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Maintenance and Malfunctions of Plate Heat Exchangers for Laymen
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Maintenance and Malfunctions of Plate Heat Exchangers for Laymen

  • Katarina Knafelj Jakovac

    January 31, 2024

Plate Heat Exchangers are a widely used type of heat exchanger between the hot fluid that needs cooling and the cold fluid that absorbs the heat, especially in facilities with limited space, due to their practical design, operational simplicity, and compact installation.

They are more expensive compared to shell-and-tube heat exchangers, but they have lower maintenance costs and spare parts expenses.

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Image: Plate heat exchangers in operation (Source)

Plate heat exchangers have been used for over 100 years to optimize processes in the chemical, food, pharmaceutical, oil, and petrochemical industries. The first recorded patent for a plate heat exchanger was filed by German inventor Albrecht Dracke, paving the way for practical heat exchange between two working fluids.

Dr. Richard Seligman was the first to commercially produce plate heat exchangers in 1923 in his company Aluminum Plant and Vessel Company Ltd .

The development of plate heat exchangers continued, gaining popularity among designers, technicians, and engineers due to their high thermal efficiency, the possibility of increasing cooling capacity by adding corrugated plates, and simplicity in disassembly and cleaning.

The technical characteristics and construction of plate heat exchangers are defined by the European Pressure Equipment Directive PED 2014/68/EU, and the ASME standard Sec VIII Div.1.

What does a plate heat exchanger consist of?

Every plate heat exchanger must have a nameplate on the external side of the frame plate.

The nameplate provides information about: the type of heat exchanger, serial number, permissible pressures [bar g], permissible temperatures [°C], test pressure [bar g], volume [L], compression dimensions Lmax. and Lmin. [mm], net mass, and year of manufacture.

The image shows a typical plate heat exchanger assembled from a frame and carrier with guides.

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Image: Parts of a plate heat exchanger (Source: Scanned image from the book Velimir Ozretić - Ship Auxiliary Machinery and Devices)

Corrugated plates are arranged in a bundle or so-called honeycomb on the guides.

The gap between two plates is usually between 1.3 and 1.5 mm.

The corrugations on the plates create a larger cooling surface, improving the efficiency of cooling by enhancing turbulent flow of the working fluid.

The choice of materials for making corrugated plates is determined by the specific operating conditions of the future user, including pressures, temperatures, working fluids, and operating modes.
Corrugated plates are most commonly made of nickel, copper, steel and aluminum alloy, or steel and titanium, depending on the purpose of the exchanger.
After a certain period of operation, deposits of calcium carbonate, calcium sulfate, and silicates, or corrosion, begin to form on the surfaces of the corrugated plates.

Corrosion is a consequence of the interaction between the material of the corrugated plate and the chemical composition of the working fluid.

The formed deposits further slow down heat transfer and impede turbulent flow, leading to a reduction in effective cooling.

This phenomenon is defined by the term fouling factor or fouling factor Rf (m2K/W). Fouling factor represents the thermal resistance of deposits on the surface of the heat exchanger plate to the transfer of heat from the working fluids.

Dirt deposited on the surface of the plate acts as an insulator and reduces heat exchange, directly decreasing the efficiency of the exchanger.

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Image: Plates of the heat exchanger with deposits of dirt (Source)

The most common type of fouling is the deposition of solid substances from the liquid onto the heat exchange surface. Other types of surface fouling include chemical fouling (e.g., corrosion) and biological fouling caused by the growth of algae.

The fouling factor on the surface depends primarily on the chemical and physical characteristics of the working fluid, the flow velocity of the working fluid, and the type of material used for the plate construction.

A flow velocity of 0.9 m/s or lower leads to faster fouling of the plate surface.

The pressure plate and frame plate press the corrugated plates into a bundle, and the entire bundle is tightened by pairs of preload bolts.

Typically, the construction of a plate heat exchanger includes a minimum of 4 pairs of bolts for pre-tensioning.

Media leakage is prevented by gaskets on the edges of the corrugated plates, allowing an alternative passage of the working fluid.

Gaskets are consumable parts sensitive to chemical, thermal, and mechanical damage.

The choice of materials for making gaskets also depends on specified pressures, temperatures, and the properties of working fluids.
Gaskets are divided into two groups: a) elastomeric gaskets with 2 or 4 eyes, attached to the plate surface by embedding or using special adhesive, depending on the heat exchanger manufacturer.

b) hard gaskets made of mineral fibers that are invariably attached with adhesive.

