Research on the calorimeter used in the hottest ce

2022-08-24
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Research on calorimeter for central heating and central air conditioning system

Abstract Taking central heating and central air conditioning system as the application object, a set of calorimeter system is developed. The system is based on single chip microcomputer MCS51, with modules composed of external crystal oscillator circuit and reset circuit, power supply module, amplification and a/d conversion module, external RAM and voltage monitoring module, external clock and flow measurement module It is composed of keyboard and display module. With the help of software system, it completes data collection, calculation, processing and storage, and realizes intellectualization. The verification experiment shows that; The system has the advantages of low starting flow rate, high precision, high degree of automation, stable operation, and the error is less than 4% when the flow rate is greater than the starting flow rate. The development and research of this system is of great significance to the market-oriented operation and management of central heating and central air conditioning systems

subject words calorimeter central air conditioning single chip microcomputer module

introduction

in the process of charging for central heating and central air conditioning, at present, it is still calculated according to the building area. This method does not meet the requirements of market-oriented management. There is an urgent need to measure the heat (cold) consumed by users, so as to safeguard the interests of users and heating (cold) sides, but at present, there is no widespread use of this similar instrument. This is due to the difficulties in heat measurement, which limits the development of such instruments. First of all, because heat belongs to process quantity, in experiment or engineering measurement, traditional measurement methods have great difficulty in measuring process quantity, and there are many problems such as large measurement error and many correction factors. In fact, traditional measurement methods can not meet the accurate measurement of heat, but with the wide application of computer and signal processing technology in thermal parameter measurement, thermal measurement instruments are developing towards intelligence and miniaturization, and the accurate measurement of heat can be realized by making full use of the advantages of the combination of microcomputer software and hardware

in theory, the measurement of heat flow rate in stable flow can be attributed to the product of fluid mass flow and its temperature difference and constant pressure specific heat, that is, in the experiment, the measurement of heat flow rate mainly adopts the direct method, and assumes that the constant pressure specific heat of fluid is constant, that is, it is simplified to the measurement of mass flow rate and its temperature difference. To measure heat, it is necessary to continuously measure the heat flow rate and accumulate it. The research of this kind of measuring instrument is of great significance to heating and ventilation, energy utilization, experimental research and other fields, but the development and research of this kind of instrument is relatively difficult. Taking the heat measuring instrument for central heating and central air conditioning system as an example, the following problems need to be solved

① in the heating system, the fluid flow velocity is low and the mass flow rate is small, so it is difficult to accurately measure the small flow rate fluid in the heating system

② the measurement of inlet and outlet temperature difference should ensure a certain accuracy, and ensure that the measurement of temperature difference and mass flow rate is synchronized and relevant data is stored; Moreover, the temperature (difference) of the system fluctuates greatly, and there are many practical problems such as the determination and installation of measuring points, which are extremely difficult to deal with

③ even if it can realize the synchronous measurement of small flow rate heat exchange fluid and temperature difference, a certain τ The heat flux at any time can be calculated by theoretical formula:

it is extremely difficult to complete the cumulative measurement of the above formula by using the traditional measurement method

based on the above problems, in order to realize the accurate measurement of heat, we must give full play to the advantages of microcomputer software and hardware, and realize a series of functions such as measurement, data storage, calculation and display for specific users with small flow and small temperature difference according to their own use conditions. This paper is filled with B. test object data: it provides users with the data of all test objects. The single-chip microcomputer system has the characteristics of easy development, strong function, small size, low price and so on. A set of calorimeter has been developed. Experiments show that the system has the characteristics of good stability, high precision, strong function, high degree of automation, easy maintenance and so on

research and development

in the research and production process of thermal engineering and material science, the indirect method is generally used for the measurement of heat. Most of these instruments only measure the heat flow. At present, the industrialized products include radiant heat flow meter, thermal resistance heat flow meter, etc. these instruments need to calibrate the instrument constants experimentally, which has the disadvantages of large error and measurement lag. This paper is based on the discretization equation of the theoretical calculation formula of heat, By making full use of the advantages of MCS51 single chip microcomputer system, which is easy to develop and combines software and hardware, the intelligent calculation of heat is realized. Combined with the difficulties of heat measurement, the intelligent instrument can well realize the following functions

(1) temperature difference measurement, which is completed by two-stage amplification circuit, a/d conversion circuit and relevant acquisition software

(2) the measurement of small flow mainly depends on the magnetoelectric induction element to convert the flow signal into the standard frequency signal, and the MCS51 single chip microcomputer and relevant acquisition software realize the cumulative measurement of the frequency signal

(3) the cumulative calculation of heat and the storage of data are mainly completed by software and corresponding registers

(4) power off protection function. Due to the external power failure of the system, important data will be written into the relevant memory and saved. The system's own power supply will start to work, and start to record the power off start time and call time. After the call, the power off time will be automatically accumulated and stored in the external RAM memory

(5) display function, whether it is used to check whether each wiring is disconnected or whether the heating company can understand the relevant data information through the display function of the instrument

