Design idea of heat exchanger
Heat exchanger is a kind of equipment that transfers part of heat from heat fluid to cold fluid, also known as heat exchanger. Heat exchanger plays an important role in chemical, petroleum, power, food and many other industrial production. It can be used as heater, cooler, condenser, evaporator and reboiler in chemical production.
Classification of heat exchanger:
Shell and tube heat exchanger can be divided into the following two types according to the structural characteristics.
1. Shell and tube heat exchanger with rigid structure: this kind of heat exchanger is also a fixed tube plate, which can be generally divided into single tube pass and multi tube pass. The advantages of the utility model are simple and compact structure, low cost and wide application, and the disadvantages are that mechanical cleaning can not be carried out outside the pipe.
2. Shell and tube heat exchanger with temperature difference compensation device: it can make the heated part expand freely. The structure can be divided into:
- ① Floating head heat exchanger: the tube plate at one end of the heat exchanger can expand freely, which is called “floating head”. It is suitable for the large temperature difference between the tube wall and the shell wall, and the tube bundle space is often cleaned. But its structure is more complex and the cost of manufacturing is higher.
- ② U-tube heat exchanger: it has only one tube plate, so the tube can stretch freely when heated or cooled. The structure of the heat exchanger is simple, but the work of manufacturing bends is large, and because the pipe needs to have a certain bending radius, the utilization rate of the tube plate is poor, it is difficult to carry out mechanical cleaning in the pipe, and it is not easy to remove and replace the pipe, so the fluid passing through the pipe is required to be clean. This kind of heat exchanger can be used in the situation of large temperature difference, high temperature or high pressure.
- ③ Stuffing box heat exchanger: it has two forms. One is that each tube on the tube plate has a separate packing seal at the end to ensure the free expansion of the tube. When the number of tubes in the heat exchanger is small, this structure is adopted, but the tube spacing is larger than the general heat exchanger, and the structure is complex. The other is to make a floating structure between one end of the pipe and the shell. The whole stuffing box is used to seal the floating part. The structure is simple, but this structure is not easy to use in the case of large diameter and high pressure. Stuffing box heat exchanger is rarely used now.
Review of design conditions:
1. For the design of heat exchanger, the user shall provide the following design conditions (process parameters):
- ① Operating pressure of pipe and shell side (as one of the conditions for judging whether the equipment is of the same class, it must be provided).
- ② Operating temperature of tube and shell side (inlet / outlet).
- ③ Metal wall temperature (calculated by process (provided by the user)).
- ④ Material name and characteristics.
- ⑤ Corrosion allowance.
- ⑥ Number of courses.
- ⑦ Heat exchange area.
- ⑧ Specification and arrangement of heat exchange tube (triangle or square).
- ⑨ Number of baffles or support plates.
- ⑩ Insulation material and thickness (to determine the extension height of nameplate base).
- ⑪ Paint:
- I. If the user has special requirements, please provide the brand and color.
- II. There is no special requirement from the user, and the designer shall choose by himself.
2. Several key design conditions
① Operating pressure: as one of the conditions for judging whether the equipment is in the class, it must be provided
② Material characteristics: if you do not provide the material name, you must provide the toxicity level of the material.
Because the toxicity degree of the medium is related to the non-destructive monitoring, heat treatment and forging level of the equipment. For the above equipment, it is also related to the classification of the equipment:
- a. GB150 10.8.2.1 (f) indicate 100% RT of containers containing extremely toxic or highly hazardous media.
- b. 10.4.1.3 the vessel with extremely toxic or highly hazardous medium indicated in the drawing shall be subject to post weld heat treatment (the welded joint of austenitic stainless steel may not be subject to heat treatment).
- c. Forgings. Forgings with extremely toxic or highly hazardous medium shall meet the requirements of grade III or IV.
