Heat Exchangers are the equipment we use in chemical industries to exchange the heat between two fluid streams via indirect contact of the fluids. In this both the fluids remain separated by a heat conductive surface. And, heat transfer takes place from hot fluid stream to cold fluid stream. Generally, we use utilities for exchanging the heat inside a heat exchanger. Based on the process requirement, these utilities can be either Hot Utility (i.e., such as steam, hot water, thermic fluid, molten salt, etc.) or Cold Utility (i.e., includes cooling water, chilled water, chilled brine, refrigerants, etc.). During the plant operation we always face fouling problem in heat exchangers, which can be either side of the fluid stream. This fouling depends upon various properties of the fluid such as, viscosity, temperature sensitivity, presence of dissolved & suspended solids, polymerization tendency, etc.
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Impact of Heat Exchanger Fouling
In chemical plants heat exchanger fouling is a big problem and this causes loss of revenue in terms of equipment replacement cost, maintenance & cleaning expanses. Moreover, fouling is responsible for productivity loss also as fouling of heat exchanger force us to continuously keep on reducing the raw material or input feed rates to the plant. In a shell & tube type heat exchanger fouling can be inside and outside surface of the tubes. Due to increase in fouling thickness, heat transfer resistance increases which continuously bring down the operating heat duty of the heat exchanger. And, finally when this operating heat duty is too low to operate the plant above its minimum turndown capacity, we need to shut down the plant to clean the heat exchanger.
Below is the figure showing shell & tube heat exchanger fouling at outside surface of the tubes:
To understand the heat exchanger fouling at inside surface of the tubes please refer to the below figure:
Most of the time we can clean the fouled surface and restart the plant for normal operation. But, is some cases where this fouling is impossible to remove, we need to replace the fouled heat exchanger with new equipment.
In a heat exchanger, deposition of fouling reduces overall heat transfer coefficient. Because of this heat transfer efficiency of the equipment reduces. According to a study, roughly build-up of a 0.6mm thick layer of fouling & scaling in tubes can reduce chiller efficiency by 20% approximately.
Causes of fouling in Heat Exchangers
We can classify various causes of fouling in heat exchangers in below three broad categories: –
Slurry & tarry material in fluid
There are fluids containing slurry or tar which we handle in our plants. This will settle & deposit on tube surface at low fluid velocity. This is generally a soft scaling or fouling and can be cleaned easily using low pressure water jets. The example of this is reaction product liquid containing catalyst fine particles or slurry of a crystallized material.
Dissolved Solid or Salts
In other cases, we can see some fluid containing dissolved solids like sodium chloride & water. When this fluid is heated in a heat exchanger (i.e., vaporizer) because of boiling at tube surface water evaporates but dissolved salt is non-volatile and remains on tube surface. In due course of time this deposition keeps on building and causes hard fouling of salts. To clean this, we need high pressure jets and wire brush. This fouling we can see in distillation column condensers (i.e., primarily operating at high condensing vapor temperatures around > 120 0C), where we use cooling water as cooling utility. At this temperature dissolved solids present in cooling water retain at inside tube surface because localized boiling phenomena. This fouling we can see mostly at the vapour inlet side of the exchanger.
Miscellaneous Reasons
Many fluids have tendency to polymerize or form tar at high temperature. When we heat or boil this type of fluid using reboilers in distillation column bottom, after a certain operating cycle tubes of reboiler chock and cleaning is required.
Overcooling of a fluid below its freezing point over a heat transfer surface causes coating of frozen fluid layer. This we type of fouling we know as a freezing fouling.
Other cause of fouling can be wrong selection of metallurgy, in such case due to corrosion metal surface get corroded and creates fouling on heat transfer surface.
Dealing with the Heat Exchanger Fouling
As we discussed above, fouling is heat exchanger is a serious problem for an efficient plant operation. Hence, as a process engineer, we need design the heat exchanger to deliver efficient working time span. So that, this can tackle fouling problem effectively. Below are some practical solutions, which we can consider:
Considerations during Design
One strategy we can opt, during heat exchanger designing is, keeping more fouling fluid inside the tubes. While, less fouling fluid we should keep in shell side. This type of arrangement facilitates us to easy cleaning of tubes from inside using water jet or wire brush to remove deposited fouling. As it is very difficult to clean the shell side fouling comparatively.
However, shell side cleaning, which is the removal of scaling or fouling from outside surface of the tubes. We can use suitable chemicals circulation methods through shell side, which can dissolve of remove deposited scale on the tubes outer surface. Chemical removal of fouling in heat exchanger we can achieve, in some cases by weak acid, special solvents, and so on.
Where, shell side fouling is too much, there we can select U-Tube bundle and shell type arrangement instead of fixed tube sheet type exchanger. After fouling we can open the heat exchanger and pull out the tube bundle to do the cleaning.
Considerations During Operation
Avoid large temperature gradients for heating with steam and thermic fluid. We should keep temperature gradients for heating around 30-35 0C in case of organics and 35-40 0C for the aqueous solution. This will minimize hard scaling on the process side due to over heating of the chemical on the tube surface. Also, we should use saturated steam into the reboilers.
In condensers and coolers tube side velocity (i.e., cooling water side) should be sufficiently high (around 6 – 8 ft/s minimum) to ensure the self-cleaning of the tube surface.
For viscous, sticky & slurry type materials we can use scrap surface heat exchangers. For example, a super saturated fluid is likely to crystallise on cooling & degree of fouling is very high. We can use scraped-surface heat exchangers in which a rotating element has spring-loaded scraper blades which wipe the surface of the tubes for efficient heat transfer.
Where fouling is the part of the process and unavoidable, we can consider a standby equipment in line. So, when one gets fouled take other in line and clean the fouled heat exchanger for next cycle. This way we can achieve uninterrupted plant operation.
Selection of compatible metallurgy will avoid the corrosion fouling of the heat exchanger tubes. We should provide temperature interlocks and controls to eliminate the possibility of freezing fouling, wherever we are dealing with possible below freezing temperatures.
Cooling Tower Parameters Monitoring
In case of condensers & coolers, where we use cooling water, there we need to monitor and maintain the cooling tower parameters (i.e., TDS & pH) regularly. Cooling tower blowdown, side stream filter and chemical dozing are the measures to control the cooling water quality. These measures help us to control the total dissolved solid, suspended solids, microbiofoulants such as slime and algae and macrobiofoulants such as snails and barnacles in circulation cooling water.
Conclusion
Finally, we understand that fouling in heat exchanger is the inherent part of the chemical process operation in heat transfer surfaces, especially in heat exchangers. Therefore, as a process engineer and plant operator it is very important to understand the causes and nature of possible fouling. This understanding will enable us to handle the fouling deposition effectively and we can keep our plant running for a longer span of time. In many cases we can avoid the damage of costly equipment altogether.
I assume this article will help you to understand & effective dealing with fouling of heat transfer surfaces.
Thanks for reading..