With the promulgation of a series of national energy-saving policies and regulations, central cooling public buildings are more and more commonly used in central air conditioning household metering technology, through measurement on the one hand, users can be advocated to use air conditioners rationally, actively save energy, and avoid waste; on the other hand, through reasonable measurement of cost allocation, the problem of difficult fees for property management companies is solved, and users "use more and pay less", so that the charges are based on evidence. The deeper significance is to evaluate the actual cooling capacity of the refrigeration unit through the readings of the energy meter, establish a database based on historical data, and study the energy-saving potential of large public buildings through actual data analysis, so as to implement energy-saving transformation of large public buildings.
Although "cold" and water, electricity and gas have the same commodity attributes in the market economy and society, in terms of measurement, it is more complex and difficult than the metering and charging of water, electricity and gas that have already entered the market and succeeded. The principle of air conditioning in a large area (the same owner) is mainly based on energy meter measurement, the principle is as follows: install a temperature sensor (paired use) on the air conditioning supply and return pipe in a certain metering area, measure the temperature of the supply and return water, install a flow meter on the return pipe, and measure the temperature and flow rate of the refrigerant - water inlet and outlet respectively, and then obtain the "cold" value consumed by the compensation and integral calculation of density and enthalpy value. At present, in terms of energy meter application, there are three main types according to the selection of flowmeters, namely mechanical, ultrasonic and electromagnetic. Their main difference lies in the principle and construction of flow sensors.
1. Application of mechanical flowmeter in household energy meter
The mechanical energy meter measures the flow rate of chilled water through the speed of the impeller, and the mechanical energy meter is mainly divided into single beam type and multi-beam type according to the internal structure of the flow sensor, and is divided into dry type and wet type according to whether the counter is in contact with hot water. The impeller speed of the dry sensor is usually transmitted to the counter through magnetic coupling, and the current imported mechanical energy meter part adopts the method of induction conduction, that is, the non-magnetic way, which can reduce the impact of rust on the meter in the water.
Due to technology and price reasons, most of our country used various mechanical energy meters in the early days, including imported and domestic brands. However, the use and research practice in recent years have shown that there are a series of problems in measuring the cooling capacity of air conditioners using mechanical energy meters, such as the problem of narrow range and too large starting flow, and the difference in chilled water flow coefficient. Due to the general problems of air conditioning and chilled water, such as many impurities, scale, and unstable flow state, it is easy to produce large measurement errors or non-measurement, resulting in measurement disputes and bringing trouble to property management. At the same time, mechanical energy meters have a large amount of maintenance, and usually need to stop the air conditioning system to drain water, clean the flowmeter, and replace the flowmeter when the wear is large. In addition, the current operation of air conditioning systems in our country also puts forward more stringent requirements for energy meters than foreign heat measurement, such as poor water quality of circulating water; There are many reasons why water contains not only a large amount of harmful chemicals, but also a variety of small particles and impurities that are damaging to flow sensors. These problems will still exist for a long time. Therefore, due to the inherent limitations of mechanical energy meters in terms of structure and principle, it is a very serious challenge to operate normally and accurately within a verification cycle (five years).
2. Application of ultrasonic technology in household energy meters
In air conditioning measurement, some items use ultrasonic energy meters. Ultrasonic energy meters measure the time difference between the propagation of ultrasonic waves in the thermal medium (when sound waves propagate in the fluid, the propagation speed of the sound wave in the downstream direction will increase, and the countercurrent direction will decrease, and the propagation time will be different for the same propagation distance). The relationship between the difference in propagation velocity and the flow velocity of the measured fluid is used to obtain the flow rate and derive the flow rate. Since there are no moving parts inside the measuring chamber, there are no strict requirements for the composition or impurity content of the medium. Therefore, in terms of service cycle cost, reliability and accuracy, ultrasonic energy meters are better than mechanical energy meters. However, before 2000, due to price, mechanical energy meters dominated the field of household energy meters. Now with the recognition of the defects of mechanical energy meters, they have been basically eliminated.
However, in the process of practice, some people have entered a misunderstanding, that is, simply use imported ultrasonic heat meters to measure the cooling capacity of air conditioners. From the perspective of engineering practice, the effect is very unsatisfactory, and even the system is not running well. The reason for this is that the imported heat energy meters (most of which are European brands) are mainly used in foreign countries for heat supply and household measurement, and the working conditions of supply are "large temperature difference (temperature difference is generally 15 °C) small flow", while the working condition of cooling supply is "small temperature difference (maximum temperature difference of 5 °C) large flow". In the supply environment, ultrasonic heat meters have been widely used, the technology is also very mature, and when applied to cooling, the condensate will be attached to the surface body, and the integrated structure of the ultrasonic heat meter makes it easy for the condensate to penetrate into the surface body, destroy electronic components, consume a large amount of batteries, and use up the battery in half a year (ultrasonic heat meters generally come with batteries, which can be used for 6~10 years). The larger the caliber of the ultrasonic energy meter, the higher the price, sometimes the larger the caliber of the energy meter, its price will be much higher, so the investment cost is a factor to consider using an ultrasonic energy meter. In addition, the selection of ultrasonic energy meters is more complicated, the main parameter of the energy meter is the system flow rate rather than the system pipe diameter, according to the flow size to determine the model of the energy meter (so that the minimum flow rate of the selected energy meter is less than the minimum flow rate of the system pipeline, the maximum flow rate of the energy meter is greater than the maximum flow rate of the system pipeline), which makes the installation complicated and requires the need to take reducer measures.
3. Application of electromagnetic induction technology in household energy meters
Mechanical energy meters are generally considered unsuitable for air conditioning measurement, and electromagnetic energy meters appear at the same time as the application of ultrasonic energy meters. The electromagnetic energy meter mainly uses an electromagnetic flowmeter. Its working principle is that based on Faraday's law of electromagnetic induction, when the conductive liquid flows through the measuring tube surrounded by the magnetic field, an induced electromotive force E proportional to the average flow velocity V will be generated in the direction perpendicular to the flow direction and the magnetic field. Electromagnetic flowmeters include pipe type and plug-in type.
Summary: With the adjustment and increase of basic energy demand and its price, the operating costs of high-energy-consuming equipment such as central air conditioning are also increasing. Therefore, in appropriate occasions (such as the use of central air conditioning in a large area), the energy meter is used to scientifically measure the central air conditioner, so as to realize the reasonable allocation of central air pressure operating costs, and promote users to use it on demand and pay according to the usage.
