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A selection guide for process, refrigeration and HVAC engineers — sealing, cleaning access, pressure, gasket life, standards and total cost, compared side by side.
| November 12, 2026 | Aiprel Engineering Team |
In short: A gasketed plate heat exchanger seals every channel with an elastomer gasket and opens completely for mechanical cleaning. A semi-welded plate heat exchanger (SWHE) laser-welds plate pairs so one circuit has no gasket at all, while the other circuit stays gasketed and openable. Choose gasketed when both fluids are mild and the pack must be opened often; choose semi-welded when one fluid is aggressive, hazardous or high-pressure and the other still needs service access.
Ask three suppliers to quote the same duty and the prices can differ by a third. The thermal calculations usually agree. What differs is the sealing method — and with it the temperature ceiling, the fluids you can run, how the unit is cleaned, and what the next fifteen years of maintenance will cost. The gasketed vs semi-welded plate heat exchanger choice is decided by that difference, not by the heat transfer coefficient.
A gasketed plate heat exchanger — also called a gasketed plate-and-frame heat exchanger or GPHE — is a pack of corrugated metal plates compressed between a fixed frame plate and a movable pressure plate. An elastomer gasket sits in a groove around each plate’s perimeter and ports. The gaskets seal alternate channels, keeping the two fluids apart while heat crosses the thin plate wall.
Because the pack is held together by tightening bolts rather than welds, it can be opened in roughly an hour. Plates can be brushed or jet-washed individually, a damaged plate can be replaced, and capacity can be changed by adding or removing plates within the frame’s tightening range. That serviceability is why gasketed units dominate the installed base in HVAC, district heating, food and beverage, and general process cooling. Our gasketed plate heat exchanger range covers those duties from 0.1 to 4,500 m³/h.
The limitation is also the gasket. Every gasket is a wear item and a potential leak path. Elastomer compatibility has to be checked against both the process fluid and the cleaning chemicals — a detail covered in our guide to NBR, EPDM, HNBR and FKM gasket materials.
A semi-welded plate heat exchanger (SWHE) is a hybrid construction. Two plates are laser-welded along the sealing groove to form a cassette. The demanding fluid flows inside the welded cassette, where the boundary between the two fluids is metal. The cassettes are stacked in the frame and sealed to one another with gaskets, so the second fluid runs through conventional gasketed channels.
The result is a deliberate split: no elastomer touches the aggressive medium, while the utility side remains fully serviceable. This is a well-established arrangement in industrial heat transfer: because the aggressive fluid is separated by metal rather than elastomer, welded channels can be designed for higher pressures than a fully gasketed frame. Typical duties include ammonia refrigeration, sulphuric acid and caustic processing, petrochemical streams and industrial heat pumps. Aiprel’s semi-welded plate heat exchanger design uses the same cassette principle with plate options in 304, 316L, titanium, 254 SMO, Hastelloy and nickel.
If neither fluid can tolerate an elastomer, semi-welded is no longer enough — the answer is a fully welded plate-and-frame unit or a welded plate-and-shell exchanger.
Both constructions move heat the same way. Fluids run counter-current on opposite sides of a corrugated plate; the corrugations generate turbulence; thin metal keeps resistance low. Published reference ranges put plate-to-plate liquid duties broadly in the 1,000–4,000 W/(m²·K) band depending on fluid pair, velocity and fouling allowance — see the overall heat transfer coefficient table. The sealing method does not move that number materially.
What it changes is the number of gasketed sealing points and which fluid sees them. A semi-welded pack roughly halves the gasketed interfaces of a similarly sized gasketed pack, and — more importantly — places the remaining gaskets on the benign side. That is the whole logic of the design.
| Decision factor | Gasketed plate heat exchanger | Semi-welded plate heat exchanger |
| Channel sealing | Elastomer gasket in every channel | Laser-welded plate pairs on one circuit; gaskets between cassettes |
| Gasket exposure to aggressive medium | Direct | None on the welded circuit |
| Gasketed sealing points | One set per plate pair | Roughly half — between cassettes only |
| Pack opening | Whole pack opens | Gasketed side opens; welded cassettes stay closed |
| Mechanical cleaning | Both sides, plate by plate | Gasketed side only; welded side CIP/backflush |
| Capacity change | Add or remove plates | Add or remove cassettes |
| Typical industry pressure range quoted | About 10–16 bar for standard frames; higher figures with heavy construction | Commonly quoted 25–30 bar; higher with special designs |
| Typical industry temperature ceiling | Around 150°C with standard elastomers; higher with FKM/HNBR | Around 200°C on the welded side; gasketed side still elastomer-limited |
| Aiprel published envelope | 0.1–4,500 m³/h; DN32–DN500; up to 3.5 MPaG; up to 180°C; 60 MW | 0.1–4,500 m³/h; DN32–DN500; up to 3.5 MPaG; up to 180°C; 60 MW |
| Leak signature | Weep at frame edge; internal leak possible | Gasketed side visible; welded side metal-tight |
| Relative purchase cost | Lower | Higher |
| Best fit | Mild fluids, frequent cleaning, variable duty | One difficult fluid, one serviceable utility side |
A caution on the numbers. Industry ranges and individual manufacturers’ datasheets do not always agree. Aiprel lists the same pressure and temperature envelope for both families because the frame and plate design set those limits. That does not make the two interchangeable. The envelope tells you what the unit survives; it does not tell you which fluid touches rubber, or what a leak on that circuit would cost. Always ask which circuit carries the gasket.
