Shell & Tube Heat Exchanger

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Transfer heat between two streams without letting them mix. The tube-side stream enters the channel head, is turned through the tube bundle by the pass partition plate and leaves at the temperature the downstream duty requires; the shell-side stream is driven across the bundle by the baffles rather than along it. The tube walls, the tube-to-tubesheet joints and the bolted girth joint are the containment boundary — between the two streams, and between both of them and atmosphere. Which stream is the hotter one and which sits at the higher pressure is a property of the service, not of the geometry: this reference model cannot tell you, so read both off the unit's datasheet before working any case here, and read the deviations below in those terms rather than assuming the shell is the cold, low-pressure side. Tagged as the exchanger E-101. The exchanger itself is what a P&ID tags — its shell, bundle, baffles, tubesheet, gasket and bolting carry no tags of their own — and the instrument and relief tags used on the safeguards below are conventional illustrations of the protection, not tags read from a drawing.

Components

• Shell

  Contains the shell-side stream and forms the outer pressure boundary around the bundle. Its design pressure and temperature belong to the shell-side service, and are frequently not the same as the tube side's.

• Tube bundle

  Carries the tube-side stream through the shell. The tube wall is both the heat transfer surface and the barrier between the two streams, so it is the one part where a loss of containment mixes the streams instead of releasing them. One tubesheet and a pass partition plate are modelled, which is the arrangement of a two-pass — most probably U-tube — bundle; no rear tubesheet, floating head or return bends are separately named, so the rear-end type cannot be read off this model. Neither can the model say which side is hotter or at the higher pressure. Both are properties of the service.

• Baffle set

  Directs the shell-side stream across the bundle rather than along it, and supports the tubes against sagging and flow-induced vibration. Four are modelled.

• Tubesheet

  Anchors the tube ends and separates the shell side from the tube side. The tube-to-tubesheet joints are the boundary between the two streams, and are where thermal cycling, crevice corrosion and vibration do their damage.

• Pass partition plate

  Divides the channel head into inlet and outlet compartments, so the tube-side stream is made to pass through the bundle instead of short-circuiting from the inlet nozzle straight to the outlet nozzle.

• Channel head, inlet compartment

  Receives the tube-side stream from its inlet nozzle and distributes it across the first pass of the bundle.

• Channel head, outlet compartment

  Collects the tube-side stream returning from the bundle and passes it to the outlet nozzle.

• Nozzle flanges

  The flanged connections through which the two streams enter and leave, together with any vent and drain points. Six are modelled; the geometry alone does not say which serve the shell side, which the tube side, and which are vents or drains.

• Girth joint gasket

  Seals the bolted joint between the channel, the tubesheet and the shell flange. At that joint it is the last barrier between the process and atmosphere, and it is the joint broken every time the bundle is withdrawn.

• Flange bolting

  Clamps the girth joint, holding gasket compression against internal pressure and against the movement of the joint through temperature.

• Saddle supports

  Carry the weight of the shell and its contents into the supporting structure, and locate the exchanger. On a horizontal unit one saddle is conventionally fixed and the other allowed to slide so the shell can grow axially; the model does not distinguish the two.

Operating states

• Startup and warm-through

  Both sides vented and filled, then brought up together. The colder stream is established first and the hotter one admitted gradually, so the shell and the bundle change temperature at a rate the joints can carry rather than one dragging the other. High-point vents on the channel and on the shell are closed only once liquid is confirmed: gas trapped in a channel compartment or under the top of the shell leaves part of the surface dry, and a dry surface drifts toward the temperature of the stream on the other side of it instead of sitting between the two. Bolted joints are re-checked warm, because bolt load moves as the metal expands.

