capacity for the overall plant. Improvements can result from heat exchanger services and upgrade heat exchanger engineering, which involve identifying the optimal process conditions for new equipment design, and, the assessment of the resulting benefits for achieving those conditions, and finally optimally designing upgrade heat exchanger equipment to accomplish the plant desired goals for improvement.Gradual Fouling Buildup, Harming Heat TransferDepending on the service, certain heat exchanger equipment is more likely to develop fouling substances buildup, causing decreasing heat transfer, which may also harm plant or equipment efficiency, capacity, or both, and may also cause continuous higher plant energy use. Heat exchanger fouling exists for many heat exchanger services, but it is particularly troublesome in process cooling, where recycled cooling water from a
cooling tower is used for cooling. In such equipment, fouling can result from large temperature differences, accumulation of or precipitation of solids on heat transfer surface, as well as other thermal-mechanical design considerations of the heat exchanger equipment. Upgrading of process coolers through optimized heat exchanger engineering can achieve improved resistance to fouling, through careful control in design of fluid
velocities and temperatures, as well as from proper selection of materials of construction and use of the best heat exchanger type. Decreasing heat transfer resulting from equipment fouling usually lowers both plant capacity and efficiency over time, and damaged performance is usually only improved by plant shut-down for
the heat exchanger to be cleaned. All plants want to minimize this down time, so optimum exchanger designs in heat exchanger engineering should minimize the effects of fouling, to fully maximize production and run time. When cleaning becomes necessary, usually hydro-blasting or chemical cleaning procedures can be successful in restoring the best performance for the heat exchanger. In rare cases, specialized cleaning techniques may
be necessary for hard to clean applications.Excessive Stream Pressure LossWhen heat exchanger equipment is originally supplied in a new plant, optimal size and selection in the heat exchanger engineering should have been performed to support efficient plant operation. That is, a range of heat exchanger equipment sizes (designs) should have been studied to specify the most favorable economic design with attractive economic performance for the equipment, considering energy costs during that time.
Sometimes this is not the case. When large plant contractors develop "standard plant designs" to offer clients, they reuse major sections of previous plant designs which will achieve the venture client's requirements, usually for higher capacity.
Also, over time, plants may be gradually upgraded with improvements in subsections of the plant site, resulting in higher overall plant production and greater stream pressure loss for heat exchangers that are still original equipment, that have not been upgraded. In these various cases, heat exchanger equipment may have high pressure loss for a stream because of the original design supplied, or from subsequent plant expansion and
improvements over time. Usually the excessive stream pressure loss prevents further plant expansion at some higher than original production capacity. To achieve further plant capacity increases, such fluid flow limiting, or heat transfer limiting heat exchanger equipment may need to be replaced, using larger or different designs.Fluid pressure drop is controlled by a wide variety of design variables during heat exchanger engineering,
when the process fluid flow and conditions are decided. For the tube side stream, the controlling variables are shell diameter, tube length, tube geometry (straight or U-tubes), number of tube passes and number of shells in series or parallel. Channel nozzle sizes have a significant influence in the design pressure drop. Shell side fluid pressure drop is much more complicated and has many more variables of influence. The equipment
design variables which control shell side fluid pressure drop include tube diameter, tube pitch, tube layout, shell diameter, baffle type, baffle spacing, number and size of shell side nozzles and the exchanger shell type.Excessive pressure loss also can result in significant to large energy use, compared with original design or
operation at lower plant capacity, depending on the purpose and operating conditions of the heat exchanger service. In many instances, economic justification exists for replacement of a perfectly normal performing (non-fouling, non-failing, reliable) heat exchanger, based upon energy savings or plant capacity improvement,
or both. This is usually true for older equipment which is no longer economically optimally sized for required production or for minimizing energy use. Capacity and efficiency optimization studies of sections of plants, or complete plants in replacement heat exchanger engineering can identify the heat exchangers that are the most
suitable for economic replacement, providing the highest economic savings.Failing Heat Exchanger EquipmentHeat exchangers can fail for a wide variety of reasons, and each case must be analyzed carefully to fully understand the cause and correct it. The most common heat exchanger failure is a tube leak, where the higher pressure stream passes through the leaking tube to the other stream, causing deteriorated thermal
performance, as well as contamination, and sometimes serious plant capacity loss and increased energy use. The magnitude of the economic harm to the plant from a heat exchanger failure depends on a great many factors, including the type of plant, the cost of energy, and the purpose of the heat exchanger. But, a mechanical failure resulting in a heat exchanger tube leak can in some cases result in a plant almost immediately being shut down. In other cases, the plant can continue to operate until the leak reaches a certain
unacceptably high magnitude. The loss of plant production capacity, efficiency, and lost profits on potential higher production are the usual drivers to make the decision to shut down and make a repair, or swap a heat exchanger.Tube vibration is a leading cause of tube leaks, where tubing comes into contact with adjacent tubes, or wears
at baffles or other supports until leakage begins. Heat exchanger engineering software can design new heat exchangers to avoid tube vibration, through careful design of tube thicknesses, arrangements and the spacing of tube supports. The best software can also be used to identify critical design flaws in the mechanical design of existing heat exchangers that are known to have had tube failures resulting in leakage of one stream into the other, eliminating further replacement heat exchanger failures.
