What are the advantages of using plate-and-frame heat exchangers over tube type?

Tube-type heat exchangers are superior to plate-and-frame (shell-and-tube) models in the majority of applications for hydraulic cooling because they provide more efficient heat transfer for each unit volume, have 30% to 40% less space and are more easily maintained via disassembly, rather than the pulling of tube bundles, and are able to be adjusted in capacity by simply the addition or removal of plates. For hydraulic systems in which space, thermal load variation, and maintenance downtimes are constant considerations, plate-and-frame designs typically have the highest durability, but shell-and-tube models still have advantages in high-pressure and high-fouling conditions.

They're a quiet but vital component in any hydraulic power system. Each horsepower not converted to productive work is converted into heat and has to be disposed of before it deteriorates the fluid, causes damage to seals, or causes increased wear rates throughout the entire system. Selecting the correct heat exchanger's design directly impacts the reliability, energy costs, and maintenance load during the lifetime of the device.

Understanding the two design concepts

What is the process by which do tube type (shell-and-tube) exchangers function?

A shell-and-tube exchanger transports the hydraulic fluid via a series of tubes that are housed in the larger cylindrical shell. Cooling media, typically air or water, circulates through the outer walls of the tubes, while heat transfer occurs through the tube's walls. This type of design has been a reliable industrial tool for a long time because it's durable, resistant to extreme pressure, and easy to fabricate in large dimensions.

How do exchangers for plates and frames work?

The plate-and-frame exchanger is the stack of corrugated and thin metal plates that are clamped within frames. Hot hydraulic fluid as well as cooling media flow in alternating channels between the plates and are separated by gaskets and welds. The corrugated pattern causes turbulent conditions even at very low flow rates. This dramatically increases the area of surface available for heat transfer compared to the size of the exchanger overall.

The primary advantages of frame-and-plate designs

More efficient efficiency of thermal energy per unit

The corrugated pattern pushes the fluid to enter a turbulent flow very quickly, as opposed to the more laminar, smoother flow that is typical of round tubes. The turbulent flow disrupts that boundary of the stagnant liquid that usually adheres to a transfer surface and then insulates it. Because the insulating layer gets thinner and less shattered by a plate-exchanger, heat travels across the wall of the plate more easily. In terms of practicality, the plate-and-frame model can typically have the same cooling power as a shell-and-tube unit but more than twice its size.

Compact footprint

Machine builders that work using mobile equipment such as skid-mounted power units or retrofit installations typically face difficult space limitations. Plate-and-frame exchangers typically provide the same or more cooling capacity within less than the shell-and-tube envelope, which is important when the cooler must fit into an enclosure already in place or beside an existing reservoir that has no space to spare.

Easy capacity adjustment

A major important change is evident following installation. When a hydraulic system's temperature load grows -- for example after an upgrade to the pump or a duty cycle change the plate-and-frame exchanger may be rearranged through the addition of plates on top, which will increase the area of the exchanger without having to replace the whole unit. Exchangers made of tubes and shells do not offer mid-life adjustment. The tubes and the shells are set when they are manufactured.

Cleaning and maintenance are simplified.

Plate packs open like a book. Technicians are able to separate the plates, examine each surface separately, and then clean off any scale or fouling deposits using a pressure washer. Shell-and-tube units require the removal of all the tubes out of the shell. This is a heavier and more labor-intensive job that usually requires special tools and longer downtime, especially on larger units.

A lower volume of fluid and a faster response

Since the channels for flow between the plates are comparatively narrow, a frame-and-plate exchanger can hold a lesser amount in hydraulic fluids at any time compared to a tube-and-shell unit with the same capacity. This means that the system reacts quicker to temperature fluctuations, which allows for more precise thermal control in unpredictable load conditions typical in industrial and mobile hydraulic applications.

