Heat exchanger is the equipment that transfers part of the heat of the hot fluid to the cold fluid, also known as the heat exchanger. Heat exchangers in chemical, petroleum, power, food and many other industrial production plays an important role in the chemical production of heat exchangers can be used as heaters, coolers, condensers, evaporators and reboiler, etc., widely used.

Heat exchanger tube

Classification of heat exchanger tubes
Heat exchanger tube is usually divided into: stainless steel boiler tube, stainless steel heat exchange tube, stainless steel condensation tube, with high temperature steam resistance, impact corrosion resistance, ammonia corrosion resistance; Anti-scaling, not easy to stain, anti-oxidation corrosion; Long service life, reduce maintenance time, save costs; Good tube loading process, direct tube change, uniform tube wall, safe and reliable, the overall thermal conductivity is better than copper tube; It is an essential heat exchange product for the transformation of old units and the manufacture of new equipment.

Main material of heat exchanger tube
The main materials of heat exchanger tube are: 304, 304L, 321, 316, 316L, 310S, 347H, 2205, 2507, 904L, etc. Stainless steel heat exchanger tube production standard: GB3087-1999 "Seamless Steel tubes for low and medium pressure boilers", GB5310-1995 "Seamless steel tubes for high pressure boilers", GB13296-2007 "Seamless stainless steel tubes for boilers and heat exchangers", ASMESA213 "Seamless ferrite and austenitic alloy steel tubes for boilers, superheaters and heat exchangers".

Heat exchanger box
Common heat exchanger tube box structures can be roughly divided into the following four categories

Type A tube box can be used for single tube and multi-tube, the advantage is easy to clean the tube of the heat exchanger; The disadvantage is that the structure of the pipe cover has more materials, and when the size is larger, forging is required, so it is recommended that the A-type pipe box should be used for DN≤900mm occasions.

B-type tube box is used for single tube and multi-tube, the advantage is simple structure, easy to manufacture; The disadvantage is that when repairing and cleaning the heat exchange tube of the pipe, it is necessary to disassemble the nozzle flange and the equipment flange on the pipe box, and remove the whole pipe box.

Type C pipe box The pipe box is a return pipe box of a multi-pipe heat exchanger.

Type D tube box This tube box is used for the inlet and outlet tube box of a single tube heat exchanger.

The flat cover of the tube box of the heat exchanger is an important part of the tube box, and different flat covers can be selected according to the use, material consumption, convenient cleaning and so on.

Heat exchanger tube Heat exchanger tube

Working principle of heat exchanger tube
The working principle of the heat exchanger tube is that the tube is generally fixed on the support. Two different media can flow in reverse (or in the same direction) in the tube to achieve the purpose of heat transfer.

Structure: Two tubes of different diameters are set together to form a concentric casing, each section of casing is called "one leg", the inner tube (heat transfer tube) of the process is connected by a U-shaped elbow tube, and the short tube of the outer pipe is connected in turn and fixed on the support. Heat is transferred from one fluid to another through the inner tube wall. In general, the hot fluid (fluid A) is introduced from above, while the cold fluid (fluid B) is introduced from below. Both ends of the casing and the outer pipe are welded or flanged with the inner pipe.

The structure of this type of heat exchanger is relatively simple, compact and cheap, but the tube can not be mechanically cleaned. The tube bundle of the heat exchanger is connected with a tube plate, which is welded at both ends of the shell, and is connected with a top cover, and the top cover and the shell are equipped with a fluid inlet and outlet nozzle. A series of baffle plates perpendicular to the tube bundle are usually arranged outside the tube.

At the same time, the connection between the pipe and the tube plate and the shell is rigid, and the inside and outside the pipe are two kinds of fluids with different temperatures. Therefore, when the temperature difference between the tube wall and the shell wall is large, due to the different thermal expansion of the two, a large temperature difference stress is generated, and the tube is twisted or the tube is loosened from the tube plate, and even the heat exchanger is destroyed.

Standard for heat exchanger tubesheet
The structure of heat exchanger is complicated. The heat exchanger tube with different structure forms is affected by load, supporting condition, boundary constraint condition and other factors, so the strength calculation process is complicated and the method is not uniform. Most standard formulas for calculating the strength of tubesheet generally simplify the tubesheet into an axisymmetric circular plate with open holes placed on an elastic foundation supported by the heat exchange tube, subject to uniform distribution load and uniform weakening of the tube holes. On this basis, different degrees of simplification and assumptions are made, and the basic assumptions are as follows.

