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Capillary Tube Thermal Mass Flow Meters & Controllers: a User's Guide

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Capillary Tube Thermal Mass Flow Meters & Controllers: a User's Guide Capillary Tube Thermal Mass Flow Meters & Controllers A User’s Guide By John G. Olin, Ph.D., Sierra Instruments, Inc. / 9.24.13 A SIERRA WHITEPAPER www.sierrainstruments.com NORTH AMERICA 5 Harris Court, Building L / Monterey, CA 93940 / USA 800.866.0200 / 831.373.0200 / fx 831.373.4402 EUROPE Bijlmansweid2 / 1934RE Egmond aan den Hoef The Netherlands +31 72 5071400 / fx +31 72 5071401 ASIA-PACIFIC Second Floor Building 5 / Senpu Industrial Park 25 Hangdu Road Hangtou Town / Pu Dong New District Shanghai, P.R. China Post Code 201316 +8621 5879 8521/22 / fx +8621 5879 8586 © 2013 Sierra Instruments, Inc. / All rights reserved. A USER’S GUIDE TO CAPILLARY TUBE THERMAL MASS FLOW METERS AND CONTROLLERS John G. Olin(1) Abstract Capillary tube thermal mass flow meters directly measure the mass flow rate of clean gases and gas mixtures in lower flow ranges. A capillary tube thermal mass flow controller adds an integrally mounted flow control valve to the flow body of the mass flow meter and both monitors the mass flow rate and controls it to be equal to a set-point value selected by the user. This paper describes capillary tube mass flow meters and controllers for use in general purpose industrial and laboratory applications and in the fabrication of semiconductor devices. It provides a detailed description of the major components-- -flow body, flow conditioner, bypass, capillary sensor tube, control valve, and electronics. Flow ranges and specifications for which this technology is best suited are presented. The paper explains the principle of operation; describes gas conversion factors that provide Fig. 1 Classifications of mass flow meters multi-gas capability in a single instrument; lists best practices for users, including the selection, installation, and operation of the instruments; and concludes with a discussion of preferred 1 Introduction methods of flow calibration. Capillary tube thermal mass flow meters and Keywords: flow meter, gas flow meter, mass controllers were first commercialized in the flow meter, mass flow controller, thermal mass early 1960’s. The space industry was one of the flow meter, capillary tube thermal mass flow first users, but before long the industry that meter, capillary tube thermal mass flow fabricated solid state semiconductor devices controller recognized their usefulness. When integrated circuit semiconductor devices began their long and continuing period of exceptional growth, the market for capillary tube thermal mass flow controllers grew with it. In the 1970’s and 1980’s, general industry recognized the advantages of using capillary tube mass flow meters and controllers in a broad range of 1 of 28 applications, and several new companies were because it eliminates errors associated with formed to serve this growing market. The secondary measurements and requires only one advantages of accuracy, compactness, reliability, penetration of the process line. and cost-effectiveness continue to make capillary tube thermal instruments the choice for Thermal mass flow meters are of two types--- monitoring and controlling smaller mass flow thermal dispersion mass flow meters and rates of clean gases in general industry and in capillary tube thermal mass flow meters. the fabrication of semiconductor devices. Thermal dispersion instruments are used for general industrial gas flow applications in pipes The primary virtue, and the source of and ducts. Capillary tube instruments are used their prominence, is the fact that capillary for lower flows of clean gases in a wide range of tube thermal instruments directly measure mass process and laboratory applications. Both types flow rate, as opposed to, for example, of instruments measure mass flow rate via the volumetric flow rate. This is important because heat carried away by the gas flowing over the most industries need to measure and control surface of a heated element. Beyond that, their the flow of the molecules, i.e., the mass, of principles of operation are quite different. the gas entering their process. In the case of the thermal dispersion, or Fig. 1 shows the classifications of mass flow immersible, type of mass flow meter, heat is meters---a “tree” with “mass flow meters” as its transferred to the boundary layer of the gas root and capillary tube thermal mass flow meters flowing over a cylindrical heated sensor and controllers as its top branches. immersed in the main flow stream. The heat carried away by the gas provides the As shown in Fig. 1, two kinds of flow meters measurement of mass flow rate. Thermal directly measure the mass flow rate of fluids--- dispersion mass flow meters have two major Coriolis mass flow meters and thermal mass configurations---in-line and insertion. Their flow meters. Coriolis mass flow meters directly applications, description, principle of operation, measure the mass flow rate of most fluids, both and usage are discussed in references [1] to [3]. liquids and gases, and do