Sometimes known as the Formula One race cars of the heat exchanger world, gasketed plate heat exchangers can help you cross the “finish line” of your process.



Some liken gasketed plate heat exchangers to Formula One race cars. Able to provide approach temperatures close to 1°F (~0.5°C) and capable of handling a range of flow rates from 5 to 20,000 gal/min, gasketed plate heat exchangers are high performance units. The range of possible plate materials allows them to be used to handle such disparate media as sulfuric acid and yogurt. Their use is so widespread in so many industries that they often are more commonly known by their function than by the generic name “gasketed plate heat exchanger.” For instance, the chemical processing industry calls them process coolers; the dairy industry calls them pasteurizers; and the power generation industry knows them as lube oil coolers or closed loop coolers.

Unfortunately, many who think they know about heat exchanger selection and maintenance apply information based on older and substantially different shell-and-tube heat exchanger technology to gasketed plate heat exchangers. This can lead many design and plant maintenance engineers to make decisions that are counter-productive and even raise operation and maintenance costs. It has been said that a Formula One race car has as much in common with a jet fighter as it does with an ordinary race car.1 Likewise, a gasketed plate heat exchanger is very different technology than its shell-and-tube counterpart. To prevent problems and get the most from a gasketed plate heat exchanger, users must understand this high performance piece equipment.

A plate heat exchanger consists of stacked, corrugated metal plates with gasket seals around the perimeter. The hot and cold media flow through alternating gaps between the plates, transferring heat from one media to the other.

A gasketed plate heat exchanger consists of corrugated metal plates stacked atop each other, with gasket seals around the perimeter. This forms gaps between the plates, and the hot and cold media flow through alternating gaps, transferring heat from one media to the other as they pass by the plates. Although a range of corrugation depths is possible (from 0.059 to 0.95" [1.5 to 24 mm]), the most common corrugation depth is about 0.1181" (3 mm) from top to bottom of the “wave.”

As the media flows through the gaps, the corrugations create a great deal of turbulence, which produces two effects. First, it enhances the heat transfer from one media to the other, making it efficient. This is why a plate heat exchanger can be smaller than a shell-and-tube heat exchanger for the same heat transfer rate. Second, the turbulence creates a self-cleaning effect that slows clogging and fouling rates. This can help gasketed plate heat exchangers operate at “like new” efficiency for a longer period than shell-and-tube heat exchangers.

Gasketed plate heat exchangers can be designed for large volume applications such as power plant cooling tower isolators and large-scale desalination.

Common Problems and How to Prevent Them

Clogging. As a general rule, a gasketed plate heat exchanger can pass particles through the gaps as long as they are smaller than half of the corrugation depth. This means that a round particle that is 0.059" (1.5 mm) or smaller will pass through a gasketed plate heat exchanger with a 0.1181" (3 mm) corrugation depth without getting stuck. If the particle size is larger, it turns the gasketed plate heat exchanger into a giant filter. This rapidly decreases the heat transfer rate and causes the pressure drop to increase to the point of non-function. In extreme cases, clogging has caused heat exchanger failure within half an hour of commencing operation.

One way to prevent clogging is to add a strainer or filter upstream. Only the particles that are too large to pass through the heat exchanger must be filtered out of the stream.

It also is important to know your media. If the particle size is too big, change your exchanger specification to a plate gap design that can handle that size particle. Also, designing for a higher velocity increases the turbulence in the gaps, which helps to reduce clogging.

If the heat exchanger is already installed, add a strainer or filter if possible. Also, be sure your monitoring system is measuring pressure drop, and open the heat exchanger for cleaning when it exceeds a certain point. If one of your media is heat sensitive, use temperature measurement as well, and use that as your primary factor in deciding when to maintain the heat exchanger.

Fouling. Fouling is the gradual buildup of material on the plate surface. Fouling adds an insulating layer on the surface of the plate, reducing the heat transfer rate. Fouling causes can be complex, containing one or more of the following types:
  • Crystallization (e.g., precipitation and deposition of salts).
  • Sedimentation (e.g., mud or sand buildup).
  • Reaction (e.g., “baking on,” seen with media containing proteins that are overheated.).
  • Corrosion (e.g., formation of an oxide layer)
  • Biological (e.g., mussels, algae, bacteria)
To prevent fouling or address existing fouling, it is most important -- and often most difficult -- to understand what kind of fouling is occurring. Once the cause is identified, multiple solutions -- as wide ranging as the types of fouling -- are possible. Here are a few examples:
  • Changing the plate material to one that is more resistant to corrosion-based fouling for the media in question.
  • Adding a modest oversizing of the total heat transfer surface area and building in more frequent maintenance to combat crystallization-based fouling.
  • Adding a filter or strainer to address mussel infiltration.
  • Adding a settling tank to remove sand and mud, addressing sedimentation.
In most cases, taking advantage of the easy-cleaning design of the gasketed plate heat exchanger is a good solution at the maintenance level.

Specifiers often oversize heat exchangers to compensate for the efficiency loss caused by fouling, and this is a common and accepted practice. Many use Tubular Exchanger Manufacturer Association (TEMA) recommendations. While useful in shell-and-tube heat exchangers, following the TEMA-recommended method for gasketed plate heat exchangers can result in gross oversizing and cause a significant drop in performance with a high rate of fouling -- exactly the opposite of the desired result. Oversizing a gasketed unit to compensate for fouling is best done in consultation with the plate heat exchanger manufacturer.

Erosion. The gradual abrasion from the constant flow of hard particles through the heat exchanger causes erosion. The classic example is sand-filled water from a lake being used to cool a power plant.

To prevent erosion, specify plate materials that are harder and resistant to the erosive particles. Also, specify a filter or strainer upstream to reduce the erosive content in the media. If the heat exchanger is already installed, add a filter or strainer if possible. The addition of an inspection protocol that looks for indicators of erosion-based plate failure during planned maintenance intervals can be helpful as well.

Gasketed plate heat exchangers are suitable for liquid-to-liquid applications in the chemical, power, sugar, renewable energy, food and other industries.

Maintenance: Benefits and Tips

A major benefit of a gasketed plate heat exchanger is that it is easy to open, perform maintenance or repair, and close again. The most user-friendly designs even might be repaired without opening the unit or taking it offline. Plant engineers can take advantage of design benefits such as these to minimize equipment downtime on the exchangers. Other tips include:

Plan your Outage Properly. The smart plant operator will not only have spare parts for emergencies but also will have a set of refurbished plates with new gaskets ready for the planned maintenance. Although gasket replacement is easy, plate damage and cracks require detailed inspection, which takes time. The best way to address this and shorten downtime is to have a fully inspected set of gasketed plates waiting when the heat exchanger is taken offline.

Treat Equipment with Respect. This means avoiding pressure shocks, and opening and closing valves gradually. Pursue less invasive treatments first if you can. Just as a doctor will try therapy for a back problem before pursuing surgery, so you should try simpler adjustments before opening the unit. Other less-invasive procedures include cleaning in place (CIP), where the heat exchanger is flushed with a cleaning chemical, or backflushing, where the original media is temporarily routed backward through the same channel to dislodge clogging elements.

Finally, measure temperatures, flow rates and pressures to be sure the heat exchanger is being subjected to proper design conditions. A frequent cause of heat exchanger failure is that they are subjected to conditions they were never designed for.

By pursuing these measures, both specifiers and maintenance staff can minimize maintenance costs and maximize plant efficiencies.


Reference
1. http://www.formula1.com.

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