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Quotes - Engineering - SalesFruit puree
Heating caustic inline for a puree pasteurizer
Fruit puree is thick, sweet and acidic, and it is pasteurized in tube-in-tube or scraped-surface exchangers at 190°F or more. Where the product film slows against the hot wall it caramelizes: sugar browns, pectin sets around it, and fiber and seed fragments from the puree get locked into the layer. The result is a soil that behaves like candy on the hottest few feet of the pasteurizer and like a gel everywhere else. Fruit acid keeps the mineral low, but calcium from the water still deposits under the caramel on hard-water sites.
Puree lines are often cleaned once-through rather than from a hot tank. The pasteurizer, the balance tank and the aseptic or hot-fill filler form a single 3-inch circuit at about 70 gpm, and the plant pushes caustic through it from a makeup tank, heating it inline so the chemical reaches the pasteurizer at full temperature from the first minute. Sodium hydroxide runs at 1.5–2% and 160–165°F for 30–45 minutes; phosphoric at 1% and 130°F follows a few times a week.
Taking 70 gpm from a 100°F makeup to 165°F inline is 65°F of rise, about 2.3 MBtu/h, for the length of the caustic step and then nothing.
Caustic at 165°F from the first minute, not after the tank warms
An inline heater on a 70 gpm puree circuit delivers 2.3 MBtu/h the moment the step begins, so the caramel on the pasteurizer sees hot caustic for the whole 30–45 minutes. A tank heater that is still climbing gives away the first third of the wash.
The circuit: caramel, pectin gel, 165°F once-through
The puree pasteurizer wash is a long caustic step aimed at one place: the hottest section of the exchanger where caramelized sugar bonds to the wall. Caustic at 1.5–2% and 160–165°F dissolves the sugar and swells the pectin around it over 30–45 minutes; the scraped-surface units are run with the dasher turning so the blades help. Below 155°F the brown layer survives the wash, and the next run cooks a fresh layer onto it.
Once-through cleaning means every gallon of caustic passes the heater exactly once at line flow before it reaches the pasteurizer. The return goes to drain or to a recovery tank at 140–150°F, where a plate exchanger can hand that heat to the next batch of makeup water. The acid step uses the same inline path at a lower set point.
The heater: a U-tube sized for line flow and a wide turndown
We recommend a U-tube shell-and-tube with steam in the tubes and caustic on the shell side, sized for 2.3 MBtu/h at 70 gpm with the solution-side fouling allowance of 0.001–0.002 for pectin carried in the return. Steam at 100 psig condenses at 338°F, and the large approach to 165°F keeps the bundle short enough to fit beside the pasteurizer. Shell, bundle and heads are 316L; the head gasket is EPDM, or FKM where nitric replaces phosphoric. Tri-clamp connections to 4 inches and a drainable shell suit the sanitary loop most puree lines run.
Inline duty needs turndown. The same heater that lifts 65°F for the caustic step lifts 30°F for the acid step and almost nothing on a warm rinse, so the control valve is specified for the full range and a temperature sensor sits at the heater outlet, not at the pasteurizer.
What to send for a puree line heater
An inline heater is defined by it's flow and its two temperatures. Send these and we size the bundle, name the control valve range and tell you the steam the step will draw.
- Circuit flow in gpm and the line size
- Makeup water temperature entering the heater and the target at the pasteurizer
- Caustic strength and the acid used
- Steam pressure at the heater location
- Once-through to drain, or return to a recovery tank
- Tri-clamp or flanged ends, and the finish the loop is held to
Call and talk it through with an engineer: 1-805-484-2992
Puree line questions
A puree pasteurizer soil is concentrated on a few feet of hot tube, and the wash works best when caustic arrives at full temperature from the start. A tank heated to 165°F cools 10–15°F in a long line and a cold pasteurizer before it recovers; an inline heater just upstream of the pasteurizer holds the set point at the inlet. Plants with short lines and a hot tank do fine either way.
Not on steam, in our view. The caustic step starts and stops every cycle, and steam-side temperature swings of 200°F or more at each start shorten plate gasket life. A U-tube bundle with steam inside the tubes has no gasket in the steam path. The plate pack is the right tool on the recovery side, where 140–150°F return water warms the next makeup batch.
Some does, as pectin gel and fines carried back from the pasteurizer after the pre-rinse. On the shell side of a U-tube it collects on the first rows and in the baffle corners, and the fouling allowance covers it between pulls. A strainer on the heater inlet catches seeds and skin; the removable bundle lets you clear the rest. Narrow plate channels would not forgive the same solids.
About 2.3 MBtu/h, or roughly 2,500 lb/h at 100 psig, for the 30–45 minutes of the caustic step, and then little or nothing until the acid step asks for a third of that. The boiler sees a step load each cycle. If several puree lines wash at once, tell us, and we size each heater knowing the header may sag.
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