Viscosity and Particulates

Coefficients, Pressure Drop and Particle Passage

Viscosity attacks the coefficient twice. It thickens the laminar film at the wall, and it lowers the Reynolds number so the flow stops being turbulent at all. A gasketed plate that gives 1,000 Btu/h-ft2-F on milk will give 300 on a 200 cP syrup at the same flow, and 150 on a 500 cP one, and to hold even that the pressure drop rises several times over. Somewhere around 500 cP the pump head needed to keep a plate channel working exceeds what a sanitary centrifugal pump delivers and a positive displacement pump is needed, and at that point the plate has lost its main advantage. Where the flow curve of a product is not known, a sample can be tested; see rheology testing on our tube-in-tube site.

A wide-gap plate buys some room. Its channels are two to three times the width of a standard plate, its coefficient is lower, and it passes pulp, seeds and small fibers that would bridge a standard channel. It is the right answer for citrus juice with pulp, fruit purees without pieces, and moderately viscous products in the low hundreds of cP where the plate's counterflow and close approach are still wanted. It is not the answer for diced product or for anything above a thousand cP.

Tube-in-tube handles what plates cannot because the inner tube is a single open passage, typically 1 to 3 inches, with nothing to bridge and nothing to shear the product against. The coefficient on viscous product is 100 to 250 Btu/h-ft2-F, sometimes less on a paste, so the surface is large for the duty and the unit is long. That length is the price of passing the product intact and cleaning it with a CIP flush that goes end to end with no dead spots. A corrugated inner tube adds turbulence at the wall and can raise the coefficient by 30 to 50% on products that tolerate the mixing.

Particulates are sized on their largest dimension, not their average. A sauce with quarter-inch diced peppers and the occasional half-inch piece needs an inner tube that passes the half-inch piece with room to spare, which in practice means an inner tube of 1.5 inches or more. Multi-tube modules, with several smaller tubes in one shell, increase surface per foot but each small tube must still pass the largest piece, so the piece size sets the minimum tube diameter and the surface follows from that.

Pressure drop is a budget, and on viscous product it is the tightest budget on the sheet. Across a plate pack 10 to 15 psi is normal on thin product; on a cooler that finishes at 2,000 cP it may not be achievable at any plate count. On a tube side 5 to 10 psi is the target, and on a viscous product it is reached by shortening the series path and adding parallel modules rather than by opening the tube up, because a larger tube at the same flow drops velocity and coefficient together. Give us the discharge pressure of the pump feeding the exchanger and the pressure the downstream equipment needs, and the unit is sized inside that window.

Fouling and viscosity go together. Thick product moves slowly at the wall, the wall runs closer to the utility temperature, and burn-on starts. On a steam-heated tube-in-tube the fix is a lower utility temperature or a hot water loop, and a longer unit; on a plate the fix is usually to leave the plate behind. Products that gel, set or crystallize as they cool, starches, pectin, some fats, need the outlet temperature held above the set point or a scraped surface unit, and are worth a call before anything is sized.

The common mistake on viscous product is to size the exchanger for the liquid and forget the pieces, or to size it for the room-temperature viscosity and be surprised at the cold end. The second common mistake is to reject tube-in-tube because it looks large, then spend the difference on plate gaskets and a positive displacement pump that was not in the budget. State the viscosity at both ends, the largest piece, and the pump you have, and the construction follows. Call and talk it through with an engineer: 1-805-484-2992