Design and calculation of column tube (shell and tube) heat exchanger
1. The choice of fluid flow path which fluid flows through the heat exchanger tube, which fluid flows through the shell, the following points can be selected for reference (in the case of a fixed tube heat exchanger)
(1) Unclean and fouling fluids should be removed from the tubes to facilitate cleaning of the tubes.
(2) The corrosive fluid should be taken away from the pipe to prevent the casing and the pipe from being corroded at the same time, and the pipe can be easily cleaned and overhauled.
(3) The fluid with high pressure should be taken away from the tube to avoid pressure on the shell.
(4) Saturated steam should be taken away from the pipe to facilitate the timely removal of condensate, and the steam is relatively clean. The condensation heat transfer coefficient has little to do with the flow rate.
(5) The fluid to be cooled should be taken away from the tube and the heat dissipation from the outer shell can be used to enhance the cooling effect.
(6) Fluids that require increased flow rates to increase their convective heat transfer coefficients should be routed within the pipe, since the pipe flow area is often smaller than the shell process, and multiple pipe passes can be used to increase the flow rate.
(7) Large-viscosity liquids or fluids with small flowrates should be run between tubes. Since fluids flow on the shell side of baffle baffles, the flow rate and flow direction are constantly changing, and at low Re (Re>100). Turbulence can be achieved to increase the convective heat transfer coefficient.
When selecting the fluid flow path, the above points can not always be taken into account at the same time. The main contradiction should be grasped depending on the specific situation, for example, the fluid pressure, anti-corrosion and cleaning requirements should be considered first, and then the convective heat transfer coefficient and pressure drop should be checked so that Make more appropriate choices.
2. The choice of fluid velocity increases the fluid flow rate in the heat exchanger, will increase the convective heat transfer coefficient, reduce the possibility of deposition of dirt on the surface of the tube, that is, reduce the thermal resistance of the dirt, so that the total heat transfer coefficient, As a result, the heat transfer area of ​​the heat exchanger can be reduced. However, the increased flow rate increases fluid resistance and increases power consumption. Therefore, the appropriate flow rate can only be determined through economic balance.
In addition, structural requirements must also be considered when selecting flow rates. For example, choosing a high flow rate will reduce the number of tubes. For a given heat transfer area, longer tubes or increased number of passes have to be used. The tube is too long to be easily cleaned, and the general manager has a certain standard; a single pass into multiple passes reduces the average temperature difference. These are also issues that should be considered when selecting flow rates.
3. Determination of the temperature at both ends of the fluid If the temperatures of the cold and hot fluids in the heat exchanger are specified by the process conditions, there is no problem in determining the temperature at both ends of the fluid. If one of the fluids is only known for the inlet temperature, the outlet temperature should be determined by the designer. For example, when cooling a hot fluid with cold water, the inlet temperature of cold water can be estimated based on the local temperature conditions, and the temperature of the cold water at the outlet of the heat exchanger needs to be determined based on economic balance. In order to save water, the water outlet temperature can be increased, but the heat transfer area needs to be increased; in order to reduce the heat transfer area, it is necessary to increase the amount of water. The two are contradictory. In general, the temperature difference between the two ends of the cooling water can be 5~10°C. Water shortage areas use larger temperature differences, and water-rich areas use smaller temperature differences.
4. Pipe Size and Arrangement When selecting the pipe diameter, the flow rate should be as high as possible, but generally should not exceed the flow rate range described above. Easy to scale, the viscosity of the larger liquid should adopt a larger diameter. The series of tube heat exchangers currently tested in China have only two types of tubes: φ25×2.5mm and φ19×mm.
The choice of the director is based on the principle of convenient and rational use of pipes. Long tubes are not easy to clean and are easy to bend. The standard length of a typical steel pipe is 6m. A reasonable length of the heat exchanger should be 1.5, 2, 3, or 6m. The four standards are also used in the series of standards. In addition, the tube length and shell diameter should be adapted, generally take L / D 4 ~ 6 (smaller diameter of the heat exchanger can be larger).
As mentioned earlier, the tubes are arranged on the tube sheet in an equilateral triangle, a square column, and a square column, as shown in Figure 4-25 in Section 5. The advantages of the equilateral triangle arrangement are: high strength of the tube plate; less chance of the fluid to walk short circuit, and greater fluid disturbance outside the tube, so the convective heat transfer coefficient is higher; more tubes can be arranged in the same shell diameter. The advantage of the square in-line arrangement is that it is convenient for cleaning the outer wall of the tube, and is suitable for occasions where the shell-side fluid is prone to fouling; however, its convective heat transfer coefficient is lower than that of the normal triangle arrangement. The square staggered arrangement is between the above two, that is, the convective heat transfer coefficient (arranged in more straight lines) can be appropriately increased.
The distance between the tubes arranged on the tube sheet (referring to the center distance between two adjacent tubes) varies with the method of connecting the tubes to the tubes. Generally, the tube expansion method takes t = (1.3 ~ 1.5) do, and the distance between the outer walls of two adjacent tubes should not be less than 6mm, that is t ≥ (d + 6). Welding method to take t = 1.25do.
