Tuesday, June 17, 2014

Thermal Mass Flow Controllers and Sensors - An Introduction

Brooks Thermal Mass Flow
Brooks Thermal Mass Flow Meter
Operating upon the thermodynamic principle of differential temperature, thermal mass flow meters combine a flow sensor, a valve, and a PID controller for very accurate and efficient means of controlling gasses and liquid over a wide range of flows.

The following are the primary elements to a thermal mass flow meter:

Body: The body provides foundation for the other elements contained in the mass flow meter, plus the primary flow path for the process media.

Restrictor: The restrictor (also called the bypass) restricts the flow path, which causes a reduced flow through the sensor. The ratio of flow through the restrictor compared to the flow through the sensor must be constant. Different bypass methods can be used depending on flow rate and application.

Sensor: The sensor uses heat and differential temperature between upstream and downstream flow which provides a signal proportional to mass flow.

Control Board: The control board interprets and manages the external inputs and outputs (I/O) to and from the device, and also the internal I/O with the sensor and control valve. It adjusts the valve signal output to satisfy the set point signal to attain the desired flow.

Valve and Orifice: The valve and orifice combination is the flow control element. Several types of valves may be used on mass flow controllers, dictated by flow rate and application. Most flow controllers use a normally closed solenoid control valve. These have a coil winding around a valve stem, that when the coil is energized, a magnetic field modulates a plunger and valve seat assembly. When energy is applied, the valve seat is raised a few thousands of an inch off the orifice and allows fluid to flow through the device.

Thermal mass flow controllers are used in a variety of industries and applications requiring very accurate and repeatable gas or liquid mass flow measurement. Thermal mass flow controllers are often used in these industries: aerospace, automotive, biotech, gas analyzers, pharmaceutical, chemical, petrochemical, electronics manufacturing, food & beverage, fuel cells, furnaces, medical, metals processing, packaging, solar, semiconductor processing.