not require knowledge of the identity, or composition, of the fluid. In the case of the capillary tube type of Thermal mass flow meters directly measure the thermal mass flow meter (hereinafter, “MFM”) mass flow rate of gases, and do require described in this paper, the flow enters the flow knowledge of its composition. Coriolis mass body and splits into two internal flow paths. One flow meters have high accuracy, high pressure path flows through a heated capillary sensor tube drop, work best with liquids, and are relatively that has a small diameter and relatively long expensive. Thermal mass flow meters have length. The second parallel path inside the flow medium to high accuracy, low pressure drops, body flows through a bypass consisting of a work best with gases, and are relatively laminar flow element. This creates a pressure inexpensive. drop that forces a small fraction of the total mass flow rate through the adjacent capillary sensor Both Coriolis and thermal technologies tube. The ratio of the flows through the bypass “directly measure” mass flow rate because they and the sensor tube is a constant. The capillary require no secondary measurements to deliver sensor tube measures its internal mass flow rate their mass flow rate output. Several kinds of by means of the heat capacity of the gas that flow meters directly measure volumetric flow carries heat from an upstream resistance- rate, such as ultrasonic, vortex, turbine, positive temperature-detector winding to a downstream displacement, and differential pressure winding, both on outside of the sensor tube. The producing devices. To deliver a mass flow rate difference in the electrical resistances of the two output, they must become multivariable flow windings provides the measurement of the mass meters that require the secondary measurements flow rate through the sensor tube, and thereby of fluid temperature and pressure. Direct the total mass flow rate in the flow conduit. measurement of mass flow rate is preferred 2 of 28 A capillary tube thermal mass flow controller (hereinafter, “MFC”) adds an integrally mounted flow control valve to the flow body of the MFM and both monitors the mass flow rate and controls it to be equal to a set-point value selected by the user either remotely or on the MFC itself. More MFCs are manufactured than MFMs because most users want to control the mass flow rate of the gas in their process rather than just monitor it. Capillary tube thermal MFCs offer a cost-effective solution for controlling the flow of gases because they are compact, require only one penetration of the process line, and have a built-in optimized control system. This paper applies to gas flow, and not liquid flow, because gas flow constitutes the vast majority of the applications for capillary tube thermal MFMs and MFCs. This is true because the measurement sensitivity for gases is much greater than for liquids. Nevertheless, this technology has been used to measure and control very low flow rates of liquids (less than approximately 1000 grams per hour) in Fig. 2 Typical general purpose mass flow specialized applications in the semiconductor, controller chemical, food, and pharmaceutical industries, as well as in analytical laboratories. Many of the (2) Semiconductor manufacturing and other principles of operation stated here apply to some high purity vacuum processes liquids. For the remainder of this paper the fluid is a gas. Both of these applications are shown in the mass flow meter tree of Fig. 1. Semiconductor It is common practice to express the mass flow applications almost always use MFCs, and not rate measured by capillary tube thermal MFMs MFMs. Semiconductor MFCs are covered in and MFCs in units of slpm (standard liters per considerable detail in a series of standards minute) and, for low flows, in units of sccm published by the Semiconductor Equipment and (standard cubic centimeters per minute). The Materials International [4]. This paper focuses on units slpm and sccm may appear to be general purpose MFMs and MFCs designed to volumetric flow rate units, but indeed they are meet the needs of general industry and mass flow rate units, as described in para. 6. laboratories. Here, the terms “MFM” and “MFC” apply generically to both kinds of capillary tube thermal mass flow meters and controllers, and the 2 General Purpose and term “instruments” applies collectively to both. In Semiconductor Applications cases where the two major applications must be distinguished, the prefixes “general purpose” and Capillary tube thermal mass flow meters and “semiconductor” are used. controllers have two broad fields of application: Fig. 2 shows a typical general purpose capillary (1) General purpose industrial and laboratory tube thermal MFC that operates in the medium applications flow range of approximately 50 to 300 slpm. 3 of 28 In serving these applications, general purpose MFMs and MFCs monitor and control the flow of clean gases and gas mixtures, such as: air, nitrogen, oxygen, hydrogen, argon, carbon dioxide, carbon monoxide, methane, helium, nitrous oxide, some semiconductor fabrication gases and gas mixtures.
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