True counter-flow configuration

The plate-and-frame design is naturally designed to work with true counter-current flow. This is where hot and cold fluids travel in opposite directions along all of the exchange surfaces. This arrangement allows for a higher than average temperature of the approach—meaning that the temperature at the outlet of the cooling fluid can be closer to the temperature of the inlet of the medium cooling it more than the majority of shell-and-tube configurations can reach.

In the case of shell-and-tube, it still wins.

Frame-and-plate exchangers aren't the best choice for all applications. A fair comparison must take into account the differences.

High-pressure applications

Exchangers made of shell and tube generally withstand greater working pressures due to their shell geometry, which is cylindrical and disperses stress more evenly than a plate stack that is gasketed. In circuits that run at extremely high system pressures, where the cooler is located within a high-pressure line, shell-and-tube may be the most secure and established alternative.

Fluids with high-fouling or heavy debris

The small channels between plates, which create beneficial turbulence, could also trap debris and are susceptible to blockage when cooling water contains large amounts of particulate matter or when process fluids are sticky or dirty. Exchangers made of tube and shell, due to their more flow passages, tend to manage media prone to contamination with less blockage problems.

Chemical compatibility and the life of the gasket

Gasketed plate exchangers are based on elastomer seals that connect plates. These gaskets have a finite life span and a set of chemical compatibility limitations. Extremely aggressive fluids or temperature fluctuations could shorten the life of gaskets, while tubes with brazed or welded joints in a shell-and-tube design tend to have fewer limitations on compatibility, however at the expense of the advantages of serviceability mentioned above.

Very huge cooling loads of cooling

At the highest industrial scale (think condensers in power plants or massive processing cooling loads), the shell-and-tube method is cheaper and more durable than trying to increase the size of plates to fit.

The decision to choose hydraulic systems

The majority of industrial and mobile hydropower units in which there is a limited space for maintenance, access is important, and pressures on the cooling loop are low, plate-and-frame exchangers are usually the best choice. Their small size, ease of use, and efficiency in heat transfer are in line with how the majority of hydraulic systems are designed as well as maintained on the job.

Systems with extremely high line pressures in the cooler, liquids susceptible to fouling, or massive cooling requirements are the reasons shell-and-tube designs are still earning their spot. In several fleets, the correct solution isn't always the same, but it is contingent on the particular requirements, such as the frequency of operation, cleanliness of the fluid and space availability, as well as the maintenance capabilities of the facility.

Whatever design you choose, the correct sizing, basing it on actual load calculations, proper flow rates for both the cold and hot sides, and a realistic plan for maintenance will provide more long-term reliability than the selection of an exchanger model alone.

Are heat exchangers made of plate and frame more expensive than tubes and shells?

The cost of the initial purchase is based on dimensions and materials, but frames and plates typically cost less than cooling capacity due to the fact that they use less material and have a smaller frame. But replacing gaskets with time is a constant maintenance cost that shell and tube designs generally do not have to worry about.

Can exchangers with plates and frames manage pressures from high hydraulic systems?

Standard gasketed plate exchangers generally have lower pressure ratings for maximum pressure than tubes and shells. If high pressure is required, the brazed or welded plate exchangers may be able to extend the range of pressure, but generally at a lower level of serviceability.

What is the frequency at which frame-and-plate exchangers require gaskets replaced?

The lifespan of gaskets depends on the type of fluid used, the operating temperature, and pressure cycling. However, many industrial applications require replacement intervals that span the range of a few years with normal use and earlier replacements required when working in harsh chemical or thermal conditions.

Which is the best design for particulate- or dirty-cooling water?

Shell-and-tube exchangers are able to deal with fouling-prone or debris-laden fluids better because of their larger circulation channels that are less susceptible to blockage than the narrow channels found in plates.

Can a shell-and-tube exchanger be replaced by an exchanger that is plate-and-frame?

In a lot of retrofit situations it is true, as long as the flow rate, pressure rating, and fluid compatibility are matched. A thorough analysis of hydraulic and thermal sizing should be performed prior to any exchanger conversion to verify that the performance requirements have been fulfilled.