How Does a Plate-and-Frame Heat Exchanger with PTFE Gaskets Compare to a Shell-and-Tube PTFE Unit?

Shell-and-tube exchangers reign supreme in corrosive service, but plate-and-frame exchangers with fluoropolymer gaskets provide a compelling alternative. Their basic geometry leads to distinct performance characteristics which may be useful in some applications. For clean, moderate pressure process fluids, a plate-and-frame design with PTFE gaskets can offer better heat transfer efficiency and a dramatically lower footprint than a standard PTFE shell-and-tube exchanger.

The Plate and Frame vs Shell and Tube Design Overview
Plate-and-frame heat exchangers are made up of a succession of corrugated metal plates pressed together in a frame. Each plate has a gasket (in this case PTFE or any other fluoropolymer) that seals the flow channels and directs the hot and cold fluids into alternate routes. The fluids flow through the thin tortuous spaces between plates in a counter-current or cross-flow direction. The corrugations encourage turbulence even at low flow rates, which improves heat transmission considerably.

PTFE shell-and-tube heat exchangers (as detailed in other articles) are made of a bundle of PTFE tubes housed in a metal or plastic shell. The service fluid flows on the other side and the corrosive fluid flows through the tubes (or around them). The PTFE tube walls are somewhat thick so heat transfer is by conduction. Flow is generally laminar or low-turbulence unless high velocities are used .

The main difference is the substance of the heat transfer surface. In the plate-and-frame construction, the plates are metal (usually stainless steel, Hastelloy, titanium or other alloys) and only gaskets are PTFE. The process fluid is in contact with the metal plates and the PTFE gaskets. The PTFE shell-and-tube unit has all wetted surfaces made of PTFE, which has universal corrosion resistance but substantially poorer thermal conductivity.

Thermal Performance: Higher U-values achieved due to plate geometry
The plate and frame exchanger has the greatest advantage in its overall heat transfer coefficient (U-value). This is due to a number of causes.

Thin metal plates - The thickness of the plates is normally 0.5 to 1.0 mm against PTFE tube walls of 1.0 to 2.0 mm. Metals (stainless steel ~15 W/m·K, titanium ~17 W/m·K, Hastelloy ~10 W/m·K) transmit heat orders of magnitude better than PTFE (~0.25 W/m·K). The metal plate is a far better conductor of heat even taking into account the resistance of the PTFE gasket seals (which represent only a small fraction of the total area).

High turbulence - The design of the corrugated plate causes high local turbulence. This disrupts boundary layers and significantly reduces convective resistance on both sides of the fluids. In shell-and-tube PTFE systems, flow in small-diameter PTFE tubes is usually laminar unless significant pumping power is used, and the shell-side flow is also relatively mild.

True counter-current flow - Plate-and-frame systems can produce near perfect counter-current flow, maximising the log mean temperature difference (LMTD). Shell and tube PTFE exchangers are typically set up in cross flow or mixed flow configurations that are less thermodynamically efficient.

In practice, for the same pair of fluids, a plate-and-frame heat exchanger with PTFE gaskets can be expected to have an overall U-value three to five times higher than that of a PTFE shell-and-tube unit. In water-water service a plate exchanger can obtain 3,000-5,000 W/m²·K, while a PTFE shell-and-tube unit may only reach 200-400 W/m²·K. In corrosive chemical services the benefit is still large but absolute values depend upon qualities of the fluid and fouling tendencies.

Footprint & Weight: Plate Design Compactness
The U-value is much larger, hence a plate-and-frame exchanger requires far less surface area to transmit a given heat duty. The resulting footprint is typically one-third to one-fifth that of a PTFE shell and tube heat exchanger with the same thermal performance. The weight is also considerably reduced as the plate pack is much lighter than a bundle of PTFE tubes with a metal casing.

The compactness of the plate-and-frame design is a distinct benefit for applications where space is at a premium, such as retrofitting into existing process skids, offshore platforms or modular chemical facilities. A PTFE shell-and-tube exchanger of similar duty could be several metres in length and occupy a considerable floor area, whereas a plate exchanger can be installed vertically or horizontally in a fraction of this space.

