The Research Of Heating Systems Suitable For Low Temperature Water Heating Terminal Devices

In order to recycle and make use of the large amount of low-grade heat comingfrom the nature or modern industrial, China has developed a series of lowtemperature water heating terminal devices, the water supply temperature of whichis between30℃and60℃. Compared to the conventional radiators, the lowtemperature water heating terminal devices are more expensive, and the watersupplied to them is of lower temperature, smaller temperature difference and biggerflow. So as to ensure a high operating efficiency, getting the largest economiceffects and taking full advantage of the low-temperature heat energies, we shouldfind out low temperature heating systems that are suitable for the new terminaldevices.Firstly in this paper, according to the devices’ requirements for the watersupply temperature, water quality and the stability of heat sources, it analyzed thepossible forms of the constituent elements of the low temperature heating systems,which contained the combination forms of the heat sources, the connection formsbetween the devices and the pipe network and the connection forms among thedevices. Based on the above analysis, it put forward three types of effective lowtemperature heating systems and gave principles to design a low temperatureheating systems.Secondly, with the method of mathematical analysis, it calculated theeconomic specific frictional resistance of the low temperature heating system pipenetwork, according to the principle of minimizing the pipe network annualcalculated expense. And then it compared the diameters and the specific frictionalresistances in the actual project chosen according to the range of economic specificfrictional resistances with those inquired from the table according to the traditionalrecommended pecific frictional resistances.Thirdly, for different low temperature water heating terminal devices, itoptimized the design conditions of the three low-temperature heating systemsaccording to the principle of minimizing the annual calculated expense of the wholesystem, obtaining the best water supply temperature and the best temperaturedifference between supply-water and backwater. Through the extra investmentpayback period method, it analyzed the application scope of each low temperature heating system, by selecting the coal-fired boiler heating system with supply andreturn water temperature95℃/70℃as the comparison datum. The applicationscope contained not only the lowest water supply temperature and maximum heattransportation distance of the low-temperature direct supply system and the boilerupgrading system, but also the required minimum parice ratio between the coal andelectricity when using the heat pump upgrading system.At last, with the energy analysis method and exergy analysis method, itestablished the gray-box analysis model of the entire low-temperature heatingsystem. And then it analyzed and evaluated the energy efficiency and exergyefficiency of different low temperature heating systems. Through the comparasionof the energy efficiency and exergy efficiency between the the low-temperaturedirect current system and the conventional coal-fired boiler heating system, itanalyzed the energy-saving effect of using the low-temperature energy directly toheating. For different temperature ranges of low-temperature heat source, it putforward two methods to calculate the heating energy efficiency and exergyefficiency of the heat pump upgrading system. And then through comparing themwith those of the low-temperature direct current system, it analyzed the reasonabletemperature range of the low-temperature heat source. lastly, it further analyzed theapplicable conditions of the three low-temperature heating systems from the viewpoint of energy efficiency.