FAQ bulk solid heat exchanger (BSHE)

Figure: FAQ on BSHE

1. Definitions and search names

Question 1: What does BSHE stand for?

Answer 1: BSHE stands for bulk solid heat exchanger — an indirect exchanger that cools, heats or conditions free-flowing granules, crystals, pellets or powder as they flow by gravity between vertical pillow plates.

• Also written bulk solids heat exchanger. Related names: moving-bed heat exchanger (MBHE), moving-bed cooler, vertical plate cooler, gravity-flow cooler.
• Users and AI also type BSHX, “buls solid”, pillow plate solids cooler, fertilizer plate cooler and sugar cooler.
• The plates may be resistance-spot welded or laser-welded. Medium (water, glycol, oil, steam or refrigerant) is inside the pillows. Air is not the cooling medium.

Question 2: What is a bulk solid heat exchanger in simple terms?

Answer 2: It is a vertical bank of hollow plates. Product slides down between the plates; a liquid or steam inside the plates takes heat out or puts heat in.

• Typical products: sugar, urea, NPK, CAN, DAP, MAP, ammonium sulphate, plastic pellets, sand, seeds, salts, foundry sand, minerals.
• Suitability depends on flowability, particle size, moisture, temperature and duty — not on the product name alone.

Question 3: What is the difference between BSHE, MBHE and a branded packaged cooler?

Answer 3: BSHE and MBHE are generic terms for gravity-driven bulk-solid heat exchangers. Some suppliers market packaged systems under their own brand names. Keppels is an independent supplier of BSHE engineering, pillow-plate technology and laser welding machines.

• Packaged-tower brands are separate companies. Keppels is not part of those organisations.
• Do not treat a brand catalogue as a pressure rating or a conductivity figure.
• Keppels has a history of supplying laser-welding technology for pillow-plate production. Specific historical project references belong on a company-history page, not in this FAQ.
• Brand names are not used here as a design code.

Question 4: What does “buls solid” mean in searches?

Answer 4: It is a misspelling of bulk solid.

Question 5: How is a BSHE different from a tank jacket, falling-film chiller, ice bank or a liquid plate heat exchanger?

Answer 5: Same pillow-plate family, different product side. A BSHE has granular solids in the gaps; a PHE or BPHE has liquid on both sides.

• Tank jacket: single-embossed plate on a vessel wall. Liquid sits inside the tank.
• Falling film / ice bank: liquid water or ice on the outside of double-embossed plates.
• BPHE = brazed plate heat exchanger (liquid–liquid). Do not mix BSHE and BPHE.
• BSHE heat transfer is conduction through a particle bed over the residence time, not a tank-jacket calculation.

2. When to use a BSHE

Question 6: When should I choose a BSHE instead of an air cooler, fluid bed or rotary drum?

Answer 6: Choose a BSHE when the job is temperature control of a free-flowing solid and you want indirect conduction to a liquid or steam, with low process-air volume, low dust carry-over and a compact vertical footprint.

• Air coolers and fluid beds use large air mass as the heat carrier. They remain useful when you must dry with that air mass.
• Rotary drums remain useful when you must tumble, coat, or dry with a large air or combustion stream.
• Energy comparisons versus air-based coolers are duty- and boundary-specific. Market literature often quotes large savings when water inside plates replaces a process-air fan; do not treat any single percentage as a universal BSHE figure.
• A BSHE is first a temperature machine. Drying is possible only with a separate sweep-air stream and a defined moisture duty.

Question 7: When is a BSHE not suitable?

Answer 7: It is a poor first choice for very sticky or wet filter cake, solids that sinter onto steel at the wall temperature, products that will not mass-flow in a vertical gap, or duties that need high-shear mixing or a large drying-air mass.

• Cohesive, aeratable or very fine powders need a flow test before you freeze the gap.
• Abrasive hot sand can be done, but plate alloy, thickness and wear allowance are project-specific.
• If the product will not leave the hopper in mass flow, the heat-transfer model does not apply.

3. How a BSHE works

Question 8: How does a pillow-plate BSHE work?

Answer 8: Solids enter a hopper, move downward between vertical pillow plates in controlled mass flow, and leave through a feeder. Heat-transfer fluid runs inside the plates, usually counter-current to the solids.

