In the chemical processing industry, some of the major problems faced by heat exchangers are fouling, vibration, leakage, and increased energy consumption. All of these issues have a direct impact on the performance of the heat exchanger and affect the performance of downstream equipment.
By Mrinal Das – Senior Vice President, Project & Engineering Procurement, Dorf Ketal Chemical
Many of these issues are interrelated. For example, many heat exchanger leaks are caused by flow distribution issues. If the flow through the heat exchanger is not uniform, then the high flow rate can cause an additional problem, i.e., vibration. This vibration can increase the erosion effect of the heat exchanger, which can lead to frequent leaks, which can cause problems with maintenance and associated costs. Therefore, it is critical to use analytical tools to solve the problem of flow distribution inside the heat exchanger. Out of all the above problems, the most troublesome and common problem is scaling.
Fouling in the heat exchanger
Fouling is a gradual process that can be defined as the deposition of unwanted material on the surface of a heat exchanger. Unwanted materials may be inorganic salts, polymers, sludge, sediments, crystals, coking products, biological deposits, etc. Fouling in a heat exchanger has a direct impact on heat transfer and flow. Fouling is caused by the synergistic result of transient mass momentum and heat transfer phenomena between the fluid and the surface. The fouling process is highly dependent on operating conditions such as temperature, pressure, viscosity, density, thermal conductivity, etc. Fouling in a processing plant can lead to a significant decrease in thermal performance, an increase in pressure drop, the promotion of corrosion, and ultimately heat exchanger failure.
»Fouling is a gradual process that can be defined as the deposition of unwanted material on the surface of a heat exchanger.
Common types of dirt
In general, there are about six types of liquid-side fouling mechanisms observed in processing plants, namely corrosion fouling, biofouling, particulate fouling, chemical reaction fouling, precipitation or crystallization fouling, freezing/solidifying scaling. Common types of fouling include chemical, biological, sedimentary, and corrosive. Chemical fouling is caused by a chemical reaction in a fluid stream that causes material to be deposited on the surface of the heat exchanger. This type of fouling is common for chemically sensitive materials when the fluid is heated to a temperature close to its decomposition (degradation) temperature. Coking of hydrocarbon materials on heat transfer surfaces is also a common chemical fouling problem. Biofouling occurs when a biological organism grows on a heat transfer surface. This is a common scaling mechanism in which untreated or improperly treated water is used as a coolant.
Corrosion fouling is caused by a chemical reaction involving the surface material of the heat exchanger. Many metals, such as copper and aluminum, form an adherent oxide coating that is used to passivate the surface and prevent further corrosion. Many metal oxides are corrosion products with a fairly low thermal conductivity, and even a relatively thin oxide coating can significantly affect the performance of a heat exchanger.
Deposition fouling occurs when fluid particles settle into the inorganic surface area. This is usually the case when the fluid velocity is below a critical level. This type of fouling can be helpful with proper management of the combined particles/fluids. In fact, there is more than one fouling mechanism in many processes, and their combined effects are staggering and can be much more severe than expected.
In the diagram below, the various deposition and removal processes in the scaling process can be seen at a glance.
While it is not possible to eliminate fouling in a heat exchanger, it is absolutely possible to control and reduce fouling.
How to monitor your heat exchanger
Therefore, it becomes important to have data points and information to accurately monitor the heat exchanger. Parameters that can be measured for monitoring include inlet and outlet temperatures of cold fluids, inlet and outlet temperatures of outlet fluids, mass flow of cold and hot fluids, and hot and cold fluid pressure changes throughout the heat exchanger.
In order to keep the heat exchange system running efficiently, it is advisable to consider the following: 1) be aware of all available data related to pressure, flow, and temperature indications; 2) identify and evaluate the flow, pressure and temperature recorded by the operator in his daily work; 3) Continuous Pressure Check – Differential Pressure Tracking
This can be very valuable information on the shell side or tube side of a heat exchanger – if the pressure differential builds up over a period of time, it means that something is restricting the flow; 4) To record temperature data information, it is best to use a handheld temperature gun, the infrared temperature gun can help you effectively monitor the temperature of the system throughout the operation.
By utilizing today's manual tracking system technology, heat exchangers can be tracked in any process industry. Acid-base chemical cleaning methods, combined with flushing, can be used to remove dirt from heat exchangers. Chemical cleaning can effectively clean dirt without disassembling the heat exchanger. This process enables the heat exchanger to operate in less time without having to take it offline.
Anti-fouling design
During the design phase, fouling factors are taken into account. Manufacturers employ different approaches to heat exchanger construction to help increase the heat transfer area that may be affected by fouling. During the manufacturing process, a number of important factors are considered to minimize the occurrence and build-up of fouling: a) dispensing more fouling fluid to the tube side; b) Design the heat exchanger so that the fouling fluid velocity is 3 ft/s on the shell side and 5 ft/s on the tube side; c) allows easier cleaning of the heat exchanger; d) When servicing, make sure that the pipe wall temperature is not kept too hot, as it may create unnecessary salt deposits or invalidate any chemicals.
The effects of fouling can lead to deposits on the channel walls on the shell side of the pipe or/and heat exchanger. This reduced channel area results in increased pressure drop and reduced heat transfer. Heat transfer and pressure drop are inversely proportional to the channel diameter. The pressure drop will increase to the cube root of the diameter. The pressure drop in the contaminated heat exchanger channel is very high compared to the clean channel of the heat exchanger.
Fouling can cause significant production losses and lead to reduced efficiency. This loss of production can be related to unplanned and planned operational downtime due to fouling,
Various deposition and removal processes during scaling.
The maintenance costs for the removal of heavy dirt deposits with chemicals/antifouling devices, the maintenance costs associated with replacing clogged or corroded equipment, the industrial cleaning costs for each heat exchanger range from €46 to €55,000. Fouling can be determined by the nature of the scaling process fluid, the heat transfer process, or a combination of both. The heat transfer mechanism is the main cause of most fouling. High temperatures increase the rate of fouling. By lowering the temperature, the fouling rate can be reduced, which in turn reduces the cleaning cycle. Therefore, it is important to reduce fouling by drastically increasing the heat transfer efficiency and changing the flow conditions within the heat exchanger tube.
conclusion
We can conclude that the fouling effect in a heat exchanger can be greatly reduced with proper design, control of the materials that cause fouling, good selection of building materials, proper protective coating of the heat exchanger surface, and regular cleaning. A detailed and effective analysis of the operating parameters of the scale heat exchanger is essential as it is essential to calculate the correct fouling coefficient for the product and service fluids. The process and mechanical design of the heat exchanger should ensure the correct fluid velocity, temperature, and other operating parameters.
Preventative maintenance should be carried out, particularly through system inspections and cleaning, to prevent fouling and keep the heat exchanger operating efficiently. While fouling in a heat exchanger cannot be eliminated, it is absolutely possible to control and reduce fouling to maintain the performance of the heat exchanger without adversely affecting process performance. It is advisable to read and refer to case study articles from publications such as Heat Exchanger World to gain more knowledge and awareness.