③ Pipe specification:
- Common carbon steel pipe: φ19×2，φ25×2.5，φ32×3， φ38×5
- Stainless steel pipe: φ19×2，φ25×2， φ32×2.5，φ38×2.5
Arrangement of heat exchange tubes: triangle, corner triangle, square, corner square.
★ when mechanical cleaning is required between heat exchange tubes, square arrangement shall be adopted.
Determination of basic design parameters
1. Design pressure, design temperature and welding joint coefficient.
- For cylinder with DN < 400, steel pipe shall be used.
- Cylinder with DN ≥ 400 shall be made of steel plate.
- 16 ” steel pipe – steel plate rolling is adopted in consultation with users.
According to the heat exchange area and the specification of heat exchange tube, draw the layout and determine the number of heat exchange tubes.
If the user provides the pipe layout, check whether the pipe layout is within the pipe layout limit circle.
★ Principle of pipe laying:
- ① The pipe shall be covered in the pipe laying limit circle.
- ② The number of multiple passes shall be as equal as possible.
- ③ The heat exchange tubes shall be arranged symmetrically.
Forgings shall be used when the tube sheet itself has a shoulder and is connected to the cylinder (or head). Because the tubesheet with this structure is generally used in high pressure, flammable, explosive, and extremely toxic, highly hazardous situations, the requirements for tubesheet are high, and the tubesheet is also relatively thick. In order to avoid slag addition and delamination at the shoulder and improve the stress condition of the fiber at the shoulder, reduce the processing amount and save materials, the integral forging directly forged from the shoulder and the tube sheet is used to manufacture the tube sheet.
Connection mode of heat exchanger and tube sheet
The connection of tube to tube plate is an important part in the design of shell and tube heat exchanger. It not only has a large amount of processing work, but also must make every connection in the operation of the equipment to ensure that the medium has no leakage and can bear the medium pressure.
There are three main ways to connect the tube and tube plate:
- a. Expansion joint;
- b. Welding;
- c. Expansion welding.
Expansion joint is used for the case that the leakage of medium between the shells will not cause adverse consequences, especially for the case of poor weldability of materials (such as carbon steel heat exchange tube) and excessive workload of the manufacturer. Because of the plastic deformation and residual stress in the welding process, the residual stress disappears with the increase of temperature.
In this way, the sealing and binding force at the pipe end are reduced, so the expansion joint structure is limited by pressure and temperature, which is generally applicable to the design pressure ≤ 4MPa, design temperature ≤ 300 ℃, and there is no severe ground motion, excessive temperature change and obvious stress corrosion in operation.
Welding connection has the advantages of simple production, high efficiency and reliable connection. Through welding, the pipe has a better effect on the tube plate; it can also reduce the requirements of tube hole processing, save processing time, convenient maintenance and other advantages, so it should be preferred.
In addition, when the medium is very toxic, the mixture of medium and atmosphere is prone to explosion, or the mixture of materials inside and outside the tube will have adverse effects, in order to ensure the joint sealing, the welding method is often used. Although the welding method has many advantages, because it can not completely avoid “crevice corrosion” and stress corrosion of welded joints, and it is difficult to get reliable welds between thin tube wall and thick tube plate.
Although the temperature of welding method is higher than that of expansion joint, under the action of high temperature cyclic stress, the welded junction is prone to fatigue crack, and there is a gap between the pipe and the pipe hole. When corroded by corrosive medium, the damage of the joint will be accelerated.
Therefore, the method of using welding and expanding at the same time is produced. This can not only improve the fatigue resistance of the joint, but also reduce the crevice corrosion tendency, so its service life is longer than that of single welding. At present, there is no uniform standard for the suitable welding and expansion joint method.
In general, when the temperature is not too high and the pressure is very high or the medium is very easy to leak, strength expansion and densification seal welding is used (seal welding refers to the welding which is only used to prevent leakage, and strength is not guaranteed). When the pressure and temperature are very high, strength welding plus expansion (strength welding refers to the welding when the strength of the weld is equal to the strength under the axial load of the pipe, which can ensure that the joint has a large pulling force even if the weld has tightness).