Tube plugging rate of heat exchanger
Under the design situation, the basic heat exchange area margin is mostly about 20%, but with the long-term operation of the heat transfer coefficient decreases, the heat exchanger plugging rate can not exceed 20%.

Heat exchanger tube specification
Pipe specifications mm Pipe diameter mm Distance between the outer walls of adjacent pipes (tubesheet
Minimum hole bridge width) mm Minimum extension length after cutting the tube head mm material
Φ25*2.0 21 4 0.5~1.0 304, 316, 321
Φ25*1.5 22 4 0.5~1.0 304, 316, 321
Φ21.3*2.0 17.3 3 0.5~1.0 304, 316, 321
Φ21.3*1.5 18.3 3 0.5~1.0 304, 316, 321
Φ21.3*1.0 19.3 3 0.5~1.0 304, 316, 321
Φ17.2*2.0 13.2 3 0.5~1.0 304, 316, 321
Φ17.2*1.5 14.2 3 0.5~1.0 304, 316, 321
Φ17.2*1.0 15.2 3 0.5~1.0 304, 316, 321
Φ15*1.0 13 2 0.5 to 1.0 304, 316, 321
Connection mode of tube and tube plate of heat exchanger
Because the heat exchanger tube and tube plate are the only barrier between the tube and shell of the heat exchanger, the good quality of the joint between the heat exchanger tube and tube plate is the most important factor for the failure of the tube and shell heat exchanger. The type of joint between the tube plate and the pipe of the heat exchanger can be divided into expansion, welding, expansion and welding according to the different use conditions of the heat exchanger.

Expanded joint
First of all, the tube expander is put into the pipe, so that the diameter of the pipe becomes larger, plastic deformation occurs, and it is tightly attached to the tube plate. And the tube plate in contact with the tube mouth will be deformed elastic as the tube mouth becomes larger. When the tube expander is pulled out, the elastic deformation of the tube plate will recover the previous size, but the tube mouth will still remain in a larger state after plastic deformation, and will not return to the original state, so that the two are tightly connected together.

weld
The welding process is simple, the connection strength is good, and the tightness of the connection and the tensile ability can be ensured at high temperature and high pressure. The welding method should be used to fix the pipe and the thin tube plate. Residual thermal stress and stress concentration in tubesheet and pipe after welding may cause stress corrosion and fatigue failure during operation.

Expansion and welding
When the temperature and pressure are high, and under the action of thermal deformation, thermal shock, thermal corrosion and fluid pressure, the connection between the heat exchange tube and the tube plate is easy to be destroyed, and it is difficult to ensure the connection strength and sealing requirements by expanding or welding.

At present, the method of expansion and welding is widely used. Expansion and welding structure can effectively damp the damage of tube bundle vibration to the weld, can effectively eliminate stress corrosion and gap corrosion, improve the fatigue resistance of the joint, thus improving the service life of the heat exchanger, than the simple expansion or strength welding has higher strength and sealing.

Why should the heat exchanger tube use 304 stainless steel tube
304 stainless steel pipe production features Light production equipment, less investment, fast construction speed, simple welding method, wide product specifications, high dimensional accuracy, small wall thickness deviation, smooth surface, high yield. In terms of cost, it is about 20% lower than seamless tubes, so the heat exchanger tubes are now used in 304 stainless steel tubes.

What is the standard of heat exchanger pipe diameter?
The pipe diameter depends on the flow rate of the medium, the pipe length depends on the heat transfer area calculation, the design should consider the pipe length diameter comprehensively, to meet the heat transfer requirements, but also to ensure the safety of the heat exchanger, such as to prevent vibration, such as stress intensity calculation is also essential, but also to calculate the cost of the heat exchanger, in short, the choice of these two parameters is based on many factors

The outer diameter of the heat exchanger tube is divided into A and B two series, the outer diameter of the A series heat exchanger tube is generally with a decimal point, and the B series is the common standard series products in China. HG/T20592-2009 standard, there are A, B series of heat exchanger tube size table, corresponding to different internal diameters of nominal size flanges.