The purpose of a plate heat exchanger is to transfer heat from one medium to another. Heat passes easily through the thin wall separating the two media from each other. But this high heat flow can be seriously reduced by the formation of deposits on the wall surfaces.

Heat exchanger fouling is unwanted and can, under certain circumstances, be harmful to the heat exchanger. Regular maintenance can help reduce fouling.

In plate heat exchangers, heat is transferred from one medium to another through thin metal plates that have been pressed into a special pattern. When a package of channel plates is pressed together, the holes at the corners form continuous tunnels, or manifolds. These manifolds direct the media, which perform the heat transfer process, from the inlets into the plate pack, where they are distributed into the narrow passages between the plates. Because of the arrangements of the plates, the two liquids enter alternate passages. For example, the warm liquid passes between the odd-number passages while the cold liquid flows in the even-number passages. The media are separated by a thin metal wall. In most cases, the liquids flow in opposite directions.

The purpose of the equipment is to transfer heat from one medium to another, and heat passes easily through the thin wall separating the two media. The pattern into which the plate material has been formed gives strength and rigidity. Also, the pattern helps increase the rate of heat transfer from the warmer medium to the metal wall, and from the metal wall to the other medium. However, this high heat flow through the walls can be seriously reduced by the formation of deposits on the wall surfaces. The deposits may increase the total wall thickness substantially, and they often consist of materials that have a much lower thermal conductivity than the metal plate. Consequently, a layer of deposits can severely reduce the overall heat transfer rate and increase fluid pressure drop.

Pressure Drop. Pressure drops are wasted energy. All pipe systems, and the equipment included in them, offer resistance to the media flowing through them. Some pressure drop is unavoidable, but for given equipment, it should be kept as close as possible to the designed value.

The formation of deposits on the heat transfer surfaces instantly leads to a reduction of the free space between the plates. This means that more energy is required to get the desired flow through the exchanger. It is clear that the fouling of the surfaces is undesirable. Larger particles and fibers also may be drawn into the heat exchanger and clog it if strainers or other means of protection have not been used. A reduced ability by the heat exchanger to hold the desired temperatures -- in combination with an increased pressure drop on any of the media -- indicates that fouling or clogging is taking place. Proper maintenance and cleaning are the corrective steps you want to take to return your exchanger to design conditions.

In a plate heat exchanger, the heat transfer media are distributed into the narrow passages between the plates.

Maintenance And Cleaning

Cleaning the unit may be accomplished by either backflushing, circulating cleaning agents or by disassembly. If the water contains suspended foreign material, deposits can accumulate in the flow passages. Under these circumstances, backflushing may be adequate to clean the unit. If this method is ineffective for the removal of hard scale, you may want to circulate buffered cleaning agents in the unit to remove scale, or disassemble the unit for manual cleaning.

Plates can be cleaned while hanging on the frame or removed, depending on how much work is required.

Cleaning in Place. In applications where extremely corrosive fluids are being used, a clean-in-place system is recommended to lengthen the life of the thermal plates. To clean in place, follow these steps:

  • Close main isolating valves and completely drain the unit through the clean-in-place system.
  • Rinse the unit with warm water (100 to 110oF {38 to 43oC]) until water becomes clear and all traces of the process fluid are gone.
  • Completely drain the rinse water from the system.
  • Refill the system with water and
    then add the appropriate cleaning
    solution.
  • Circulate the cleaning solution at approximately 180oF (82oC) for 3 to 6 hr.
  • Drain cleaning solution from the unit and rinse with water as described previously.
  • Close the clean-in-place system and open the main isolating valves.

Backflushing. Many applications utilize fluids that may contain solids and sediments that can be trapped and plug up a plate heat exchanger. If this problem is anticipated, it is beneficial to make provisions in your piping for backflushing the unit. Backflushing should be done with water at 1.5 times the process flow rate.

It is recommended that fluids containing particulate enter at the bottom of the unit and exit at the top. This will cause some solids to collect in the bottom of the inlet nozzle instead of going into the plate pack and clogging. As an alternative to backflushing, consider arranging piping so that both fluids can be periodically reversed.

With proper maintenance, a plate heat exchanger provides effective heat transfer in a relatively compact footprint.

For more information, contact ITT Standard. Call (800) 447-7700 or visit www.ittstandard.com.

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