This paper introduces a new kind of evaporators in which a self-rotating spiral and a driving wheel with spiral flange are installed in each heat transfer tube in heating chamber. The spiral can prevent and remove fouling at low flow velocity and the overall flow resistance is less than 10kPa. The modified boiling chamber has great depth, large cross-area and the area becomes bigger gradually. When the circulating mother solution flows though it at low flow velocity, it can begin to vaporize at greater depth and stay for a longer time. The sensitive heat got in the heating chamber can be changed into vaporizing heat as much as possible. Thus the kinetic energy loss can be reduced greatly to make the driving power of natural circulation reach 10similar to15kPa. The force-cycle pump can be removed, which will consume a great deal of energy. So, the new evaporator has great value to enhance heat transfer and save energy.
The smooth twisted strip, which can not only self clean away fouling in condensing tubes but also enhance heat transfer, cannot be directly used for condensers of low flowing velocity. It is mainly because the force movement of self-rotation is too small. For this reason, the authors have successfully developed another kind of twisted strip, i.e. the twisted strip with oblique teeth, according to the principle of momentum movement of particle system. The operation movement can be enhanced by times by the action of the teeth. Thus, the modified twisted strip can self-rotate and remove fouling reliably even at low flowing velocity. All the structure parameters of the twisted strip with oblique teeth have different effect on its performance, such as the teeth shape, oblique angle, intersection angle, etc, and their interrelationship is quite complex. So, many experiments were conducted including the tracer determination of flowing distance of an eddy after a tooth and comparison experiments of different parameter structure. Among many kinds of twisted strips with oblique teeth, the strip "Q" can increase the self-rotation movement by 277% over the smooth twisted strip. The overall flowing resistance of equipment is not great although the resistance coefficient of the strip with teeth is much greater than the smooth strip. Therefore, the twisted strip with teeth has good development prospects in condensers and other shell-tube heat exchangers of low flowing velocity.
Based on the problem that the self-rotating force moment is weak for the present plastic smooth twisted strip, the authors propose a method to enlarge it. Oblique teeth are attached on both surfaces of twisted strip dissymmetrically at a regular interval. The fluid directed by the teeth acts dissymmetrical reacting forces on the twisted strip. The forces form an added moment of forces couple. This makes the total force moment be increased by 75%similar to101%. So, the twisted strip with oblique teeth can be used widely in heat exchangers of low flow velocity of no less than 0.5m/s to prevent and remove fouling automatically, online and continuously. Furthermore, the heat transfer coefficient is raised by average 186% over the smooth twisted strip because strong vortexes must be induced behind the teeth. Though the coefficient of flow resistance is great, the overall flow resistance of equipment is not large because the flow velocity is low. Such resistance is in the range of industry permission.
The authors have developed a new fluidized-bed technique for vertical water-cooled condensers that substitutes bubble caps for sieve plate, in order to solve the problem that some of sieve holes will be blocked up after long operation. The technique key is that the internal circulation must be uniform and fluidized-bed must be stable after using the bubble caps. The experimental research shows the fluidized-bed layer at the bottom chamber must be enough thick and in the range of 260similar to280mm; the cooling water spurts from the bubble cap at the flow velocity of 5m/s; each bubble cap can ensure the fluidized-bed uniform in the range of less than the diameter of 345mm. Its industry application shows that the cooling capacity is increased by nearly once and the result of saving water and electricity is considerably higher using the new technique.
calculating model;driving force of natural cycle;evaporator;disequilibrium process
There has been no design calculating methods of driving force of natural cycle for evaporators. As the essential theoretical basis of the calculating model, the temperature difference (power) is grasped from so many factors whose interrelationship is complex. After the effect of disequilibrium evaporation process in evaporating chamber is analyzed thoroughly, the correction model is proposed which involves the effect coefficient K-1 of evaporating temperature of disequilibrium process and another effect coefficient K-2 of evaporating depth where vaporization begins. By calculation analyses of the flowing velocity and kinetic energy at the outlet of evaporating chamber, it is indicated that there is great effect of kinetic energy consumption on the driving force value. For this, the concept of effective driving force and the corresponding calculation model are proposed, which are convenient. for the calculation of cycle flow path. The engineering calculating formula with the comprehensive effect coefficient K is produced in order to consider the effect factors such as unlinear process in all-round way, make calculating results more reliably, avoid too much quantity of experimental work and make design calculation easy and convenient. The theoretical system proposed by this paper will be beneficial to develop the deep research on the driving force of natural cycle of evaporators.
[Yu, Tianlan; Yu, Xiumin] Research Institute of Mechanical Cleaning, Hunan University of Technology, Zhuzhou, China;[Yu, Tianxiang; Peng, Deqi; Wei, Biao; Zhi, Xiaoheng] Mechanical Engineering School, Xiangtan University, Xiangtan, China
2011 Asia-Pacific Power and Energy Engineering Conference, APPEEC 2011
March 25, 2011 - March 28, 2011
(1) Research Institute of Mechanical Cleaning, Hunan University of Technology, Zhuzhou, China; (2) Mechanical Engineering School, Xiangtan University, Xiangtan, China
Asia-Pacific Power and Energy Engineering Conference
International Conference on Energy and the Environment
Shanghai, PEOPLES R CHINA
Zhuzhou Inst Technol, Zhuzhou, Hunan, Peoples R China.
enhance heat transfer;fouling;prevent;plastic twisted strip;wear and tear
This paper discusses briefly the damage of fouling on heat exchangers. The damage is more serious, especially when the heat transfer coefficient is higher in the side where there is no fouling. The vacuum condensers equipped for steam turbines in power plants are taken as a typical example to analyze quantitatively. The structure principle of fouling preventing with the dynaflow twisted strip in each heat transfer tube is introduced. The experiments result shows that the ability of the technology to clean away fouling on line, continuously and automatically is great enough to meet the demand of removing or preventing fouling of heat exchangers, in which fouling will grow easily. Compared with the other advanced cleaning technologies such as using rubber balls, reciprocating brushes, etc., this technology has another advantage, i.e. to enhance heat transfer, because the fluid flows spirally led by the twisted strip. The enhancement is 9%-15%. The materials of the strips have been optimized to avoid the wear and tear on the heat transfer tubes wall. Application in large cooling and condensing equipments for over 3 years shows that there is no wear and tear, and the benefit is pretty high, e.g. there is average about 300 million tones water to be saved for each heat exchanger per year.