Heat Exchanger Selection —
Shell and Tube, Plate
and Spiral Compared

Heat exchanger selection is typically framed as a thermal design problem — LMTD, NTU-effectiveness, area estimation, pressure drop. But the thermal design only confirms whether a particular type will work for a given duty. The selection decision — which type to use at all — is made earlier, and it is driven by factors that do not appear in the heat transfer equations: the nature of the fluids, fouling behaviour, maintenance access, available footprint, operating pressure and temperature, and whether the process allows the two streams to intermix if there is ever a leak.

Getting the selection wrong before the thermal design begins produces a heat exchanger that works thermally but fails operationally — it cannot be cleaned, it blocks within weeks of commissioning, it corrodes from the outside, or it requires a shutdown every six months that the process cannot accommodate. This article covers the principal types, their characteristics and limitations, and a systematic basis for selection.

The Principal Types

Shell and Tube

The dominant type across the process industries. One fluid flows inside a bundle of parallel tubes, the other flows across the outside of those tubes within a cylindrical shell. Baffles inside the shell direct the shell-side fluid across the tubes in a series of passes, improving heat transfer coefficients and maintaining velocity. Multiple tube-side passes can be arranged by fitting an internal partition in the channel head.

Shell and tube is the default choice for demanding applications: high pressure, high temperature, high fouling duty, or where the process cannot tolerate the two streams mixing in the event of a tube failure. It is the most robust, the most extensively coded (TEMA, ASME VIII, EN 13445), the most maintainable in the field, and the most configurable — virtually any combination of operating conditions can be accommodated by appropriate tube material, shell geometry, and tubesheet design.

The tradeoff is size and cost. A shell and tube unit for a given duty will typically be physically larger, heavier, and more expensive than a plate heat exchanger of equivalent thermal duty under comparable conditions. Where the application allows it, plate heat exchangers are almost always the more economical choice.

Plate Heat Exchangers (PHE)

A stack of thin corrugated metal plates clamped in a frame, with alternating channels carrying the two streams. The corrugated plate geometry creates highly turbulent flow at low velocities — heat transfer coefficients 3–5× higher than a shell and tube unit per unit area. The result is a far more compact unit for the same duty, typically one-fifth to one-tenth the footprint of an equivalent shell and tube exchanger.

Plate heat exchangers come in three main configurations:

• Gasketed (GPHE) — plates sealed with elastomeric gaskets, bolted in a frame. Fully accessible for cleaning — the plates are pressed apart and each one can be inspected and cleaned. Plates can be added or removed to adjust thermal capacity. The gaskets limit operating temperature (typically 180°C maximum for NBR, 200°C for EPDM) and pressure (typically 25 bar maximum for standard frames, higher for reinforced designs). Gasket material compatibility with the process fluid must be verified — nitrile is incompatible with many ketones and esters; EPDM with mineral oils; PTFE-encapsulated gaskets extend chemical compatibility significantly.
• Brazed (BPHE) — plates vacuum-brazed together with copper or nickel, no frame, no gaskets. Compact, inexpensive, suitable for refrigeration and HVAC duties. Not cleanable — if it fouls, it is replaced. Limited to relatively clean fluids. Copper-brazed units incompatible with ammonia.
• Welded / semi-welded — alternating channels are welded (on the more aggressive fluid side) and gasketed (on the other). Extends the operating envelope of plate exchangers to higher temperatures and more corrosive fluids without the full mechanical complexity of a shell and tube. Used in ammonia chillers, chemical duty, and higher-temperature process applications.

Spiral Heat Exchangers

Two flat strips of metal wound into concentric spirals, forming two continuous channels — one flowing inward, one flowing outward. The geometry produces pure countercurrent flow (the theoretical maximum for heat recovery), very high wall shear stress (self-cleaning behaviour with fibrous or particulate fluids), and a compact footprint with very low pressure drop per unit of heat transferred.

Spiral exchangers are the specialist choice for difficult fluids: slurries, fibrous streams, viscous products, fluids with suspended solids, and biological process streams that would block a plate or tube bundle quickly. They are more expensive than equivalent gasketed plate units and less configurable than shell and tube, but for viscous or fouling duty they often outperform both. The single-channel geometry also means that if one stream leaks, there is nowhere for it to go except to mix with the other — this must be considered for cross-contamination-sensitive applications.

Other Types Worth Knowing

Air-cooled (fin-fan) heat exchangers — tubes with extended fins on the outside surface, air driven across them by fans. The default choice when cooling water is unavailable, expensive, or where environmental discharge limits restrict once-through cooling. High operating cost (fan power), large footprint, and dependent on ambient air temperature — duty falls in hot weather. Common in refineries, power generation, and gas processing.

Double-pipe (hairpin) — one tube inside another, in a hairpin configuration. The simplest possible shell and tube arrangement. Used for small duties, high-pressure applications, or where the very close temperature approach available in true countercurrent flow is required and neither a shell and tube nor a plate unit can achieve it economically.