The main function of sticking expansion is to eliminate the crevice corrosion and improve the fatigue resistance of the weld. The specific structural dimension standard (GB / t151) has provisions, which will not be detailed here.
Requirements for surface roughness of pipe hole:
- a. When the heat exchange tube is welded with the tube sheet, the surface roughness Ra of the tube shall not be greater than 35um.
- b. When the single heat exchange tube is connected with the tube sheet by expansion joint, the surface roughness Ra of the tube hole shall not be greater than 12.5um. When the tube hole is connected by expansion joint, there shall be no defects affecting the tightness of the expansion joint, such as through longitudinal or spiral marks, etc.
1. Calculation of shell wall thickness (including the calculation of the wall thickness of the short section, head and shell side barrel)
The wall thickness of the tube and shell side barrel shall meet the requirements of GB151 for the minimum wall thickness. For carbon steel and low alloy steel, the minimum wall thickness is considered as the corrosion allowance C2 = 1mm. For C2 greater than 1mm, the minimum wall thickness of the shell shall be increased accordingly.
2. Opening reinforcement calculation
For the shell made of steel pipe, it is recommended to adopt overall reinforcement (increase the wall thickness of the cylinder or use thick wall pipe); for the relatively thick pipe box with large holes, comprehensive economy shall be considered
Several requirements to be met without additional reinforcement:
- ① Design pressure ≤ 2.5MPa.
- ② The center distance between two adjacent holes shall not be less than twice the sum of the two hole diameters.
- ③ Nominal diameter of nozzle ≤ 89mm.
- ④ The minimum wall thickness of nozzle shall meet the requirements of table 8-1 (the corrosion allowance of nozzle is 1mm).
When using standard flange for equipment flange, attention shall be paid to the matching of flange, gasket and fastener, otherwise the flange shall be calculated. For example, the gasket of type a flat welding flange in the standard is non-metallic soft gasket; when spiral wound gasket is used, the flange shall be recalculated.
4. Tube plate
The following issues should be noted:
① Design temperature of tubesheet: according to GB150 and GB / T151, the metal temperature shall not be lower than the element’s, but the medium effect of shell side cannot be guaranteed in tubesheet calculation, and the metal temperature of tubesheet is difficult to calculate, so the design temperature of higher side is generally taken as the design temperature of tubesheet.
② Multi pass heat exchanger: in the range of pipe laying area, the area which can not be supported by the heat exchanger due to the need of setting diaphragm groove and pull rod structure ad: calculated according to the formula of GB / t151.
③ Effective thickness of tube sheet.
The effective thickness of the tubesheet is the tubesheet thickness at the bottom of the pass divider plate groove minus the sum of.
- a. The part of the tube side corrosion allowance beyond the depth of the tube side diaphragm groove.
- b. The maximum planting of shell side corrosion allowance and structural slotting depth of tubesheet on shell side.
5. Setting of expansion joint
In the fixed tubesheet heat exchanger, because there is a temperature difference between the fluid in the tube side and the fluid in the tube side, and the heat exchanger and the shell are fixedly connected with the tube plate, there is a differential expansion between the shell and the tube in the use state, and the shell and the tube are subject to axial load. In order to avoid damage of shell and heat exchanger, instability of heat exchanger and pulling of heat exchange tube from tube sheet, expansion joint should be set to reduce axial load of shell and heat exchanger.
Generally, when the temperature difference between shell and heat exchanger wall is large, expansion joints shall be considered. In tube sheet calculation, σ T, σ C and Q shall be calculated according to various common conditions with temperature difference. If one of them is unqualified, expansion joints shall be added.
σ T – axial stress of heat exchange tube.
σ C — axial stress of shell side cylinder.
Q — pull off force of connection between heat exchange tube and tube plate.
