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Design limiting flow rates, and velocity contribute to efficiencies. Due to the high flow the pressure gradient results in turbulence, required for thermal transfer. It also may increase the pumping charges. A compromise has to be made to ensure the performance of the HE does not suffer. As a Shell & Tube Heat exchanger is not just a circular pipe, to calculate the pressure drop across tube and shell sides respectively, following numerical equations are followed,
Pressure drop in shell and tube heat exchanger
Shell Side
L = tube length, m
l B
= baffle spacing
N p
= number of tube-side passes j f
= friction factor u t
= tube velocity m / s u s
= shell velocity m / s
D s
= diameter of shell( m) d e
= equivalent diameter of shell
ΔP s
= shell side pressure loss
ΔP t
= Tube side pressure loss
Tube Side
How to optimize pressure drop in process design of heat exchangers When designing the shell and tube for clients, the Kinam team tries to manage and limit the pressure drop. They prepare a configuration suitable for the plant operations onsite. This section will also address the question on the optimal performance of the fluid inside the heat exchanger.
To reduce the pressure drop on the shell side the following ways are applicable:
• Baffle selection: The purpose of providing baffles in the exchanger is to create more turbulence and to support the tube bundle. The turbulence created due to baffles leads to higher pressure drop, hence the right choice of baffle is important. For example, when there is a liquid in the shell, based on pressure drop perspective, the following are the baffle choices: 1. Segmented baffle( single)- offers high turbulence, high pressure drop and is easy to manufacture.
2. Double segmented- offers moderate turbulence, moderate pressure drop and is a little difficult to manufacture.
3. Rod baffle- offers low turbulence, low pressure drop and is difficult to manufacture.
4. No tube in window- it’ s like single segmental baffles with no tubes in the baffle cut out, provided to reduce vibrations in the exchangers.
• Baffling space increases the cross-flow. As the area is large, the pressure drop is limited to the minimum. Higher the baffle spacing, lower the turbulence and pressure drop.
• By increasing the baffle cut, the window flow area is enlarged. The limitation does not exceed 45 % of the diameter inside the shell.
Apart from the baffles the shell side pressure drop depends on the following factors,
• Tube pitch and pattern: Pitch is the center to center distance between two adjacent tubes. Higher the pitch, lower will be the pressure drop and turbulence. A triangular pitch offers higher pressure drop and turbulence. Square pitch offers lower pressure drop and turbulence. Designer has to select the right pitch and pattern.
• The shell diameter is increased, and the tube length is cut. This may affect the transfer coefficient which leads to lower pressure drop in the shell.
• Nozzle size: Proper nozzle size selection is important to ensure less pressure loss. For liquid, velocity in the nozzle should be equal to average velocity in shell or equal to the liquid line size. It is similar in the case of gases. Important factor is ρV ² which needs to be as low as possible.
To reduce the pressure drop on the tube side the following ways are applicable:
• Tube velocity: The higher the velocity, the higher is the pressure drop.
• Number of passes: The higher the number of passes, the higher is the pressure drop.
• Nozzle Sizes: Proper nozzle size selection is important to ensure less pressure loss. For liquid, velocity in the nozzle should be equal to avg. velocity in the tubes or equal to the liquid line size( provided line size is selected properly). It is similar in case of gasses. Important factor is ρV ² which needs to be as low as possible.
Thermal design fundamentals of shell and tube heat exchangers As we return to the drawing board, the thermal performance is integral to any shell and tube design. With customized plans, we take into account the surface area and calculate the hydraulic analysis to determine the pressure drop of flowing fluids. The important
Pressure drop measuring station. www. heat-exchanger-world. com Heat Exchanger World April 2025
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