ONW Shell and Tube

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Natural Convection Oil Flow
Unifin's first introduction to transformer cooling problems was more than half a century ago. It began by supplying coiled tube assemblies, which were placed inside transformer tanks. Unifin's initial expertise was the manufacture of "Integral Finned Tube" which when applied to tank coolers meant increased capacity over formerly plain tube coolers. A smaller cooler resulted and the transformer rating was often increased. The internal ONW was born and Unifin's developing leadership in transformer cooling was started.
Industry advances meant higher ratings. Tank coolers were too small so the demand for a larger capacity external water operated cooler with natural oil convention was created. Unifin designed and tested prototypes and developed ratings for a series of sizes designated "Type ONW". Industry requirements for maximum safety led to Unifin's development of the Double Concentric Tube, which combined with Double Tubesheets yield what Unifin calls its "Leak Detector Design". The ONW's latest format continues in use today.
The most efficient transformer oil cooler, the water cooled "Type ONW" was conceived by Unifin as larger demand/capacity for external water operated coolers with natural oil convection. Industry requirements for maximum safety led to Unifin's development of the double concentric tube combined with double tube sheets, Unifin's superb leak detector design. This shell & tube heat exchanger uses natural oil convection on the shell side and water through the tube. It is available with Unifin's exclusive leak detector feature, which immediately detects any leak before water can get into the oil and endanger the apparatus.
Unifin International's External "Type ONW" has largely replaced coolers mounted inside a transformers oil tank. It is mounted near the top of the tank where hot oil enters the cooler. Water in the tubes cools the oil as natural convection carries oil downward through an external pipe to enter the bottom of the transformer tank.
 Features and Options:

  • No internal water piping subjected to hazardous leaks, erosion of bends and corrosion inside the tank
  • Cooler location high in the tank and piping to the bottom of the tank increases natural convection and prevents mixing of hot and cold oil
  • Cooler arrangement of tubes and increased oil flow gives higher velocity and increased heat transfer and efficiency
  • Unifin's prototype testing provided a reliable basis for the rating of industry coolers
  • Double Tube sheet construction eliminates the hazard of leaking tube joints
  • Straight tubes reduce the risks of corrosion/erosion of bends
  • Inspection and cleaning of the tube side is made possible
  • Coolers are water drainable to prevent damage during freezing weather shutdown
  • ONW's design using Double Concentric Leak Detector Tubes removes risk of a tube leak, a hazard always present when water pressure exceeds oil pressure

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