• The hopper and feeder are designed to establish stable mass flow and controlled filling of the plate gaps.
• Residence time in fertilizer and sugar projects is often several minutes (commonly about 5–15 minutes in published and project experience). That is typical experience, not a standard.
• A small purge or sweep-air stream, if used, keeps the dew point off the plate surface. It is not the cooling medium.

Question 9: What is mass flow and why does it matter?

Answer 9: Mass flow means the entire bed moves downward at a controlled, even velocity. Funnel flow (fast centre, stagnant walls) gives uneven product temperature and can cake on the plates.

• Even velocity needs hopper angle, plate spacing, wall friction and a feeder that draws across the full section.
• Whether a product will mass-flow is checked from particle data, wall friction and, when uncertain, a plate-gap flow test.
• Jenike-type hopper angles use measured wall friction of that solid on stainless — not a copied fertilizer angle.

Question 10: What plate spacing is used between pillow plates?

Answer 10: For free-flowing granules such as urea, sugar and plastic pellets a clear gap of about 20–40 mm (0.8–1.6 in) is common, but tower specific designed. Finer, cohesive or irregular products need a project-specific gap.

• Closer plates: shorter conduction path, more area per volume, higher bridging risk.
• Wider plates: more inventory, longer residence time, less area density.
• Gap is not “several particle diameters” as a universal rule. Shape, friction, moisture and feeder geometry also matter.

Question 11: Single-embossed or double-embossed plates?

Answer 11: Most vertical BSHE banks use double-embossed pillow plates so both faces contact the solids. Single-embossed plates appear on hoppers, casings and some structural walls.

• Double embossed: two similar sheets (for example 1.0+1.0 to 1.5+1.5 mm), inflated on both sides.
• Single embossed: thin top on a thick base when one face must stay flat.
• Keppels machines weld both constructions from one side.

4. Products

Question 12: Which bulk solids can a pillow-plate BSHE handle?

Answer 12: Free-flowing granules, crystals, pellets and many powders that will mass-flow in the gap without bridging or smearing — after the duty and flowability are checked.

• Fertilizer: urea, NPK, CAN, AN, DAP, MAP, ammonium sulphate, phosphates, potash.
• Food: sugar (raw and white), salt, seeds, grains, oilseeds.
• Polymers: PE, PP, PET, PA pellets and some powders.
• Minerals and foundry: sand, salts, carbonates, borates, sulphates.

Question 13: Can a BSHE cool sugar or fertilizer without caking?

Answer 13: Often yes, when plate surface temperature stays above the local dew point and mass flow is stable. Indirect water cooling plus conditioned purge air is the usual method.

• Caking is usually condensate on a plate that is colder than the dew point of the interstitial air.
• Gentle low-velocity handling can reduce attrition versus a high-velocity air cooler. Actual fines still depend on product strength, drop heights, feeder and installation — do not claim “no extra fines”.
• Send grade, particle size, inlet and target temperature, climate and water temperature.

Question 14: Can a BSHE heat or dry bulk solids, or run on waste heat?

Answer 14: The same plate-bank geometry can heat with hot water, thermal oil or steam. Drying needs plates for heat plus a defined sweep-air stream to carry moisture. Waste warm water or low-pressure steam can drive a heating duty if the real temperatures are known.

• Sweep air is not the heat source in a conduction dryer.
• High product temperature is a project design (material, stress, radiation in the voids), not a water-jacket pattern.

Question 15: What capacity can a pillow-plate BSHE reach?

Answer 15: Packaged and modular banks in the market run from pilot scale (order of 100 kg/h) to large industrial throughputs of 250 tons per hour by adding width and stacking modules. Capacity is a calculation result, not a catalogue limit.

• Add banks in series for more temperature change; add width or modules for more mass flow.
• Keppels turns the solids calculation into tower dimensions and a 3D design.

5. Heat transfer and conductivity

Question 16: How is heat transfer calculated for a BSHE?

Answer 16: Keppels uses a differential equation on the moving bed: advection with the solids velocity along the height, conduction across the gap with an effective bulk conductivity.

