ELECTRONIC FLOWMETERS FOR THERMAL ENERGY MEASUREMENT. By Dr. Crainic Monica Sabina

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1 ELECTRONIC FLOWMETERS FOR THERMAL ENERGY MEASUREMENT By Dr. Crainic Monica Sabina Luxten Lighting Company AEM Branch Office Gas and Water Meters Research Department 26 Calea Buziaşului Timişoara Romania Tel: /172Fax: In Physics Conference TIM 03, Timişoara November 28-29, 2003, Annals of ABSTRACT West University of Timişoara Physics Series vol. 44, (2003), p. 78, Thermal energy is mainly distributed by fluid media (clean hot water or steam) to the points of consumption. The energy manager is not only interested in the total energy required but also in the consumption of the individual heat consumers and the flow of energy in the plant in general. Measurements of the heat volumes in circulation and under a transmission system of thermal energy as these measurements form the basis for assigning the large tariff sums. There are therefore essential requirements which must be in large heat metering devices. Heat metering devices known also as heat meters are installed directly in the transmission pipeline and therefore permit an exact physical measurement of the heat. Heat consumption is calculated from the difference between supply and return temperature of the heat medium as well as the flow rate. The measurement of volumetric flow rate of heat transfer medium is based on the electronic flowmeters like as electromagnetic flowmeters, vortex flowmeters and ultrasonic flowmeters. In this paper we describe the electronic flowmeters and we make a comparation between different electronic flowmeters used for thermal energy measurement. KEY WORDS flow measurement, smart instrumentation, technology, heat meters, physical effects. 1. INTRODUCTION Thermal energy is mainly distributed by fluid media (clean hot water or steam) to the points of consumption. The energy manager is not only interested in the total energy required but also in the consumption of the individual heat consumers and the flow of energy in the plant in general. Heat meters are metering device for thermal energy consumption. The components of heat meters are: resistance temperature sensors, flowmeter and heat energy calculator. Heat consumption is calculated from the difference between supply and return temperature of the heat medium as well as the flow rate, namely Q k i 1 0 Q v (1) 1

2 where: Q v - volumetric flow rate, 1 inflow temperature, 0 outflow temperature, k I heat coefficient for the medium defined by the specific enthalpy and the density of the heat carrying fluid. The measurement of volumetric flow rate of the heat transfer medium is based on the electronic flowmeters, which are non intrusiveness in the process stream and doesn t create significant pressure drop like traditional flowmeters with moving parts subjected to wear like for example bearing and rotors of turbine meters. 2. ELECTRONIC FLOWMETERS FOR HEAT METERS[1-10] The electronic flowmeters includes the following product categories: conventional flowmeters, smart flowmeters and multivariable flowmeters. Conventional electronic flowmeters normally have a 4 20 ma output and do not have the capability of remote configuration or communication. Smart flowmeters means a microprocessor based flowmeter which are capable of remote two way communication. Multivariable flowmeters are flowmeters that measure more than one process variable. Typically, multivariable flowmeters measure differential pressure, absolute or gauge pressure, process temperature. These meters use the measured variable to calculate density and to compute mass flow. Electronic flowmeters for thermal energy measurement are: magnetic flowmeters, vortex shedding flowmeter and ultrasonic flowmeters Magnetic flowmeters operate on the principle of Faraday s law of electromagnetic induction, an electrical voltage is induced in a conductor that is moving through a magnetic field and at right angles to the field. The faster the conductor moves through the magnetic field, the greater the voltage induced in the conductor. The vortex flowmeter measures fluid flow rates using the principles of vortex shedding. Vortices are caused by flow passing an object such as a cylinder. Vortices are shed of alternate sides of the object creating a street of vortices This alternating pattern of vortices is called a Von Karman vortex trail. It is this increase and decrease of pressure is measured to determine the frequency of vortex formation. The frequency f of the vortex generation is directly proportional to the fluid velocity v, and volumetric flow rate Q v Q f K (1) where: 2