(6) reset function: the heating company can reset the instrument at the end of the heating cycle for convenience of management

in order to realize the above functions of the instrument, the system hardware is mainly composed of the following modules: basic module, power module, amplification and a/d conversion module, external RAM and voltage monitoring module, external clock and flow measurement module, keyboard and display module, which is mainly composed of single-chip microcomputer MCS51 and additional external crystal oscillator circuit and reset circuit. See Figure 1 for the composition of system components, The composition and main functions of each module are as follows:

the basic module mainly composed of MCS51 single chip microcomputer is the core part of the system, which mainly completes the relevant processing of the data collected by the system, coordinates the work of other modules, and makes the whole system work in step. The chip selected is 8051 single chip microcomputer, with 5 internal interrupts and 4K ROM program memory, which is very convenient to use. The external crystal oscillator is 12Hz, The reset circuit is mainly designed for the convenience of calorimeter operation and management, and is connected with the function reset key of the keyboard

system power supply module: it is mainly used to supply 5V standard DC working voltage to the system, including the working voltage of single chip microcomputer, operational amplifier, LCD display, a/d conversion and standard comparison voltage in the system. The precision of the power supply has a great impact on the whole system, which is mainly composed of transformer, rectifier circuit, regulator and comparison circuit, The output voltage of the power supply is measured by the 6.5-bit KEITHLEY2000 multi-function meter, and its output range can be stabilized at: 4 0001v, its accuracy is extremely high, and the error caused to the system as a reference voltage can be ignored

amplification and a/d conversion module: the main function is to amplify the signal of the thermocouple and send it to the corresponding register through a/d conversion for relevant calculation. The accuracy of this module directly affects the temperature measurement accuracy of the system and is the main source of temperature measurement error. Therefore, the selection of amplification devices mainly considers its precision, ability to suppress zero drift, self calibration and other performance. The chip selected in the system is tlc40502. The error caused by zero drift when the chip is amplified by 5000 times during debugging is not greater than 0.4 ℃. Thermal potential of copper thermocouple in the range of 0 ~ 100 ℃ 36 μ V/℃, it can be concluded that the error caused by zero drift is not greater than 0.4 ℃. Tlc0831 is selected as the a/d converter, and the working temperature range of the chip is 0 ~ 70 ℃. It is an 8-bit serial control analog-to-digital converter, which is easy to connect with the microprocessor interface. The resolution and quantization error of the device are important reasons that affect the temperature measurement accuracy. It can be seen from the copper constantan thermocouple and the measurement magnification that the maximum error caused by the resolution and quantization error is not greater than 0.2 ℃, Therefore, the total temperature measurement error caused by amplification and a/d conversion is not more than 0.6 ℃. Compared with the design temperature difference of 20 ℃ in general heating system, the maximum error caused by the above reasons is not more than 3%, which is relatively high accuracy

external RAM and voltage monitoring module: external RAM is mainly used to store important data, especially when the system is powered down, to store the collected heat value and remember the power down time, which is convenient for management. Its main chip is X24C45, which is nonvolatile and easy to write. The main functions of the voltage monitoring circuit are: automatically connect the backup battery to the circuit when the main power supply fails, and disconnect the backup battery when the main power supply is restored, so as to save the system data. The main chip is inp708, which is equipped with a watchdog timer and a step-down detection μ P monitoring circuit

external clock and flow measurement module: it mainly completes the operation of the single chip microcomputer, provides time counting and continues to work with the battery power supply in case of power failure, and provides a clock for recording the time of power failure. The main chip is dsi302, which belongs to the trickle charging timing chip. The flow measurement circuit mainly completes the conversion and measurement of flow signals, and the fluid flow is converted into frequency signals through magnetoelectric sensors, optocouplers, etc, This part is one of the main sources of error in heat measurement. The key is the calibration of the corresponding constant between frequency signal and flow and the influence of minimum flow in the process of flow signal conversion

keyboard and display module: the main functions of the keyboard include resetting, adjusting the magnification, querying the important real-time value of the register, etc. the display function is to display the accumulated heat value calculated by the single chip microcomputer, which is the main source of system power consumption. Therefore, LCD is selected, which has cc14544 chip with low power consumption and easy connection with the single chip microcomputer

(1) under the condition of normal power on and starting to work, first carry out the system self-test. After the self-test is completed, read the data converted from a/d, convert it into the corresponding temperature difference, read the frequency value of the counter (reset the counter immediately after reading the value) and convert it into the corresponding flow value, calculate the heat, read the accumulated stored heat value from the external RAM, accumulate it with the measured heat value, and return it to the external RAM for storage after accumulation, Complete a measurement cycle

(2) when the sudden power failure is abnormal, the voltage monitoring circuit starts to work, and provides a short-term power supply to enable the single chip microcomputer to record the important data and the value of the external clock into the external RAM. At the same time, the system starts to record the time, so that when the power supply returns to normal, the system will carry out the work of (1) and record the cumulative value of the power failure time

(3) the software automatically measures and eliminates the temperature drift and time drift. Set the zero value of each sensor in the software to coexist as a data file, and subtract the zero value of the corresponding sensor in the heat measurement calculation, which can effectively eliminate the influence of temperature drift and time drift, and improve the measurement accuracy of the sensor and the overall accuracy of the system

Experimental verification and application of accuracy

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