Control of thermal energy consumption in the central heating station

Modern individual heating points (IHP) are complex engineering systems that provide efficient heat supply to buildings. One of the key elements of such systems are flow meters, which play a decisive role in the accurate measurement and control of heat energy consumption. A variety of flow meter types allows you to choose the optimal solution for specific operating conditions, which significantly increases the accuracy of metering and helps optimize the operation of heating points. This material was prepared by us, a research team from the Moscow Power Engineering Institute. In this article, we will consider the main types of flow meters used in IHP, their operating principles, advantages and disadvantages.

Flow meters in the IHP primarily provide the ability to regulate the amount of coolant entering the heating system. These control devices are installed on the supply pipeline. The following types of flow meters are distinguished:

• electromagnetic – used to measure the volumetric flow rate of an electrically conductive liquid by recording the voltage generated by a magnetic field between two electrodes. In this type of device, the coolant transport rate remains constant due to the absence of additional parts that change the cross-sectional area of ​​the pipeline. They can operate in a large dynamic range, are widely used to measure the flow rate of aggressive environments, and have a low relative error of 0.2%, 0.5%. The main disadvantage is sensitivity to magnetic and conductive deposits in the liquid.

• ultrasonic – calculate the volume flow rate of liquid based on the difference in the time of passage of an ultrasonic signal proportional to the speed and quantity of the coolant passing through. They can be installed on pipelines with a large nominal bore, have an average dynamic range of measurements, and are produced with a limit of relative flow error of ± 1%, ± 2%, ± 2.5%. Disadvantages include increased sensitivity to vibrations, flow turbulence, and the need to monitor deposits in the pipeline at the working site.

• tachometric – in such devices, the primary flow converter can be an impeller, turbine, ball, etc., the rotation speed of which is directly proportional to the measured volumetric flow. They are widely used at the household level, do not require electricity, are simple and cheap to maintain. The main disadvantages include increased metal consumption when used on large-diameter pipelines, high hydraulic resistance, small dynamic range, and relatively low metering accuracy.

• Coriolis – directly calculate the mass flow rate of liquids depending on the change in the phases of mechanical vibrations of U-shaped tubes. Suitable for measuring the flow rate of both liquids and gases regardless of their electrical conductivity, pressure, viscosity, temperature, do not require the presence of straight sections of the pipeline before and after installation of the device, are not sensitive to the presence of sediment or contamination, are produced with a limit of relative error of liquid flow rate of ± 0.1%, ± 0.15%, ± 0.2%, for gas ± 0.75%, ± 1%. This type of flowmeters has limited application, since their production is highly expensive and technologically complex, it is necessary to ensure increased accuracy of installation, in addition, distortions of measurements are possible with strong vibrations.

• vortex – in such devices, flow measurement occurs by determining the frequency of oscillations occurring in the flow during the bypass of the flow body. There are vortex-acoustic flow meters, in which the frequency of vortex formation is measured by an ultrasonic transducer. Their advantages include the ability to work with various media, a wide dynamic range, and ease of operation. But at the same time, they have low efficiency at low flow rates, significant pressure losses due to the presence of additional resistance in the form of a mechanical obstacle.

• differential pressure flow meters – their operating principle is based on measuring the differential pressure that occurs when a flow of a moving medium passes through a narrowing device. This type of device is easy to manufacture and has no moving parts. The disadvantages include the presence of a mechanical obstacle in the form of a narrowing device, the need for individual calibration for real measured media, and sensitivity to the presence of polluting particles in the flow.

The system error is calculated by the flow measurement channel. The flow measurement channel is a complex of several devices that allow calculating the exact flow rate of the substance. It is customary to maintain the flow rate at a certain level in order to avoid flooding the heat and hot water supply system to the premises. The estimated flow rates of the coolant are determined depending on the purpose of the heating network, the type of heat supply system (open or closed), the accepted temperature schedule, and the connection diagram of hot water heaters in closed heat supply systems.

The flow rate in the pipeline is measured using several devices. First of all, it is a narrowing device that creates a pressure difference in the straight pipeline. In this case, a diaphragm is used, which is installed in the pipeline using a flange connection of the pipe parts. It is necessary to measure the pressure difference created by the narrowing device. For these purposes, any primary flow converter (differential pressure gauge or flow meter) can be used. A vortex-acoustic flow converter is installed in the system.

The principle of operation of vortex acoustic flowmeters is based on the formation of vortex tracks, breaking away from opposite sides of the body in turn, when a liquid flow flows around a solid body. Registration of vortex flows and determination of the frequency of their formation occurs using ultrasonic transducers. This is what allows determining the volumetric flow rate of the medium, which the device converts into a secondary unified current signal and sends to the recorder.

Example: Information about the flow converter and recorder manufactured by Metran, popular in use in IWPs in Russia

Authors of the material: Temrina D.N., Guzhov S.V.