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Quotes - Engineering - SalesSame Duty, Three Very Different Machines
Give the same 1.16 MMBtu/h milk heating duty to the three sanitary constructions and you get three answers that differ by a factor of four in surface, a factor of ten in footprint, and a factor of two in price, and any of the three can be the right one depending on what else is on the sheet. The plate wins on coefficient and approach. The shell-and-tube wins on pressure, steam and pharma documentation. The tube-in-tube wins the moment the product has pieces in it or will not flow through a plate channel.
The six questions below settle most selections. Answer them in order and the construction is usually clear by the third.
Six questions, in order
- Is the product below about 500 cP everywhere in the unit, with no pieces above an eighth of an inch? If not, plates are out
- Does the duty need an approach under 10 F, or a temperature cross? If yes, plates or tube-in-tube, not a single shell
- Is either side above about 150 psig, or is the utility steam? If yes, shell-and-tube or tube-in-tube
- Is the hygiene class ASME BPE, or is a double tubesheet required? Shell-and-tube, or double-wall plates for food duty
- Will the unit be opened for inspection between CIP cycles? Plates open in minutes; tubes are inspected by borescope
- How much floor space is there? Plates take a fifth to a half of the area of tubes for the same duty
The milk duty in tube-in-tubeRunning near 300 Btu/h-ft2-F, a tube-in-tube needs about 60 square feet in a rack of modules for the same milk heater, which is why it waits for cream with fruit in it.
The Milk Heater Three Ways
Ten thousand lb/h of milk from 40 F to 165 F, hot water at 190 F returning at 175 F, LMTD about 65 F. In a sanitary gasketed plate at a service coefficient of 900 Btu/h-ft2-F the unit is about 20 square feet: a small frame, a few dozen plates, tri-clamp ports of 2 inches, and a pressure drop around 12 psi. In a sanitary shell-and-tube at 250 it is about 72 square feet: a 6 inch shell around 8 feet long with two tube passes, or an 8 inch shell around 5 feet, at 6 to 8 psi on the tube side. In a tube-in-tube on this product it would run near 300 and need about 60 square feet, which is a rack of several modules; it is the wrong choice here and the right one the moment the product becomes cream with fruit.
Change the utility to 150 psig steam and the plate is out because of wall temperature, the shell-and-tube shrinks to a 4 inch shell, and the wall has to be managed with a throttled valve.
- Plate: highest coefficient, closest approach, smallest footprint, opens for inspection, expandable by adding plates
- Plate limits: viscosity, particulates, gasket temperature and pressure, and a single gasket between product and utility
- Shell-and-tube: pressure and temperature capacity, steam service, double tubesheet, seal-welded tubes, full ASME and BPE documentation
- Shell-and-tube limits: wider approach, larger footprint, no temperature cross in a single multi-pass shell
- Tube-in-tube: passes pieces and viscous product, true counterflow, drains and cleans end to end, condenses and evaporates
- Tube-in-tube limits: low surface per foot, long for its duty, usually not the cheapest on thin product
Ask the six questions in order; the construction shows by the third.
- Same milk duty, 20 sq ft of plate or 72 of tube
- Plate gaskets cap pressure at 150 to 230 psig
- Above about 150 psig or on steam, shell-and-tube or tube-in-tube
What Each Construction Costs You and Gives You
The gasketed plate is the default for thin sanitary product because nothing else comes close on coefficient per square foot or on approach. Corrugated plates force turbulence at velocities that would be laminar in a tube, so the coefficient on milk, juice, beer or water sits at 800 to 1200 Btu/h-ft2-F. True counterflow means a 2 to 3 F approach and a temperature cross, which is what makes 90% regeneration a modest frame rather than a room full of shells. The unit opens by backing off the tie bolts, so every square inch of product-contact surface can be seen, which is why plants that inspect between CIP cycles prefer it.
The plate's limits are the gasket and the channel. Elastomer gaskets cap the temperature, typically 300 F for EPDM and less for nitrile, and cap the pressure, usually 150 to 230 psig depending on frame. Each gasket is one barrier between product and utility, and although the vented double gasket around each port makes a port leak visible, a plate crack is not visible until the product tests positive. The channels are narrow, so viscosity above about 500 cP and pieces above an eighth of an inch are out, and wide-gap plates only stretch that so far. Plates are also the construction most sensitive to fouling, because a deposit that is a nuisance in a tube is a blockage in a channel.
The sanitary shell-and-tube is the construction for pressure, temperature and documentation. Tubes are seal-welded or expanded and welded into the tubesheet, the shell takes steam at any plant pressure, and the whole vessel is code-stamped, hydrotested and documented in a way that a validation engineer can file. A double tubesheet, a sloped shell with low-point drains, a 20 Ra electropolished tube side and a full ASME BPE package are all available on the same basic geometry. This is why WFI coolers, clean steam condensers, point-of-use coolers and CIP heaters are almost always tubular.
The shell-and-tube's cost is thermal. Coefficients of 150 to 350 mean three to four times the surface of a plate for the same thin-product duty. A single shell with two or more tube passes is partly parallel flow, carries a correction factor, and cannot deliver a temperature cross, so a 10 F approach is economic and a 3 F approach is not. It takes more floor length, and its product-contact surface is inspected with a borescope rather than by eye. On thin product with a tight approach it is the wrong choice; on steam, pressure, pharma water or anything where the second barrier matters, it is the right one.
The tube-in-tube is a shell-and-tube reduced to its simplest form: one product tube, or a few, inside a jacket, in modules connected by return bends. Because the product path is a single open tube it passes anything that fits through it, and because the modules are in series it is true counterflow and can run a tight approach. It is drainable by design, cleans end to end with no dead legs, and the corrugated version raises the coefficient on products that can take the mixing. It also condenses and evaporates, which is useful in extraction and solvent recovery.
Its cost is length. Surface per foot of a single 2 inch tube is under a square foot, so a 60 square foot duty is a rack of modules, and on thin product it will never be the cheapest answer. But on yogurt with fruit, sauce with pieces, egg, tomato, and anything that would either block a plate or need a positive displacement pump to get through one, it is not competing with plates at all; it is competing with a scraped-surface unit at several times the price.
The sensible procedure is to size the thin-product duties in plates and check the pressure and hygiene questions, size the pressure, steam and pharma duties in shell-and-tube and check the approach, and send everything with pieces or above a few hundred cP to tube-in-tube. Where a duty sits near a boundary, between 200 and 500 cP, or at a 5 to 10 F approach, it is priced both ways. Send the sheet as it is and the comparison comes back with it. Call and talk it through with an engineer: 1-805-484-2992
Plates win on coefficient and approach, shells on pressure and paperwork, tube-in-tube the moment the product has pieces.
Request a QuoteCommon FAQs
At small duties, yes, because a stock 4 inch sanitary shell-and-tube costs less than a plate frame with its gaskets, and at any duty where the plate would need double-wall plates or a heavy fouling allowance. Above a few hundred thousand Btu/h on clean product the plate is usually lower.
Only with shells in series, or a single-pass counterflow shell, which is unusual in sanitary service. A single shell with two tube passes cannot. If the process needs a cross, plates or tube-in-tube are the direct route.
Plates, because the pack opens and every plate can be seen and swabbed. A shell-and-tube is inspected with a borescope through the heads, and a tube-in-tube through its return bends, both of which are routine but not a visual of the whole surface.
Shell-and-tube with steam on the shell side, or tube-in-tube with steam in the jacket, both throttled to control wall temperature. Plates are used on steam only at low pressure with a gasket rated for it, and are not the first choice.
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