In nature, water always flows from high to low, and heat always transfers from high temperature to low temperature. People can use water pump to lift water from low position to high position through electric energy, and can also use heat pump technology to transfer heat from low temperature to high temperature through electric energy, thus upgrading low-temperature heat source to high-temperature heat source. Therefore, the heat pump is essentially a heat lifting device. The function of heat pump is to absorb heat from the surrounding environment and transfer it to the heated object (working medium with higher temperature). Its working principle is the same as that of the refrigerator, which works according to the inverse Carnot cycle, but the difference is that the working temperature range is different.
The first characteristic of heat pump technology is that it can make long-term large-scale use of low-temperature heat energy in rivers, lakes, oceans, urban sewage, industrial sewage, soil or air. As we all know, from focusing light with a lens, to lighting a fire, to solar water heaters, low-temperature heat energy is upgraded to high-temperature heat energy, but it cannot run on the ground all day. Heat pump technology breaks through this limitation, and can upgrade the low-temperature heat energy that we usually waste in production, life and nature to high-temperature heat energy for utilization. Obviously, the use of high-temperature heat sources, such as boilers, can achieve heating, refrigeration or other industrial purposes, so the application of heat pumps covers all industries and fields.
The second characteristic of heat pump technology is that it is the most energy-saving heating system in the world (i.e. coal, oil, natural gas, etc.). It can convert a large amount of low-temperature heat energy into high-temperature heat energy with a small amount of non-renewable energy (such as electric energy). For example, electric heating consumes 1 kwh of electricity and can only provide 1 kwh of heat at most, while a well-designed heat pump system can provide 4 kwh of heat when consuming 1 kwh of electricity. For example, it also provides 10 kWh of heat, and uses resistance heating to consume 34 kWh of primary energy (including 24 kWh of power generation and transmission and distribution losses); If high-efficiency oil-fired and gas-fired boilers are used for heating, 29. 1 kwh of primary energy will still be consumed if the pipeline heat loss is not calculated; Using electric heat pump for heating only consumes 2.7 kWh of electric energy, and with the loss of power generation and transmission and distribution of 3.3 kWh, it only consumes 6 kWh of primary energy. The primary energy consumption of heat pump technology is only 1/5 or nearly 1/6 of the first two heating methods.
The third feature of heat pump technology is that it can be used in reverse under certain conditions, which can be used for heating and cooling, without the investment of two sets of equipment. A paper company in Shanghai is a professional enterprise that produces high-grade coated white paperboard, with fixed assets of 400 million yuan and sales income of 268 million yuan from 65,438 to 099. The quality of white cardboard produced by the company can replace imported products.
The company's production steam is provided by the thermal power plant. The dryer of paper machine originally used three-stage steam ventilation, but there are the following problems in actual operation: first, the steam heat transfer efficiency of dryer is low, which leads to a large consumption of steam per ton of paper; Second, it is difficult to speed up the paper machine and improve the output; Thirdly, when the variety of paperboard production changes, equipment failures are frequent and paper breakage is more serious; Fourth, the operation and control are complicated. At the end of 1997, the company adopted the jet heat pump control system of an American company to transform the original system. The construction period of the project is 1 month. The new system makes full use of the heat energy of steam, the steam consumption per ton of paper is obviously reduced, the paper machine speed is increased by about 20%, and the equipment maintenance is greatly reduced. The output of 1999 increased by 70.7% compared with 1998, and the economic benefit was very remarkable. After installing the heat pump control system, it not only simplifies the operation and control, but also meets the frequent changes of production varieties.
The steam consumption before transformation is 9.0 109 joules/ton of paper. After the transformation, the steam consumption was reduced to below 7.5 109 joules/ton of paper, with a decrease of 16.7%, and the average saving per ton of paper was10.5109 joules. From 1998, 65438+ 10 to1August 999, the accumulated steam was saved 1439, 32 109 joules, with an average annual steam saving of 86359 109.
The total investment of the project is 3.645 million yuan, and the payback period is 1. 1 year.