Printed circuit heat exchangers (PCHE) — chemically etched microchannels in metal plates, diffusion bonded. Extreme compactness and very high heat transfer coefficients. Used in LNG liquefaction, hydrogen processes, and offshore/aerospace where size and weight are critical. Very high cost and not field-maintainable — a specialist product for specialist applications.

TEMA — The Design Standard for Shell and Tube

The Tubular Exchanger Manufacturers Association (TEMA) standard defines the mechanical design requirements for shell and tube heat exchangers and, crucially, a three-letter nomenclature that fully describes the exchanger geometry:

• First letter — front end stationary head type: A (channel with removable cover), B (bonnet/integral cover), C (channel integral with tubesheet), N (fixed tubesheet), D (special high-pressure closure)
• Second letter — shell type: E (one-pass shell — most common), F (two-pass shell with longitudinal baffle), G (split flow), H (double split flow), J (divided flow), K (kettle reboiler), X (crossflow)
• Third letter — rear end head type: L (fixed tubesheet), M (fixed tubesheet bonnet), N (fixed tubesheet integral), P (outside packed floating head), S (floating head with backing device), T (pull-through bundle), U (U-tube bundle), W (externally sealed floating tubesheet)

So an AES exchanger has a channel with removable cover at the front, a one-pass shell, and a floating head with backing device at the rear — a pull-through bundle floating head, the most common configuration for fouling or high-temperature duties where tube bundle removal for cleaning is required. A BEM exchanger has an integral bonnet, E-shell, and fixed tubesheet — the simplest and cheapest construction, used for clean duties where the bundle does not need to be removed.

TEMA additionally defines three classes of mechanical design severity — R (severe process requirements, refinery service), C (generally moderate service, commercial and general process), and B (chemical process service, intermediate between R and C) — which govern tolerances, corrosion allowances, and testing requirements.

Fouling — The Dominant Practical Consideration

Fouling is the deposition of material on heat transfer surfaces, progressively degrading thermal performance and increasing pressure drop. It is the most important single practical factor in heat exchanger selection and the most common cause of operational problems — a correctly sized, correctly selected heat exchanger that fouls within months of commissioning is a more common failure mode than one that was thermally undersized.

Fouling types and their implications:

• Particulate fouling — suspended solids in the fluid settling and adhering to surfaces. Worst at low velocity. Shell and tube with low shell-side velocity is vulnerable. Spiral exchangers have the highest wall shear stress and are most resistant.
• Biological fouling (biofouling) — microbial growth on surfaces, common with cooling water. Gasketed plate exchangers are particularly susceptible because the narrow channel gaps trap biological material. Chlorination of cooling water towers, spiral exchangers, and regular mechanical cleaning are the mitigation routes.
• Chemical fouling (scaling) — precipitation of dissolved salts (calcium carbonate, calcium sulphate, silica) on hot surfaces. Worst above 60°C on the hot surface. Hard water in cooling systems is the most common cause. Gasketed plates are cleanable; brazed units are typically replaced. Acid descaling is effective for carbonate scale; sulphate and silica scale is harder to remove.
• Corrosion fouling — oxide and corrosion product deposition on surfaces. Carbon steel in untreated water service produces iron oxide deposits. Stainless and titanium plates significantly reduce this.
• Polymerisation fouling — process fluids that polymerise or coke at elevated temperatures deposit insulating films. Shell and tube with removable bundle is the standard choice; temperatures must be controlled to keep below the threshold.

TEMA provides fouling resistance (Rf) values for common fluids used in thermal design calculations. These values add a thermal resistance to the calculation to account for the expected fouling layer, resulting in a larger area specification that provides clean-condition headroom. Common values: cooling water (once-through) 0.000176 m²K/W, river water 0.000352, process streams 0.000176–0.000528, crude oil 0.000528–0.000881. The fouling resistance has a disproportionately large effect on the required area for exchangers with high heat transfer coefficients — in a plate exchanger where the clean overall U is 5,000 W/m²K, adding fouling resistances of 0.000176 per side reduces the effective U to approximately 2,174 W/m²K, more than doubling the required area. This effect is often underappreciated and leads to underspecification of the fouling allowance.

Thermal Basics — LMTD and NTU

Two methods are used for heat exchanger thermal analysis. Both give the same result — they are two routes to the same answer:

LMTD Method

Q = U × A × LMTD × F

Where Q is the heat duty (W), U is the overall heat transfer coefficient (W/m²K), A is the heat transfer area (m²), LMTD is the log mean temperature difference, and F is a correction factor for non-ideal flow arrangement (F = 1.0 for true countercurrent, <1.0 for multipass or crossflow arrangements).

LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂) where ΔT₁ and ΔT₂ are the terminal temperature differences at each end of the exchanger.