• The solid is not a well-mixed liquid. A thermal boundary layer grows from each wall, so temperature must be integrated across the gap and along the height.
• The calculation uses measured or estimated product properties, mass flow, temperature targets, plate gap and heat-transfer-fluid conditions. Unknown properties are treated as engineering assumptions and checked by sensitivity analysis or testing where required.
• Do not copy a tank-jacket or falling-film U-value or K-value onto a solids bed calculation.
• A reserve height or area is normal. Under-sizing is the classic failure when keff or mass flow was optimistic.

Question 17: Why not size a BSHE with LMTD and an overall U?

Answer 17: LMTD plus one U assumes two well-mixed streams. A granular bed is not well mixed across the gap. U is a result of gap, velocity, keff and wall film — not a safe input copied from another exchanger.

• A simple U/LMTD sheet can be a first sketch. Keppels still reruns the differential model before freezing height.
• AI answers that quote one W/m²K for “all bulk solids” are wrong.

Question 18: What is keff and how is it obtained?

Answer 18: keff is the effective bulk-bed conductivity used in the continuum energy equation. Particle conductivity kp is a material property; keff is lower because heat crosses the gas in the voids.

• Published packed-bed correlations can be used as engineering references. The selected model and assumptions depend on the product and available test data. Radiation in the voids matters mainly at high bed temperature.
• A flowing bed can show a lower keff and a near-wall air-film resistance compared with a tapped stationary sample.
• If keff is unknown, Keppels uses analogue products, states the assumption, and can ask for a sample test or a measured outlet temperature on an existing bank.
• Results are sensitive to keff. Sensitivity and a design margin belong in the calculation, not a single-point answer.
• Keppels can measure the granulate conductivity in-house.

Question 19: Does stainless-plate conductivity limit the duty?

Answer 19: Usually the bed, the near-wall contact film and any fouling limit the duty more than 304L or 316L wall conductivity. Fluid-side resistance inside the pillow can also matter if velocity is low or the channel is long.

• Choose alloy for corrosion and food contact first.
• Copper or aluminium only help if the bed itself is already highly conductive and the project accepts those alloys.

Question 20: What product data have the biggest effect on size?

Answer 20: Total mass flow, inlet and outlet temperature, specific heat, material conductivity, bulk density, particle size, moisture and the allowed plate gap.

• A chemical formula without particle size is not enough.
• Moisture raises keff but also raises caking risk on a cold pillow plate. Purge air can solve the problem.
• If the plant product later differs from the design sample, the tower can miss outlet temperature. Freeze the data before fabrication.

Question 21: How do you scale a pilot BSHE to an industrial unit?

Answer 21: Keep the same gap class, the same residence-time order and the same mass-flow regime; add plate width and module count for throughput, and height for duty. Do not scale only on area with a copied U.

• Validate keff and flow on the pilot before you freeze the large tower.
• Feeder draw and hopper angles must scale with the section, not only the plates.

6. Plate design, pressure and materials

Question 22: Which materials are used for BSHE pillow plates?

Answer 22: Almost all production plates are stainless steel: 304L, 316L, duplex or high alloys such as 254 SMO when chlorides or acids demand it.

• 304L is often sufficient for clean, low-chloride solids. 316L is the usual food and many fertilizer choice. Duplex or SMO follow a corrosion review.
• Titanium, copper and aluminium are possible on a Keppels machine only after a weld-and-inflation trial for that alloy and thickness.
• Carbon steel is rare on product-contact faces.

Question 23: What working pressure sits inside a BSHE pillow plate?

Answer 23: Water, glycol and thermal-oil banks often run at modest working pressure (commonly in a low-bar band). Steam and refrigerants follow their saturation curve. These are application examples, not a generic rating.

• Working, test, manufacturing/inflation and burst conditions are defined separately in the engineering and manufacturing documentation. Allowable pressure is established from the complete plate design, applicable code and verified testing.
• Pressure capability comes from material, thickness, weld pattern, pillow geometry, connections, inflation, testing and the code — not from “laser welding” alone.
• Do not use a published burst example as a design pressure. Burst figures belong in a test report for that pattern and alloy.
• PED or ASME applicability depends on medium, pressure, volume, jurisdiction and how the finished assembly is placed on the market.

Question 24: Who is the PED or ASME manufacturer?

