Compact heat exchangers -  Performance and service

By Jaswant Choudhary, Head - Parts & Service Division, Alfa Laval India

Heat exchanger technology has come a long way since the plate heat exchanger made its first appearance over 80 years ago.  In this article the latest compact developments, featuring ease of installation, maintenance and cleaning are described in detail by the author. 

Since the plate heat exchanger (PHE) was introduced in the early 1930s it has improved considerably, becoming both cost-effective and versatile. One of the most important improvements is the welded concept introduced in the mid-1980s. Today, products based on the corrugated plate concept are used widely as coolers, heaters, steam heaters, condensers, vacuum condensers, re-boilers and evaporators in a wide range of process industries, not least petroleum and petrochemical. Unplanned production-stops tend to be extremely costly - and very annoying.  Costly - in that they represent a loss of revenue and perhaps even of goodwill for the user.  And annoying because they are avoidable through good design work, preventive maintenance and quality service. To keep the heat exchangers at peak performance and ensure that production levels are maintained and that the plant operates economically it is necessary to consider operating conditions and maintenance requirements already at the design stage, as well as to be aware of service methods and trouble-shooting techniques. Basically, there are two necessary things to do with a compact heat exchanger when it comes to service: cleaning to restore original performance, and replacing the rubber gaskets at the end of the lifetime for gasketed PHEs.

Sooner or later, depending on media, temperature and pressure, the heat exchanger will begin to show signs of needing service. In general terms they are performance failures due to fouling and clogging, or mechanical failures, eg internal or external leakage. Service ranges from onsite cleaning and emergency repair, replacing gaskets, new plates or complete plate packs, to offsite full reconditioning. The gasketed plate heat exchanger consists of a pack of corrugated metal plates with portholes for the passage of the two fluids between which the heat transfer will take place. The plates are fitted with a gasket, which seals the channel and directs the fluids into alternate channels. The plate pack is carried and guided by bars, pressure retained by carbon steel frame plates and compressed with tightening bolts.

Every second gasket can be replaced with a laser weld and a ring gasket, thus forming twin-plate cassettes (semi-welded PHE) or all gaskets can be replaced with laser welds or other welding methods to form a fully welded PHE. Based on the given flow rates, physical properties of the media, allowed pressure drop and temperature program, the design is optimised by using different plate geometry, corrugation, pressing depth, pass arrangement and co-current or counter current flow. For two-phase duties, the condensing curve or heat curve is used to calculate the mean temperature difference. The plate corrugations support the plates against differential pressure and promote fluid turbulence enabling high heat transfer and wail shear stress (frictional force of the fluid against the heat transfer surface). Plate pressing depths and patterns determine the degree of tur bulence. The turbulence and the shear forces reduce fouling during operation.

Plates can be manufactured in all materials, which can be cold formed. The most common materials are:
Stainless steel, like AISI 304, AISI 316, A1SI 316L and 254 SMO Titanium and palladium-stabilised titanium High nickel alloys, like Incolloy and Hastelloy, and pure nickel.
When evaluating plate material fluid properties and operating conditions, including pH levels and chloride content, the operating temperature and pressure, and whether the process is batch or continuous must be considered. Types of service, potential fouling problems and required cleaning procedures must also be evaluated.

Gaskets are available in a wide range of elastomers such as:
Nitrile rubber, including hydrogenat- ed nitrile for hydrocarbons with high sulphur content EPDM, including ALEPDM for polar fluids with some hydrocarbon content Viton (FPM) Neoprene (chloroprene)
It is the elastomer gaskets that should be considered as the plate heat exchanger’s only replacement parts even though individual plate or frame components also are replaceable. Gasket life expectancy depends on such process variables as operating temperature, temperature variations, differential pressure, and compatibility with the fluids involved and room environment. Operating temperature is one of the most important parameters for the gasket lifetime. Operating at 10°C above the maximum will halve the gasket life, while reducing the temperature with the same amount will double it. 

As a general guideline, the performance limits for compact heat exchangers can be given as: 
Pressure <25 bar <30 bar <40 bar Temp <180°C <180°C <350°C

Duties
Coolers: A traditional plate heat exchanger duty, where the process fluid can be cooled efficiently using a “hot” cooling medium due to the high heat transfer coefficient, the possibility of temperature cross and close temperature approach.

Heaters: A traditional plate heat exchanger duty, where the process fluid can be heated efficiently, using a “cold” heating medium due to the high heat transfer coefficient; the possibility of temperature cross and close temperature approach.

Steam Heaters: Plate heat exchangers make very compact steam heaters, due to the high heat transfer coefficient. Venting on the steam side is normally integrated in the steam trap outside the unit, and the reason for venting is only to remove inert gases, which could disrupt the condensation process from the steam chamber. It is recommended to either use primary condensate two-port control (by controlling the liquid level) or primary steam two-port control (by controlling the steam pressure).