The LMTD method is best for rating an existing exchanger or where both the inlet and outlet temperatures are specified.

NTU-Effectiveness Method

The NTU (Number of Transfer Units) method is better suited to sizing problems where the outlet temperature of one or both streams is not known. Effectiveness ε is defined as the actual heat transfer divided by the maximum possible heat transfer. NTU = UA/C_min where C_min is the smaller of the two fluid capacity rate products (ṁCp). For a given exchanger type and NTU, the effectiveness is determined from standard charts or equations, giving the outlet temperatures directly without iteration.

Selection Guide — Matching Type to Application

Criterion	Shell & Tube	Gasketed Plate	Brazed Plate	Spiral
Max pressure	Very high (700+ bar with special design)	~25 bar (standard)	~30 bar	~15–25 bar
Max temperature	Very high (>600°C with alloy)	~200°C (gasket limited)	~225°C (brazed)	~400°C
Fouling duty	Good (removable bundle)	Good (gasketed, cleanable)	Poor — not cleanable	Excellent (self-cleaning)
Viscous fluids	Moderate	Poor (>~5 Pa·s)	Poor	Excellent
Slurries / fibrous	Possible (shell side)	Poor — blocks	Poor — blocks	Excellent
Phase change (boiling/condensing)	Excellent	Good (rising film condensation)	Good (refrigerants)	Limited
Close temperature approach	Moderate (multipass limits F)	Excellent (<1°C approach achievable)	Excellent	Excellent (true countercurrent)
Leak between streams acceptable?	Yes (double tubesheet for no)	No (gasket failure = mixing)	No	No
Relative cost (same duty)	Highest	Low–medium	Lowest	Medium–high
Footprint	Large	Very compact	Very compact	Compact
Capacity flexibility	Fixed	Add/remove plates	Fixed	Fixed

Common Selection Scenarios

Process-to-cooling water (clean duty)

Default recommendation: gasketed plate heat exchanger, titanium or stainless plates, EPDM or NBR gaskets depending on process fluid. Reasons: high heat transfer coefficient reduces area and cost, fully cleanable on cooling water side (where biofouling and scaling are expected), compact footprint. Shell and tube only if the process pressure exceeds plate frame capability (>25 bar) or if the cooling water chemistry is severe enough to attack gasket materials.

Process-to-process heat recovery

If both streams are clean and temperatures are within plate limits: gasketed plate. If the close-approach temperature difference of true countercurrent is required: plate or spiral. If pressures or temperatures exceed plate limits, or if one stream is a gas at elevated pressure: shell and tube.

Reboilers and vaporisers

Shell and tube kettle reboiler (K-shell) is the standard. The oversized shell provides vapour disengagement space above the tube bundle. Natural circulation thermosyphon reboilers use E-shell with vertical or horizontal orientation. Plate heat exchangers can be used for falling-film or rising-film evaporation but are not the standard choice in conventional distillation service.

Condensers

Shell and tube (E-shell or X-shell) for large-scale condensation duty. Gasketed plate for compact refrigeration condensers and moderate-duty process condensers. Air-cooled fin-fan for gas processing where cooling water is constrained.

Viscous fluid heating or cooling

Spiral heat exchanger first choice — the self-cleaning geometry and high wall shear handle viscous, fouling process streams that would block plate channels or foul tube bundles rapidly. For very high viscosity (>50 Pa·s) or highly non-Newtonian fluids, scraped-surface or jacketed agitated vessel rather than a conventional heat exchanger.

High-pressure gas service

Shell and tube, sized for the tube-side pressure. Plate units at standard frame rating (25 bar) are not suitable for high-pressure gas streams — the risk of a gasket failure in gas service is not acceptable in most process plants. Double-pipe hairpin for small duties where true countercurrent flow is required at high pressure.

Biological and food processing

Gasketed plate (sanitary design, Tri-Clamp connections, electropolished plates) or spiral (for product streams with fibrous or pulped content). Hygiene-in-place (HIP) and clean-in-place (CIP) compatibility must be specified — not all gasket materials or plate geometries are CIP-compatible without disassembly.

Summary

Heat exchanger selection precedes thermal design. The type determines the feasibility of the application; the thermal design determines the size. Shell and tube is the universal fallback — any duty can be accommodated, at a cost in size and price. Gasketed plate is the preferred first choice for clean and moderate-fouling duties within its pressure and temperature envelope — compact, cleanable, and cost-effective. Spiral is the right answer for viscous, fouling, or fibrous process streams that would cause rapid operational problems in any other geometry. Brazed plate is the economical choice for clean, moderate-pressure refrigeration and HVAC duties where cleanability is not required.

The fouling specification is the single most important input to heat exchanger design that is most often underweighted. An exchanger sized for clean conditions in a fouling service will underperform within months of commissioning. Size for the expected fouled condition and design the installation to allow cleaning — either CIP provision for plate units or bundle pull space for shell and tube.