A Look at Plate Heat Exchangers

Plate heat exchanger designs have evolved to satisfy the changing needs of process industries. Take another look at these versatile units when your application demands effective heat transfer.

Depending on the heat exchanger's configuration, it may be better suited to particular services.

The plate heat exchanger is well known as one of the most efficient heat exchanger designs. Its basic concept -- using corrugated plates for heat transfer -- was introduced in the early 1930s for the food and dairy industry. Since then, plates have gone through tremendous development and now are used as standard equipment in many chemical processes.

In its most basic form, the plate heat exchanger consists of corrugated metal plates compressed in a frame. Hot and cold media flow on either side of the plate and transfer heat in a fully countercurrent flow arrangement. Each plate is equipped with a double sealing system that keeps the fluids between the channels. The sealing system is selected based on the application and can be anything from elastomer gaskets to laser welds.

The hallmarks of plate heat exchanger operation are high heat transfer efficiency, compact size, design flexibility, low fouling (as a result of plate pattern turbulence) and low lifecycle cost. A typical heat transfer coefficient for a plate heat exchanger may be four to five times higher than that of a shell and tube design. The compact heat transfer area in a plate heat exchanger results in an efficient design with space requirements as low as 10% of a typical shell and tube design. Some designs employ a modular approach that allows each part of the plate heat exchanger to be changed as needed. With these designs, adding or removing plates for a new duty or changing frame parts to accommodate expanded capacity requirements can be performed on site by the user.

The main limitation of the basic plate heat exchanger design is its sealing system. When using elastomer gaskets, the upper temperature limit is approximately 350

^oF (177

^oC). In addition, some chemicals are not compatible with elastomer gasket materials. For these applications, alternative sealing systems such as nonelastomer gaskets or laser welding can be used to seal the plate channels. Laser welding can be performed on every channel, or on alternating channels, depending on application. Semiwelded plate heat exchangers are welded on every other channel and can handle applications where one of the two fluids is aggressive. Fully welded plate heat exchangers are welded on every channel and should be used if both fluids are aggressive or temperatures exceed the gasket limitation.

Developments in the last 15 years have expanded the applications in which plate heat exchangers can be used. New approaches include wide-gap designs for fouling services, graphite plates for extremely corrosive services, double-wall plates for increased safety, and plates designed specifically for condensing and evaporating duties. All of these designs are based on the advantages of the original plate but are modified to handle specific applications. With these new designs, there are few applications that cannot be handled by plate heat exchangers.

The plate heat exchanger, often called the plate and frame heat exchanger, consists of a frame which holds heat transfer plates. A plate pack of corrugated metal plates with portholes for the media is aligned in a frame and compressed by tightening bolts. The plates form a series of channels for the two media.

Gasketed Plate Heat Exchangers

A gasketed plate heat exchanger consists of a number of thin corrugated metal plates that are compressed together by bolts within a frame. Each plate is equipped with a double gasket system that seals the fluids from the surrounding environment as well as between the two media. The sealing system also guides the fluids into a set of parallel channels, so that one of the fluids flows in the odd-numbered channels while the other fluid flows in the even-numbered channels in a countercurrent flow pattern. The plates' corrugation pattern gives the plate its strength and enhances surface area while inducing turbulence of the fluids flowing across it. When the plates are compressed together until there is metal contact between the corrugation patterns, a large number of contact points are formed and give the design its strength. In this way, high pressures can be handled with a relatively thin plate thickness. Other features include:

Design pressure of vacuum to 450 psig ASME.
Design temperature from -10 to 350^oF (-23 to 177^oC).
Product range (flow rate) of 0 to 20,000 gal/min.

Advanced designs of frames, plates and gaskets allow gasketed plate heat exchangers to be used for virtually any application within its pressure and temperature range. Plate designs allow for both fouling and clean fluids to be handled, and a wide range of plate materials, in combination with high performance gasket materials, allow processors to handle almost most any fluid.

Welded Plate Heat Exchangers

As the gasketed plate heat exchanger technology matured, it became apparent that a gasketed design restricted plate technology in many industrial processes. To satify this demand, a wide range of welded plate heat exchanger designs were developed to meet different process requirements. Welded plate heat exchangers use the same principles as their gasketed counterpart but employ a fusion weld to seal the fluid flow channel instead of the gasket. The result is a fully welded heat transfer plate pack.

While incorporating the advantages of a corrugated plate design, welded plate heat exchangers offer other benefits that extend the operating range of plate technology. They include:

Design pressure up to 600 psig (50 bar) and design temperature up to 650^oF (343^oC).
Low maintenance costs as gasket replacement is not required.
Configuration allows easy access and cleaning on both sides.
High resistance to thermal and mechanical fatigue for cyclic applications.
Improved safety and reliability.

Welded plate heat exchangers are being used more frequently in chemically aggressive processes where the base chemicals or entrained contaminants in the process stream preclude the use of gaskets. For this reason, welded plate heat exchangers are used increasingly in all facets of process industry, including oil and gas production and refineries; hydrocarbon processing units such as primaries, intermediates and polymers; pharmaceutical and specialty fine chemicals; and inorganic chemicals production such as chlor-alkali or hydrogen peroxide. One of the best uses for welded heat exchangers is as an interchanger for heat recovery or absorption-stripping and distillation unit operations.