1. Pipe box
(1) Length of pipe box
A minimum inside depth
- ① The minimum depth at the center of the opening of the single pass pipe box with axial opening shall not be less than 1 / 3 of the inner diameter of the nozzle.
- ② The inside and outside depth of the tube side shall ensure that the minimum flow area between the two sides is not less than 1.3 times of the flow area of each heat exchange tube.
Bmaximum inside depth
Consider whether it is convenient to weld and clean the internals, especially for the multi pass heat exchanger with smaller nominal diameter.
(2) Split partition
The thickness and layout of diaphragm shall be in accordance with table 6 and Figure 15 of GB151. For the partition with thickness greater than 10 mm, the sealing surface shall be cut to 10 mm; for the tubular heat exchanger, tear holes (drain holes) shall be set on the diaphragm, and the diameter of drain holes is generally 6 mm.
2. Shell and tube bundle
① Bundle grade
For the first and second grade tube bundles, only for carbon steel and low alloy steel heat exchange tubes, there are still “higher” and “ordinary” standards. Once the domestic heat exchange tube can be made of “higher grade” steel tube, the heat exchange tube bundle of carbon steel and low alloy steel need not be further divided into grade I and grade II.
The difference between tube bundles I and II mainly lies in the difference of the deviation of the outer diameter and wall thickness of the heat exchange tube, and the corresponding difference of the hole size and deviation.
For the stainless steel heat exchange tube, there is only grade I tube bundle.
a. Deviation of pipe hole size.
Pay attention to the difference between grade I and II tube bundles.
b. Partition slot.
- I. Groove depth is generally not less than 4mm.
- II. Groove width of split partition: carbon steel 12mm; stainless steel 11mm.
- III. The chamfer at the corner of the split diaphragm groove is generally 45 degrees, and the chamfer width B is approximately equal to the fillet radius r of the split gasket.
③ Baffle plate
a. Hole size: according to tube bundle level.
b. Notch height of bow baffle.
The notch height shall be such that the flow velocity of the fluid passing through the notch is similar to that across the tube bundle. The notch vertigo is generally 0.20-0.45 times of the inner diameter of the fillet. The notch is generally cut below the centerline of the tube bundle or between the small bridges of the two rows of tube holes (convenient for threading).
c. Notch orientation
- One way cleaning fluid, the gap is arranged up and down;
- A small amount of liquid is contained in the gas, and a liquid port is opened at the lowest part of the baffle plate with the notch upward;
- There is a small amount of gas in the liquid, and the air port is opened at the highest part of the baffle plate with the notch downward;
- When gas and liquid coexist or solid materials are contained in the liquid, the gap is arranged left and right, and the liquid port is opened at the lowest place.
d. Minimum thickness of baffles; maximum unsupported span.
e. The baffles at both ends of the tube bundle shall be close to the inlet and outlet nozzles of the shell side as much as possible.
④ Pull rod
- a. Diameter and number of tie rods: the diameter and quantity shall be selected according to table 6-32 and table 6-33. On the premise that the cross-sectional area of the tie rod given in table 6-33 is greater than or equal to, the diameter and quantity of the tie rod can be changed, but the diameter shall not be less than 10 mm and the quantity shall not be less than 4.
- b. Pull rods shall be arranged at the outer edge of the tube bundle as evenly as possible. For large diameter heat exchanger, appropriate number of pull rods shall be arranged at the pipe laying area or near the gap of baffle plate, and no baffle plate shall be less than 3 support points.
- c. Pull rod nut, some users require the following nut to be welded with baffle plate.
⑤ Scuff plate
a. The setting of the baffle plate is to reduce the uneven distribution of the fluid and the erosion of the heat exchange tube end.
b. Fixing method of anti impact plate.
Fixed on the distance pipe or the first baffle near the pipe plate as far as possible. When the shell side inlet is located on one side of the pipe plate of the non fixed pull rod, the impact plate can be welded on the cylinder.
