Malfunctioning in VHS-4 high vacuum diffusion pump

Hi-Agilent technologies expert. I have installed a VHS-4 high vacuum diffusion pump in my furnace system. Recently the heater(1450w) of the pump stopped working and I replaced it with a new one . Also, I have changed the oil. But after giving the necessary maintenance treatment when I turned back the system on the High vacuum pump VHS-4 was not working suction to 10^-6 torr. However, the mechanical pump working properly, but the heater is getting hot too as compared to the previous one. By viewing through the glass site window It was observed that the oil was splashing or boiling violently during the heating of the pump and showed a change in color. what caused it to malfunction and how to rectify it. I am waiting for your kind suggestions and remedial action Thanks.

  • Hi,

    The VHS-4 pump uses a 1,450 watt heater no matter which voltage is used.  Obviously the heater part number is different for each voltage.  If you are running 120V, the heater part number is 647304205. The 208V heater is 647304210 and the 240V heater is 647304250.  It is important to label the wires when you change the heater.  

    Is there adequate water flow through the cooling coils?  Insufficient water flow can cause overtemperature conditions.  The max inlet water temp is 60-80 deg F.  The maximum outlet water temp is 120 deg F.  The flow rate should be .15 gpm on the VHS-4.  The inlet water fitting is at the top of the pump, near the inlet flange.

    If there is too little water flow I would think you would see the thermal switches pop.

    I would check the water flow first, as well as the temperature.

    Second I would just confirm what the base pressure of the rough pump is.

    Let me know what you find.



    Info and manual link:

    Manual can be found here:,%20VHS-6%20and%20VHS-250%20Diffusion%20Pumps.pdf

    The instructions from the manual state:

    1. Turn off the power to the diffusion pump and disconnect the power supply.

    2. Remove the heater cover from the bottom of the diffusion pump.

    3. Label the heater wires for proper location during installation.

    4. Disconnect the terminal leads. To prevent excessively torquing and possibly breaking the heater terminals, for:

           ❑ Tubular elements, hold heater terminal pads with pliers when loosening Philips head screws.

           ❑ Platen heater elements Use two wrenches when loosening (one on each nut).

    5. Remove the heater insulation from the bottom of the diffusion pump.

    6. Remove the nut holding the heater clamp.

    7. Lower the entire heater unit from the pump and replace the defective heater.

    8. Coat the boiler stud with an anti-seize compound such as FEL-PRO C5A or common milk of magnesia.

    9. Support the heater unit by the heater clamp, line up the hole with the boiler stud, and push the unit up against the boiler plate. Refer to Figure 3-7 on page 43, Figure 3-8 on page 44 or Figure 3-9 on page 45.

    10. Finger-tighten the nut to hold it in place.

    11. Tighten the heater clamp bolt to a torque of 250 inch-pounds.

    12. Replace the heater insulation. Refer to Figure 3-10 on page 46 or Figure 3-11 on page 46.

    13. Reconnect the heater wires and replace the cover

    The manual also has some good pictures.

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