Questions about heat exchange equipment

Shell and tube heat exchangers, sleeve heat exchangers, water immersed heat exchangers, spray heat exchangers, rotary (serpentine) heat exchangers, plate heat exchangers, plate fin heat exchangers, tube fin heat exchangers, waste heat boilers, etc.

 

 

In the most common wall to wall heat exchangers, there are mainly two heat transfer methods: conduction and convection.

 

The hot fluid first transfers heat to one side of the tube wall through convection, then transfers heat through conduction from one side of the tube wall to the other, and finally transfers heat to the cold fluid through convection on the other side of the tube wall, thus completing the heat transfer process of the heat exchanger.

 

 

The higher the flow velocity of the medium in the heat exchanger, the greater its heat transfer coefficient. Therefore, increasing the flow rate of the medium in the heat exchanger can greatly improve the heat transfer effect, but the negative impact of increasing the flow rate increases the pressure drop through the heat exchanger and increases the energy consumption of the pump, so there must be a certain suitable range.

 

 

The use of specially designed surface structures for heat exchange tubes, such as finned tubes, nail head tubes, threaded tubes, etc., on the one hand increases the heat transfer area, and on the other hand, the turbulence effect of the special surface greatly increases the turbulence level of the fluid outside the tube. Both can improve the overall heat transfer effect of the heat exchanger, so these surface structures have better performance than the surface of the bare tube.

 

 

Adopting structures that increase the heat transfer surface, such as finned tubes, nail head tubes, threaded tubes, corrugated tubes, etc; Mechanical processing of pipe surfaces: spiral ring pipes, spiral groove pipes, threaded pipes, etc; By using small-diameter pipes, the number of pipes arranged on the same tube plate area can be increased, thereby increasing the heat transfer area.

 

Increasing the flow velocity of the fluid in the heat exchanger can greatly improve its heat transfer coefficient, such as by adding turbulence elements; Insert a spiral band into the tube, and set up baffles, fake tubes, etc. outside the tube; Increase the number of tube or shell processes. In addition, using materials with good thermal conductivity to manufacture heat exchangers, implementing anti-corrosion and anti scaling measures, and timely cleaning are all means to improve heat transfer efficiency.

 

 

Corrosion perforation of individual tubes in the bundle can be blocked with processed metal plungers with a cone degree of 3 ° to 5 °. Generally, within the same pipeline, the number of blocked pipes does not exceed 10% of the total pipes, but it can be increased appropriately according to the requirements of the process.

 

 

Because the flange fastening bolts on both sides of the pipe plate are the same bolt, the specific pressure applied to the gaskets on both sides of the pipe plate is the same. If different materials are selected for the gaskets on both sides, it will inevitably result in insufficient pressure on one side of the gasket, causing seal failure, or excessive pressure on the other side of the gasket, causing seal failure. Therefore, the gaskets on both sides of the tube plate must be made of the same material.

 

 

Scale is formed by the crystallization of dissolved salts in water, which adhere to the walls of heat exchanger tubes. Its characteristics are dense, hard, firmly attached, and difficult to remove. The large amount of suspended particles in water can become crystal seeds, while other impurity ions, bacteria, rough metal surfaces, etc. have strong catalytic effects on the crystallization process, greatly reducing the supersaturation required for crystallization precipitation. Therefore, cooling water heat exchangers are prone to scale formation.

 

 

Tube bundle, baffle plate, anti-collision plate, pull rod, fixed distance tube, shell, tube box, tube plate, inlet flange, outlet flange, floating tube plate, floating head flange, floating head cover, floating head hook ring, floating head gasket, outer head cover flange, outer head cover side flange, outer head cover gasket, vent port, drain port, tube box flange, tube box side flange, tube box gasket, tube box side gasket, fixed saddle, movable saddle.

 

 

Tube bundle, baffle plate, pull rod, fixed distance tube, shell, tube box (top cover), tube plate, inlet flange, outlet flange, tube box flange, tube box gasket, fixed saddle, movable saddle, ear support, expansion joint.

