Fluid Power Journal

Oil-heater Problems with a Hydraulic Power Unit (HPU) Located in an Unheated Shed

figure it outA metal fabrication shop has a 30-year-old bending press that can bend wide flange beams to a semi-circular shape. The machine was up-graded to bend larger beams and required additional floor space. The contractor located the HPU just outside the building in an unheated shed. The contractor told the customer that the unheated shed would not cause problems because the contractor added a Chromalox brand electric heater to the reservoir, and there was a 230-volt 3-phase electrical supply close buy.

Within a month, a fork truck damaged the heater where it projected from the tank. The customer could read most of the part number and ordered another 230-volt unit. During the upcoming winter months in the Chicago area, when they started up the pump that was mounted on top of the unit, the pump started to cavitate badly. They determined the Chromalox heater was not heating the oil. They confirmed that the heater was getting 230 AC volts to the heater connection so they un-screwed the heater and had difficulty getting it out. The unit looked like a 4 to 5-inch lead pencil from the threads to the tip of the elements. The black coating was brittle and broke off into chunks that an old timer said looked like chunks of coal.  They checked the name tag, and everything seemed to be connected properly, as before.

Any idea what went wrong?

See the Solution

The system that had problems with a tank heater was generating 45 Watts per square inch and hydraulic oil should not be subjected to anything above 15 Watts per square inch. If you purchase a 3-phase 460-volt heater and run it on 230 volts with the three elements wired in a Delta configuration, you would get ¼ of the normal 45 Watts per square inch or 10.25 Watts/S.I. The rebuilder did not inform the owner that the heater was ordered as a 460-volt unit.

The new heater was ordered the same as the old as far as they knew but 230-volt 3 phase was specified causing the heater to over-heat the oil, turning it into a carbon solid surrounding the heater elements.


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