Elastomer life is a function of temperature, fluid chemistry and cleaning regime, not a fixed number. A commonly cited service interval is roughly three to eight years before hardening, compression set or cracking makes replacement necessary. Short intervals are not automatically a design fault — a planned replacement is maintenance; an unplanned one is a leak.
Three choices determine how predictable that cycle is: the compound, the attachment method and the closing dimension. Clip-on gaskets remove the adhesive curing step and shorten every future shutdown. The cost of a gasket set is worth modelling over ten years rather than at the purchase order stage. And where a leak appears, external vs internal leaks have different signatures: an internal leak can cross-contaminate two circuits with nothing visible on the floor.
On a semi-welded unit, this entire discussion applies to one side only.
A gasketed pack can be opened and every channel mechanically brushed or jet-washed. A semi-welded pack cannot. The welded channels are cleaned by circulating chemical solution — CIP, soak or backflush — and hard scale, crystals or baked-on fouling may not come off. A damaged welded cassette is replaced as a pair, not as a single plate.
This is why the classic selection rule is more precise than “semi-welded is stronger”:
If the aggressive stream is also the one that scales heavily, stop and reconsider. A gasketed unit, a wide-gap plate pack or a different exchanger family may serve better than a semi-welded unit you cannot clean.
Ammonia is the reference case for this construction. It is an efficient natural refrigerant with a well-established safety discipline — the International Institute of Ammonia Refrigeration is the central technical authority — but it is toxic, and any copper in the circuit is a problem. Nothing copper-bearing should touch the refrigerant: no brazing filler, no copper ports, no brass instrument fittings. That check belongs in the bill of materials, including small items such as gauge snubbers and thermowells.
A semi-welded evaporator or condenser keeps ammonia inside welded channels and puts water or glycol on the serviceable side. Several design details matter more than the headline duty:
For a deeper look at the design details, we have written separately about plate evaporators and about marine plate heat exchangers in seawater service.
The same split applies in process plants. Sulphuric acid cooling, caustic soda service, petrochemical condensation and metallurgical process streams all benefit from a welded barrier on the aggressive side, while the cooling-water side stays open for descaling. Plate material is selected independently for each circuit — stainless 316L, titanium, 254 SMO, Hastelloy or nickel — so the alloy budget goes where chlorides or acid actually attack.
For high-purity or severely corrosive media on both sides, a fully welded construction is more honest than a semi-welded one. For duties where one side needs frequent hygiene inspection, gasketed remains the practical choice.
| Fluid A — welded side | Fluid B — gasketed side |
| Hazardous, toxic or expensive | Benign utility water or glycol |
| Corrosive acid, caustic, solvent | Fouling but cleanable stream |
| High pressure or deep vacuum | Low-pressure service side |
| High-purity stream that must not cross-contaminate | Stream requiring periodic inspection |
| Ammonia, CO₂ cascade, refrigerant | Cooling water, brine, chilled water |
A mistake here defeats the purpose of the construction. If the fouling stream is assigned to the welded side, the maintenance advantage disappears. If the aggressive stream is assigned to the gasketed side, the safety advantage disappears.
A gasketed unit fails at the gasket: a weep at the frame edge, a drifting pressure test, a drip on the floor. It is the cheapest failure to detect and the cheapest to fix, which is why gasketed units remain preferred wherever a small external leak is tolerable.
A semi-welded unit fails either on the gasketed side, in the same visible way, or at a weld or plate crack on the welded side — rarely, but after years of thermal cycling or chloride attack, and expensively. You are trading a frequent, cheap, visible event for a rare, costly one. That is a risk decision, not simply an upgrade.
