and discharged under pressure.
- The inner, external gear has completed a full cycle and has now advanced one place around the teeth of the outer, internal gear.
For continuous dosing applications, the pump is operated by turning the pump shaft at a constant speed. For dispensing and precise dosage, the pump shaft is turned incrementally to serve up the required number of pulses.
What are the main features and benefits of a micro annular gear pump?
Micro annular pumps are compact, perhaps only a few centimetres long, and simple with only a few moving parts. They offer a smooth and adjustable flow proportional to the rotational speed of its gears. This makes them ideal for applications requiring accurate delivery or metering. For continuous delivery, micro annular pumps can provide accurate flow rates (from 1µl/h to 1 l/m) with a differential pressure up to 200 bar. For metering applications, a micro annular pump can provide accurate and reproducible doses as low as 2 µl with a precision of 1% or greater.
In continuous delivery, pulsation is low compared with other types of positive displacement pumps and is a function of the number of gear teeth. So, for example, a pump consisting of inner and outer gears with 6 and 7 teeth respectively may produce a flow pulsation of 6%. The same pump design with gears of 10/11 teeth will produce a smoother flow with pulsation of perhaps only 1.5%.
Micro annular pumps can be hermetically sealed and driven by means of powerful magnetic drives, eliminating leaks on the drive end. The gears and wetted parts can be constructed of nickel-based carbides, stainless steel, ceramics based on aluminium oxide, zirconium oxide or silicon carbide, and plastics such as PTFE, PEEK and FKM so the pumps are suitable for use with corrosive substances. Micro annular pumps can be fitted with Ex approved drives for use in hazardous environments.
Frictional wear and pump noise is small because the relative speed of the gears is low (the number of teeth on the inner rotor is only one less than that of the outer rotor). This minimises shearing forces making them suitable pumps for shear-sensitive liquids such as paint and soaps.
What are the limitations of a micro annular gear pump?
The close tolerances between the gears (typically only about 2µm) mean that micro annular pumps are susceptible to wear particularly when used with abrasive liquids or feeds containing entrained solids. Pumps should always have a filter installed on the suction side for protection from large, potentially damaging, solids.
For high temperature applications, it is important to ensure that the operating temperature range is compatible with the pump specification. Thermal expansion of the casing and gears reduces clearances within a pump and this can also lead to increased wear, and in extreme cases, pump failure.
Despite the best precautions, all gear pumps eventually succumb to wear of the gears, casing and bearings. In a micro annular pump, wear, and consequently leakages, can occur around the gear teeth, across the face of the gears and around the bearings. As these clearances increase, there is a gradual reduction in efficiency and increase in flow slip: leakage of the pumped liquid from the discharge back to the suction side. Flow slip is proportional to the cube of clearances within a pump so internal wear can have a significant effect until a critical point is reached, from which performance degrades rapidly.
Micro annular pumps continue to pump against a back pressure and, if subjected to a downstream blockage will continue to pressurise the system until the pump, pipework or other equipment fails. It is always advisable to fit relief valves in the system to protect downstream equipment.
Cavitation can occur under some conditions, particularly if the inlet tubing or upstream filter restricts flow, and this can affect dosing or flow accuracy as well as potentially damaging the pump and associated equipment.
Generally, a micro annular pump relies on the pumped liquid for lubrication and should not be run dry. It is therefore necessary to prime the pump before operation and to ensure a continuous supply of liquid.
What are the main applications for micro annular gear pumps?
Micro annular pumps are used for the continuous delivery and discrete dosage of water, watery solutions, solvents, methanol, oil, lubricants, adhesives, inks and paints as well as other high viscosity liquids. The pumps suit applications requiring the targeted dosing of small volumes of liquid, for example: the transfer of crystallizing, air-sensitive or outgassing media, fuel cell technology, the delivery of urea solution (AdBlue) to remove NOx from diesel engine exhaust gas, or dialysis and analysis technologies.
Micro annular pumps are available for liquids with viscosities between 0.2 and 1,000,000 mPa and a differential pressure range of 0 to 200 bar. They offer flow rates from 1µl/h to 1 l/min or dosages as low as 2µl at up to 100 per minute with 1% precision.
Typical applications include:
- Chemical processing
- Industrial and plant engineering
- Packaging
- Medical and pharmaceutical industry
- Mini plant technology
- Spraying
- Dispensing of adhesives
- Ink and paint dosage
- Vacuum applications
Summary
A micro annular pump (also called a gerotor or trochoid pump) is a type of internal gear pump. It consists of inner and outer gears assembled eccentrically within the pump casing. When the inner gear turns, it rotates the outer gear in the same direction. Since the number of teeth and the centre for each gear are different, the volume of the gaps between the two gears changes throughout a cycle. Close tolerances between the gears allow the pump to develop suction at the inlet and prevent liquid from leaking back from the discharge side.
Micro annular pumps are small and compact. The pumps offer a smooth and adjustable flow which is proportional to the rotational speed of its gears. In addition, incremental rotation of the gears allows precise dosing.
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