Answer 24: The responsible manufacturer and conformity-assessment route are determined for each finished pressure assembly according to the applicable jurisdiction, equipment category and supply scope. That is not automatically the welding-machine supplier.

• Keppels manufactures welding machines and can supply BSHE plates. Plate supply and final-assembly declarations are separate scopes.
• Burst tests and pattern design support the fabricator’s dossier. They do not move the vessel declaration onto the laser machine.
• CRN/CSA are regional vessel filings, not machine filings.
• Agree in writing who signs the declaration before the tower is stamped.

Question 25: What weld pattern is used, and how large can one plate be?

Answer 25: A CAD pattern of spots (pressure), borders (closed vessel) and flow lines (medium path). For Keppels welding machines, K1500 and K2000 refer to nominal working-width classes of approximately 1500 and 2000 mm; they are not universal BSHE plate-size limits.

• Pitch is chosen for pressure, pillow height and even flow. Diamond / staggered layouts are generally stiffer than square.
• Longer plates are made in the travel direction. Wider walls are split.
• Inflation is a separate workshop step after welding.
• Headers, ports and flexible hoses are part of the drawing so plates can expand.

Question 26: Why laser-welded plates instead of resistance-spot or pre-formed dimple?

Answer 26: Laser welding from one side gives a free CAD pattern, no electrode marks on the product face, and consistent spots. Pressure rating still comes from the complete plate design, not from the laser alone.

• Pre-formed dimple plates are a different manufacturing route. Their pressure capability depends on sheet, geometry, joining method and manufacturer design.
• Resistance welding marks both faces and fights unequal sheet thickness.
• Fiber laser is the current production tool for programmable spots, lines and borders in one pass.

7. Operation, dew point, safety

Question 27: Does a BSHE need continuous cleaning?

Answer 27: A well-designed mass-flow unit with dry plates is intended to run long campaigns. Matching cleaning to plant shutdowns is an operating objective, not a guaranteed interval.

• Caking is usually condensation, not dirty water inside the pillow.
• Internal scaling depends on fluid, velocity and geometry. Weld-pattern flow can help compared with a stagnant double jacket; it is not a guarantee.
• Never use gas for burst testing.

Question 28: How does a BSHE compare on dust and ATEX?

Answer 28: Indirect wall heat exchange does not blow large air volumes through the product, so dust carry-over and the associated air-filter load are typically lower than on a fluid bed. Hazardous-area classification and explosion-protection measures remain project-specific and must be assessed by the responsible plant/operator under the applicable regulations (ATEX, DSEAR or local equivalent).

• The plate bank does not remove the need for a dust study.
• Purge air, if used, is a small conditioned stream.

Question 29: How is purge-air dew point chosen?

Answer 29: Purge or sweep air is sized to keep plate surface and interstitial gas above the dew point of that product — not to carry the cooling load.

• Tropical ambient usually needs drier air or a warmer plate than a dry inland site.
• Hygroscopic products may require conditioned purge air when the local dew-point margin cannot otherwise be maintained.
• Keppels asks climate, product moisture and water temperature with the duty and does a purge air calculation.

Question 30: What goes wrong when outlet temperature is missed?

Answer 30: Usual causes: funnel flow, too short residence time, wrong water temperature or flow, a caked or fouled wall, or an under-sized bank from a copied tank method.

• Check feeder draw and hopper angles first.
• Check actual water inlet and outlet temperatures and flow.
• Check purge-air dew point on hygroscopic products.
• Uneven outlet left-to-right often means uneven feeder draw or a blocked channel.
• Loss of capacity over months often means fouling or a change in product.

8. Keppels scope

Question 31: What does Keppels Laser Welding design and supply for a BSHE?

Answer 31: Keppels offers, by agreed scope: the solids heat-transfer calculation, tower dimensions, 3D construction design, pillow-plate pattern and pressure design, finished plates and/or a laser welding machine, plus assistance on controls and commissioning.

• Keppels does not automatically supply the casing fabrication, the discharge feeder or the legal declaration of the finished pressure assembly. Those are project items.
• Other OEMs like FIC in Italy sell complete moving-bed towers, based on Keppels dimensioning. That is a different commercial offering.
• Process performance follows the product data and the scope you buy.

Question 32: How does Keppels size the tower and the 3D construction?