• Normal operation

  Tube-side fluid enters the inlet compartment, is turned into the bundle by the pass partition plate, returns to the outlet compartment and leaves. Shell-side fluid crosses the bundle on the path set by the four baffles, which also hold the tubes against sagging and vibration. Heat passes through the tube wall; both outlet temperatures sit where the duty requires and the pressure drop on each side is steady — and it is the two readings together, not either alone, that say the exchanger is clean. Read the geometry for what it is: one tubesheet and a pass partition plate is consistent with a two-pass tube side, most probably a U-tube bundle, with the two channel-head nodes as the compartments either side of the partition. A fixed-tubesheet or floating-head unit cannot be ruled out from the geometry alone, and the model says nothing whatever about which stream is hot or which is at the higher pressure. Treat all three as indicative and confirm them against the unit.

• Reduced throughput

  The unit runs below design flow on one side or both. Velocities fall, so solids that were previously swept through settle in the shell-side baffle windows and along the bottom of the shell, and deposits build on the tube bore — fouling accelerates at exactly the point where the duty is easiest to hold. With less flow to heat or to cool, the outlet temperature approaches the other stream's inlet temperature more closely than it does at design, so the exchanger still looks as though it is performing while the fouling resistance grows underneath. Pressure drop is the reading that gives it away.

• Controlled shutdown

  The stream supplying heat is taken off first, and the other kept running until the bundle has come down with it, so the exchanger cools together rather than one side being quenched. Both sides are then depressurised and drained through the low-point connections. Stopping the cooling stream first leaves a hot bundle blocked in, where trapped liquid expands against closed valves; stopping both at once puts the whole differential across the tube-to-tubesheet joints in a single step. Neither side is left liquid-full and isolated while the other is still at temperature.

• Standby and isolation

  Isolated, drained or blanketed, and idle — between campaigns, or waiting for the bundle to be withdrawn for cleaning and inspection. Liquid left standing in the shell or in the channel is the condition under which under-deposit and microbially influenced corrosion do most of their damage, so a wet lay-up is a decision taken deliberately, not a default. As the joint cools, bolt load relaxes and the girth and nozzle gaskets can weep on the next pressurisation; the joint is remade with a new gasket and controlled tightening whenever it has been opened, and the exchanger is proved isolated, depressurised and drained before any bolt is slackened.

Failure scenarios and safeguards

• NOFlow

  Loss of flow shows first as the tube-side outlet temperature running away from its normal value — upward where the other stream is hotter, downward where it is colder — and the operator restores flow or takes the other stream off. Where the side has been blocked in, the thermal relief inside the isolation holds the pressure while that is done; and the isolation sequence is what should have prevented a liquid-full side being shut in against heat in the first place.

• LESSHeat transfer

  The duty loss is visible as a drifting outlet temperature together with a rising pressure drop, and the exchanger is taken off line and cleaned at a planned opportunity rather than run to failure. Inspection at that opportunity is what establishes whether the tubes have thinned as well as fouled — the process data cannot show it.

• OTHER THANContainment (stream to stream)

  Pressure falling on one side while it rises on the other is the signature, and it is alarmed: the trip takes out the higher-pressure source, the exchanger is shut in and isolated, and where the unit was designed with margin across the interface, that margin is what the lower-pressure side is holding while it happens. Where there is no margin, the relief device is carrying the rupture flow on its own.

• AS WELL ASComposition

  Composition monitoring or routine sampling of the receiving stream picks the ingress up while it is still small — pressure instrumentation will not, and should not be credited here — and the exchanger is isolated and the leaking tubes located, plugged or replaced. Where the pressure hierarchy was set deliberately, the leak has been running into the stream that can tolerate it rather than out into a utility.

• MORERate of temperature change

  Controlled warm-through and cool-down, at a rate that is monitored rather than assumed, keeps the differential inside what the joints can carry; the outlet temperature alarm is what tells the operator that one stream has gone while the other is still running, which is the cause that arrives without warning. Any cycle known to have run too fast is followed by inspection aimed specifically at cracked tube-to-tubesheet joints and relaxed bolting.

• OTHER THANContainment (to atmosphere)

  The weep is found by detection or on an operator round while it is still a weep; the exchanger is proved isolated, depressurised and drained before a bolt is touched, and the joint is remade with a new gasket and controlled tightening rather than nipped up in service.