In the case of primary steam two-port control, it is very important that the control components are correctly matched. Any type of on/off control would inevitably lead to thermal fatigue. That means if liquid-level control is used, the condensate control valve must operate in its upper range, so that is does not act as an on/off valve. Gasketed, semi-welded and fully welded plate heat exchangers welded in two dimensions only are in general more resistant to fatigue than shell and tube heat exchangers, while other types of fully welded plate heat exchangers are less resistant. For these types, cyclic  opera tions, eg batch heating, should be avoided.

Process Condensers:
Due to the corrugated plate pattern breaking the liquid film, plate heat exchangers are extremely efficient as process condensers in some cases even at vacuum conditions. For some fully welded plate heat exchangers, the condensate and the non-condensable gases are separated inside the unit. The non-condensables are extracted or sucked via a separate connection at the top of the unit and can in some cases be sub-cooled within the same unit, thus avoiding the need for a separate heat exchanger for sub-cooling.

Reboilers and Vaporisers: Specially designed semi-welded or fully welded plate heat exchangers with a large outlet connection are ideal for reboiling duties due to the high heat transfer coefficient and close temperature approach.

Fouling
Plate heat exchangers are less prone to fouling than conventional heat exchangers, such as shell and tube, due to a number of reasons, but mainly because of a uniform flow distribution and a high wall shear stress, which increases the fouling removal rate considerably. Even at low fluid velocities, a high wall shear stress can be achieved due to the corrugated plates, this way extending the time between cleanings from months to years. Using a smooth and corrosion resistant wall surface decreases the fouling deposition rate. The risk of coking is reduced due to the low hold-up volume, which gives a rapid process control, eliminating the risk of strange operating conditions during startup and shutdown, as well as a close temperature approach, since low temperature heating media can be used.

To minimise fouling, design engineers must consider the media and any contaminants present in the process environment. Size, quantity and characteristics of particles must be considered. Filters or wide-gap plate heat exchangers are used for fluids with particles larger than 3mm.

Maintenance and Service
A heat exchanger is a static piece of equipment which cannot create pressure surges, energy, temperature, plugging solids or scale. If properly designed for the service and operated within its original design parameters, it will provide many years of trouble-free, high- efficiency service. When it is abused and operated outside its design parameters, problems may occur. In addition, proper operating procedures, simple preventative techniques such as periodic external visual inspections, performance monitoring and appropriate cleaning procedures can help further to extend the operating life of the exchanger. The service needs for a plate heat exchanger can generally be divided into spare parts and cleaning.

Frame Parts
Parts are replaceable but will in general under proper conditions last as long as the lifetime of the heat exchanger (20-40 years).

Plates: The plates are also replaceable. If properly designed, correct choice of material and the PHE is handled properly etc, the lifetime can be as long as the heat exchanger’s (20-40 years). In some aggressive processes it is accepted to replace plates after some five to 10 years.

Gaskets:  The only part considered necessary to replace. Lifetime varies from process to process and can typically be anything from two to 20 years.

Cleaning:  Depending on the process, the heat exchanger may need cleaning anything from every few months to never during its life.

Inspection and Monitoring
Crack Detection
The exchanger and the area immediately adjacent to it should be inspected periodically for external signs of problems. Leaks, drips and/or dry solids accumulated on the outside of the exchanger or the surrounding area are indications that there may be problems with the sealing system, the gaskets or the welded seams.

A plate heat exchanger does not need to be opened for inspection since all internal parts are made of corrosion- resistant material. If there is a risk for macro fouling (eg mussels in seawater), the moveable frame plate can be equipped with inspection covers for easy access to the ports. The heat exchanger’s performance should also be monitored. Flow, pressure and temperature gauges or monitors should be installed in the inlet and outlet piping of the exchanger. These gauges should be checked on a regular basis and any changes noted. Changes in exchanger flow rates, temperature differences and changes in pressure are primary indicators that a fouling problem may be developing.

Cleaning
Depending on the nature of the liquids and the application, performance of the heat exchanger may decline over a period of time. The decline in performance is typically due to the build up of scale, sediment and/or biological mass on the plates. Fouling of the exchanger manifests itself as a decrease in thermal performance, an increase in pressure drop across the exchanger and/or a reduction in the flow through the exchanger.

There are two methods usually used for cleaning the exchangers: The plates are removed from the exchanger and mechanically or chemically cleaned out side the PHE, or they are chemically cleaned while still installed in the exchanger. The first method can be done either at the site by or under the guidance of qualified field service personnel or off-site in a qualified PHE service centre. The second method, called cleaning-in-place (CIP) is an economical way of maintaining the exchanger at peak performance and extending its operating life.