5. Determination of tube and shell number When the flow rate of the fluid is small or the heat transfer area is large and the number of tubes needs to be large, the flow velocity in the tube is sometimes low, and the convective heat transfer coefficient is small. In order to increase the flow rate in the tube, multiple tubes can be used. However, the excessive number of passes leads to increased fluid resistance in the tube and increases the cost of power; at the same time, multiple passes reduce the average temperature difference; in addition, the multi-channel partitions reduce the area available on the tube sheet, and these problems should be taken into account when designing. The series of tube heat exchangers have four types of tube passes: 1, 2, 4 and 6 cycles. When using multiple passes, the number of tubes per pass should generally be approximately equal.
The number of pipe runs m can be calculated as follows:
(4-121)
The appropriate velocity of the fluid in the u??? tube in the formula, m/s;
u'??? The actual velocity of the fluid in the tube, m/s.
When shell side fluid velocity is too low, shell side multiple passes can also be used. If a baffle parallel to the tube bundle is installed in the housing, fluid flows through the housing twice. This is called a two-shell process. However, since the longitudinal baffle has difficulties in manufacturing, installation and overhaul, it is generally not used. Instead of a multi-pass heat exchanger, several heat exchangers are used in series instead of a shell-side multi-pass. For example, when a two-shell process is required, the total number of tubes is divided into two parts, which are respectively installed in two equal-diameter and smaller diameter shells, and then these two heat exchangers are used in series as shown in the figure.
6. Baffle baffle
The purpose of installing deflector baffles is to increase the velocity of the shell-side fluid and increase the degree of turbulence in order to increase the convective heat transfer coefficient of the shell side.
Figures 4-26 of the fifth section have shown various baffle forms. The most commonly used is a circular lack of baffle, cut out the arch height is about 10 ~ 40% of the inner diameter of the shell, generally take 20 ~ 25%, too high or too low are not conducive to heat transfer.
The distance between two adjacent baffles (plate spacing) h is (0.2 to 1) times the inner diameter D of the housing. The h values ​​used in the series of standards are: fixed tube plate type 150, 300 and 600mm; floating head type 150, 200, 300, 480 and 600mm five. The plate spacing is too small to be easily manufactured and overhauled, and the resistance is also large. If the distance between the plates is too large, it will be difficult for the fluid to flow vertically through the tube bundle and the convective heat transfer coefficient will decrease.
The influence of the height of the bow cut off by the baffle and the distance between the plates on the fluid flow is shown in Figure 3-42.
7. Determination of Shell Diameter The inner diameter of the heat exchanger shell should be equal to or slightly larger (for floating head heat exchangers) than the diameter of the tube sheet. According to the calculated actual number of tubes, diameter, tube center distance and the arrangement of the tubes, etc., can be used as a rule to determine the inner diameter of the shell. However, when the number of tubes is large and calculations are repeated, the mapping method is too troublesome and time-consuming. Generally, in the preliminary design, the flow rates of the two fluids may be selected separately, and then the required cross-sectional area of ​​the tube path and the shell path may be calculated. , Detect the diameter of the housing in the series of standards. After all the designs are completed, the pipe arrangement drawing is still applied by drawing. In order to make the tubes evenly arranged and prevent the fluid from going "short-circuiting", some tubes may be appropriately increased or decreased.
In addition, the inner diameter of the housing can also be calculated using the following formula in the preliminary design, ie: (4-122)
Type D???? shell diameter, m;
t???? tube center distance, m;
Nc??? - the number of tubes crossing the centerline of the tube bundle;
The distance from the center of the outermost tube on the centerline of the tube bundle to the inner wall of the shell is generally b'=(1~1.5)do.
The nc value can be calculated by the following formula.
When the tubes are arranged in a regular triangle: (4-123)
When the pipes are arranged in squares: (4-124)
Where n is the total number of heat exchangers.
The calculated shell diameter should be rounded to the standard size, as shown in Table 4-15.
8. The main components of the head seal are square and round, square for small diameter shell (usually less than 400mm), round for large diameter shell.
The buffer baffle can prevent the impact of the shell-side fluid on the tube bundle when entering the heat exchanger. A buffer baffle can be installed at the feed nozzle.
There must be a space (dead angle) where the fluid cannot flow between the inlet, outlet and tube plate of the shell side fluid. In order to improve the heat transfer effect, a guide tube is often added outside the tube bundle to make the fluid enter and exit the shell. Must pass this space.
Ventilation holes, drain holes The heat exchanger shell often has bleed holes and drain holes to exclude non-condensable gases and condensate.
The pipe diameter of the inlet and outlet of the fluid in the heat exchanger is taken into account as follows:
The volume flow of Vs--fluid in the formula, /s;
u - The flow rate of the fluid in the pipe, m/s.
