Semi-Welded Plate Heat Exchangers for Ammonia Secondary Cooling: A Design Review
Keeping ammonia near the machinery and sending glycol or brine out to remote loads changes where people encounter the refrigerant. The plate exchanger sits exactly at that change. Inside, ammonia may boil as the secondary liquid gives up heat. The liquid next door must keep moving without freezing, even at low load. A tight temperature approach looks good on a quotation; for the maintenance crew, the drawing showing seals and accessible passages may matter more.
A semi-welded plate pack can be a good candidate. It is also easy to misunderstand. The International Institute of Ammonia Refrigeration’s plate-evaporator material gives context for this equipment in refrigeration service. The model and the plant design still have to be checked together.
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What the cassette changes
Look at a section drawing before looking at a capacity number. In a semi-welded unit, two plates are joined to make a refrigerant passage. Gaskets separate those pairs and guide the secondary fluid through the neighboring passages. Grano’s semi-welded product page describes laser-welded plate pairs, a clamped frame and an annular seal on the welded side. The exposed gasket route differs from a fully gasketed water-to-water pack.
Welding a pair reduces the long gasket exposure of a conventional plate channel. It does not erase the annular seal or the joints elsewhere in the system. On the sectional drawing, follow the ammonia path past every seal. Mark the pressure each sees. A defect at one location may release gas outside the frame; at another, the concern could be migration into coolant. Different leak paths call for different detectors and isolation valves.
Grano says the semi-welded assembly can be opened and washed. Ask a mechanic to show what would actually be exposed after the clamping plate moves. The gasketed coolant passage may be accessible, while the ammonia channel remains inside a welded pair. A chemical flush of that closed route needs its own compatibility and acceptance checks. The exact cassette’s service instructions should settle what the crew may do on site.
One simple drawing exercise catches ambiguity: trace the route from ammonia inlet to outlet in one color, coolant in another. Where is each seal? Could a defect at that location reach the room or the other fluid? If the drawing lacks the detail to answer, ask for a section or seal map. A sales photograph, however clear, cannot show the leak path.
Freeze the duty sheet before the model choice
“Ammonia exchanger” could mean an evaporator, condenser or liquid subcooler. Here the duty is evaporation against a secondary coolant. A condenser quote from the same plate family would say little about this job: distribution and drainage change with phase. Give the evaporator supplier low, normal and pull-down loads, the intended evaporating conditions and the feed and oil-return arrangement. Beside the capacity, write the outlet condition needed by the separator or compressor. It narrows the design more than the product name does.
For the secondary circuit, write the fluid name and mixture concentration on the duty sheet. Add supply and return temperatures, flow range and available pump head. “Glycol” alone is too vague. More concentrate may protect a cold loop but thicken the liquid; a water-based pressure-drop estimate then tells the pump engineer very little.
Where will the ammonia and oil go at low load?
Start at the outlet the compressor or separator expects. Work back through the cassette: at low load, could some passages run dry while others carry liquid? Oil can collect in poorly drained regions, so the project needs an explicit return path. Distribution depends on this model’s inlet and the surrounding piping. Show the expansion device, separator and compressor protection with the exchanger on one diagram; a generic family description leaves too many blanks.
Pressure documentation belongs to the quoted cassette. The older IIAR 2-2008 Addendum B covers plate evaporators, including constructions with paired welded plates and the testing basis for the equipment. It is useful background. The project engineer still has to establish the current applicable code and local paperwork; the vendor then supplies the matching design and test-pressure records for its model.
An advertised maximum for an entire product family is not the rating for this particular cassette. The proposal should state limits for the selected plate, weld, gasket, frame and connections, at the actual design temperature.
Give the secondary loop its own review
Take the coldest likely plate surface, not merely the mixed outlet, as the freeze concern. The fluid concentration, minimum flow and lowest evaporating temperature must appear in the same calculation. If the pump slows, local freezing risk can rise before the mixed outlet probe reacts. The supplier’s minimum-flow assumption and the plant’s trip setting should meet on the same page.
The pump curve has to be read with coolant properties from the coldest operating case. Otherwise a thermal rating can pass while a distant cooler gets too little flow. Include filter loss on the same sketch. If head is short, include the replacement pump in the estimate; discovering that shortage during commissioning is expensive.
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The submittal should explain the cassette, not just name it
The supplier’s section should let a reviewer trace both fluids past the welds and seals. Next to it, place two rated cases with their pressure losses, temperatures and material grades. Connection orientation, pressure-test basis and opening clearance belong with the same model number. Have the supplier write down its oil-return and distribution assumptions. A feature shown on a different model’s brochure is no help here.
Grano’s product range includes detachable, brazed, fully welded and semi-welded exchangers. Its wider offering includes plates and gaskets. On this quote, the useful spares list is narrower: cassette, annular seal, gasket between pairs, closing dimension and tightening instructions, all tied to exact parts. Show where the frame opens after safe isolation. The coolant-side service route should be clear, and the weld-enclosed ammonia channel should be identified as a different repair problem.
This matters during a short outage. If the coolant side is fouled, can the team open it in place? Is there room to move the clamping plate? Can the cleaning chemical be used on the gasket and weld? How is the ammonia side made safe? These are purchase questions. Leaving them for commissioning can make a nominally compact exchanger awkward to own.
The spare plan has two timescales. A common gasket could be stocked locally against routine replacement. A damaged welded cassette may require a different lead time and a different repair decision. Ask Grano to identify which parts are individually replaceable on the chosen frame, and which arrive only as a pair or assembly. Record the tightening dimension alongside the spare list. The product information for its detachable units describes plates aligned in a frame and clamped with bolts; the mechanic needs the model’s actual closure value before reopening it, not a value copied from a similar unit.
Startup checks at the ammonia boundary
At commissioning, match every pipe to the approved port drawing and verify the pressure-test record. Relief, isolation and detection devices should be checked under the site’s ammonia procedures. Run the secondary pump and prove the low-flow trip before bringing up cooling duty. Take a set of temperatures, flows and pressures once the plant holds a known load. If someone investigates a drifting approach next year, that page will be more useful than a clean test made under unspecified conditions.
Suppose coolant pressure loss climbs months later. The strainer and fluid condition are quicker checks than opening a frame. If approach temperature has moved, compare the refrigeration set point and both flows with the startup sheet. Suspected ammonia in the secondary circuit is an isolation event under the site’s emergency procedure. The question is where it escaped; another turn on the clamping bolts would not answer it.
The package to Grano can be short if it is specific: refrigeration diagram, ammonia envelope, secondary-fluid analysis, required code and a sketch of the space available to open the frame. Request the selected model’s rating and seal map together. They let a refrigeration reviewer judge the proposed cassette as installed, rather than relying on a broad product description.
FAQ
Where are the ammonia-side seals in a semi-welded pack?
At least the annular seal and other connections need review. Their locations vary by model; the section drawing should show them.
Which passage can technicians open for cleaning?
Do not assume it can. A welded passage and a gasketed passage have different access. Follow the selected unit’s service instructions.
