Dosing pumps precisely control liquid flow rates by dispensing exact amounts of liquid per revolution or stroke. They are used in medicine, manufacturing, and commerce. There are two types of dosing pumps: variable displacement constant speed and fixed displacement variable speed. Peristaltic dosing pumps are also widely used in medicine and laboratory work.
A dosing pump precisely controls the flow rate of a liquid by pumping it in a continuous succession of small doses. They can only be used for liquids because liquids are incompressible; they occupy the same volume regardless of their pressure. For this reason, dosing pumps are positive displacement pumps: the pumping mechanism moves only the exact amount of liquid it is set to dispense in one revolution or stroke. When functioning properly, a dosing pump will not have leaks or slippage from inlet to outlet or vice versa.
Dosing pumps are widely used in laboratories, chemical plants, medicines and commercial distributors. When used medicinally, they can deliver exact amounts of medication to patients over a period of time. For manufacturing, they ensure that the right quantities of ingredients are metered into the manufacturing processes. In commerce, dosing pumps ensure that customers receive all of the liquid they have purchased. Dosing pumps are often used to dispense oils, syrups and food products, such as a steaming cup of hot chocolate from a vending machine.
There are two basic types of dosing pumps and a number of different technologies that can be used to facilitate both types. The first type is the variable displacement constant speed pump. Its motor runs at a constant speed, but the amount of liquid pumped per revolution can be adjusted. The other type, fixed displacement variable speed, varies the speed of the motor to vary the pumping rate.
The variable displacement dosing pump works on the same hydraulic principle used in hydrostatic transmissions. Four to eight small hydraulic cylinders are arranged in a circle, with pistons with roller balls protruding from the open ends of the cylinders. An adjustable swash plate or wobble plate rides on a shaft that spins at a constant speed. As the shaft turns, the lowest point on the swashplate lowers each piston in sequence by a precise amount, causing it to expel a precise amount of liquid with each revolution. Increasing the angle of swing increases the amount of movement of each piston and with it the amount of liquid delivered by each piston and cylinder at the outlet per engine revolution.
The variable speed type pump simply increases or decreases a non-adjustable swashplate shaft speed. It can have a radial arrangement of cylinders and pistons around a central crankshaft. This type is usually less expensive to produce and is by far the more popular of the two.
Another type of dosing pump, the peristaltic dosing pump, is widely used in medicine and laboratory work, also due to its ease of use. A peristaltic pump holds a length of flexible plastic tubing in a circular track while it is alternately squeezed and relaxed by a number of rollers on a rotating disc in the center of the track. This rolling action divides the length of the tube into equally sized movable segments of fluid which are drawn in from one end of the tube and discharged from the other.
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