The empirical value of the flow rate u is:
For liquid u=1.5 to 2 m/s
For steam u=20~50 m/s
For the gas u = (15 ~ 20) p / Ï (p is the pressure in units of atm; Ï is the gas density in units of kg / )
9. The materials used for the material selection of the tubular heat exchanger should be selected according to the operating pressure, temperature, and fluid corrosivity. The mechanical properties and corrosion resistance of general materials decrease at high temperatures. At the same time, there are few materials that have heat resistance, high strength, and corrosion resistance. Currently used metal materials are carbon steel, stainless steel, low alloy steel, copper and aluminum; non-metallic materials are graphite, PTFE and glass. Although stainless steel and non-ferrous metals have good corrosion resistance, they are expensive and scarce and should be used as little as possible.
10. Fluid flow resistance (pressure drop) calculation
(1) Pipeline fluid resistance The resistance of the pipe can be calculated according to the general formula of frictional resistance. For multi-pass heat exchangers, the total resistance Δpi is equal to the sum of the straight pipe resistance, the bending resistance, and the inlet and outlet resistances. In general, the inlet and outlet resistances are negligible, so the total resistance of the pipe is calculated as:
(4-125)
Wherein, Δp1, Δp2 ------ are the pressure drop caused by frictional resistance in straight pipe and return bend pipe respectively, N/ ;
Ft - scaling correction factor, no dimension, for φ25 × 2.5mm tube, take 1.4, for φ19 × 2mm tube, take 1.5;
Np - number of pipe runs;
Ns - number of shells in series.
In the above equation, the pressure drop of the straight pipe Δp1 can be calculated according to the formula described in Chapter 1; the pressure drop Δp2 of the return bend is estimated by the following empirical formula:
(4-126)
(2) Shell-side fluid resistance Although there are many formulae for calculating the fluid resistance of shell-side fluids, the resulting fluids are quite different due to the complex fluid flow conditions.
The following formula is used to calculate the shell pressure Δpo by Esso:
(4-127)
Where Δp1'--pressure drop across the tube bundle, N/ ;
Δp2′-------fluid pressure drop through the baffle notch, N/ ;
Fs -------- Shell pressure reduction scaling factor, no dimension, for the liquid desirable 1.15, desirable for gas or condensable vapor 1.0
And (4-128)
(4-129)
The F----pipe alignment method has a correction factor for pressure drop. For the normal triangle arrangement, F=0.5, for the square, 45 degrees is 0.4, and the square arrangement is 0.3;
Fo----The friction coefficient of the shell fluid, when Reo>500,
nC--the number of tubes crossing the centerline of the tube bundle;
NB----the number of baffles;
h ---- baffle spacing, m;
Uo---- The flow rate calculated from the flow cross-sectional area Ao of the shell side flow, and.
In general, the pressure drop of the liquid flowing through the heat exchanger is 0.1 to 1 atm and the gas is 0.01 to 0.1 atm. When designing, the process size of the heat exchanger should be weighed between the pressure drop and the heat transfer area so that it can meet the process requirements and be economically reasonable.
Third, the choice of tube heat exchanger and design calculation steps
1. Trial and primary equipment specifications
(1) Determine the flow path of the fluid in the heat exchanger.
(2) Heat load Q is calculated based on the heat transfer task.
(3) Determine the temperature of the fluid at both ends of the heat exchanger, select the type of the tubular heat exchanger; calculate the qualitative temperature, and determine the nature of the fluid at the qualitative temperature.
(4) Calculate the average temperature difference, and determine the number of shells based on the principle that the temperature correction coefficient should not be less than 0.8.
(5) According to the empirical value range of the total heat transfer coefficient, or according to the actual production situation, select the total heat transfer coefficient K selected value.
(6) From the total heat transfer rate equation Q = KSΔtm, initially calculate the heat transfer area S, and determine the basic dimensions of the heat exchanger (such as d, L, n and the arrangement of the tube on the tube sheet, etc.), or according to series standards Select device specifications.
2. Calculate the pressure drop of shell and shell process. Calculate the flow rate and pressure drop of the tube and shell side fluid according to the initial equipment specifications. Check the calculation results are reasonable or meet the process requirements. If the pressure drop does not meet the requirements, adjust the flow rate, and then determine the number of tubes or baffle spacing, or select another size of equipment, recalculate the pressure drop until the requirements are met.
3. Calculate the total heat transfer coefficient calculation tube, shell side convection heat transfer coefficient αi and αo, determine the dirt thermal resistance Rsi and Rso, then calculate the total heat transfer coefficient K, compare the initial value and calculated value of K, if K/K=1.15～ 1.25, the primary equipment is suitable. Otherwise, you need to set another value for K. Repeat the above steps.
In general, when selecting or designing heat exchangers, other issues should be considered under the premise of satisfying heat transfer requirements. They are often contradictory to each other. For example, if the overall heat transfer coefficient of the designed heat exchanger is large, the pressure drop (resistance) of the fluid passing through the heat exchanger will increase, and the power cost will be increased accordingly; if the surface area of ​​the heat exchanger is increased, it may make the total The heat transfer coefficient and pressure drop are reduced, but they are limited by the size of the heat exchanger that can be installed, and the cost of the heat exchanger is also increased.
In addition, other factors (such as the amount of heating and cooling media, overhaul and operation of heat exchangers) cannot be ignored. In short, the designer should comprehensively consider the above factors and give careful judgment in order to make a proper design.
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