Brooks Instrument SLAMf Series - Manual (Page 35)

3-1
Section 3 Operation Installation and Operation Manual
X-TMF-SLAMf-MFC-eng
Part Number: 541B032AAG
April, 2013
Brooks ® Digital MFC's and MFM's
3-1 Operating Procedure
After the flowmeter has been properly installed in the process, it is ready
for operation. When initiating flow, slowly open the valve to avoid a flow
surge. Bypass is a help in bringing the flow on smoothly. Avoid starting a
pump to supply the flowmeter without the use of a valve upstream of the
flowmeter.
After the flowmeter has been properly installed in the process, it is ready
for operation. In most cases, particularly in high flow (> 100 slpm) and/or
high pressure (> 150 psig) applications it is recommended to slowly initiate
flow in order to reduce servere pressure surges. Where possible, use a
bypass piping arrangement to bring flow smoothly on-line. When starting
pumps to supply the flowmeter, use an isolation valve between the pump
and the flowmeter in order to prevent pressure surges.
3-2 Theory of Operation for Flow Measurement
The thermal mass flow measurement system consists of two components:
the restrictor and the flow sensor. Figure 3-1 contains a diagram of the flow
stream through the MFC/MFM with an enlarged view of the flow sensor.
Gas flow entering the MFC/MFM is separated into two paths; one straight
through the restrictor and the other through the flow sensor. This is
represented in Figure 3-1 where the total flow A+B enters the MFC/MFM
and is separated into streams A and B. The streams are joined again at the
far side of the restrictor.
The separation of the flow streams is caused by the restrictor. During flow
conditions there will be a pressure differential across the restrictor which
forces gas to flow in the sensor.
The pressure difference caused by the restrictor varies linearly with total
flow rate. The sensor has the same linear pressure difference versus flow
relationship. The ratio of sensor flow to the flow through the restrictor
remains constant over the range of the MFC/MFM (A/B = constant). The
full scale flow rate of the MFC/MFM is established by selecting a restrictor
with the correct pressure differential for the desired flow.
The flow sensor is a very narrow, thin-walled stainless steel tube. Onto this
tube are built upstream and downstream temperature sensing elements on
either side of a heating element. Constant power is applied to the heater
element, which is located at the midpoint of the sensor tube. During no-
flow conditions, the amount of heat reaching each temperature sensor is
equal, so temperatures T1 and T2 (Fig. 3-1) are equal. Gas flowing
through the tube carries heat away from the upstream temperature sensor
and toward the downstream sensor. The temperature difference, T2 - T1,
is directly proportional to the gas mass flow. The equation is: