Brooks Instrument 5851E - Manual (Page 50)

4-18
Brooks ® Model 5851E
Section 4 Maintenance Installation and Operation Manual
X-TMF-5851E-MFC-eng
Part Number: 541B104AAG
November, 2008
5. Locate the Nitrogen equivalent flow rate (sccm Nitrogen) on the vertical
line marked "Q
Nitrogen
" (Point B).
6. Draw a line connecting
Δp and Q
Nitrogen
and extend it to the baseline.
Mark this point (Point C).
7. Locate inlet pressure (psia) on the vertical line marked "P
in
" (Point D).
8. Draw a line connecting P
in
(Point D) and baseline (Point C) and then
extend this line to the vertical line marked D
o
(orifice diameter, inches)
(Point E).
9. This point on the line is the minimum orifice size for the given
conditions. If this point is between two orifice sizes, select the next
largest size orifice to ensure adequate flow. If the orifice selected falls
below .0013, choose .0013 size orifice.
For the example in Figure 4-4, the .007 size orifice would be selected.
4-7 Restrictor Sizing
The restrictor assembly is a ranging device for the sensor portion of the
controller. It creates a pressure drop which is linear with flow rate. This
diverts a sample quantity of the process gas flow through the sensor. Each
restrictor maintains the ratio of sensor flow to restrictor flow, however the
total flow through each restrictor is different. Different restrictors (micron
porosity and active area) have different pressure drops and produce
controllers with different full scale flow rates. For a discussion of the
interaction of the various parts of the meter, you are urged to review
Section 3-1 (Theory of Operation).
If the restrictor assembly has been contaminated with foreign matter, the
pressure drop vs. flow characteristics will be altered and it must be cleaned
or replaced. It may also be necessary to replace the restrictor assembly
when the mass flow controller is to be calibrated to a new flow rate.
Restrictor assembly replacement should be performed only by trained
personnel. The tools required for the removal/replacement procedure are
as follows:
Appropriate size wrench for the removal of the inlet process connection
Restrictor removal tool (Refer to Table 5-2)
Restrictor O-ring, refer to the spare parts Section 5, for the correct part
number.
The Model 5851E Mass Flow Controller utilizes porous metal restrictor
assemblies for all full scale flow rates. Restrictor elements with porosities
of 40 and 60 microns are used in different combinations. Up to 3 restrictor
elements can be placed in one assembly. These restrictors are assembled
by pressing the porous metal elements into the header plate.
When sizing a restrictor assembly for gases other than Nitrogen, the
sensor conversion factor must be included as the restrictor is used to divert
a portion of the process flow through the sensor. Use the following
equation to obtain the desired output in air equivalent units. Refer to Table
4-3 for the conversion factor list.