Brooks Instrument 5851i - Manual (Page 48)

4-20
Installation and Operation Manual
X-TMF-5851i-MFC-eng
Part Number: 541B109AAG
September, 2009
Brooks ® Model 5851 i
Section 4 Maintenance
& Troubleshooting
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 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
controller, you are urged to review Section 3-1 “Theory of Operation.”
If the restrictor assembly has been contaminated with foreign matter, the
pressure drop versus 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 hex wrench for the removal of the inlet block.
Restrictor removal tool may be required (contained in Service Tool Kit
P/N S778D017AAA)
Restrictor O-ring, refer to the spare parts Section 5 for the correct part
number.
The 5851 i 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 three 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 nitrogen equivalent units. Refer to
Table 4-3 for the conversion factor list.
Example:
The desired gas is cyanogen (C
2
N
2
).
The desired full scale flow rate is 30 slpm.
Sensor conversion factor is 0.499 from Table 4-3.
Nitrogen equivalent flow = 30/0.499 = 60.1 slpm Nitrogen.
In the example above, the restrictor should be sized for a 60.1 slpm flow
rate (P/N S613E528BMT).
If a restrictor assembly is being replaced because the original has become
contaminated, the original may be used as a guide to select the
replacement assembly. The porosity of the original element or elements is
marked on the calibration sheet which was shipped with the flow controller.
The replacement assembly should be replaced in the same orientation as
the original restrictor.