Flow meters- Heat Quantity Ultrasonic Flow Meter

Thermal energy is mainly distributed by fluid media to the points of consumption. The energy manager is not only interested in the total energy required, but also in the consumption of individual heat consumers and the flow of energy in the plant in general. The EESIFLO™ EF portable ultrasonic flowmeter with integrated heat quantity calculator has been developed to compliment permanently installed devices.
The flowmeter EESIFLO™ EF is especially appropriate for measurements in large variable supply networks, e.g. to register the heat distribution in a large complex of buildings or to review the heat balances in a process engineering facility. This device is particularly useful in situations where temporary, non-intrusive inspections of heat consumption and distribution need to be made quickly. The advantages of this portable instrument are its flexibility, enabling it to be used in a wide range of applications, and the low installation and running costs.

The differential method is the basis for the precise measurement of heat quantity. This method considers the enthalpy that enters and leaves a system. The difference between the two values gives the heat consumption. Since the enthalpy difference cannot be measured directly, the value is calculated from the volumetric flow, the inflow and outflow temperatures and the heat coefficient for the medium.
Assuming constant conditions, the heat flow can be calculated with the following formula:

The heat coefficient ki is defined by the specific enthalpy and the density of the heat carrying fluid. These two quantities depend on the temperature and pressure of the medium. In incompressible media however, the variation with pressure is insignificant and can be ignored. Consequently, flowmeters to measure the quantity of heat consist of devices for measuring volumetric flow and temperature. A microprocessor is necessary to compute the quantity of heat flow.

The newly developed flowmeter EESIFLO™ EF incorporates all these features and in contrast to conventional flow meters, it allows the user to measure heat flow and distribution from the outside of pipes without the necessity of disrupting the process in the plant. This is achieved by using a clamp-on ultrasonic flow meter together with two surface temperature sensors.

The EESIFLO™ EF ultrasonic flowmeter features two input channels to connect resistance temperature sensors Pt100 in four wire circuit. This sensor type has been chosen because of its popularity in industrial applications and ready availability in a variety of versions. Two surface sensors are supplied with the unit to measure the temperature of the inflow and outflow. The user may, however, connect other types of sensors of a compatible type according to specific application requirements. This is particularly advantageous where temperature sensors are already installed in the pipe. In such cases, an input correction for each sensor is required to obtain a linear resistance temperature curve. These correction values can be stored in the non-volatile memory of the flowmeter and are therefore always available. When using the supplied sensors, the ability to correct may serve to compensate for the temperature gradient of the pipe.

The so-called energy temperature, which represents the temperature for the transportation of energy, is of special interest for measuring heat flow. According to Adunka[1], this temperature corresponds to the temperature in the middle of the pipe in case of turbulent flow. Under laminar flow conditions, it is more difficult to determine this temperature and the energy temperature is calculated as the mean of the temperatures of the wall and the centre of the pipe.

When using surface temperature sensors, it is the pipe wall temperature which is measured not the energy temperature. In practice however, the temperature difference is important in the calculation of heat flow not the absolute temperatures. The absolute temperatures are only required to determine the heat coefficients. Studies at the University of Rostock[2] showed that the difference between the surface temperatures approximates to the difference between the energy temperatures. The pre-condition is, that the pipe has sufficient insulation to limit the heat loss through the pipe walls. Both the inflow and outflow temperatures should always be measured with the same type of sensor.

The flow measurement of the heat carrying fluid is based on the ultrasonic transit time technique. This method utilises the transmission of sound waves in the fluid. Sound pulses are sent alternatively downstream and upstream through the liquid. The ultrasonic signal has different transit times for the two directions comparable to a swimmer in a river who swims faster downstream than upstream. The resolution of the signal time difference is 0.1 ns with a transit time of the sound from 16 µs and 1.6 ms. If these values together with details of the profile of the pipe section are known, the volumetric flow rate can be calculated.

The transducers for coupling the sound signals through the pipe clamp from the outside onto the pipe ensuring that there is no disturbance to the flow nor any expensive installation costs. This method of flow measurement implies that the pipe diameter and tolerances are part of the measuring conditions. Often the inner diameter and wall thickness of the pipe are unknown although this information is required to calculate the volumetric flow from the flow velocity. The input of incorrect pipe parameters will result in measurement errors. For this reason, an device for measuring the wall thickness of the pipe was incorporated into the flowmeter.

Measurement of flow from the outside of a pipe with the EESIFLO™ using magnetic clamps

The microprocessor within the flowmeter computes the heat flow from the measured inflow and outflow temperatures and the volumetric flow rate. The specific enthalpy and the density of the fluid can be internally calculated depending on the measured temperature.

As various liquids may be used as heat carriers, the portable ultrasonic flowmeter EESIFLO™ can be adapted for specific tasks using an in-built database. The database contains information on pipe materials and fluids frequently used and requiring measurement. As well as information on sound velocity and viscosity, the database also stores the coefficients necessary for calculating the heat quantity.

The database can be specifically adapted and extended by the manufacturer to meet specific customer requirements. It is also possible for the customer to enter set-up values and make changes to the stored data. Special software has been designed for use with a Personal Computer to generate the coefficients used for calculating the heat flow and to transfer them via a serial interface to the flowmeter where they are stored in non-volatile memory. These data are available even when the instrument has been repeatedly switched off, the batteries have been changed or a cold start has been performed.

The EESIFLO™ EF can measure volume flow, flow velocity, mass flow or heat quantity of liquids within a temperature range from -30 °C up to 130 °C. With specially designed high temperature transducers, the temperature range can be extended up to 250 °C, and for short periods up to 300 °C. The ultrasonic sensors are small, lightweight and very robust. Pipe diameters may range from 10 up to 3,000 millimetres.

The instrument can always be used where the pipewall and the liquid to be measured are sonically conductive. This is true for pipewalls consisting of homogeneous material, such as steel, synthetic material, glass or copper, and for liquids which carry not an excessive amount of solid particles or gas bubbles. There is no dependency on electrical parameters of the fluid such as conductivity or dielectric constant.

To assist the user in obtaining a complete profile of the flow conditions in the plant, the EESIFLO™ EF features an in-built data logger which can record up to 150,000 measuring values and up to 15 different sets of site parameters. The data can either be transferred to a Personal Computer (PC) or to a printer as numerical values or in graphic format.

The device allows the operator dialogue in different languages and guides the user through the menus for parameter set-up, measurement or data storage.

The instrument can feature an integrated measuring point multiplexer which allows for the connection of up to four independent flow sensor sets with one transmitter. EESIFLO™ automatically recognises the connected sensors through Intelligent Sensor Identification. This means that all calibration parameters are stored in the sensor and automatically transferred to the instrument at the time when the sensors are connected.

EESIFLO™ can also be fitted with various process inputs and outputs. The instrument can be equipped with a maximum of four temperature inputs whereby the temperatures can be freely assigned to the available flow channels. This makes it possible to configure, for example, a 3-channel heat flow measuring system with a common inlet temperature and three independent outlet temperatures

Multi-channel heat flow measurement of 3 thermal energy consumers