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Means to Increase Heat Transfer from an ESP Motor Disclosure Number: IPCOM000015479D
Publication Date: 2003-Jun-20

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• Disclosure: This invention relates to a means to increase the rate of heat transfer from a submersible electric pump motor to the fluid flowing past the motor. • Background: Electric submersible pumps (ESP) are widely used to pump water and oil from well bores. ESP's used in the petroleum industry are typically constructed as combined unites wherein a long (20 feet or more), small diameter (7.25 inches or less) electric motor is directly connected to and drives a similar long, small diameter, multi-staged centrifugal pump. The ESP is lowered into the well bore and submerged in the fluid being pumped. The pumped fluid flowing over the outside of the motor cools the electric motor. Typical ESP motors operate at high power densities and without this cooling they are quickly damaged by overheating. When the fluid being pumped has a low viscosity, a high specific heat and a high thermal conductivity (such as light oil, water or water cut oil) adequate cooling is seldom a problem. When the fluid has a high viscosity, a low specific heat and a low thermal conductivity (typical of highly viscous crude oils), the ESP motor is forced to operate at a much higher temperature in order to reject internally generated heat. This high operating temperature reduces motor life and results in short run times. Heat transfer, from the motor's outside surface to the fluid flowing last the motor, occurs according to the question: Q = hc A (Ts – Tf) Where: Q = rate of heat transfer hc = rate of transfer coefficient A = surface area Ts = temperature of surface Tf = temperature of fluid For a given amount of heat generated by the motor, increasing the surface area and/or the heat transfer coefficient can lower the surface temperature. The heat transfer coefficient represents the complex interaction of the fluid's thermophysical properties, the temperature differentials, the velocity of flow and, to a lesser extent, the geometry of the flow path. The thermophysical properties of a fluid, at any given temperature, are relatively fixed and unalterable. Increasing the velocity of flow has only a small effect upon the heat transfer coefficient of highly viscous fluids. This leaves increasing the surface area as the most promising way of lowering surface temperatures of ESP motors operating in highly viscous fluids. The addition of "cooling fins" to a heat transfer surface is a widely used means of increasing the surface area. The design criteria, predictability of performance and effectiveness of cooling fins have been widely investigated and are indisputably established. The most effective cooling fins are relatively thin and somewhat fragile. The challenge is how to utilize such cooling fins in the harsh deployment and operating environment of well bores. • The Invention: The following description will be best understood by referring to the enclosed sketch. A sheet of thin, corrugated metal is wrapped around the outside of the ESP motor. An outer tube is then placed over the outside of the corrugated metal wrap. The height of the corrugations and the inside diameter of the outer tube are such that the corrugations are forced into intimate contact with the motor's outer surface and the outer tube's inside surface. Those skilled in mechanical design will recognize several means of accomplishing this arrangement. The fluid being pumped from the well now flows through the finned annular space between the pump and outer tube and the annular space between the outer tube and the well casing. • Heat Transfer Analysis: Two computer based heat transfer analysis were made to establish the effectiveness of the invention. The first analysis was for an ESP motor surrounded by an outer tube or shroud. The second analysis is for an ESP motor surrounded by a corrugated metal wrap and an outer tube or shroud. The data for the first analysis is as follows: Thermophysical Properties Of The Well Fluid: Viscosity---------------------------------------------------1000 centipoise 180 F Thermal Conductivity-----------------------------------0.07 btu / hr ft F Specific Heat---------------------------------------------0.50 btu / Ibm F Density----------------------------------------------------60 lbm / ft3 Volume Coefficient of Thermal Expansion----------0.00038 1/F Flow Rate-------------------------------------------------1000 BPD Motor Horsepower--------------------------------------100 Motor Efficiency----------------------------------------85% Motor Length--------------------------------------------20 feet Motor Outside Diameter-------------------------------5.62 inches Shroud Inside Diameter--------------------------------6.094 inches Shroud Outside Diameter------------------------------7 inches Shroud Length-------------------------------------------20 feet Temperature Of Motor Outer Surface Is 580 F (see enclosed computer printout sheets) For the second analysis all data remains the same except a corrugated metal sheet is now in the annular space between the motor's outside surface and the shroud's inside surface as described in "the invention". The corrugated sheet is 16 gauge (.062 inches) corrugated with a sine wave having an amplitude of .119 inches and a pitch of .5 inches. Temperature of Motor Outer Surfaces Is 354 F (see enclosed computer printout sheets) The motor is now unning 226 F coolor when the fins are in place.