 

 

U-shaped tube bundle, baffle plate, anti-collision plate (inner guide tube), pull rod, fixed distance tube, shell, tube box, tube plate, inlet flange, outlet flange, tube box flange, tube box side flange, tube box gasket, tube box side gasket, fixed saddle, movable saddle.

 

 

The characteristics of fixed tube plate heat exchangers are compact structure, simplicity, low cost, the highest number of tubes arranged within the same shell diameter, easy replacement and maintenance of single tubes, and convenient cleaning inside the tubes. However, cleaning outside the tubes is difficult, and there is a large temperature difference and stress between the tubes and the shell.

 

The characteristics of U-shaped tube heat exchangers are relatively simple structure, no temperature difference stress problem, high fluid flow rate, low metal consumption, suitable for high temperature and high pressure fluids, and the tube bundle can be extracted for easy cleaning of the shell side and between the tubes. However, the elbows inside the tubes are not easy to clean, the number of tubes arranged on the tube plate is small, the tube spacing is large, there is a gap at the center of the tube bundle, and the fluid outside the tubes is prone to short circuit.

 

The characteristics of a floating head heat exchanger are that the tube bundle can move freely, without temperature difference stress problems, and the tube bundle can be freely extracted, making it easy to clean the outside of the tube and the tube bundle. However, the floating head structure is complex, the cost is high, and the sealing requirements at the floating head are strict. During operation, the floating head is prone to leakage and difficult to detect.

 

 

Triangular arrangement and square rotation at a 45 ° angle each have their own advantages and disadvantages. The advantages of triangular arrangement are compactness, high heat transfer efficiency, and the highest number of tubes arranged on the same tube sheet area, about 15% more than square arrangement, but it is not easy to clean the outer surface of the tubes; Square arrangement at a 45 ° angle is more convenient for cleaning the outer surface of pipes, but the number of pipes arranged is much less than triangular arrangement.

 

 

The size of the pipe diameter directly affects the performance of the heat exchanger. Small pipe diameter, high heat transfer coefficient, and large effective heat transfer area within the same volume. This can make the structure compact and save materials. However, a small pipe diameter can also have adverse effects. For fluids with the same flow rate, the smaller the pipe diameter, the greater the resistance encountered during flow, and the pressure loss also increases accordingly.

 

 

The bolt holes on the fixed support are circular in order to tightly fix the shell. The bolt holes on the movable support are long and circular, designed to allow the shell to expand and contract freely during temperature changes, avoiding significant stress and protecting the equipment.

 

 

There are oil resistant asbestos pads, iron clad pads, corrugated pads, and metal pads.

 

 

The baffle (baffle rod) in the heat exchanger can change the flow direction of the fluid in the shell side, increase the flow velocity of the fluid in the shell side, increase the turbulence level of the medium, improve heat transfer efficiency, and support the tube bundle.

 

 

When the total number of tubes in the heat exchanger is the same, increasing the number of tube passes can increase the flow velocity in each tube, thus increasing the heat transfer coefficient and reducing the required heat transfer area. But at the same time, it also increases the pressure drop and prevents the fluid from exchanging heat entirely in a countercurrent manner, and the structure of the heat exchanger is more complex. So the number of tube passes generally used is not less than 2 and not more than 8, and the specific selection should be based on the actual process requirements.

 

 

Corrosion, perforation or fracture of heat exchange tubes; Leakage caused by corrosion and thinning of the pipe mouth; The expansion joint between the heat exchange tube and the tube sheet is loose; Cracks, porosity, or corrosion perforation occur at the welding point between the heat exchange tube and the tube plate; Loose or broken small floating head bolts; The small floating head gasket is damaged; The small floating head or floating tube plate seal is damaged.

 

 

The purpose is to check whether the heat exchanger has the ability to safely withstand the design pressure (i.e. compressive strength), tightness, quality of interfaces or joints, welding quality, and tightness of the sealing structure. In addition, the residual deformation of the base metal welds of the container and pipeline under pressure can be observed to promptly detect any problems with the materials.