Onsite Cleaning
CIP is an efficient and economical method to do onsite cleaning. Small amounts of chemicals are used due the low holdup volume and high turbulence. Stronger acids can be used since corrosion-resistant material is used and the efficiency is very high because of the uniform flow and high wall shear stress. CIP connections should be incorporated in the piping in order to easily connect the hoses from the CIP unit.

CIP Cleaning
Hydrochloric acid should not be used to clean stainless steel plates, nor should hydrofluoric acid be used to clean titanium plates. If these acids are used on these plates, the plates will corrode and need to be replaced. Quality water of known makeup should be used in preparing all cleaning agents. Water with a chloride content of 300ppm or higher must not be used. For environmental reasons, care must be taken to properly dispose of all materials used in the cleaning process. This can sometimes complicate onsite cleaning. Mechanical onsite cleaning can be difficult, messy and not necessarily the safest or most reliable method. The cleaning, as well as the disassembly and reassemble of the exchanger, should be done under the direction of an experienced service engineer. If not, it should be done by a service organisation, which is specifically qualified in plate exchanger cleaning. Non-metallic brushes, high-pressure washing and various cleaning agents can be used to clean the plates onsite. The combination used will depend on the nature and degree of fouling.

Common cleaning agents for encrusted scales and sedimentation include:

• Hot water
  
• Nitric, sulphuric, citric or phosphoric acid
  
• Complexing agents such as EDTA or NTA
  
• Sodium polyphosphates.

It is generally recommended that the concentration of these agents should not exceed 4 per cent and that a temperature of 60°C to 80°C be used. For biological growth and slime, alkaline cleaning agents such as sodium hydroxide and sodium carbonate are usually effective. The recommended maximum concentration and temperature for these agents are respectively, 4 per cent and 80°C. Cleaning can some times be enhanced by the addition of small quantities of complex forming agents or surfactants. High-pressure water jet can usually be used to remove the growth immediately after opening the plate heat exchanger. When cleaning it with high-pressure water jet, the cleaning should start at the top of the plate and slowly work downward. During the cleaning process care must be taken not to damage the gaskets. All gaskets should be thorough ly inspected after cleaning and any damaged gaskets replaced. If more than a couple of gaskets need to be replaced, all gaskets should be replaced to ensure uniform gasket hardness, sealing force and extended operation. If the exchanger has been in operation for a number of years and/or the unit is opened frequently, all the gaskets should be replaced at the same time.All plates should be visually inspected for signs of corrosion, erosion and deformation.

Offsite or Service Centre Cleaning
Sometimes the plate heat exchanger will require a more extensive service than can be carried out onsite. In this case a high quality service centre with established well-proven multi-step routines should be used. Visual assessment of the plate packs general condition upon arrival to a service centre

Gasket Removal: The plates are immersed in a liquid nitrogen bath at minus 196°C. After one minute at this temperature, the gaskets become brittle, shrink and separate cleanly from the plate Plate cleaning: the deposits to be removed from the plate naturally vary in composition. Most types can be removed by immersion in a series of chemical baths and or in combination with power washing

Inspection: Use in corrosive applications may cause minute pinholes or cracks not visible to the naked eye. To find these flaws, the plate is sprayed with a dye penetrant and is inspected under ultraviolet light or with a developer

High-Pressure Cleaning: After inspection the plates are cleaned by high-pressure water jet.

Regasketing: date-stamped high quality gaskets are bonded to the plate with a two-component epoxy resin adhesive

Oven Curing: The glue is oven cured to form an immensely strong bond to the plate

Pressure Testing: The plates and the whole unit may be pressure tested either before dispatch to their owner or onsite.The end result should be a guarantee that the heat exchanger plates meet “as well as new” performance standard.

Reconditioning intervals All elastomers are slowly deteriorating in their environment. The time is decided by several factors, such as temperatures, aggressiveness of the media, service intervals etc. and range from a few years to more than 20 years. An indication that the end of gasket life is approaching is often experienced as leakage at low temperatures, eg cold startup, or if the hot medium is shut off. Different users may have different expectations or demands on operating lifetime of gaskets and safety against leakage. The demand level may depend on such events as costs of breakdowns or of changing gaskets, on expensive or dangerous media, or tolerance on drop leakage or occasional leakage - such as cold startups.

Emergency Repair
A problem often may be related only to a few plates. If so, and it is not at the end of the gasket lifetime, these plates should be removed in pairs to maintain the critical plate pattern in the unit. In general, removing a few plates will only effect the heat transfer marginally.

