Shell & tube heat exchangers have no moving parts, make no noise and generally go about their jobs in an unassuming manner. But there is a lot happening inside them and a failure in one heat exchanger may shut down an entire operation whether it be a large power plant, an industrial complex, a ship or an automatic transmission on a truck.
Besides transferring heat from one fluid to another, shell and tube heat exchangers also act as coarse strainers or filters, and are constantly at risk of fouling from trapped foreign objects.
Periodical internal inspections and a regular cleaning regime should be implemented and, if fouling is found or leaks detected, the unit should be thoroughly cleaned, assessed and repaired or replaced. An investigation should also be carried out to locate the source of the fouling and steps taken to inhibit or remove the cause entirely.
Some fouling is a natural process that occurs during the operation of all shell & tube heat exchangers. Materials deposited by fluids onto the heat transfer surface increase the thermal resistance to heat transfer, decrease its effectiveness and increases the hydraulic resistance to fluid flow. The thermal effectiveness and hydraulic efficiency of the heat exchange process can become compromised eventually rendering it inefficient.
To prevent this from happening and to ensure your heat exchanger performs as it should, look out for the following 3 signs your shell and tube heat exchanger needs attention.
1. Decreasing temperature change on either side of Heat Exchanger
The most common sign of impaired heat exchange is when temperatures have changed from their historical values.
A small decrease in performance can occur naturally over the lifetime of a heat exchanger and as contaminants build up the performance will deteriorate accordingly. But a sudden change in temperatures is often a sign that foreign material has entered the unit in the form of either heavy particulate (Figure 1a), corrosion and/or marine growth (Figure 1b).
Using a hand held heat sensor at the oil inlet and outlet pipes can identify if your heat exchanger is doing its job. If there is no or very little change in the temperature of the fluids at those points, it is likely that the unit is not performing as designed and investigation as to the cause should be undertaken immediately. Keeping records of all temperature readings will assist in assessing the performance of your heat exchanger.
2. Increasing Pressure Drops across Heat Exchanger
Unlike temperature changes, increasing pressure drops across a heat exchanger are difficult to observe and track without instrumentation. The pressure drop across a heat exchanger is affected by a number of factors and passage geometry plays a significant role. Increased pressure drops, whether observed over a short or long period of time, invariably point to partial or complete system blockages. (Figure 2b).
On the other hand, a sudden or gradual decrease in pressure drop across one side of the heat exchanger, either shell side or tube side, usually points to internal leakage caused by either failure of static seals (gaskets or “O” rings), damaged tubesheet/tube joints, or punctured or otherwise damaged tubes – as in Figure 3.
Such failures usually result in contamination of fluids and, owing to the serious implications of fluid contamination (both environmental and commercial), changes in pressure drop readings should always be investigated until the cause is identified and resolved.
3. Contaminated fluids
Contaminated fluids, where one fluid passing through the heat exchanger has escaped into the other, is a sure sign of heat exchanger failure. After all, the essential job of a heat exchanger is to keep two fluid apart while only their thermal properties are exchanged.
Generally the fluids travelling through a heat exchanger are at different pressures and a failure of the barrier between those fluids (the heat exchanger) will generally result in the higher pressure fluid flowing into the lower pressure fluid.
Cross contamination of fluids can be as a result of tube failure (puncture, rupture or corrosion), failure of gaskets or ‘O’ rings or a failure of the connection or bond between the tube end where it joins the tube sheet.
Eventually tubes may become so thin they can no longer contain the fluid pressures and rupture.
The connection between the tube and the tube sheet can be compromised by abrasion and high flow rates and the tube ends can become worn away or crack. Gaskets and ‘O’ rings can also deteriorate or become damaged over time and fail.
Contamination between fluids in a shell & tube heat exchanger is a critical issue and must be avoided and promptly remedied.
Cleaning and Maintenance of Shell and Tube Heat exchangers
Keeping plant equipment clear of debris and deposits is essential to maintaining efficiency and reducing running costs. The external and internal cleaning of heat exchangers is just one aspect of routine maintenance which needs to be performed regularly but which, if done incorrectly, can compound problems with equipment.
Excessive or negligently performed rodding or bulleting of tubes can damage and weaken tube walls and cause damage. Incorrect, worn or deteriorated gasketing and badly installed or pinched ‘O’ Rings are also cause for concern and should be checked.
Trying to clean hidden parts of a heat exchanger, such as the inside of a tube bundle, can be a difficult task and ultrasonic cleaning has proved successful in removing contaminants that are difficult to reach on both the tube and shell sides of tubes stacks.
The challenges of keeping heat exchangers clean depend on the application. Some processes contain hard to remove substances such as oil, carbon and sticky polymers and others contain dirty water carrying debris, abrasive solids, scale or marine and biological growth. Depending on the nature of the fouling, some heat exchangers require specialized cleaning services to remove foreign objects including ultrasonics, bulleting, rodding, high pressure water jet cleaning or a combination of these processes.
For any of these services on shell and tube heat exchangers please contact the friendly staff at Fluid Dynamics.