Elastomeric gaskets age over time and become brittle.

Hard gaskets do not have the property of compensating for changes in their structure due to changes in temperature loading, for example, during startup from a cold state, which can lead to leakage of the working fluid.

The service life of gaskets is influenced by the usage mode of the heat exchanger.

In the case of improper use, a shortened service life is inevitable, leading to higher maintenance costs as the user needs to replace them prematurely.

When gaskets are completely replaced, the required compression dimension L is equal to the value Lmax. imprinted on the nameplate.

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Image: Side view of a plate heat exchanger with the indicated compression dimension L (adapted from Source)

When the plates of the heat exchanger are only cleaned, and the gaskets are not replaced, it is necessary to measure and record the existing compression dimension L before opening the plate heat exchanger and tighten the bundle to the recorded value L.

If there is a case of changing the number of plates in the heat exchanger bundle, whether adding new plates or reducing the number of existing ones, the new Lmax. compression dimension must be agreed upon and confirmed with the original equipment manufacturer (OEM).

Then it is necessary to update the data, create and install a new nameplate with the new value of Lmax.

During the installation of the plate heat exchanger, it should be installed according to the instructions of the manufacturer in a way that avoids transferring forces of high intensity, large moments, or increased vibrations over pipes and connections with the exchanger.

Good practice is to have an authorized representative of the manufacturer or supplier present during the installation of the plate heat exchanger and commissioning to confirm the correctness of the installation with a handover protocol.

If you want to avoid problems with clogging the plate heat exchanger or its connecting pipes during the initial commissioning, be sure to install a filter or bypass pipe for the first flushing of the entire pipe system.

The filter or bypass pipe should be removed after the plate heat exchanger reaches stable operating parameters.

How does a plate heat exchanger work?

An eye or opening at each corner of the corrugated plate allows simultaneous entry of 2 working fluids of different <u>temperatures</u>, which then enter the corrugations, i.e., narrow channels on the surface of the plates.

The channels direct the flow of the working fluid.

The channels direct the flow of the working fluid. Gaskets on the edges of the plates are mounted so that the working fluid passes through an alternative path. Fluid "A" enters through odd passages, while fluid "B" enters between even passages.

The image illustrates the operation of a classic plate heat exchanger.

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Image: Operation of a plate heat exchanger (Source: Scanned image from the book Velimir Ozretić - Marine Auxiliary Machinery and Devices)

The flow diagram of working fluids in the next image shows that the cold fluid (blue) and the hot fluid (red) alternately flow through the surfaces of the plates.

Cold and hot fluids are completely separated from each other on the plates of the heat exchanger.

Gaskets on the edges of the plates ensure that there is no mixing of fluids.

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Image: Flow diagram of cold and hot working fluids in a plate heat exchanger (Source)

Maintenance and troubleshooting

Plate heat exchangers should always be in a functional state to avoid premature wear of specific parts due to unfavorable environmental conditions.

The quality of maintenance directly affects the frequency of failures.

Mandatory maintenance and monitoring activities include:

  • Regular visual inspections of the plate heat exchanger during operation, checking for leaks, condensation of the working fluid on the frame and/or pressure plate, and signs of evaporation.
  • Monitoring of pressures and temperatures.
  • Verification of the use of the correct working fluid.
  • Regular sampling of the working fluid and sending it for laboratory analysis.
  • Operation within minimum and maximum operating parameters specified on the nameplate, in the data sheet, in the technological manual, and in standard operating procedures.
  • Avoiding sudden changes in pressures and temperatures during operation.
  • Planned cleaning of the exchanger from the outside with a high-pressure washer (miniwash).
  • Planned cleaning "in place" when instead of a fluid, cleaning chemicals flow through the heat exchanger. The solution dissolves deposits and removes them from the corrugated surface, further enhanced by turbulent flow.

Cleaning chemicals must be compatible with the material of the corrugated plates and the gasket material to avoid damage and corrosion.

The cleaning solution should contain a maximum of 5% by volume of caustic soda or 0.5% by mass of acid at a maximum temperature of 70°C during rinsing.

  • Planned lubrication of the threads on the tightening bolts and checking the tightness of all nuts.

  • Regular painting of the frame and pressure plates by applying a new protective coating.

When a plate heat exchanger is taken out of operation for an extended period, it must be completely drained and depressurized from the working fluid.

It should then be thoroughly cleaned, preserved, and properly stored for the next commissioning.

