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Canned Motor Pumps for High-Temperature Applications in Solar Thermal Power Stations

Thursday, April 28, 2011 by HERMETIC

There are numerous technical and industrial processes which generally absorb or emit energy. Water and steam are often used to transmit heat within a temperature range of 0°C to 200°C, because of their high specific heat value. Machines and plant manufacturers and end users predominately make use of heat transfer oils for heating or cooling in temperature ranges from 200°C to 450°C. The following information describes the design and mode of operation for seal-less canned motor pumps, including specifics for high-temperature applications.

There are two design principles available for high-temperature applications:

a) Canned motor pump with externally cooled motors

In this design, the pump is spatially separated from the canned motor by an intermediate piece, which acts as a thermal barrier. This prevents heat transfer from the conveyed fluid to the motor. A circumferential gap equalises the pressure differential between the hydraulics and the rotor cavity. There is an auxiliary impeller installed in the motor itself which recirculates the fluid in the rotor cavity through an external cooler. This creates two pump circuits with different temperatures.

The operational circuit can be rated at temperatures of up to 450°C, while the conveyed fluid in the secondary cooling / lubricating circuit has much lower temperatures between 60°C and 80°C. Due to the pressure equalisation in the thermal barrier's circumferential gap, there is hardly any fluid exchange between the two temperature levels.

b) Canned motor pumps with internally cooled motors

If there is not enough quality coolant, or no coolant at all, available for cooling the motor, it is necessary to turn to different design principles. In addition, no matter what conveyed fluid is used, the fluid will always need to be heated before initial operation; in the pump as well as in the canned motor.

The temperatures required for this are generally in a range which exceeds the allowed maximum temperature of the aforementioned Insulation Class H. That is where the canned motors furnished with special Insulation Class C windings come into play. Windings of this type are able to withstand constant temperatures of 450°C at the winding ends. They are rated for economical motor loads of up to 400°C (temperature of the conveyed fluid). Fins at the centre of the motor's casing improve heat dissipation through natural convection.

New solar energy applications

Solar thermal power stations, particularly those placed on the Earth's 'sunbelt', are regarded as suitable industrial scale technology for creating inexpensive electricity from solar power. These concentrated systems come under the heading concentrating solar power (CSP) technology.