Certification follows the installation location, not the factory. Units installed in North America are commonly assessed to ASME BPVC Section VIII; European installations to the Pressure Equipment Directive; and liquid-to-liquid gasketed units may carry AHRI 400 performance certification. Refrigeration systems are commonly referenced to EN 378, ISO 5149 or IIAR practice for charge and machinery-room rules. AHRI’s certification programme is described on the AHRI Liquid-to-Liquid Heat Exchanger certification page.
Decide this at enquiry stage, because the answer changes plate thickness, test regime and paperwork. Material certificates, pressure and leak test records, the assembled pack dimension and any third-party inspection should be treated as deliverables. On export projects, inspection is normally witnessed before crating, so the test schedule must be agreed early enough to book an inspector.
Per square metre of installed area, gasketed is the cheaper purchase and semi-welded sits above it. Over ten years the order can reverse. The gasketed unit adds gasket sets and opening hours, but capacity can be extended without replacing the frame. The semi-welded unit starts more expensive, removes gasket risk from the difficult circuit, and then mostly stops costing anything there — provided that circuit is kept clean.
The number to model is downtime cost multiplied by expected frequency, plus the cost of the consequence if the leak is not visible. On benign water duty that arithmetic favours gasketed. On ammonia or concentrated acid it usually favours semi-welded regardless of purchase price.
| Duty | Starting point | Reason |
| Chilled and hot water, HVAC | Gasketed | Close approach, openable, expandable |
| District heating and heat recovery | Gasketed | Small driving temperature difference; plates can be added |
| Ammonia evaporation or condensation | Semi-welded | No elastomer in the refrigerant circuit; moderate charge |
| Sulphuric acid, caustic soda | Semi-welded | Removes aggressive medium from gasket contact |
| Petrochemical and metallurgical processes | Semi-welded | Hazardous streams with one serviceable side |
| Industrial heat pumps and cascade systems | Semi-welded | Higher design pressure and low refrigerant charge |
| Food, dairy, beverage | Gasketed, hygienic design | Daily CIP plus periodic inspection |
| High-purity or severely corrosive media both sides | Fully welded | No elastomer in the pack |
| High-pressure gas or condensing duty | Welded plate-and-shell | Pressure-vessel containment |
| Fibrous or particle-laden fluids | Gasketed wide-gap | Larger channels resist blinding |
Send the heat duty in kW or flow rate in kg/h, inlet and outlet temperatures on both sides, fluid properties (density, viscosity, specific heat, concentration, solids content), allowable pressure drop per side, design pressure and temperature, applicable design code, material and gasket preference, connection sizes and orientation, cleaning method and frequency, and installation limits including plate-pack withdrawal space.
Our plate heat exchanger range covers gasketed, semi-welded and fully welded plate-and-frame units, welded plate-and-shell, wide-gap packs and marine configurations, with replacement gaskets and replacement plates for equipment already in service. Dimensions and materials are in the catalogue. If you would rather start from the duty than the hardware, send us the process data and we will tell you which construction fits — including when the honest answer is gasketed rather than semi-welded.
A gasketed unit seals every channel with an elastomer gasket and opens completely. A semi-welded unit laser-welds plate pairs so one circuit has no gasket; only the other circuit remains gasketed and openable.
No. Semi-welded keeps gaskets between welded cassettes on one side. A fully welded unit has no elastomer in the plate pack and cannot be opened for mechanical cleaning.
Yes. Ammonia evaporation, condensation, desuperheating and cascade duties are core applications. Welded channels contain the refrigerant, reducing both charge and the risk of leakage to atmosphere.
Does semi-welded allow higher pressure than gasketed?
Usually yes, because the welded cassette removes the gasket from the high-pressure circuit. The exact rating depends on plate pattern, material and design code; confirm it on the datasheet for your model.
No. They can be cleaned in place or backflushed, but not opened and brushed. Hard scale or fibre deposits are the main reason to reconsider the construction.
It depends on compound, temperature, fluid and cleaning regime. A commonly cited range is three to eight years; confirm the grade and its continuous limit for your duty.
On benign duties, usually yes. Where the gasket is the limiting component, avoided downtime and avoided leak consequence typically outweigh the higher purchase price of semi-welded.
No. Plate pairs, grooves and frames are different. The decision belongs at specification stage.
Gasketed when both fluids can live with a gasket and the pack must be opened; semi-welded when one fluid cannot and the other side still needs access; fully welded when neither fluid can. Everything else — brand, plate count, price per square metre — is a second-order decision made after that fork is right.
Aiprel builds gasketed, semi-welded and fully welded constructions from one production base, so the recommendation does not depend on what happens to be in stock. Send the duty and the constraints, and the selection can be argued on media compatibility, pressure, cleaning and total cost rather than on preference.
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