⑥ Setting of expansion joint.
a. Between baffles on both sides
In order to reduce the fluid resistance of the expansion joint, if necessary, a liner can be set inside the expansion joint. The liner should be welded with the shell along the fluid flow direction. For vertical heat exchanger, when the fluid flow direction is upward, a drain hole should be set at the lower end of the liner.
b. Protection device of expansion joint to prevent the equipment from being damaged during transportation or use.
⑦ Connection between tube plate and shell
- a. Extension part as flange.
- b. Pipe plate without flange (GB151 appendix g).
3. Pipe flange:
- ① If the design temperature is more than or equal to 300 ℃, the counter flange shall be used.
- ② For the heat exchanger that can not be abandoned and discharged by using the nozzle type interface, the vent port shall be set at the highest point of the tube and shell side, and the drain port shall be set at the lowest point, with the minimum nominal diameter of 20 mm.
- ③ Vertical heat exchanger can be provided with overflow port.
According to Article 5.20 of GB151.
5. Other accessories
① Lifting lug
Lifting eyes should be set for the official box and the pipe box cover with a mass of more than 30kg.
In order to facilitate the removal of the pipe box and the cover of the pipe box, the top wire shall be set on the official plate and the cover of the pipe box.
Requirements for manufacturing and inspection
1. Tube plate
- ① Butt joint of splicing tube sheet shall be subject to 100% radiographic inspection or UT, qualification level: RT: Level II ut: Level I.
- ② Stress relief heat treatment of spliced tubesheet except stainless steel.
- ③ Width deviation of tube plate hole bridge: calculate the hole bridge width according to the formula: B = (S-D) – D1
- Minimum width of hole bridge: B = 1 / 2 (S-D) + C.
2. Heat treatment of pipe box:
For the pipe boxes made of carbon steel and low alloy steel welded with split diaphragm, and the pipe boxes with side opening more than 1 / 3 inner diameter of cylinder, stress relief heat treatment shall be conducted after welding, and the sealing surface of flange and diaphragm shall be processed after heat treatment.
When the shell side design pressure is lower than the tube side pressure, in order to check the quality of the connection between the heat exchange tube and tube plate
- ① Raise the shell side pressure to the same as the tube side test pressure for hydrostatic test to check whether the pipe joint leaks. (but it is necessary to ensure that the primary membrane stress of the shell is ≤ 0.9rel Φ during the water pressure test.)
- ② If the above method is not suitable, the shell can be tested as per the original pressure and then tested for ammonia leakage or halogen leakage.
Some problems that should be paid attention to on the drawing
- 1. Indicate the tube bundle level;
- 2. The standard number shall be written on the heat exchange tube;
- 3. The contour line of tube sheet layout is set with closed thick solid line;
- 4. The notch position of baffle plate shall be marked in the assembly drawing;
- 5. Whether the standard expansion joint drain hole, pipe joint and pipe plug on the vent hole should be mapped.
Source: China Heat Exchanger Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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What is heat exchanger design?
The heat exchanger is a thermal heat transfer device that exchanges (hence their name) the thermal energy from one source and transfers it to another at different temperatures. In most heat exchanger designs, the fluids or gases used to transfer the heat are separated and do not mix.
What is the work of heat exchanger?
A heat exchanger is a device that allows heat from a fluid (a liquid or a gas) to pass to a second fluid (another liquid or gas) without the two fluids having to mix together or come into direct contact.
What is the application of heat exchanger?
Heat exchangers use containment vessels to heat or cool one fluid by transferring heat between it and another fluid. Users of heat exchangers include chemical, petrochemical, oil & gas, power generation, refrigeration, pharmaceuticals, HVAC, food & beverage processing and pulp & paper industries.
Can the exchanger be repaired or replaced?
The best way to repair a cracked heat exchanger is to remove it and put in a new one. This doesn’t require replacing other parts of the furnace. However, it’s an expensive repair.