Welded plate heat exchangers can handle temperature cross in a single compact unit and provide greater heat recovery than other designs. Also, welded units are used as a condenser or reboiler because they offer large cross-flow area, short flow path (low pressure drop) and flexible connection sizes.

Spiral heat exchangers are well suited to handle sludges, slurries and viscous fluids.

Spiral Heat Exchangers

The spiral heat exchanger was introduced in the 1930s as a solution to plugging and fouling problems in the pulp and paper industry. Since that time, the spiral has been used in many difficult applications such as PVC slurry, ammonia liquor in coke plants and catalyst slurries in refineries.

The spiral heat exchanger consists of two long, flat plates wrapped around a mandrel or center tube, creating two concentric spiral channels. Hot fluid enters at the center of the unit and flows from the inside outward in one of the two individual channels created by the plates. The cold fluid enters at the periphery and flows toward the center in the other channel.

The single, curved channel is the key to the spiral heat exchanger's performance. The curving channel keeps the flow in the turbulent regime and prevents solids from adhering to the heat transfer surface by continually impinging upon the exchanger's walls. The single channel prohibits plugging due to solids settling in the channel. If solids begin to settle, the cross-sectional area of the channel is decreased. Because the fluid has no other channel in which to flow, the velocity at the location of the solids is increased, creating a scrubbing effect that re-entrains the solids and flushes them out of the heat exchanger.

The spiral heat exchanger can be customized to handle many problem applications. When used as a condenser, the spiral can provide low pressure drops. The unit's compact size also allows the spiral condenser to be mounted directly on top of a column or reactor.

For hard-to-seal fluids such as heat transfer fluids or oleum, the spiral can be welded completely closed. This eliminates contact with gasket surfaces, which can lead to leak problems in these types of duties. The spiral heat exchanger also is useful in cyclic services where thermal fatigue is a problem. Its relatively thick heat transfer plates and clock-spring type design allow the spiral element to coil and uncoil in order to accommodate thermal expansion and reduce fatigue associated with cyclic duties.

Typical applications and industries include pellet water coolers in polyethylene production, phosphoric acid coolers in fertilizer production and molten sulphur coolers in refineries.

A wide-gap heat exchanger is designed to handle fluids with high percent solids, fibers or suspended solids.

Specialty Plate Heat Exchanger Designs

For applications that require an extra assurance of safety or reliability, a double-wall or graphite plate heat exchanger can be used.

Double-Wall Heat Exchanger. Just as its name indicates, a double-wall plate heat exchanger uses double plates (rather than single plates) between the fluids to increase safety. The plate pack consists of a set of double plates where each double plate has two identical plates stacked on top of each other. They are joined together by laser welds around the ports. A thin air gap between the plates acts as a safety zone in case either of the plates were to fail. With this design, should one of the fluids leak through the first plate in a wall, it is prevented from going any further due to the air gap and the second plate. Leakage due to the failed first plate would show up as a peripheral leak to atmosphere and be visible from the outside. Used with a standard double gasket system, the double plates offer maximum safety against cross-contamination.

Graphite Heat Exchanger. The graphite plate heat exchanger uses the same working principle as the conventional plate heat exchanger, but the plates are made of a highly corrosion-resistant graphite composite material. This material can handle applications previously too corrosive for traditional metal alloys such as HCl, HF and H₂SO₄ at certain concentrations.

A fully welded heat exchanger is engineered for efficient heat transfer in high fouling or aggressive applications. A semiwelded heat exchanger is designed to handle aggressive media in the welded channels and nonaggressive media flow in the gasketed channels.

Brazed Plate Heat Exchanger

The brazed plate heat exchanger design is a variation of the gasketed plate heat exchanger. Available in standard and custom designs, a brazed plate heat exchanger can be used in applications with temperature ranges from -256 to 450^oF (-160 to 232^oC) and up to 500 psig.

Designed to provide turbulent flow and high heat transfer coefficients in a compact size, the brazed plate heat exchanger is well suited for industrial and refrigeration applications. It can be used as an evaporator, condenser, subcooler, desuperheater or oil cooler. It also performs with high efficiency in heat recovery applications for liquid-to-liquid and gas-to-liquid duties.

Thin plate material, corrugated plate patterns and turbulence caused by liquid being forced into the plate's channels all contribute to the exchanger's high heat transfer coefficients. In general, this heat exchanger design occupies less space than other heat exchange designs used for the same duties. Other benefits include lower holdup volumes and simple installation.

A brazed plate heat exchanger consists of channel plates, front and back cover plates, seal plate and connections. The channel plates typically are AISI 316-type stainless steel plates stamped with a corrugated herringbone pattern. They are arranged so that the pattern of adjacent plates points in opposite directions. A criss-cross arrangement of support points is created where the ridges of each plate meet. Plates are brazed together using copper or nickel, sealing the contact points and forming continuous channels. Brazing around the plates' outer edges and connections creates a completely sealed unit. In operation, media enters the inlet connections and is distributed into the narrow channels between the plates. Plates are arranged so the media can travel in either co-current or countercurrent flow.

So, the next time you're in the market for a heat exchanger, consider a plate design. Its flexibility in sealing systems, ability to handle corrosives and high heat transfer may be just what you need.