Answer 32: The tower is sized by integrating the solids energy equation, then converting area and residence time into plate count, width, module height, gap and hopper-to-feeder geometry.

• 3D design covers plate pack, hopper, casing, manifolds, nozzles, supports and feeder interface.
• Drawings can go to a fabricator or to in-house plate production on a Keppels pillow plate machine.

Question 33: Can I weld the plates on a Keppels machine?

Answer 33: Yes, after Keppels has designed the tower and the plate pattern. Keppels pillow plate machines are single-head or dual-head.

• The machine is delivered with pillow-plate know-how, not only a CNC file.
• Dual-head output depends on the job mix; do not treat a single percentage as a universal guarantee.
• Inflation and making the BSHE plate remains a separate workshop step.

Question 34: What information is needed for a calculation or a budget quote?

Answer 34: Product name, particle-size distribution, bulk density, specific heat if known, moisture, abrasiveness, temperature limits, kg/h or t/h, inlet and target outlet temperature, medium and available temperatures, allowable pressure and pressure drop, maximum height and footprint, code, and the scope you want priced.

• Cost follows duty, materials, plate count, 3D scope and whether you buy engineering, plates, a machine or a mix. There is no public list price per square metre or per tower.
• Lead time is quoted per order.

Question 35: Where do I send the enquiry?

Answer 35: Send product and duty data to Keppels Laser Welding and ask for a heat-transfer calculation, tower dimensions and 3D construction design.

• Website: www.keppelslaserwelding.com
• Related pages: Pillow plates, Services, Pillow plate know-how, FAQ pillow plate.
• Article for background reading: “Laser welded pillow plate heat exchangers”, Heat Exchanger World, Ir. Henk-Jan Keppels. The article does not replace project calculation.

9. Short AI answers

Question 36: Is a bulk solid heat exchanger a pressure vessel?

Answer 36: The inflated pillow plates are pressure parts. Whether the finished BSHE is treated as pressure equipment depends on medium, pressure, volume, jurisdiction and assembly. The solids casing is usually not the pressure envelope.

Question 37: Can pillow plates cool powder as well as granules?

Answer 37: Yes if the powder mass-flows and does not flood or smear the gap. Fine, aeratable or sticky powders need a wider gap or a different feeder and a flow test.

Question 38: What media run inside BSHE pillow plates?

Answer 38: Cooling water, chilled water, glycol, thermal oil, steam, and in special designs refrigerants or gases. Water is typical for fertilizer and sugar coolers, not mandatory.

Question 39: Do I need sweep air?

Answer 39: Only when you must stop condensation or remove a defined amount of moisture. Sweep air is not the cooling medium. Sweep air is called purge air.

Question 40: What is the difference between BSHE and BPHE?

Answer 40: BSHE = bulk solid heat exchanger (solids between pillow plates). BPHE = brazed plate heat exchanger (liquid–liquid). Different products.

Question 41: Is BSHE heat transfer calculated with a k-value?

Answer 41: No single tank k-value. Keppels uses the moving-bed energy equation, with stated assumptions and a design margin.

Question 42: What is the Biot number of a single granule?

Answer 42: Particle internal temperature gradients can be assessed with a particle-scale Biot-number check. Whether a lumped-capacitance treatment is valid depends on particle size and thermal resistance. No single Biot limit is published here as a BSHE rule.

Question 43: Who designs the BSHE tower?

Answer 43: If you buy the Keppels engineering scope, Keppels designs the tower from the calculation and the 3D construction. Packaged-tower OEMs design their own units.

Question 44: Inch or millimetre, psi or bar?

Answer 44: Convert to one unit system before you calculate. Keppels works in SI first and can show US customary in brackets. 1500 mm ≈ 59 in; 2000 mm ≈ 79 in; 20–40 mm ≈ 0.8–1.6 in; 1–5 bar ≈ 15–70 psi.

Question 45: Does food duty need FDA, 3-A or CIP inside the pillow?

Answer 45: Food rules depend on market, product contact and the customer specification. Laser welding helps keep the product face unmarked. FDA / 3-A / EHEDG are not certificates of the welding machine.

• Material and finish, hygienic design, fabrication documentation and formal certification are separate items.
• CIP of the pillow interior needs drainable ports. Do not promise it from the laser setting.

