Heat Exchanger Baffles
Shell-and-tube exchangers contain several types of baffles to help direct the flow of both tube-side and shell-side fluids. Pass partition baffles force the fluid to flow through several groups of parallel tubes. Each of these groups of tubes is called a “pass,” since it passes the fluid from one head to another. By adding pass partition baffles on each end, the tube-side fluid can be forced to take as many passes through the exchanger as desired.
Transverse baffles support the tubes that pass through holes in the baffle. The transverse baffle cannot go all the way across the cross-section of the shell, because the fluid that is in the shell has to be able to comeover the top of the baffle and under the bottom of the next baffle, etc., as it passes across the tubes that are in the heat exchanger. When it is desired that a fluid pass through the shell with an extremely small pressure drop, these will usually be half-circle, 50% plates that provide rigidity and prevent the tubes from sagging.
Transverse baffles can help maintain greater turbulence for the shellside fluid, resulting in a higher rate of heat transfer. The transverse baffles cause the liquid to flow through the shell at right angles to the axis of the tubes. This can cause considerable turbulence, even when a small quantity of liquid flows through the shell if the center-to-center distance between baffles, called baffle spacing, is sufficiently small. The baffles are held securely by means of baffle spacers, which consist of throughbolts screwed into the tube sheet and a number of smaller lengths of pipe that form shoulders between adjacent baffles.
Transverse baffles are drilled plates with heights that are generally 75% of the inside diameter of the shell. They may be arranged, as shown in Figure 3-3, for “up-and-down” flow or may be rotated 90° to provide “side-to-side” flow, the latter being desirable when a mixture of liquid and gas flows through the shell. Although other types of transverse baffles are sometimes used, such as the orifice baffle shown in Figure 3-4, they are not of general importance.
Impingement baffles are placed opposite the shell-side inlet nozzle. The flow into the shell hits the impingement baffle and is dispersed around the tubes, rather than impinging directly on the top tubes. This keeps the full force of the momentum of the flow from impinging on and eroding the top tubes.
Longitudinal baffles force the shell-side fluid to make more than one pass through an exchanger. With no longitudinal baffle, such as in Figure3-1, the shell-side fluid makes one pass from inlet to outlet. With a longitudinal baffle, and with the nozzles placed 180° around the shell, the shell-side fluid would be forced to enter at the left, flow to the right to get around the baffle, and flow to the left to reach the exit nozzle. This would be required to approximate true counter-current flow, which was assumed in the heat transfer equations of Heat Transfer Theory.
Categories: Shell and Tube Exchangers | Tags: Baffles | Leave a comment