Preventive Maintenance
In most cases the PHE is a workhorse which operates for years without any special maintenance. However, there are some basic rules that will ensure a longer operating life. Do not open a plate heat exchanger for periodical inspections. Ensure that the heat exchanger works according to specification and avoid vibration and water hammer. During startup and shutdown, take special care to open and close valves and pumps slowly. Drain the heat exchanger completely and keep dry during longer shutdowns. If the heat exchanger has been used for corrosive media it must be flushed with clean water during short shut downs to avoid corrosion.

Any hydro pressure test with the heat exchanger should be done with clean water (no chlorides). This avoids corrosion from chloride concentrations during evaporation before the heat exchanger is taken into operation, as the exotic material does not yet have a protective oxide film. Keep tightening-bolts clean and well greased.

Fully Welded Plate Heat Exchangers
Fully welded heat exchangers combine the flow and heat transfer benefits of traditional plate heat exchangers with the mechanical and structural advantages of other heat exchangers. Since there are no gaskets between the plates, the unit can operate at higher temperatures in comparison with the plate heat exchanger and still obtain high heat transfer efficiency in contrast to the shell and tube heat exchanger. High wall shear stress due to the corrugated plate pattern in combination with a high allowable design temperature and design pressure makes these units suitable for high fouling duties.

Construction and Design
Fully welded PHEs can be manufactured in many ways, from a simple construction where the exact design of a traditional PHE is retained, to designs in which the plate channels can be accessed by removing the cover. Since most fully welded PHE constructions differ a lot from traditional PHEs, there is a great need for extensive tests and field experience in order to make accurate thermal designs. This is especially true for two-phase designs.

Cleaning
In general, fully welded heat exchangers should be used in clean duties only. Chemical cleaning is the only possible way to clean a fully welded heat exchanger. Cleaning-in-place is an efficient and economic way of doing this, as already described. In order to maintain accessibility for cleaning purposes it is important to make sure that deposits are never allowed to accumulate on the heat transfer area. Some types of fully welded heat exchangers have cover panels, eg the Compabloc, which can be removed and which gives access for cleaning with high pressure water jet cleaning. In special cases, it may be advisable to use a combination of CIP and high-pressure cleaning. This, of course, means that these types can be used for fouling duties.

Operation and Monitoring
Every precaution must be taken to ensure that fully welded heat exchangers are protected against pressure surges. Smooth temperature adjustments during startup, shutdown and in different operating modes will extend the service life. Spontaneous vaporisation in the plate pack or unwanted vaporisation of the constituent parts of the liquid can result in malfunction, impaired performance or even damage. For this reason, it is important to comply with the specified composition of the product and to maintain the service pressure, particularly when starting up, shutting down or changing the load. The operating pressure on the liquid side should permanently be sufficiently high to prevent boiling in the unit. Extreme and fluctuating temperature shocks will cause metal fatigue in metallic materials. The consequence of this may appear in the form of cracks, accompanied by internal or external leaks. The longer time allowed for the material to adjust to changes in temperature, the longer the service life of the unit. To prevent temperature and pressure shocks all control devices should be opened and closed slowly. This really holds true for any equipment, and good operating practice will lead to long and trouble-free operation. Inspection, preventive maintenance and repair Weld joints may be impossible to inspect for cracks etc, unless they are accessible through removing cover panels. Then it is a relatively simple matter to carry out inspections.

The guidelines for plate heat exchangers regarding preventive maintenance are valid for fully welded heat exchangers as well. When there are panel gaskets they should be replaced when the heat exchanger is opened for a scheduled internal inspection. Due to their robust construction, these types are seldom subject to damage. If damage occurs, it may be attributable to a number of causes: wear on the panel gaskets, corrosion, cracking due to metal fatigue etc.

Damage is indicated by internal or external leaks. By running water in one circuit only and removing the panels or piping from the other circuit for inspection, the location of any damage may be detected. If the panels can be removed, then repairing of isolated cracks can be carried out relatively easily by localised welding.  On types with no access to the welds it is impossible to make a repair. Never apply more than 1 bar over pressure on the closed circuit when the other circuit has been opened (even by loosening the bolts on the cover panel).

Alfa Laval in Brief:
Alfa Laval is a leading global provider of specialized products and engineering solutions based on its key technologies of heat transfer, separation and fluid handling. The company’s equipment, systems and services are dedicated to assisting customers in optimizing the performance of their processes. The solutions help them to heat, cool, separate and transport products  in industries that produce food and beverages, chemicals and petro-chemicals, pharmaceuticals, starch, sugar and ethanol. Alfa Laval’s products are also used in Food   processing, power plants, aboard ships, in the mechanical engineering industry, in the mining industry and for wastewater treatment, as well as for comfort climate and refrigeration applications. Alfa Laval’s worldwide organization works closely with customers in nearly 100 countries to help them stay ahead in the global arena.

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Alfa Laval
www.alfalaval.com