AIR VOLUME CALCULATION SYSTEM FOR VARIABLE SPEED OR CONSTANT SPEED FAN FIELD OF THE INVENTION [0001] The invention relates to an air volume calculation system for a variable speed or constant speed fan of a large-scale boiler fan equipment in a thermal power plant, belonging to the field of mechatronics. BACKGROUND [0002] Currently, air volume measurement devices are generally mounted in inlet ducts or outlet ducts in large-scale boiler fan equipment in thermal power plants at home and abroad. The main types of the air volume measurement devices comprise (1) a wing air volume measurement device, (2) a venture air volume measurement device, (3) a Pakistan series air volume measurement device, (4) a uniform speed air volume measurement device and (5) a hot wire air volume measurement device, etc. Most of the measurement devices take the method of firstly measuring pressure difference or thermoelectric force, and then converting the pressure difference or the thermoelectric force into air volume signal by an air volume transmitter. The method has disadvantages of complex system, low measurement accuracy, poor reliability and increased air volume resistance loss, and being unable to visually monitor operating conditions of the fan. [0003] It has not been reported that how to solve the above problems. SUMMARY OF THE INVENTION [0004] Provided is an air volume calculation system for a variable speed or constant speed fan, aiming to solve the problems of complex system, low measurement accuracy, poor reliability and increased air volume resistance loss, and not being able to visually monitor operating conditions of the fan, etc. - 1 - [0005] The object of the invention is achieved by the following solution. [0006] The air volume calculation system for a variable speed or constant speed fan is characterized in that the system comprises a central processing control system, a host processing system, a fan movable blade rotation angle indication transmitter, a fan inlet pressure transmitter, a fan inlet air temperature transmitter, a fan outlet pressure transmitter, a fan motor current transformer and a fan DCS control system; wherein the signal input terminal of the central processing control system is respectively connected to the fan movable blade rotation angle indication transmitter, the fan inlet pressure transmitter, the fan inlet air temperature transmitter, the fan outlet pressure transmitter and the fan motor current transformer; and the signal output terminal of the central processing control system is connected to the fan DCS control system. [0007] The central processing control system is a programmable logic controller (PLC) or a single chip. [0008] The host processing system is an upper computer or a fan DCS control system. [0009] The fan movable blade rotation angle indication transmitter and the fan motor current transformer are disposed inside the fan, the fan inlet pressure transmitter and the fan inlet air temperature transmitter are provided at the inlet of the fan, and the fan outlet pressure transmitter is provided at the outlet of the fan. ADVANTAGE AND EFFECT [0010] Provided is an air volume calculation system for a variable speed or constant speed fan, comprising a central processing control system, a host processing system, a fan movable blade rotation angle indication transmitter, a fan inlet pressure transmitter, a fan inlet air temperature transmitter, a fan outlet pressure transmitter, a fan motor current transformer and a fan DCS control system; wherein the signal input terminal of the central processing control system is respectively connected to the fan movable blade rotation angle indication transmitter, the fan inlet pressure transmitter, -2the fan inlet air temperature transmitter, the fan outlet pressure transmitter and the fan motor current transformer; and the signal output terminal of the central processing control system is connected to the fan DCS control system. [0011] The invention has advantages as follows: a. Original measurement methods are simplified, and the accuracy and reliability of the measured signal are improved. b. Prior air volume measurement devices and air volume transmissions are canceled, and manufacturing, installation and maintenance costs are reduced (the costs of the device are far less than the costs of the prior devices). c. In the present air flow measurement method, the measurement accessories mounted in ducts in original measurement devices are canceled, result in reduced pressure loss of the air duct system and improved fan economy. It has a certain practical significance for energy conservation advocated by National Development and Reform Commission. d. The device can be widely produced as a product and has big benefits. BRIEF DESCRIPTION OF THE DRAWINGS [0012] Fig. 1 is a structure schematic diagram of an air volume measurement system in the prior art. [0013] Fig.2 is a main structural schematic diagram of the invention. [0014] Fig.3 is a fan operating condition online monitoring graph of the invention. [0015] in figures 1-3, the reference numerals are assigned as follows: fan inlet 1, a fan air volume measurement accessories2, movable blade rotation angle indication transmitter 3, fan 4, fan outlet electric door 5, air preheater inlet door 6, air preheater 7, air preheater outlet door 8, boiler 9, pressure difference transmitter 10, fan DCS control system 11, programmable logic controller (PLC) 12, upper computer 13, fan -3 inlet pressure transmitter 14, fan inlet air temperature transmitter 15, fan outlet pressure transmitter 16, fan motor current transformer 17. DESCRIPTION OF THE EMBODIMENTS [0016] The invention will be further explained hereinafter when taken in connection with the accompanying drawings and the embodiments. [0017] Referring to Fig. 1, in prior large-scale boiler fan equipment in a power plant, fan air volume is measured by fan air volume measurement accessories 2 mounted in the fan inlet or outlet duct to obtain control for air condition volume of boiler combustion. Air volume is sucked from a fan inlet 1, and