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Quotes - Engineering - SalesThree Numbers Decide the Surface Area
Every sanitary exchanger, whatever its construction, is sized by the same relation: surface area equals duty divided by the overall heat transfer coefficient times the log mean temperature difference. Duty comes from your flow and temperatures. The temperature difference comes from how close you can afford to run the product to the utility. The coefficient comes from the construction and from what the product does to the wall over a run.
This branch walks through those three numbers in order, with real dairy, juice and pharma cases, so that what you send for a quote is a duty and a set of constraints rather than a guess at a model number.
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What This Branch Covers
What size do I need?
- The duty equation with worked numbers, what LMTD means for a counterflow unit, and how a realistic U-value and fouling allowance turn that into square feet for plate, shell-and-tube and tube-in-tube.
Flow, temperatures and regeneration
- How to state flow so it is not misread, why the approach temperature is a cost decision, and how 85 to 95% regeneration on a pasteurizer shrinks the heating and cooling sections and the utility bill with them.
Viscosity and particulates
- Where gasketed plates stop being the right answer, the viscosity and particle-size thresholds that send a duty to wide-gap plates or tube-in-tube, and how to budget pressure drop on a product that thickens as it cools.
Surface area is duty divided by U times LMTD. Get the duty and the approach right and the construction chooses itself.
Send flows and temperatures, not a model number, and the size falls out.
Duty first, always
Take 10,000 lb/h of whole milk heated from 40 F to 165 F. At a specific heat of 0.93 Btu/lb-F the duty is 10,000 times 0.93 times 125, or roughly 1.16 MMBtu/h. That figure does not care whether the unit is plates or tubes; it is what the utility has to deliver and what the surface has to pass. Juice at 0.90, cream at 0.85 and 40% syrup near 0.75 give lower duties for the same flow and temperatures, so quote the specific heat if you know it and the product name if you do not.
The temperature difference is a choice
Log mean temperature difference is set by how close the product outlet is allowed to approach the utility inlet. A gasketed plate pack in true counterflow will run a 2 to 3 F approach, which is why a regeneration section can recover 90% of the heat. A sanitary shell-and-tube is normally sized at 10 F or more, and a hot water loop at 190 F heating milk to 165 F leaves 25 F, which is comfortable. Tighten the approach and surface area rises steeply; loosen it and the utility has to run hotter or colder.
U-values by construction
Sanitary gasketed plates on water-like product run 800 to 1200 Btu/h-ft2-F because the corrugations force turbulence at low velocity. Shell-and-tube on the same product runs 150 to 350, with the higher end on multi-pass units at good tube velocity. Tube-in-tube on viscous or particulate product runs 100 to 250, and the point of it is not the coefficient but the fact that the product gets through at all. Dairy fouling is real: a fouling factor of 0.0005 to 0.001 h-ft2-F/Btu on raw milk can take 20 to 40% off a plate unit's clean coefficient by the end of a run, so the size has to carry it.
Where sizing goes wrong
The usual failures are sizing on the clean coefficient, sizing on the lightest product in the changeover schedule, and forgetting that the utility side has a flow limit of its own. A chilled water loop that can only spare 60 gpm will not cool 10,000 lb/h of milk to 38 F no matter how much surface you buy. State the utility flow available, or the pressure and temperature of the steam, on the same sheet as the product. Call and talk it through with an engineer: 1-805-484-2992
Common FAQs
For the duty, yes. For the surface area you also need the utility temperatures, the product viscosity and the hygiene class, because those set the temperature difference, the coefficient and the construction. Send all of it and the size is a calculation rather than an estimate.
Smaller, usually. Cheaper, not always. A plate unit with a high coefficient may need more plates than expected once a dairy fouling allowance is applied, and a viscous product that gives a low coefficient in a tube may still be cheaper there because it cannot run in a plate pack at any pressure drop.
Ten to fifteen psi across a gasketed plate unit and five to ten psi on the tube side of a shell-and-tube are normal starting points. Give us the pump head you have available and we will size within it, which sometimes means a longer, narrower unit rather than a shorter, wider one.
At 90% regeneration a pasteurizer raising milk from 40 F to 165 F recovers about 112 F of the 125 F rise in the regenerator, leaving the heating section to add only 13 F or so. The heater becomes a small section on the end of a large regenerator, and the hot water or steam demand drops by the same ratio.
Size for the maximum flow at the worst-case product and check that the unit still holds temperature at the minimum. A unit that is oversized for the low case can overshoot and burn on, so tell us the turndown range and we will pick the utility control to suit it.
Related
Sizing & Selection
What Size Do I Need?
Duty, log mean temperature difference and a realistic U-value, worked through on a milk heater.
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Sizing & Selection
Flow, Temperatures & Regeneration
State the flow so it cannot be misread, choose the approach on purpose, and let the product heat itself.
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Sizing & Selection
Viscosity & Particulates
The two product properties that move a duty from plates to a wide gap to a tube.
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