1. Shell-and-tube heat exchangers are basic components.
· Shell
· Outer head
· Heat exchange tubes
· Tube box
· Tube box cover plate
· Tube plate
· Baffle
· Connection pipe
Other components include: tie rods and spacing tubes, partition plates, anti-collision plates, longitudinal partition plates, sealing strips, and supports, etc.
2. Classification based on structure
2.1 Fixed tube plate heat exchanger
In a fixed tube plate heat exchanger, the tubes are firmly connected to the tube plate at both ends, and the tube plate is welded to the shell. This structure has several forms, including removable tube box cover plates, head-type tube boxes, and integral tube plates.
The fixed tube plate heat exchanger has the advantage of low cost due to its simple structure. In fact, as long as no expansion joint is set on the shell, it is the lowest cost structural form. There are also other advantages, such as being able to mechanically clean the inside of the tubes after removing the tube box cover plate, and due to the absence of flange connections, the leakage of shell-side fluid is minimized.
The disadvantage of the fixed tube plate heat exchanger is that the tube bundle is fixed to the shell and cannot be disassembled, so mechanical cleaning cannot be performed on the outside of the tubes. Therefore, its use is limited by the requirement for clean external media. However, if a satisfactory chemical cleaning method is available, this heat exchanger can also be used in situations where the shell side has dirt.
In case there is a large temperature difference between the tubes and the shell, and the tube plate cannot absorb this temperature difference, it is necessary to add an expansion joint, which will inevitably offset its low cost advantage to a large extent.
2.2 U-tube heat exchanger
As the name suggests, the tubes of a U-tube heat exchanger are bent into a U shape, and there is only one tube plate. However, the advantage of low cost of a single tube plate is offset by the increased costs due to the bending of the U-tube and the requirement for a larger shell diameter (based on the minimum bending radius of the U-tube), making the cost of this heat exchanger comparable to that of a fixed tube plate heat exchanger.
The advantage of the U-tube heat exchanger is that one end is unconstrained, allowing the tube bundle to expand and contract freely. Additionally, since the tube bundle can be pulled out, external cleaning of the U-tube bundle is possible. Its disadvantage is that effective cleaning cannot be performed inside the tubes, so cleaning can only be done using a retractable rotating cleaning head.
2.3 Floating head heat exchanger
The floating head heat exchanger is the most widely used and most expensive type of shell-and-tube heat exchanger. One tube plate is fixed to the shell, while the other can move freely inside the shell. This structure allows for cleaning both inside and outside the tubes and permits free expansion and contraction of the tube bundle. Therefore, it can be used in situations where both the internal and external media are relatively dirty, such as in some devices in refineries.
There are many structural forms of floating head heat exchangers, among which the most commonly used are two types: hook-ring removable floating heads and removable floating heads.
3. Classification based on purpose
Generally, they can be divided into single-phase flow (heating or cooling of liquids or gases) and two-phase flow (condensation or vaporization). Since shell-and-tube heat exchangers have tube and shell sides, they can be classified according to several purposes:
· Single phase (whether shell side or tube side)
· Condensation (one side has condensation, the other side is single phase)
· Vaporization (one side is vaporization, the other side is single phase)
Condensation/Vaporization (one side is vaporization, the other side is condensation)
The following terms are also frequently used:
Heat exchanger: Both sides are single phase and both are process fluids (i.e., there is no common medium).
Heater: One side is process fluid, and the other side is a common heating medium, such as steam or hot oil.
Condenser: One side is condensate gas, and the other side is cooling water or air.
Cryogenic heat exchanger: One side is a process fluid condensed below atmospheric temperature, and the other side is boiling refrigerant or process fluid.
4. Design data
Before starting the design, let's take a look at some data that should be provided by process personnel in actual heat transfer design.
(1) Flow rates of the two fluids.
(2) Inlet and outlet temperatures of the two fluids.
(3) Operating pressures of the two fluids: This is very necessary for gas media, especially when the density of the gas is not provided, but for liquid phase media, it is not necessarily required since their properties do not change with pressure.
(4) Allowable pressure drop of the two fluids.
(5) Fouling resistance.
(6) Physical properties: including viscosity, thermal conductivity, density, specific heat, especially the physical properties at the inlet and outlet temperatures.
(7) Heat load.
(8) Type of heat exchanger.
(9) Pipeline specifications.
(10) Specifications of heat exchange tubes.
(11) Maximum diameter of the shell.
(12) Structural materials.
5. Tube side design
6. Heat transfer coefficient
7. Pressure drop