Performing vibration analysis for existing heat exchangers having tube leaks using state-of-the-art heat exchanger engineering software, can prove that tubes will vibrate and wear or destruct, resulting in the noted failures due to excessive unsupported tube spans, fluid elastic whirling, fluid velocity, cross flow inertia, vortex shedding, and turbulent buffeting, resulting as acoustic induced tube vibration. Such heat exchanger
engineering failure analysis can permanently overcome these problems, getting to the cause of the failure and providing efficient, reliable upgrade equipment.
The occurrence or prediction of tube vibration damage is the result of insufficient stiffness for regions of the
tube bundle under the influence of shell side fluid flow. Increasing this stiffness to free designs from vibration
can be achieved by: increasing tube diameter or thickness, changing tube pitch or tube layout, reducing baffle spacing, reduced tube-baffle clearance, changing the shell diameter, changing the tube length, enlarging shell side nozzles, increasing shell nozzle drop spacing to nearest rows of tubes, use of impingement plates, moving from single to double or triple segmental baffle types, changing the number of shells in series or
parallel and the use of NTIW (No Tubes in Window) baffle arrangements with additional intermediate tube supports.
One of the most common situations is that the original design of the heat exchanger, as supplied, was marginal, and then plant expansion projects were implemented, raising plant capacity and the equipment loads. The marginally designed heat exchangers become overloaded, and mechanical failures gradually occur.Individual tubes that have been determined to be leaking by testing when the equipment has been opened for
inspection and repair can be plugged. Sometimes gradual patterns of tube leaks develop, which can suggest plugging the tubes for entire rows or regions within the heat exchanger. Sometimes rows of tubes that have leaked in the past are replaced with solid rods in place of tubes. A frequent question that comes up is "How many tubes in a heat exchanger can be plugged without seriously harming the exchanger performance?" It is
difficult to generalize for all types of equipment, but a general rule that has been reasonably safely used is that about 10 percent of the tubes can be plugged, without seriously harming the heat exchanger performance. As
a practical matter, when plugged tube count reaches about 10 percent, tubes are generally failing at increasing frequency, causing increased downtime and lost production. These losses can be factored into the decision process and justification to overhaul or replace the heat exchanger. A complete re-rating of the equipment should be performed through heat exchanger engineering services to determine the cause of the mechanical
failure, so that replacement equipment will not suffer with the same failure problem. In some instances, it will be best to design and procure a new heat exchanger for the current and projected plant requirement. Consulting within the industry in similar designed plants can sometimes provide guiding solutions to fix such
heat exchanger failure problems. Another option which has been done worldwide is field replacement of part
of or all of an existing heat exchanger's tubes, to buy time to determine the best permanent fix.Other types of failures also occur, resulting in leaks, including tube seal weld cracks, and tubesheet cracks, and the very serious shell and channel cracks that may result in large leaks, and sometimes fires, or exposure of plant personnel to chemical hazzards. These issues are best dealt with through careful selection of the
materials of construction that are proper for the particular application, and appropriate construction of the equipment, including careful inspection during fabrication and all of the necessary stress relieving of components at appropriate steps of construction and assembly of the equipment. These issues should have been well documented by the fabricator and delivered as part of the equipment documents to the end user.Inadequate Heat TransferEvery process application for heat transfer is unique. So this is a difficult topic to generalize. The intent here
is to describe the given process where an optimal process temperature is known that would result in highly
stable process operation, or minimized energy use for the process, or a specific stream temperature deemed
best compatible and appropriate with interrelated plant equipment.
Frequently it is advised that the plant conduct an optimization study during heat exchanger engineering to
determine the best performance conditions for replacement heat exchange equipment that will achieve highest
economic advantage for the particular process application.
A common reason for inadequate heat transfer, related to preferred process operating temperature is that the
plant site is operating at greatly expanded production with heat exchanger equipment that is significantly or
substantially now under sized. As a result, the existing heat exchanger equipment cannot achieve the optimal
desired process temperature. A fully optimized replacement heat exchanger design can be developed which
can provide an attractive plant upgrade path with the highest economic recovery. Frequently, the economic
justification for heat exchanger replacement due to inadequate heat transfer is increased production capacity,
energy savings, or both. Plant optimization studies during heat exchanger engineering can assess the
economic recovery, and provide optimal exchanger process design operating conditions, and reliable
equipment.Inadequate Pressure or Temperature RatingIn some circumstances, existing heat exchanger equipment may not have an adequate pressure or
temperature rating to match up with the optimized process requirement. The heat exchanger equipment must
be operated at less than optimal process conditions within its mechanical rating to prevent reliability problems
and to assure plant safety. Similar to the situation of inadequate heat transfer, a plant optimization study may
indicate the opportunity to replace the marginal heat exchanger equipment with a new design developed with
higher pressure or temperature rating, promoted with a favorable economic return, with higher plant capacity or
efficiency.
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