Shell & Tube Heat Exchangers usually provide long service life with little or no maintenance because they have no moving parts. However, there are several types of mechanical failures that can occur, but with a little maintenance and understanding, they can be prevented.

Metal Erosion

Excessive fluid velocity on either the shell or tube side of the heat exchanger can cause damaging erosion as metal wears from the tubing. Any corrosion already present is accelerated as erosion removes the tube’s protective films, exposing fresh metal to further attack. Most metal erosion problems occur inside the tubes. The U bend of U-type heat exchangers and the tube entrances are the areas most prone to erosion. 

Steam or Water Hammer

Pressure surges or shock waves caused by the sudden and rapid acceleration or deceleration of a liquid can cause steam or water hammer. The resulting pressure surges have been measured at levels up to 20,000 psi, which is high enough to rupture or collapse the tubing in a heat exchanger. For example, 3/4 in. x 20 BWG light drawn copper tubing has a burst pressure of 2100 psi and a collapse pressure of 600 psi.

Damaging pressure surges can result from a cooling water flow interruption. The stagnant cooling water is heated enough to generate steam, and the resumption of the flow causes a sudden condensing of the steam and produces a damaging pressure surge, or water hammer. Cooling water flow should always be started before heat is applied to the exchanger.Fluid flow control valves that open or close suddenly also produce water hammer. 

Vibration

Excessive vibration from equipment such as air compressors or refrigeration machines can cause tube failures in the form of a fatigue stress crack or erosion of tubing at the point of contact with baffles. Heat exchangers should be isolated from this type of vibration.

Shell-side fluid velocities in excess of 4 fps can induce damaging vibrations in the tubes, causing a cutting action at support points with baffles, Fig. 3. Velocity-induced vibrations can also cause fatigue failures by work hardening the tubing at baffle contact points or in U-bend areas until a fatigue crack appears.

Thermal Fatigue

Tubing, particularly in the U-bend area, can fail because of fatigue resulting from accumulated stresses associated with repeated thermal cycling. This problem is greatly aggravated as the temperature difference across the length of the U-bend tube increases.

Thermal Expansion

These failures are most common in steam heated exchangers; however, they can occur in any type in which fluid being heated is valved off without provisions to absorb thermal expansion.

Freeze Up

These failures are most common in evaporators or condensers; however, they can occur in any heat exchanger in which temperatures drop below the freezing point of either fluid in the unit. Freeze-up results from failure to provide thermal protection, a malfunction of the thermal protection control system or protective heater device, improper drainage of the unit for winter shutdown, or inadequate concentration of antifreeze solutions.

ALTERNATIVE DESIGNS

Spiral Heat Exchanger

A Spiral Heat Exchanger consists of two sheets of metal rolled into a spiral, with space between them separating fluids by a single surface with excellent heat transfer properties. Spirals are true counter flow devices that combine many of the benefits of both Shell & Tube and Plate Heat Exchangers. They excel when a close approach or temperature cross is needed, are cleanable, reduce pressure drop and allow for thermal expansion.

Floating Tube Heat Exchanger

Several Floating Tube sheet designs are available. The most popular has TEMA designation ‘AEW’ (ITT Standard ‘CPK’ & ‘FLP’). This Straight-Tube, Removable-Bundle design with Externally-Packed Floating Head construction, features a fully cleanable design with high thermal efficiency. This design allows for differential thermal expansion between the shell and the tubes at an economical price.