The heat exchanger plays a key part in the petroleum, chemical industry, light industry, pharmacy, and so on. In these fields, the goal of energy transfer is achieved by heating the cold liquid or condensing the hot liquid, and the process is done by the heat exchanger.
The heat exchanger is a device that transfers partial of the hot fluid to the cold fluid. It can be found everywhere in the modern industry. During the heat transferring process, it has great opportunity to contact the corrosive or oxidizing materials, thus the materials used to make the parts of equipment that contact the liquid are required to be corrosion resistant. The favorable materials are graphite, ceramics and glass. There is a wide range of the heat exchangers, including the reaction kettle, pressure vessel, condenser, spiral plate type, corrugated tube type, shell and tube type, positive displacement type, floating type, tube type, steam-to-water type, graphite type, Ti metallic type, etc. For your information, the titanium zirconium and molybdenum are too expensive, as well as the stainless steel is easy to have the intergranular corrosion.
A variety of heat exchangers have appeared these years. For the purpose of adapting to the development of modern industry, China already formed the rigorous standards and requirements about the production, and we tried our best to meet these requirements.
1. The technological conditions should be one hundred percent prepared.
2. The structural design should be reliable and safe.
3. The equipment should be easy to handle, install and repair
4. Economical in both acquisition and usage
The floating head type heat exchanger has the rod like structure, with one of its ends is fixed onto the equipment’s shell, and the other one is able to move freely within the shell. This structural design has many advantages, for example, no differential temperature stress forms between shell and pipes. The removable floating end allows the ease of inserting the pipes into the shell and ease of maintenance. The lid of the floating end is not visible when the equipment is working, so we should make the job of seal done well, when installing the machine in case of the leakage.
The floating end can be designed into various types, taking the free movement of floating part and ease of maintenance and cleaning into consideration.
When design the floating head heat exchanger, the outer diameter of floating end, D0, is an important parameter which should be smaller than the shell inner diameter, D1. The advisable interval between floating head and shell marked by b1, ranges from 3~5mm. In this way, when the backing ring of floating head is removed, the pipes can be draught out from the shell for ease of maintenance and cleaning. The floating head can be lidded only after the pipes are all in, so enough space should be reserved when installing the floating head.
Backing ring plays the key part in sealing, protecting the floating head. Along with the development of floating head type heat exchanger, the design of backing ring kept changing continuously.
Generally, the backing ring is split type, requiring the good sealing property, simple structure, ease of assembly and disassembly.
The floating head type exchanger shows good reliability and adaptability. For these years, a couple of new types appeared, but the floating head type still plays the leading part.
The contact face of tube heat exchanger is formed by tubes, thus the size and shape of the tube have great influence on the heat transfer efficiency. It can get its heat exchange area enlarged with the small diameter pipes. In a word, this equipment has contact structure, small metal material consumption, high heat exchange efficiency, but difficulty in production, maintenance and cleaning. The tube with big diameter is applicable to the liquid with stickiness and dirty.
The pipe material should be chosen according to the fluid pressure, temperature and chemical corrosion. The arrangement of the tubes should be concerned with the compactness of structure and nature of liquid, as well as the difficulty of machining.
If the heat exchanger has a great number of finned tubes with big diameter, the installer should fill the tube in the shell as many as possible, so as to extend the heat exchange.
The compactness and heat transfer capability are not the only two factors we think about the design of tube arrangement. The enough space for cleaning the surface of the tube and the mechanical strength of tube plate are also very important. In addition, there are some tips for fixation of tube. The interval of tubes is not the smaller the better. E.g. when we weld and connect the tubes, the weld seam should be apart enough to ensure the good welding result. When we expand and connect the tubes, the too close interval may cause the damage under the great squeeze force. Generally speaking, the center distance of a tube is no more than 1.25 times as long as the outer diameter.
When we need to expand the heat exchange area without prolong the tube length, we should expand the size of shell for the arrangement of more pipes. For the purpose of enhancing the heat transfer efficiency, we’d better classify the pipes into different pipe passes by a certain number of clipboards。
The advantages and disadvantages of floating head heat exchanger
1. The pipes can be draught out for ease of cleaning.
2. There are not too many requirements about the temperature difference between liquid media.
3. Remarkable durability to the high temperature or pressure. (450℃/ 6.4MPa at maximum)
4. Good adaptability to the working environment with too much scaling deposit or corrosive stuff.
1. The leakage takes place easily inside the little floating head
2. The great metal material consumption causes 20% increase in production cost
3. The structure is over complicated
The quality inspection should be conducted not only before the production, but also during production. The common inspection content and method are as follows
1. The inspection on the specifications of the finished parts.
2. The breaking test on the finished product, including the analysis on the chemical components of the raw material and welding line, the examination on mechanical properties and metallographic structure.
3. The inspection on the natural flaws inside the raw material and weld seam. The way is nondestructive test, referring to the X-ray, ultrasound, magnetic particle and permeation.
4. The pressure test, including hydraulic pressure test, medium test, air-tight test and so on.
Hydraulic Pressure and Air-tight Test
The finished heat exchanger should have its connector, tube and shell tested to ensure its good sealing property. Generally, the hydraulic pressure is the first choice, but if the test conditions allow no liquid to be filled or left, we choose the air-tight test.
We must test the shell to see if it had high enough pressure resistance, before we assemble the tube plate. Further, we inspect the connector between heat exchange plate and tube to test the pressure resistance of pipe.
For the U type pipe heat exchanger or kettle-type reboiler, we usually test the shell with ring crush tester, and then inspect the contactors, and test the tubes.
The floating head type heat exchanger or kettle-type reboiler should have its floating head pressure tested. For the kettle type reboiler, we will test the tube first, and then the shell.