Limitations in Pressure and Temperature
Shell and tube units (metal shell) made of PTFE can be used for moderate to high pressures, typically 6-10 bar on the tube side, 10-20 bar on the shell side, depending on the design. The PTFE tubes are pressure limited, but the metal casing is robustly contained. PTFE tubing can tolerate temperatures of 110 to 120 °C; PFA tubing, 150 to 180 °C.

More stringent limits apply to the plate-and-frame exchangers with PTFE gaskets:

Pressure – The gaskets must be squeezed enough to seal against the plates. The clamping force required increases with the increase of internal pressure and the possibility of gasket extrusion or leakage increases. Typical maximum working pressures for PTFE-gasketed plate exchangers are 6-10 bar (85-145 psi), with some systems rated to 15 bar. Higher pressures need thicker plates, heavier frames and specific gasket shapes, adding to expense.

Temperature - PTFE gaskets start to creep and lose their sealing force beyond roughly 150 to 180 °C. This depends on the fluoropolymer grade used (PTFE, PFA or modified PTFE). Operation over 160°C is not normally recommended for continuous operation. Some PTFE shell-and-tube designs (with PFA tubes) can operate up to 180-200°C, but actual limits are frequently lower.

For corrosive services at high pressure or high temperature, the shell and tube PTFE unit may be the sole option. For mild conditions (e.g. 5 bar and 100 °C) the plate exchanger is entirely suitable.

Corrosion Resistance: Alloy vs. Full Fluoropolymer Plates
PTFE shell and tube exchangers are universally corrosion resistant. PTFE is almost completely inert to all substances except molten alkali metals and elemental fluorine at high temperature. They are therefore suited for highly aggressive mixed acids, aqua regia, strong nitric acid and other fluids that would attack most metals.

The plate-and-frame exchangers with PTFE gaskets depend on the metal plates for primary corrosion protection. The gaskets are the only PTFE part. Hence the plate material has to be chosen to withstand the particular process fluid. Here are some common options:

Stainless steel (304/316) – Only for mild acids and neutral solutions.

Hastelloy C-276 or B-3 - Excellent resistance to hydrochloric acid, sulphuric acid and reducing conditions. Attacked by powerful oxidising acids.

Titanium (Grade 2, Grade 7) - Excellent resistance to oxidising acids (nitric, chromic) and chlorides. Will be attacked by hydrofluoric acid and some reducing acids.

Zirconium or Tantalum - Exotic alloys for harsh circumstances, but very expensive.

The plate exchanger can be a cost effective solution if the process fluid is compatible with an accessible plate alloy (e.g. titanium for nitric acid, Hastelloy for HCl). However, where the fluid comprises several hostile species (e.g. mixed acid with both oxidising and reducing components) no one metal may be acceptable. In such instances the PTFE shell-and-tube unit with its universal resistance is the safer choice.

Fouling, Cleaning, Maintenance
Plate-and-frame exchangers can be simply taken apart for cleaning. The plates are removable one by one and can be cleaned mechanically (brushing, pressure washing) or chemically. This is a big plus for services that generate soft or sticky fouling. However the tiny pores between plates (usually 2-5 mm) are susceptible to filling by particles, fibres or crystallising substances. A plate exchanger needs a well built strainer or filter upstream to remove particles larger than roughly 1 mm.

PTFE shell-and-tube exchangers offer bigger flow channels (tube inner diameters often 5–15 mm) and are less prone to plugging. The smooth low friction PTFE surface also prevents the adherence of numerous deposits (see article on surface energy). But it's harder to clean." Cleaning of tube interiors is done mechanically (special instruments – tube brushes) and often chemically . The tubes cannot be removed one by one for inspection, the entire bundle must be removed from the shell.

One of the big advantages of the plate exchanger is that if a gasket goes bad, or a plate corrodes, the individual plates and gaskets may be replaced in a few hours. In a PTFE shell-and-tube unit, if one tube fails, the tube may have to be plugged (lowering surface area) or the entire tube bundle may have to be replaced, a costly and time-consuming task.