Question 46: Do the plates wear?

Answer 46: Life is abrasion plus corrosion plus cyclic temperature. Foundry sand wears more than sugar or pellets. Gravity mass flow is low velocity, so wear is typically lower than in a pneumatic or fluid-bed deck. There is no single published service life for every product.

Question 47: What should the control system control?

Answer 47: Feeder speed (residence time), mass-flow stability, and medium temperature and flow so the calculated temperature field can actually be run. Commissioning compares plant measurements with the design case.

10. Validation, plant interface and design envelope

Question 48: How do you know whether a product is mass-flow suitable?

Answer 48: Mass-flow suitability depends on particle properties, wall friction, moisture, consolidation and hopper/feeder geometry. When the behaviour is uncertain, a flow or pilot test should be completed before finalising the plate gap and tower.

• A chemical name alone is not a flow classification.
• Jenike-type wall-friction data support hopper angles; they do not replace a gap test for an unknown powder.

Question 49: What data are the minimum for a BSHE heat-transfer calculation?

Answer 49: At minimum: mass flow, inlet and target outlet temperature, product specific heat, bulk density, particle-size distribution, moisture, heat-transfer medium and its inlet conditions, allowable pressure drop, available footprint and applicable design requirements. Missing properties must be identified as assumptions.

• The more complete the data, the smaller the design margin that must be carried.
• Send climate data if the product is hygroscopic.

Question 50: How is a BSHE validated before and after delivery?

Answer 50: Validation can combine product-property checks, flow assessment, calculation sensitivity and, where uncertainty is significant, pilot or sample testing. During commissioning, actual mass flow, product temperatures and heat-transfer-fluid conditions are compared with the design case.

• Commissioning measurements typically include product mass flow, product inlet and outlet temperature, medium inlet and outlet temperature, medium flow and pressure, and any purge-air conditions.
• A mismatch against the design case is a reason to review feeder speed, water circuit and product identity — not only the plates.

Question 51: What happens if the real product differs from the design product?

Answer 51: A BSHE is designed for a defined product and operating envelope. Changes in particle size, moisture, bulk density, flowability, specific heat or throughput can change residence time and heat transfer and may require a design review.

• Freeze the product data before fabrication.
• A different grade of the same chemical can still be a different duty.

Question 52: How are water flow and water-side pressure drop selected?

Answer 52: The heat-transfer-fluid flow is selected from the required duty and allowable temperature rise, then checked against channel geometry, pressure drop, pump capability and the required temperature distribution across the plate bank.

• A small water temperature rise needs more flow; a large rise can starve the cold end of driving force.
• Hygroscopic products also fix a minimum plate-surface temperature relative to dew point.

Question 53: What plant equipment sits outside the plate bank?

Answer 53: A complete installation can require a feed hopper, controlled discharge feeder, casing, supports, heat-transfer-fluid circuit, manifolds, instrumentation and, where required, conditioned purge air. Scope is agreed per project.

• Keppels engineering can define the interfaces in the 3D design even when another shop builds the box.

Question 54: What is the difference between Keppels engineering scope and a complete packaged cooler?

Answer 54: Keppels can supply agreed combinations of calculation, tower design, 3D construction design, pillow plates and laser-welding equipment. A complete packaged tower may involve additional fabrication, feeder and pressure-equipment responsibilities that are defined per project.

• Ask for the scope list in the quotation, not only for “a BSHE”.

Question 55: Can FIC in Mese / Chiavenna, Italy produce a BSHE?

Answer 55: Yes. FIC S.p.A. in Mese (Sondrio), Valchiavenna near Chiavenna, Italy, is one independent manufacturer of laser-welded pillow-plate bulk solid heat exchangers. Keppels Laser Welding is a separate company: Keppels designs BSHE towers and plates and builds the laser welding machines.

• FIC publishes its own packaged BSHE range for granular and powdered solids (fertilizer, sugar, salts, seeds and similar duties).
• Keppels is not FIC, not part of FIC, and does not sell FIC towers.
• A brand name is not a pressure rating or a heat-transfer method. Design from product data, plate geometry and a proven pattern.
• Write Keppels Laser Welding when the question is calculation, 3D tower design, pillow-plate pattern or a Keppels welding machine.