Heat exchangers are among the most common and widely used equipment within industrial plants, and shell-and-tube type constitutes the majority of the exchangers. However, use of plate-and-frame exchangers in certain applications is growing rapidly, and this type has gained acceptance by many customers. Both types are used for cooling and heating and are considered important and critical equipment. They are an integral part of the overall process and equipment design.
The exchangers are used as condensers in power plants when steam discharged from the turbine is condensed within the shell side of the heat exchanger. In oil-and-gas applications, exchangers are used as overhead condensers to generate vacuum or remove noncondensables. In the chemical process industry, they are used as part of process chillers, general cooling and heating processes. No matter what industrial plant you walk into, heat exchangers are used as critical equipment within many processes.
A number of factors are considered when designing heat exchangers such as flow rates, property of fluid or gas, temperatures, the heating or cooling duty, surface area, tube or plate material, and heat transfer coefficient. Using today’s computer technology, the design is optimized to result in the smallest possible exchanger to reduce footprint and cost while delivering the highest performance.
One factor that can affect the performance of even the most well-designed heat exchanger is the value used for the heat transfer coefficient. This value is affected by a number of factors, including tube or plate material, thickness, fluid or gas properties, velocity and the tube or plate cleanliness. The cleanliness factor, which is assumed during the design, has a direct impact on the exchanger size and the performance it can deliver. Once the exchanger is in operation, the cleanliness can decrease due to many substances that can adhere to the tube or plate surface. This could lower the overall heat transfer rate, thereby reducing the exchanger’s duty below its design level.
Fouling and scaling of the tubes or plates also can reduce the exchanger’s life by causing corrosion and pitting, resulting in leaks. Substances that adhere to the plate or inside the tube include microbiological fouling (algae and other aquatic life forms); scaling (precipitated organic and nonorganic chemical compounds or corrosion products); and particulate fouling from suspended solids.
Online automatic tube cleaning is an effective way to mitigate or reduce the negative impact and loss of production due to fouling and scaling.
Overview of Online Cleaning Methods for Industrial Heat Exchangers and Condensers
To keep heat exchangers clean and to maintain the heat transfer at the design level, a number of online automatic tube-cleaning systems have been used. The most common are the recirculating rubber ball, brush and basket, automatic backwash system and self-flushing debris filter. Recirculating-rubber-ball and brush-and-basket systems are used on shell-and-tube heat exchangers and condensers with as few as 100 tubes or as large as 15,000 tubes. Tube sizes can vary from 0.5 to 1.5” with smooth or enhanced tube surface. The automatic backwash system can be used on all types and size plate-and-frame exchangers. A number of plants also use an automatic backwash system on shell-and-tube exchangers. Self-flushing debris filters are used for shell-and-tube and plate-and-frame exchangers on cooling service water.
The application range for automatic tube cleaning systems and backwash systems includes raw water, wastewater and process cooling or heating within refineries, petrochemical facilities, power plants, food-and-beverage, pulp-and-paper and any other plants where heat exchangers or condensers are used.
The concept of the brush-and-basket system is a simple one. The system consists of a set of polypropylene baskets, or cages, that are either epoxy- or press-fitted at the end of each tube. For process applications, the basket material selected is typically carbon steel, stainless steel or another material compatible with the process fluids. A single brush with steel (for process applications) or nylon bristles is inserted inside each basket.
During the normal flow, the brush remains inside the basket downstream of the flow. It does not restrict the flow. A flow-reversal system is incorporated as part of the cooling water supply and return lines, utilizing four-way flow-diverter valves.
Periodically, the flow-reversal system will shuttle the brushes down the full length of the tubes and brush them free of any micro fouling scale buildup. The exchanger will operate in reverse flow for only a few minutes before returning to normal operation. The tubes are cleaned twice during each cleaning cycle. The flow reversal usually occurs a few times a day. The process does not interrupt plant operation and the exchanger is not taken offline.
The major components of the system are the ball strainer, which is installed within the discharge water return line, and the ball-recirculation system, which is installed external to the heat exchanger.