This heat pump control system can be installed in the drying department of paper machines in domestic paper mills to replace the multi-stage steam ventilation mode commonly used now. Not only the steam heat energy is fully utilized, but also the condensed water in the dryer of the paper machine is discharged smoothly, and the paper machine runs safely and reliably. This heat pump control system can be an effective means for enterprises to save energy and improve efficiency.
At first, the company adopted three-stage steam ventilation, which is a series reverse heating system, that is, the direction of steam flow is opposite to the direction of paper movement, and the condensed water generated at each stage generates secondary steam for the next stage through a steam-water separator, so the steam is fully utilized in form. However, in practice, the system has a disadvantage. Because it is a series system, the pressure difference of each dryer is small, which will easily lead to poor drainage of the siphon of the dryer when the dryer is heated up, the paper machine is accelerated, the speed is adjusted and the paper is broken, which will increase the water level of the dryer, affect the heat transfer efficiency and hinder production. The high temperature in the other three parts leads to paper defects, such as paper hairiness and curl, and high back pressure in the final stage when the pressed paper is dried in the dryer. Therefore, the original system not only wastes steam, but also affects the quality and output of paper products.
In order to solve the above problems, the company adopted a jet heat pump control system. The heat pump is powered by a small amount of high-pressure steam, and the secondary steam of the steam-water separator is sucked into the heat pump by the suction generated by the high-pressure steam passing through the nozzle of the heat pump at high speed (Venturi principle). After mixing, high-grade steam is obtained and reused in the dryer. The heat pump system is controlled by flow control method. According to the production variety of paper machine, the thermal balance parameters of each drying cylinder are calculated by computer simulation, and the optimal secondary steam flow control parameters are obtained. The system uses a computer to control the orifice plate on the secondary steam pipeline by adjusting the high-pressure steam valve of the heat pump, so that the secondary steam flow is constant, the water level of the dryer is optimal, and the heat transfer efficiency of the dryer is the highest, so as to save steam and improve the product quality and output.
The new system has the following features:
(1) Due to the pumping action of the heat pump, a high drying cylinder pressure difference (up to about 70 kPa) can be maintained to ensure the smooth discharge of siphon condensate and achieve the best heat transfer efficiency.
(2) Because the control system is reliable, the surface temperature of the drying cylinder can be kept within the control range, which is convenient to adjust the working conditions such as drying cylinder temperature rise, paper machine speed increase, speed regulation and paper breakage.
(3) Non-condensable gas in the drying cylinder steam can be continuously discharged, which can prevent non-condensable gas from accumulating in the cylinder and reduce the heat transfer efficiency of steam.
(4) Ensure that the surface temperature of the wet-end dryer is 60℃ ~ 70℃, so as to prevent paper disease. At the same time, the back pressure at the end will be reduced to 0. 1 1 MPa (0.20 MPa before modification) to reduce the emission loss.
(5) 1.25 kg steam can dry 1 kg water.
Jet heat pump control system can reduce steam consumption, improve output and quality, with short payback period and remarkable energy saving benefits. The system is controlled by computer, with high degree of automation and simple operation. It can replace the multi-stage steam ventilation mode widely used in the paper industry at present, and is an ideal system equipment for steam ventilation in the drying section of paper machine in the future.
Energy saving of heating pipeline
Among many heat transfer elements, heat pipe is one of the most effective heat transfer elements known to people. It transfers heat through the evaporation and condensation of working fluid in a fully enclosed vacuum tube shell, and has a series of advantages, such as extremely high thermal conductivity, good isothermal property, freely changeable heat transfer area on the cold and hot sides, controllable temperature and so on. And can effectively transfer a large amount of heat over a long distance through a small cross-sectional area without external power.
Heat pipe heat exchanger has high heat transfer efficiency, compact structure and small fluid resistance loss, which is beneficial to control dew point corrosion. Widely used in petroleum, chemical industry, electric power, transportation, metallurgy, building materials, light industry, textile, machinery and other industries, as well as the cooling of electronic device chips, notebook computer CPU, circuit control board. Miniature heat pipes have been mass-produced. The heat pipe heat exchanger used for waste heat recovery in industrial field is a non-standard product, which will be designed and manufactured according to the scale, size, use and process conditions of various equipment.