is powered by a fan 4. Most of the air volume is heated by an air preheater 7 before being directed into a boiler 9. In case of air volume signal provided by the fan, pressure difference is measured by the fan air volume measurement accessories 2 mounted in the fan inlet or outlet duct, and is converted into air volume signal after the pressure difference is sent to a fan DCS control system by a pressure difference transmitter 10. [0018] The main solution of the invention omits the measurement system which formerly uses an air volume device, so that the system is simplified, manufacturing, processing, installation, and maintenance costs are reduced, resistance loss of air supply system is reduced, and control signal accuracy, reliability and economy is improved. [0019] The invention is achieved by the following solution. [0020] Referring to Fig. 2, the invention comprises a central processing control system, a host processing system, a fan movable blade rotation angle indication transmitter 3, a fan inlet pressure transmitter 14, a fan inlet air temperature transmitter 15, a fan outlet pressure transmitter 16, a fan motor current transformer 17 and a fan DCS control system, wherein the signal input terminal of the central processing control system is respectively connected to the fan movable blade rotation angle indication transmitter 3, the fan inlet pressure transmitter 14, the fan inlet air temperature transmitter 15, the fan outlet pressure transmitter 16 and the fan motor -4current transformer 17. These data are calculated and processed via the program which has been saved in the upper computer, and is plotted to characteristic curves which are visually reflected on the screen of the upper computer, as seen in Fig. 3. The signal output terminal of the central processing control system is connected to the fan DCS control system 11. The central processing control system may choose a programmable logic controller (PLC) 12 or a single chip. The host processing system may select an upper computer 13 or a fan DCS control system. The so-called upper computer refers to a computer which can be directly sent manipulation commands by human, screen can display various acquisition signal changes and air volume calculations in real time, and can also recall historical data. [0021] The fan inlet pressure transmitter 14 and the fan inlet air temperature transmitter 15 are provided at the inlet of the fan, and the fan outlet pressure transmitter 16 is provided at the outlet of the fan. [0022] The programmable logic controller (PLC) 12 is a computing platform composed of a single chip and some hardware, which can input analogue quantity or switching value as signal via an input interface, and transmit the signal to the fan DCS control system 11 in form of analogue quantity via an output interface. [0023] The upper computer has two functions, one is that it can establish and transfer program, namely it can establish fan air volume calculation program and transfer the established program to the programmable logic controller (PLC), the other is that it can online monitor operation, namely it can display equipment operating conditions, an accident alert list and historical parameter queries by CRT display screen. [0024] The invention has following operation steps according to the variable speed or constant speed fan control method. a. The upper computer 13 can real-time display online fan operation conditions. b. Air volume data are provided to the fan DCS control system continuously. -5 c. A fan outlet air volume abnormal alarm signal is given when the calculated fan air volume exceeds the maximum and minimum air volume curve corresponding to the movable blade rotation angle. [0025] Acquisition of input signal: the fan inlet pressure, the fan outlet pressure, the fan movable blade rotation angle, the fan inlet air temperature and the fan motor current, etc. use the signal source of the original system, when the input signal of the invention is gathered. [0026] Establishment of mathematical model: the mathematical model is established according to the similarity theory of the relationship between fan rotation speed, air volume and pressure, and the real-time air volume value of the variable speed or constant speed fan is calculated by using the mathematical model. [0027] Implementation of open-loop calculation control: several curve families are calculated by using measured data of the control object, a threshold value is set according to the variable speed or constant speed fan characteristic curve, the calculated air volume value is compared with the threshold value, and an air volume abnormal alarm is given once the calculated air volume exceeds its operating range. [0028] Set of input parameters defence program: the fan inlet pressure, the fan outlet pressure, the fan movable blade rotation angle, the fan inlet air temperature and the fan motor current are provided with a signal abnormality alarm protection, so as to prevent inaccurate air volume calculation due to malfunction of input parameters. Once an unreliable parameter is appeared, an alarm signal of input parameter abnormality is given to suggest operators to look up and handle as soon as possible, so that the accuracy of the fan air volume calculation is ensured. [0029] Real-time monitoring: the fan operating conditions are directly shown by a movable flash point when opening the screen of the variable speed or constant speed fan characteristic curve (see Fig. 3). When the air volume exceeds the design bounds, operators will obviously observe this situation, and will take a replay action in time to adjust the fan operating conditions into the design area or high efficient area. -6- [0030] The invention replaces the original air volume measurement method with the air volume calculation device. The system has advantages of reducing air volume measurement device installation complexity, resistance loss and air volume measuring error, increasing safety operation reliability and adding an air volume characteristic online monitoring function. - 7-