Discussion on Several Problems in Process Design of Fluoroplastic Heat Exchanger

Due to the difference in physical and chemical properties between fluoroplastics and metal materials, the process and structural design of fluoroplastic heat exchangers are different from those of metal heat exchangers.
This paper proposes the general principles for the process (liquid-liquid heat exchange) design of fluoroplastic heat exchangers, analyzes the factors affecting the heat transfer coefficient of fluoroplastic heat exchangers, and proposes a solution to determine and improve the heat transfer coefficient through practice.
Fluoroplastic heat exchanger
Fluoroplastic heat exchanger is a new type of heat exchanger that can still resist strong corrosion under the three key technical conditions of cold flow, difficult welding, and difficult melting when connected to the tube sheet at high operating temperature and pressure. After the domestic research on fluoroplastic heat exchangers, the manufacture and application of fluoroplastic heat exchangers has become a reality. At present, the research and application of fluoroplastics started late. In 1973 The heat exchanger developed by the former Zhengzhou Institute of Technology and the former Jinxi Chemical Plant has attracted the attention and application of many industries due to its excellent performance. Due to the differences in the physical and chemical properties of polytetrafluoroethylene (F-4) tube sheet expansion pressure heating welding and polytetrafluoroethylene and metal materials developed by fluoroplastics, the fluoroplastic heat exchanger has a different design from the metal heat exchanger in the process of ethylene propylene (Fs-46) metal melt core expansion and one-time fusion method.
The materials used to make the heat exchange control and tube sheet of fluoroplastic heat exchangers in China are polytetrafluoroethylene and polytetrafluoroethylene propylene. The structure that assembles the fluoroplastic heat exchange control and fluoroplastic tube sheet (with baffles, partitions, fixed limit rings, etc. made of fluoroplastics or other materials) to form a structure that separates the cold and hot fluids is called a heat exchange element.
General principles for the design of fluoroplastic heat exchanger process (liquid-liquid heat exchange)
(1) In general, polytetrafluoroethylene (FEP) with excellent processing performance is selected as the raw material of the heat exchange tube. When there are high requirements for the operating temperature or pressure, the heat transfer area is not large, and the fluid is highly corrosive, polytetrafluoroethylene is selected as the raw material of the heat exchange tube.
(2) In general, the corrosive fluid is selected for the tube pass.
(3) Pay attention to the relationship and restrictions between the operating pressure and temperature. Under the same conditions, the operating pressure of the small diameter heat exchange tube is greater than the operating pressure of the large diameter heat exchange tube. The limit operating pressure depends on the high operating temperature of the cold and hot fluids.
(4) When the tube pass is a corrosive fluid and it is turbid or contains solid particles, it is not advisable to use a small diameter tube or a U-shaped or coil immersed structure.
(5) When designing tanks, kettles, and towers with U When designing a heat exchanger with a U-shaped or coil or other immersed fluoroplastic heat exchanger, the designer may fail to consider and take measures to prevent the entire heat exchanger (such as a cooler used in the top of a chlorinated oil tower, where the tubes are water and the coil immersed heat exchanger with a supporting frame is made entirely of fluoroplastic or heat exchange control (such as a U-shaped immersed cooler used to replace cast iron pipe cooling) from being damaged. In 98% to 93% circulating sulfuric acid, the tube side is water, and the fluoroplastic heat exchange element) floats during operation. One of the reasons for floating is that when all the parts of the heat exchanger are made of fluoroplastics (or some of the materials are non-metallic parts), they may fall apart due to their own weight or insufficient assembly strength, causing floating during operation;

The second reason
is that when the density of the cold and hot fluids differs greatly, floating occurs. As a result, the entire heat exchanger or heat exchange control will float upward, and it will be suspended in the liquid or float on the liquid surface, affecting heat exchange. The floating result will also cause some heat exchange tubes at the folding edge of the relevant parts to bend excessively, and may also cause the heat exchange tubes to collapse or burst. For the heat exchange control that is too long for the tank, kettle, and tower, it should also be required to form a concentric woven structure and try not to use a slack structure. Therefore, planners should pay attention to the floating problem, and it is important to take corresponding measures to prevent it.
(6) Anti-collision plates should be installed at the shell-side fluid inlet pipe of the shell-and-tube heat exchanger to prevent fluid erosion and damage to the heat exchange control. Removable non-metallic diversion baffles or soft materials that are in contact with the heat exchange control should be installed at the shell-side fluid outlet pipe to prevent siphoning and damage to the heat exchange control. If an arched baffle or partition is installed at the shell-side fluid outlet pipe, the spacing should be small. At the same time, the arched notch of the arched baffle should not be in the same direction as the outlet pipe.
(7) Before the tube-side fluid enters various types of heat exchangers, it should pass through filtering equipment if conditions permit.
(8) In general, seamless steel pipes are used as the shell of shell-and-tube heat exchangers. After the shell-and-tube heat exchanger is installed, its axis should have an inclination angle of ∠3° with the ground to facilitate maintenance.
(9) After the manufacture of the tank, kettle, tower or shell made of metal or non-metal materials, all surfaces in contact with the heat exchange control are required to be smooth and free of residual welding slag (lumps) and sharp protrusions, and the edges of the contact parts are required to be rounded. For metal tanks, kettles, towers or shells, it is recommended to use anti-corrosion materials to prevent and slow down the corrosion of the wall by the corrosive fluid leaked when the heat exchange tube may be damaged. It is required to set a sampling port at the outlet of the shell or tube side for sampling and analysis to confirm whether the heat exchange tube is damaged.