3 K A f d B (2) is the meter coefficient which can be defined as pulses per unit volume. There are two basic types of ultrasonic flowmeters: transit-time and Doppler. Transit-time flowmeter measures the difference in transit times between two ultrasonic pulses transmitted upstream and down stream across the flow. Doppler flowmeters use sound pulse reflection principle to measure liquid flow rate, solids or bubbles in suspension in the liquid reflect the sound back to the receiving transducer 3. DISCUSSIONS AND CONCLUSIONS Which electronic flowmeters is best for thermal energy measurement? Magnetic flowmeters? Vortex flowmeters? Ultrasonic flowmeters? Which is the right choice? The answer (as always) is It depends. To the uninitiated electronic flowmeters for thermal energy measurement selection can seem very difficult, however it can also be a rewarding learning process. The ultimate electronic flowmeters is one that is safe, can be used in all application regardless of size and material compatibility, imparts no measurable effect on the flow stream, installs simply, is affordable and readily available in all line sizes. Unfortunately no such electronic flowmeters exists. An orderly process of elimination can be used to select the best available electronic flowmeter for some particular application. Although the weight given to each attribute may differ in some application, process selection should not over look any of the following considerations (see table 1): recommended service, rangeability, accuracy, and reliability [11]. While the application can dictate the use of certain electronic flowmeters so can the advantages and disadvantages of various meter. Electronic flowmeters are non-intrusive, have no moving part subject to wear, are more accurate than traditional flowmeters like as turbine flowmeters, positive displacement flowmeters and differential pressure flowmeters but very share. Since the electrode of magnetic flowmeters are subjected to coating and they have not a reduced need for maintenance and vortex flowmeters are affected by vibration and somewhat intrusive the most frequently used electronic flowmeter for thermal energy measurement is the transit-time ultrasonic flowmeter. Ultrasonic transit-time flowmeter have a reduced need for maintenance, is much more accurate 3

4 than ultrasonic Doppler flowmeter and suitable for hot water which is a relatively clean heat transfer medium. Table 1 Electronic flowmeter selection guide KEY Do not use in this service Best for this application OK for some application but check first OK with some exceptions Recommended service (application or media compatibility) Flowmeter type Electromagnetic Vortex Ultrasonic Transit-time Ultrasonic Doppler Vapor or gas Clean liquids Corrosive liquids Dirty liquids Viscous liquids Slurries High temperature Service Semi-filled pipe Open channel Other attribute (criteria) Attribute (criteria) Electromagnetic Vortex Ultrasonic Transit-time Ultrasonic Doppler Rangeability 40 to 1 10 to 1 20 to 1 10 to 1 Pressure loss none medium low none Accuracy (%) 0,5 of rate 1 of rate 1 to 5 5 of rate of rate Required upstream pipe 5 10 to 1 5 to 30 5 to 30 (diameters) Pipe size 1/10 to 100 in 1 /2 to 12 in 1 /2 in and up 1 /2 in and up Viscosity effect none medium none none Repeatability 0,2 % 1) 0,2 % 1) 0,5 % 2) 0,2 % 2) Maintenance medium medium low low Relative costs high high high high 1) percent of flow rate 2) percent of velocity For this reason the market of ultrasonic flow sensors is projected to reach revenues of $412,7 million in The revenues are forecast to grow with an annual growth rate of 14,6 percent. This annual growth rate is the strongest growth in the market of electronic flowmeters. 4

5 REFERENCES [1] Jesse Yoder The market for magnetic flowmeter Flow Research Wakefield USA 2003 [2] Jesse Yoder The market for ultrasonic flowmeter Flow Research Wakefield USA 2003 [3] Jesse Yoder The market for vortex flowmeter Flow Research Wakefield USA 2003 [4] Jesse Yoder The market for new technology flowmeters Flow Research Wakefield USA 2001 [5] *** Flow &Level measurement Transactions Technical Reference Series OMEGA COMPANY USA 2002 [6] Donald Ginessi Vortex shedding flowmeters Caldon Inc. Pittsburgh PA. USA 1999 [7] Donald Ginessi Electromagnetic flowmeters Caldon Inc. Pittsburgh PA, USA 1999 [8] Herb Estrada General principles of LEFM time of flight ultrasonic flow measurement Caldon Inc. Pittsburgh PA USA 2001 [9] T. Cousins, D. Augenstein Providing of multi-path liquid ultrasonic flowmeters North Sea Flow Measurement Workshop 22-nd - 25 th October 2002 [10] Walt Boys, D.W. Spitzer Two-wire magmeters: will they ever free you from the extrawires Flow Control JULY 2003 [11] Jesse Yoder Flowmeter Shootout. Part III: How users choose Flow Control Magazine March

FLOW MEASUREMENT. INC 102 Fundamental of Instrumentation and Process Control 2/2560

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