Net Oil Pressure Part II – Variables - 2012-01

Net Oil Pressure - Part II: Variables

Variables Affecting Net Oil Pressure?

Last month's Tech Tip explained what net oil pressure is and what it is not. This month we will discuss the variables that can cause the pressure to be too low. First, to recap:

Net Oil Pressure = Oil Pump Discharge Pressure - Crankcase Pressure

  • Net Oil Pressure only pertains to systems which use forced lubrication.
  • The crankcase pressure can be either positive or negative depending upon whether the crankcase is at interstage pressure or suction pressure.
  • Net Oil Pressure is a crucial component of the compressor's safety interlocks.

Net oil pressures vary from compressor to compressor, usually ranging from 20 to 40 psi. Most oil pressure safety controllers will shut the compressor down if the net oil pressure falls below 10 psi.

Compressor oil pressure is produced by a gear pump or lobe pump. In these style pumps, gears or lobes rotate and as they mesh oil is squeezed out into passages which carry the oil to the surfaces to be lubricated. Each rotation of the gears or lobes forces out a fixed volume of oil. The pressure required to move that volume of oil through the passages and across the bearing surfaces is the net oil pressure.

Variables that effect the net oil pressure include:

  • Compressor Size.
    Larger compressors need more net oil pressure because they have more surface areas to lubricate. The oil pump must also pump the oil greater distances and consequently deal with higher friction losses.
  • Temperature of the oil.
    As the oil gets hotter, its viscosity drops and the net oil pressure also drops. Hotter, thinner, oil requires less pressure to flow per unit volume.
  • Bearing Clearance.
    As a compressor wears, its clearances become greater and less pressure is required for the oil to flow through the channels and past the bearing surfaces.
  • Pump Wear.
    As the compressor's oil pump gears wear, their clearances become greater and it is less able to produce its design pressure.
  • Crankcase Pressure.
    Compressors which run with the crankcase at suction pressure can be affected by various faults within the refrigeration loop. Too low a suction pressure, below 28" or 29" Hg, could result in a net oil pressure that would satisfy the oil pressure differential switch while masking oil pump problems.

Next month we will discuss how to correctly calculate Net Oil Pressure.


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These technical tips are for use by qualified maintenance personnel who are familiar with their specific freeze drying equipment. These tips are intended only as general guidelines. Lyophilization equipment is frequently custom configured and some tips may not be appropriate for all freeze dryers. Always read and follow the directions of your equipment's maintenance manual. If you would like to discuss one of our tech tips, please contact Dave Clayton at 215-672-7800 ext-1376.


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