What Is a Mixed Flow Pump?
Mixed flow pumps are pumps in which the angle of the fluid flowing out of the impeller is in the conical plane, with the centerline of the main shaft as the axis.
The impeller attached to the shaft rotates, and centrifugal force causes the fluid flowing in from the shaft side to flow out in an oblique direction to the shaft. Pressure is recovered by the volute-shaped casing and guide vane, allowing for a higher head.
Mixed flow pumps have a relatively high head and a large flow rate. They are often used in sewage pumps for sewage systems and drainage pumps for rivers and rainwater.
Uses of Mixed Flow Pumps
Mixed flow pumps are used for urban water supply and sewage, rainwater drainage, agricultural and industrial water, and cooling water for thermal power plants. They are also used for seawater intake, general water supply and drainage, and slurry drainage in coal power plants.
Mixed flow pumps are smaller and lighter than centrifugal pumps and can operate at higher speeds than axial flow pumps. Mixed flow pumps can be broadly classified into two types: centrifugal, mixed flow pumps and diffuser mixed flow pumps.
1. Centrifugal Mixed Flow Pumps
Mixed flow pumps have a spiral casing and recover the velocity energy of the fluid into pressure. They have a relatively high head and are often used for sewage pumps and cooling water circulation.
2. Diffuser Type Mixed Flow Pumps
Mixed flow pumps have guide vanes fixed to the inner surface of the casing, and the fluid flows along the guide vanes to recover pressure. They are widely used as large-capacity pumps for river drainage, rainwater drainage, storm surge countermeasures, flood control, agricultural and industrial water intake, and factory drainage.
Principle of Mixed Flow Pumps
Mixed flow pumps, which are classified as turbo type pumps, generally have the following major components: casing, impeller, main shaft, bearings, and shaft seal.
1. Casing
The casing houses the rotor, which consists of the impeller and shaft, and has a pressure-resistant structure to expel liquid efficiently.
2. Impeller
The impeller is made up of multiple blades that rotate to expel fluid. The angle of the fluid outlet of the blades is oblique to the axial direction.
The pressure is then recovered by the casing and guide vanes to change the flow direction to perpendicular to the axis or to the axial direction.
3. Spindle
The spindle is the part to which the impeller is attached and rotates, and transmits the necessary power to the impeller.
4. Bearing
Bearings support the spindle and impeller and are essential for stable pump operation. It is the part that receives thrust force from pump operation.
5. Shaft Seal
The shaft seal is a sealing device that seals water leakage from the penetration between the shaft and casing.
Characteristics of Mixed Flow Pumps
There are three main types of pumps: turbo, positive displacement, and other. Turbo pumps are classified as centrifugal, mixed flow pumps, and axial flow pumps.
Mixed Flow Pumps are one of the turbo type pumps and have the characteristics of both centrifugal and axial flow types.
1. Efficiency
The efficiency of mixed flow pumps is slightly lower than that of volute pumps, which are representative of the centrifugal type, but higher than that of axial flow pumps.
2. Operating Range
Mixed flow pumps have a narrower operating range than centrifugal pumps, but are wider than axial flow pumps. This allows for a flexible response.
3. Head Characteristics
The centrifugal type has the smallest head variation in relation to flow rate variation, while the axial flow type has the largest head variation. Mixed flow pumps have a head characteristic that falls between the two. Even if the head is reduced to 120%, the flow rate can be maintained at about 80%.
4. Shaft Power
While the shut-off shaft power of axial-flow pumps is more than twice the rated power, the shaft power of mixed flow pumps is almost constant over the entire water flow rate. Shut-off operation is possible.
5. Noise
Low noise throughout the entire operating range.
6. Cavitation
Compared to axial flow pumps of the same head, the cavitation performance is superior. The required suction pressure of mixed flow pumps is about 1/2 that of axial flow pumps.