Typical Applications for Each Technique
Plate-and-frame exchangers with PTFE gaskets are frequently employed in clean utility services (water-water, water-glycol, low pressure steam) where the fluid is not excessively corrosive. They are seen in chemical process applications in:

Cooling of dilute acids (e.g. 10% sulphuric acid) where titanium or stainless steel is suitable.

Plating baths are used with good filtration and moderate temperatures.

Recovery of heat from relatively clean corrosive streams.

PTFE shell-and-tube exchangers are the preferred option for highly corrosive, oxidising acid or particulate-laden acid services such as:

Mixed acid pickling lines (HNO3+HF)

Hot conc. hydrochloric acid.

Aqua regia and other harsh mixes.

Treatment of waste acids with changing fluid composition

Comparison Table: Plate & Frame vs Shell & Tube (PTFE)
Feature Plate and Frame with PTFE PTFE Shell & Tube Gaskets
Overall heat transfer coefficient (U-value)High (3-5× greater than shell-and-tube)Low (PTFE conductivity limited)
Footprint (per duty)(1× baseline) SmallLarge (3–5 times larger)
Maximum sustained pressure6-10 bar (higher with specific design)6-10 bar on tube side (metal shell can be higher)
Max. continuous temperature 150–180 °C (gasket-limited)110–120 °C (tubes of PTFE); 150–180 °C (tubes of PFA)
Corrosion resistance depends on plate alloy PTFE gasketsUniversal (PTFE or PFA on all wet surfaces)
Sensitivity to dust particlesHigh (thin gaps that tend to plug)Low (bigger tube diameters; smooth surface) 
Easy to maintainVery good (the plates can be taken apart and brushed)Fair (needs chemical cleaning or tube brushing)
Repairability High (individual plates & gaskets replaceable)Limited (tube plugging or replacement of bundle)
First cost (e.g. duty)Clean, moderate services lowerBetter general corrosion resistance
Typical applications • Clean dilute acids • Plating baths • Water utilitiesAggressive mixed acids, oxidising acids, waste streams of varying composition
Decision Guide: Choosing the Right Exchanger
The decision for a plate and frame exchanger with PTFE gaskets or a PTFE shell and tube unit should depend on the cleanliness, corrosiveness, pressure, and available area for the fluid.

Select a plate-and-frame exchanger if:

The process fluid is clean and particles are less than 1 mm (or suitable filtration is provided).

The fluid is compatible with a moderately priced metal alloy (stainless steel, titanium, Hastelloy) at the operating temperature.

Operating pressure is below 10 bar and temperature is below 150°C.

Space is limited and a compact footprint is important.

Very important is ease of disassembly and cleaning.

The exchanger will be utilised for heat recovery or utility duties instead of direct contact with very reactive mixed acids.

Choose a shell-and-tube heat exchanger made of PTFE when:

The fluid is comprised of powerful oxidising acids (nitric, chromic) or combined acids which would attack any metal.

The fluid composition is unpredictable or poorly characterised, hence there is risk in alloy selection.

The fluid contains particles, fibres or crystals which could clog tiny plate gaps.

Operating pressure >10 bar or temperature >150 °C (with PFA tubing).

Universal corrosion resistance is needed without the risk of localised attack or pitting.

The plant does not have the maintenance capability to regularly disassemble and replace gaskets.

Both technologies exist side-by-side in many chemical process environments. A PTFE shell-and-tube exchanger may be chosen for a highly corrosive mixed acid heating duty, whilst a plate-and-frame exchanger with titanium plates and PTFE gaskets handles a clean, dilute nitric acid cooling duty elsewhere in the same plant.

Conclusion: Compact Efficiency or Universal Robustness
Plate and frame heat exchangers with PTFE gaskets provide high thermal efficiency and small footprint due to high turbulence and thin metal plates. Their U-values are three to five times greater than those of PTFE shell and tube units, resulting in a significantly smaller footprint and reduced weight.