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.