Net Oil Pressure Part III: Calculating Oil Pressure
Last month's Tech Tip discussed the variables which can affect net
oil pressure. This month we will discuss how to calculate net oil pressure.
First, to recap:
Net
Oil Pressure = Oil Pump Discharge Pressure - Crankcase Pressure
With
some refrigeration compressors it is possible for the crankcase pressure to be
below atmospheric pressure. In these compressors, for example the Dunham-Bush
® 20- and 30-HP
"Big 4" compressors, the suction line feeds into the bell housing of
the crankcase. There are pros and cons to this arrangement, but regardless the
effect is the crankcase pressure in these compressors is near the suction
pressure of the system.
Examples Of Net Oil
Pressure Computation
- Assume a case where the compressor crankcase (suction) pressure
gage reads 7 PSIG and the oil pressure gage reads 24 PSIG.
The net oil pressure is 24 PSIG - 7 PSIG, or 17 PSI.
- Assume a case where the compressor crankcase (suction) pressure
gage reads 18 INCHES HG vacuum (-8.84 PSIG) and the oil pressure gage reads 13
PSIG.
The net oil pressure is 13 PSIG - (-8.84 PSIG), or 21.84 PSI
In this instance the 18 INCHES HG (suction) is below 0 PSIG (atmospheric
pressure) and is converted to a negative Gage pressure. Remember that when
subtracting a negative number, you add the value.
- To illustrate there can still be a decent net oil pressure to
the compressor bearings at extreme conditions: Assume a crankcase (suction)
pressure of a 29 INCHES HG vacuum (-14.24 PSIG) and an oil pressure gage
reading of .5 PSIG.
The net oil pressure is .5 PSIG - (-14.24 PSIG), or 14.74 PSI.
Note that with an oil pressure switch set to cut-out at 12 PSI, as Dunham-Bush® recommends on the
"Big 4" series machines, even the above conditions would not cause
the compressor to shut-down on low oil pressure.
PSIG, Pounds per Square
Inch Gage; a pressure which references atmospheric pressure as Zero and
positive values as greater than atmospheric pressure.
PSIA, Pounds per Square
Inch Absolute; a pressure which references absolute vacuum as Zero. On this
scale atmospheric pressure at sea level is generally about 14.7 PSIA.
INCHES
HG (suction), inches of mercury; a measure of suction which references
atmospheric pressure as Zero and positive values as less than atmospheric
pressure. A suction given in inches of mercury can be converted to a negative
gage pressure or a positive absolute pressure.
PSI, Pounds per Square
Inch; the differential between two pressure readings given using the same
scale, both PSIG or both PSIA.
Suction Pressure Conversion: Inches Hg to PSIG
|
INCHES HG
|
PSIG
|
0
|
-0.00
|
1
|
-0.49
|
2
|
-0.98
|
3
|
-1.47
|
4
|
-1.96
|
5
|
-2.46
|
6
|
-2.95
|
7
|
-3.44
|
8
|
-3.93
|
9
|
-4.42
|
10
|
-4.91
|
11
|
-5.40
|
12
|
-5.89
|
13
|
-6.39
|
14
|
-6.88
|
|
INCHES HG.
|
PSIG
|
15
|
-7.37
|
16
|
-7.86
|
17
|
-8.35
|
18
|
-8.84
|
19
|
-9.33
|
20
|
-9.82
|
21
|
-10.32
|
22
|
-10.81
|
23
|
-11.30
|
24
|
-11.79
|
25
|
-12.28
|
26
|
-12.77
|
27
|
-13.26
|
28
|
-13.75
|
29
|
-14.24
|
|
As a quick Rule of Thumb: Every 2" of mercury equals 1 psi
|
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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.