Conventional Problems of Shell-and-Tube Heat Exchangers and Solutions

- Sep 25, 2019-

The general problems of shell-and-tube heat exchangers are common in daily life, so what are the solutions to the conventional problems of shell-and-tube heat exchangers? Today's edition gives some information as follows.


1. For shell-and-tube heat exchangers with internal pressure in both shell and tube side, under what conditions can the pressurized elements be designed with pressure difference? What other issues should be considered?


Pressure difference design can only be applied to components subjected to internal pressure in both tube and shell sides to ensure that both tube and shell sides rise and fall simultaneously. Pressure difference should also be taken into account in the process of pressure test, and the designer should propose a step-by-step procedure for pressure test.


2. Discuss the difference and function between the design temperature of tube and shell side and the temperature of tube wall and shell wall in shell-and-tube heat exchanger.


The design temperatures of tube and shell side are respectively the design temperatures of tube box and shell side shell, which correspond to the high or low values of the metal temperatures of tube and shell side compressive components (average temperature along the metal cross section of components) set by the design pressures of tube and shell side respectively. They are used to determine the allowable stresses of component materials.


The average temperature of tube wall and shell wall is the average metal temperature of heat exchanger tube and shell-side cylinder along the length, respectively. The average temperature of heat exchanger tube and shell-side cylinder formed in the process of heat transfer along the length direction is used to calculate the stress caused by thermal expansion difference between shell-side cylinder and heat exchanger tube in tube sheet, heat exchanger tube and shell-side cylinder.


These two groups of temperatures not only have different definitions, properties and functions, but also have great differences in numerical values. Therefore, we must pay attention to the calculation and not mix them up.


3. How to determine the design temperature of the elements in shell-and-tube heat exchangers which are affected by both tube and shell-side temperatures?


The design temperature of elements in shell-and-tube heat exchangers affected by both tube and shell side temperatures can be determined by metal temperature or higher side design temperature.


4. What are the principles for considering the corrosion margin of the main components of shell-and-tube heat exchangers?


Consideration Principles of Corrosion Margin for Main Components of Shell and Tube Heat Exchangers:


A) Corrosion margin should be considered on both sides of tubesheet, floating head flange, spherical crown head and hook ring.


B) Corrosion margin should be considered on the inner surface of flat cover, convex head, tube box and cylinder.


C) When slotting on the tubesheet and flat cover, the metal above the bottom of the baffle groove can be used as the corrosion margin, but when the corrosion margin is greater than the groove depth, the difference between the two should be added.


D) Corrosion margin should be considered on the inner diameter surface of pressure vessel flange and tube flange.


E) The corrosion margin is not considered for heat exchanger tubes.


F) Non-compressive components such as tie rods, pitch pipes, baffles and support plates, generally without considering corrosion margin.


5. What are the special requirements for the nozzle (or interface) design of shell-and-tube heat exchangers compared with general vessels?


(a) The nozzle should be level with the inner surface of the shell;


B) The nozzles shall be arranged as far as possible along the radial or axial direction of the heat exchanger.


C) When the design temperature is higher than or equal to 300 C, butt welding flange should be adopted.


D) Temperature meter interface, pressure gauge interface and liquid level gauge interface should be set up when necessary.


E) For heat exchangers that can not use nozzles (or interfaces) to discharge gas and liquid, vents should be set at the elevation of tube and shell, and vents should be set at the low point, with a nominal diameter of 20 mm.


F) Vertical heat exchangers can be equipped with overflow outlets.


6. How to determine the effective thickness of the integral tubesheet of shell-and-tube heat exchanger?


1. The effective thickness of the integral tubesheet is equal to the thickness of the tubesheet at the bottom of the groove of the dividing baffle, minus the sum of the following two thicknesses:


(a) The part where the corrosion margin of the pipe path exceeds the depth of the baffle groove of the pipe path;


B) The larger value of the corrosion margin in the shell side and the groove depth of the tube sheet in the shell side.


2. When the tube sheet is welded to the heat exchanger tube, the small thickness of the tube sheet should meet the requirements of structure design and manufacture, and not less than 12 mm.


3. Small thickness of clad tubesheet and corresponding requirements:


(a) The thickness of the cladding should be no less than 3mm for the cladding plate welded to the heat exchange tube. For the cladding layer with corrosion resistance requirements, the chemical composition and metallographic structure of the cladding layer with a depth of not less than 2mm from the surface of the cladding layer should also be guaranteed to meet the requirements of the cladding material standard.


B) Composite tubesheet with account connection shall have a small cladding thickness of not less than 10 mm, and shall ensure that the chemical composition and metallographic structure of the cladding at a depth of not less than 8 mm from the surface of the cladding meet the requirements of the cladding material standard.


7. What should be considered in the design of multi-pass shell-and-tube heat exchangers?


The split design of shell-and-tube heat exchangers should consider:


(a) As far as possible, the number of heat exchanger tubes in each tube should be approximately equal.


B) Layered baffle groove has simple shape and short sealing face length.


8. How to determine the inside depth of the tube box of shell-and-tube heat exchanger?


The inner depth of the tube box of shell-and-tube heat exchanger shall be determined according to the following requirements:


(a) The small depth at the center of the opening should be no less than 1/3 of the diameter of the nozzle.


B) The inner side depth of the multi-pass tube box should ensure that the small flow area between the two passes is not less than 1.3 times of the flow area of each heat exchanger tube; when operation permits, it can also be equal to the flow area of each heat exchanger tube.


9. What are the main types of connection between Tubesheet and heat exchanger tube in shell-and-tube heat exchanger?


The connection between tube sheet and heat exchanger tube of shell-and-tube heat exchanger mainly includes welding, expanding and expanding.