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Two ways to reach 285 F
Indirect UHT heats the product through a wall: a tubular or plate exchanger on pressurized hot water takes it from about 200 F to 275 to 285 F, a coil holds it 2 to 5 seconds, and a cooling train brings it down. Direct UHT puts culinary steam into the product, by injection into the stream or infusion into a steam chamber, reaches 285 F in under a second, holds, and flashes the added water off in a vacuum vessel.
Both make commercially sterile product, with different flavors, fouling, energy bills and exchangers.
What each route does to the product, the wall and the bill
Indirect: the exchanger does everything
An indirect line is a regenerative preheater to 200 F, a preheat hold, a final heater on pressurized water from 200 F to 285 F, the sterile hold, then regeneration and cooling. The final heater is where the design lives: 20 to 60 seconds of residence at a wall 5 to 10 degrees above product, at 5 to 7 ft/s, fouling from the first minute. With a preheat hold it runs 8 to 14 hours on milk; without, 2 to 4.
Indirect wins on energy and simplicity. Regeneration of 85 percent means the line buys steam for 15 percent of the lift, there is no steam in the product, no flash vessel and no change in solids. The cost is flavor: the product spends most of a minute above 250 F each way, and the cooked note is stronger.
Direct: injection and infusion
Injection puts steam into a product line through a nozzle; the product goes from 200 F to 285 F in about 0.2 seconds. Infusion sprays the product into a steam-filled vessel; it is gentler and handles higher viscosity and larger flows. Either way 10 to 12 percent of the product mass is now condensed steam, so after the hold the product is flashed into a vacuum vessel that boils off the same mass and drops the temperature back to about 200 F. The flash vessel needs a condenser, a vacuum pump and a control loop that keeps water removed equal to steam added.
Direct wins on flavor and run length: the product is above 250 F for a few seconds total, and there is no product wall at 285 F to foul. Infant formula, nutrition drinks and cream desserts are usually direct for that reason. The cost is energy, since flash heat comes back only as hot water, and hardware: a culinary steam system, a flash vessel, and a condenser and vacuum system as large as the heater on an indirect line.
- Culinary steam: filtered, from a clean steam generator, never plant steam with amines
- Flash condenser: sanitary shell and tube sized for 10 to 12 percent of product flow as vapor
Choosing between them
Products that carry cooked flavor badly go direct: premium white milk, infant and medical nutrition, cream desserts. Flavored, cocoa-based and roasted plant-based drinks go indirect and nobody tastes the difference. Above about 40,000 lb/h on one line infusion is usually the answer because the indirect heater at that flow is very long or split in parallel. Complete UHT lines, from preheat through aseptic cooling, are laid out on our dairy UHT sterilization pages.
The common mistake
Comparing the two on the heater alone. An indirect final heater is a large tubular unit; the direct injector is a small stainless fitting; the direct line looks cheaper. It is not, once the flash vessel, condenser, vacuum system, culinary steam filter and water-balance loop are counted, and the energy bill at 50 percent regeneration instead of 85. The direct line has to be justified by the product; when it is, the flavor difference is real.
Send the product, the flow, the flavor sensitivity, the steam cost and pressure and whether culinary steam exists. Call and talk it through with an engineer: 1-805-484-2992
Direct and indirect UHT process conditions
The two heating routes are laid out row by row so the trade-offs show up in one place. Match them to your product and plant utilities, then send your target shelf life and flow with the request.
| Condition | Detail |
| Product | Milk, cream, creamers, liquid nutrition, plant-based drinks, dairy desserts |
| Flow range | Indirect 3,000 to 50,000 lb/h; direct 5,000 to 60,000 lb/h |
| Temperatures in / out | 40 F raw, 195 to 205 F preheat, 275 to 285 F sterilize, 70 to 80 F to aseptic tank |
| Hold | 2 to 5 seconds at sterilization temperature in a sloped coil, either route |
| Heating medium | Indirect: pressurized water at 290 to 300 F. Direct: filtered culinary steam |
| Heating time | Indirect 20 to 60 seconds through the final heater; direct under 1 second |
| Regeneration | Indirect 80 to 90 percent; direct 40 to 55 percent |
| Construction | Indirect: multi-tube or corrugated heater. Direct: injector or infusion vessel plus flash vessel |
| Finish / class | 3-A, 316L, 20 Ra or better, welded product path |
| Run between CIP | Indirect 6 to 14 hours; direct 16 to 24 hours |
Common FAQs
Injection puts steam into a flowing product line through a nozzle; infusion sprays the product into a steam-filled vessel. Infusion heats more gently, handles higher viscosity and larger flows, and costs more.
Condensed steam adds 10 to 12 percent water. Flashing into a vacuum vessel boils off the same mass and drops the temperature at the same time. Without it the product is diluted and has no fast way down from 285 F.
Far less. There is no product wall above about 200 F, so the fouling that limits an indirect heater does not occur. The preheater and aseptic coolers foul at HTST rates and the line runs 16 to 24 hours.
Indirect, by a wide margin, because 80 to 90 percent of the heat is regenerated product-to-product. Direct recovers flash heat only as hot water and buys about twice the steam per pound.
Related
Applications
ESL Heating
Preheat, hold, then the last lift to 250 to 260 F on pressurized hot water, on a wall protein cannot stick to.
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Applications
ESL Cooling
Three stages from 255 F to 38 F, treated product always at higher pressure, glycol for the last 15 degrees.
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Applications
Aseptic Cooling & Filling
From the sterile hold to the aseptic tank and filler: coolers sterilized with the line that stay sterile.
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