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Fouling Focus a fabrication limit on baffle spacing with the minimum standard being 20 % of the shell diameter . Additionally , very little field data has been analyzed for the shell side and there is no reported pilot plant data to assign shear stress recommendations . Therefore , the general recommendation is to have shell side crossflow velocities at 0.75 m / s or higher to minimize fouling . Another factor which affects shell side deposition is the presence of low velocity circulation zones ( aka “ dead zones ”), where particles can get trapped and cannot be swept away by the force of the fluid . These zones are the physical effect of the change of flow direction around baffle edges , see Fig , 2 . Deposition in these corners can be minimized by making the crossflow and window velocities approximately equal , which minimizes the size of the dead zone . Also , a large spacing in the outlet and inlet areas leads to low flow near the tubesheets and promotes deposition .
Reboilers As explained in Part 1 , a wet-dry wall and the phenomenon of film boiling lead to deposition of insoluble material such as polymers or salts , which stay on the surface and may undergo thermal conversion to coke-like material over time . One or more of the following three conditions contribute to this mechanism - the presence of insoluble material , a surface temperature high enough so that a liquid film cannot be sustained on the tube wall , and excessive vaporization or vapor flow ( especially locally ) such that there isn ’ t sufficient liquid to keep the surface wetted ( vapor blanketed surfaces ). It may not be possible to avoid the presence of insoluble precursors as they are part of the process . However , some mitigation can be achieved by minimizing their formation or their quantity . An example is that of dienes which cause fouling in oil-refining reboilers . The composition of the feed to the process unit can be changed to minimize the component carrying the dienes , or the conversion of dienes to insoluble polymers can be limited by controlling reboiler temperatures . The phenomenon of wet-dry surfaces can be controlled by limiting the heating medium temperature and by ensuring as much as possible that liquid can reach all of the boiling surface . A liquid covered surface is less likely to allow deposition to occur and therefore keep fouling from occurring . Lowering the heating medium temperature reduces the temperature driving force for heat transfer and may cause a loss in heat duty , but it may be a smaller loss than created by fouling , or it may allow a longer run length before cleaning is required .
Fig . 3 Shell side area with minimal to no flow .
�Dead˝ Zones Fig . 2 Shell side low velocity areas at baffle corners .
Window Flow
Cross Flow
The phenomenon of excessive vapor flow can be due to two reasons . First , the operation may be such that sufficient liquid is not fed to the reboiler , and if the heat duty is fixed it necessarily means more vapor is generated . Second , the flow patterns in the heat exchanger might be such that parts of the heat transfer surface are starved of liquid and therefore are vapor blanketed . The latter also occurs in steam generators if proper liquid circulation is not maintained .
Condensers Overhead condensers typically foul due to precipitation of salts as the condensing fluid cools . Two design strategies can be effective against this mechanism . First , the design should allow for uniform condensation in the heat exchanger by ensuring that vapor reaches all the surface and condensed liquid can be available to sweep away some of the precipitated salts . Second , a wash stream can be injected in the incoming vapor to dissolve precipitated salts - for example , water for water soluble salts , assuming it is acceptable for the process . In the case of wash streams , it is necessary to ensure that the wash stream can reach all the fouling locations on the shell side , or uniformly to all tubes for tubeside condensation . Fig . 3 shows the shell side exit where the location of the nozzle is such that only very little of the vapor , condensed liquid , or the wash stream can get to the shaded area and rapid fouling occurs there . One possible solution is to place the nozzle as far to the right as possible , or to truncate the length of the tubes so that all the surface is located to the left of the nozzle .
Mitigation Economics All techniques described above and those in Part 4 , require expense in terms of capital , engineering , replacement of tube bundles , increased pressure drop , and most importantly if process changes are considered . Although a mitigation action may look expensive when only its cost is considered , it often pays off to look at the savings provided by the mitigation and evaluating a longer-term return on the expense . In a future article we will take a detailed look at the economics of fouling and fooling mitigation .
Flow
Flow Deficit
Catch up on Fouling Focus !
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Upcoming in Part 6
In the next article we will look at cleaning of fouled heat exchangers . Although it is not a direct mitigation action , it is an important aspect of minimizing the cost of fouling . The article will cover different cleaning methods , their relative costs and effectiveness , and the cost of not cleaning a heat exchanger to a fully clean condition .
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