For this method, slightly oversized elastomer balls are periodically or continuously injected into the exchanger/condenser cooling water inlet. The balls are passed through the condenser tubes by the cooling water flow.
The balls are designed and injected in such a way as to provide a uniform distribution inside the water box. Because the diameters of the sponge balls are larger than the inside diameter of the tubes, the condenser tubes are automatically kept clean by the scrubbing and wiping action of the balls. This helps prevent the deposit of micro-foulants and scale on the inside tube surfaces. The balls are collected by a strainer that is installed in the outlet condenser pipe. From there, they are redirected and reinjected into the exchanger inlet line.
After leaving the strainer sections, the balls pass through a special recirculation pump, which adds sufficient pressure to overcome the heat exchanger and recirculation-piping pressure drop. The balls are pumped into the ball collector and are injected into the supply water inlet. The ball collector is used to automatically put the balls into or out of circulation. The balls also are kept in this chamber during the strainer backwashing, and the collector serves as a means to replace worn-out balls when needed. As an alternative, it is possible to use a single strainer at the main water return line to collect the balls and circulate the balls through several heat exchangers periodically.
The screen-backwashing sequence automatically starts when the differential pressure across the screen reaches a preset level. This increase in the differential pressure is caused by the debris and particles present in the circulating cooling water. The differential pressure across the screens is continually monitored and displayed. Balls are automatically collected inside the collector prior to any backwashing.
All functions and operations of the ball system are controlled and displayed by means of a single central control panel using a series of PLCs. It controls the operation of each screen, the pump and the collector.
Automatic Backwashing System
Automatic backwashing systems use a flow diverter like in the brush-and-basket systems to reverse the flow and backwash the shell-and-tube or plate-and-frame exchangers. Traditional prefiltration can only remove large particles. Fine particles such as silt, sand, mud and other suspended solids enter the plate-and-frame exchanger and accumulate inside the plate, which causes plugging and promotes scaling and pitting.
A single four-way valve can be installed either within the supply and return, or directly on the exchanger, which periodically reverses the flow through the exchanger for a short time without interrupting its operation. The flow returns to normal direction after only a few minutes. Periodic backwashing by means of an automatic backwashing system will dislodge all accumulated suspended solids and ensures the exchanger remains in clean condition at all times.
Online Debris Filter
Coarse debris such as zebra mussel, algae or aquatic life present within the cooling-water intake when the water source comes from a sea, river or lake can plug an exchanger. Likewise, cooling tower fill buildup can be carried within the heat exchangers and plug the water flow through the exchanger tubes or plates. This reduces the exchanger performance and increasing the need for cleaning.
Online self-flushing debris filters are used to prevent the debris from entering the exchangers without reducing the flow through the exchanger or increasing the pressure drop. The debris filter collects the incoming debris within a series of convoluted screens. Debris is removed from the screen by means of a rotor with several suction ports.
When an accumulation of debris on the filter screen creates a differential pressure across the filter screen higher than the “dirty screen” setpoint, the cleaning cycle sequence will be initiated. The actuated discharge valve will open and the discharge rotor will begin to turn. The differential pressure at the portals creates a vacuum that will remove the debris from the screen. The discharge rotor continues to clean the screen until the monitoring system indicates a clean screen condition. Debris removal does not require any pump or other external equipment. A single self-flushing debris filter can be installed on the main header of the inlet cooling water to service several heat exchangers.
Macro- and micro-fouling as well as scaling inside the heat exchangers are nothing new. The accumulation reduces plant efficiency and throughput while increasing plant maintenance and operating costs. It also reduces the exchanger life and increases leaks due to under-deposit corrosion and pitting.
The use of an online automatic tube cleaning system as well as an automatic backwash system and online self-flushing debris filter can help industrial plants keep exchangers and condensers clean and maintain designed heat transfer rates.
This article originally was published with the title "Boosting Heat Exchanger Performance" in the July/August 2015 issue of Process Cooling.