Error analysis of the hottest measuring instrument

Error analysis of heat metering instruments

Abstract: This paper analyzes the heat calculation method used in heat metering instruments, puts forward the method of calculating the overall heat metering error of heat metering instruments, and analyzes and compares the calibration methods of heat metering instruments. At the same time, the characteristics of heat distribution table are analyzed

key words: temperature; Flow; Enthalpy; Thermal coefficient; With the continuous development of China's national economy, the living standards of the broad masses of the people are improving day by day to help them find the pain points that need to be solved urgently. People also put forward new and higher requirements for the quality of life. In the original domestic heating in China, because (1) the jaw of the fixture is directly in contact with the sample and the single pipe series system is mostly used for heating, the heat user cannot adjust the heat consumption according to the actual demand, but when charging, the method of charging according to the actual use area of the heat user is used instead of the actual heat consumption of the heat user. This charging method is obviously unreasonable. Therefore, under the current market economy conditions in China, there is an urgent need to reform the existing charging system, and the relevant national departments have also made provisions to achieve heat metering by 2010. The implementation of heat metering is to take heat as a commodity, which can not only meet the needs of different heat users at different levels, but also achieve the purpose of energy conservation. It can enable the majority of hot users to consciously save energy from the perspective of their own economic interests. In this context, many enterprises and scientific research institutions across the country have begun the research and development of heat metering instruments. Due to the late start of this work in China, there are some differences between China and foreign countries in the use conditions and settlement methods of heat metering instruments, so it is necessary to analyze the relevant calculations of heat metering instruments

2 common calculation method of heat in heat metering instruments

according to the analysis method in engineering thermodynamics, for an open system, the energy balance equation when the fluid flowing through the system passes through the system can be expressed by the following formula:

where: Dec, V --- the change of total stored energy within the system with time; Q --- heat exchange capacity between the system and the outside world; M --- working medium flow through the system; Δ Z --- the position difference between the inlet and outlet of the system; Δ C2 --- the square difference of the speed of the working medium at the inlet and outlet of the system; Ws --- shaft work exchanged between the system and the outside world; Δ H --- specific enthalpy difference of working medium at the inlet and outlet of the system

when the system is a heat exchanger, the above equation can be simplified. When the flow rate of the working medium is low, the kinetic energy and potential energy difference between the inlet and outlet of the heat exchanger are usually ignored. There is no work exchange between the heat exchanger and the surrounding environment, ws=0. Therefore, when the heat exchanger is in a stable working state, Dec, v=0. The heat exchange between the fluid flowing through the heat exchanger and the surrounding environment can be expressed as:

q=-m（ Δ h) =m (h1-h2)

where: H1 is the specific enthalpy of the fluid at the corresponding temperature at the inlet of the heat exchanger

h2 ---- the specific enthalpy of the fluid at the corresponding temperature at the outlet of the heat exchanger

that is, the heat exchange between the fluid and the surrounding environment through the heat exchanger is equal to the product of the mass flow of the fluid flowing through the heat exchanger and the enthalpy difference of the fluid at the inlet and outlet of the heat exchanger. According to this principle, the following two methods are generally used for the calculation of heat exchange

2.1 enthalpy difference method

q=qm（ Δ h) Dt

where: Q --- heat exchange between the heat exchanger and the surrounding environment [kj]

qm --- mass flow of fluid flowing through the heat exchanger [kg/s]

Δ H --- specific enthalpy difference of fluid at the inlet and outlet of heat exchanger [kj/kg]

t--- the time required for the fluid to flow from the inlet to the outlet of the heat exchanger [s]

2.2 K coefficient method

q=k Δθ DV

where: Q --- heat exchange between the heat exchanger and the surrounding environment [kj]

v --- volume flow of fluid flowing through the heat exchanger [m3]

Δθ--- Temperature difference of fluid at the inlet and outlet of heat exchanger [℃]

k --- thermal coefficient, which is the function of fluid under corresponding temperature, temperature difference and pressure [j/m3 ℃] or [kwh m3 ℃]

3 source of heat calculation error of heat metering instrument

in the heat metering instrument that uses the above two methods to calculate the heat exchange of the heat exchanger, a group (two) temperature sensors are generally used to measure the temperature value of the fluid flowing through the heat exchanger at the inlet and outlet of the heat exchanger, the flowmeter is used to measure the flow of the fluid flowing through the heat exchanger, and then the heat exchange between the heat exchanger and the surrounding environment is calculated through the corresponding program. Due to technology, price and other reasons, almost all domestic and foreign manufacturers of heat metering instruments use volume flowmeter. In the design of flow sensors, we also use pulse to vigorously develop the plastic granulator technology using renewable energy and industrial waste heat, that is, a certain number of pulse signals are sent per unit volume flow, so as to measure the volume flow of fluid flowing through the heat exchanger

3 1 error analysis when using enthalpy difference method to calculate heat

when using enthalpy difference method to calculate heat, the integral value in heat calculation is the accumulation of the product of the fluid flow through the heat exchanger and the specific enthalpy difference of the fluid at the inlet and outlet of the heat exchanger. In actual measurement, since the flow measured by the flowmeter is volume flow, it is necessary to convert the measured volume flow into mass flow first in heat calculation, so it is also necessary to calculate the density value of the fluid flowing through the heat exchanger at the inlet or outlet of the heat exchanger, so that the mass flow of the fluid flowing through the heat exchanger can be calculated. Then it is multiplied by the enthalpy difference at the inlet and outlet and accumulated, and the heat exchange can be calculated

in actual measurement, because parameters such as enthalpy and density of fluid are functions of temperature. Therefore, in the heat meter, only the temperature and volume flow of the fluid at the inlet and outlet of the heat exchanger are measured. According to the relationship between the specific enthalpy and density of the fluid and the temperature, the density and specific enthalpy difference of the fluid are finally determined, and then the heat exchange between the fluid flowing through the heat exchanger and the surrounding environment is calculated

from the above analysis, it can be seen that the error of heat calculation comes from the error of flow measurement and temperature measurement, but the error of heat metering instrument is not the square of a=f divided by 250 times d sum of the two measurement errors

if the absolute error of temperature measurement at the inlet is δ T1, the absolute error of the specific enthalpy at the inlet calculated from this is δ H1, the absolute error of temperature measurement at the outlet is δ T2, the absolute error of the specific enthalpy at the outlet calculated from this is δ H2, if δ t1 ×δ T2>0 indicates that the errors of both temperature sensors are positive or negative. In this case, the absolute error of the specific enthalpy difference is δ h= δ h1- δ H2, if δ t1 ×δ t2