Planned maintenance mainly consists of regular cleaning of the corrugated plates using a high-pressure washer from both sides.

The cleaning interval is determined by monitoring the condition of the exchanger, i.e., by monitoring the temperature of the working fluids on the pressure gauges installed on the inlet and outlet pipelines or by monitoring the pressure difference on a differential pressure gauge.

The smaller the temperature difference at the inlet and outlet, the dirtier the exchanger, and the lower the heat transfer efficiency.

Plate surfaces commonly accumulate deposits of sludge, mud, sand, or algae. Algae growth is a common occurrence in ship heat exchangers that cool fresh water using seawater.

Exchangers cooling water are more susceptible to sludge deposits, while those cooling lubricating oil or fuel are prone to coke deposits.

Disassembling a plate heat exchanger is done by first closing all supply and discharge valves of the two working fluids and allowing the exchanger to cool thoroughly.

Then, the exchanger must be vented and drained, cleaned, and lubricated for the supports and guides, as well as lubricating the threads and nuts of the tightening bolts.
If the nuts are stuck or corroded, use WD 40 or a similar-purpose chemical.
Measure and record the distance between the pressure plate and the frame plate at the top and bottom on both sides, i.e., the compression dimension L. Four length measurements will be needed, two for each side, at the top and bottom.

Then, release the nuts of the tightening bolts, upper left and lower right, or vice versa. It is important that the release is performed on opposite sides.

Remove the tightening bolts and the frame plate, then proceed sequentially to remove the corrugated plates and label them with numbers one after the other for the same order during assembly.

Wash each plate thoroughly on both sides using a water jet under pressure and clean with a soft brush. Wire brushes made of steel fibers should not be used because they will damage the plate surface. If there are thick deposits, the plates need to be immersed in an appropriate chemical bath.

Inspect all gaskets on all plates for damage.

Check if all plates are well cleaned. Mount them on the supports by stacking them on the guides so that the last plate removed goes first, and so on until mounting the pressure plate.

Install the tightening bolts and gently tighten the nuts of the opposite positioned bolts, then tighten to the end.

Re-fastening the clamping parts of the plate heat exchanger can only be done when the exchanger is completely emptied and there is no pressure from the working fluid, as otherwise, the compression dimension Lmin. may be smaller.

It is said that the plate heat exchanger must be unloaded.

Check if the corrugated plates are adequately pressed between the frame and the pressure plate by re-measuring the distance L between them on several sides, the same way as before disassembly.

When everything is correctly set up, the corrugated plates look like a honeycomb from the side.

The most common failures of plate heat exchangers are leakage of the working fluid outward or inward due to damaged gaskets, excessively high temperatures of the working fluids, improper tightening of bolts, or the occurrence of cracks on the plates due to corrosion.

A large pressure drop results from a higher amount of deposits on the plate surfaces, while bent plates or excessive deposits of impurities prevent tightening of the tightening bolts.

The occurrence of high pressure must be avoided.

High pressure will result from incorrect compression dimension L and problems in the flow of working fluids or their inappropriate physical and/or chemical properties.

Fast connections such as replacing fittings on pipes connected to the plate heat exchanger or disruptions in the process, such as accelerated formation of condensate or evaporation, also lead to the occurrence of high pressure in the working fluid.

Pipe connections and other fittings connected to the plate heat exchanger can sometimes cause malfunctions because they pull the exchanger sideways and lead to the bending of the corrugated plates.

This happens in cases where there is too much load on the pipe connection due to the pipe's mass, the sealing ring is dirty or does not fit properly, and the flange connection is not properly tightened.

Every plate heat exchanger must be properly grounded.

Welded joints on the pressure and frame plates, on the support, and on the guides must not have cracks.

In case of cracks, repair welding is necessary using silver electrodes with a minimum 45% silver content, and the temperature during welding must not exceed 650°C.

Katarina Knafelj Jakovac
Katarina Knafelj Jakovac social media icon
January 31, 2024

Katarina Knafelj Jakovac holds Master degree in Mechanical engineering with long term work experience in Oil industry. She is Certified Reliability Leader specialized for mechanical equipment and operational excellence. Author of blog Strojarska Radionica (Mechanical Workshop) where she shares professional knowledge and personal experience in maintaining various rotating machines, machine systems and process equipment. Adores mechanics, thermal engineering and internal combustion engines. She is dedicated to the continuous improvement of machine maintenance and quality management of physical assets.