Clean Steam Generators

Specifying and operating a clean steam generator

Why a U-tube reboiler

Plant steam at up to 150 psig condensing inside the tubes runs 40 to 100 F hotter than the boiling feedwater around them, and the unit cycles from cold every time it is started. A U-tube bundle takes that differential without a bellows and takes the cycling without fatigue, and it can be withdrawn from the shell for inspection of the boiling surface, which is where scale would form if the feedwater ever went out of spec. The tubes are 316L, seal welded to the tubesheet on the clean side, and the shell interior is polished because it is the product-contact surface.

Feedwater is the whole game

The steam is only as clean as the water it was made from. 3-A 609 accepts softened or RO water for culinary steam and lists the additives that may and may not be present; the generator concentrates whatever the feedwater carries, so blowdown is sized to hold shell-side conductivity below the point where carryover becomes a concern. Pure steam needs purified water or WFI, because USP judges the steam by its condensate: conductivity, TOC and endotoxin must meet WFI limits. That in turn means 316L throughout the clean side, electropolish where specified, and a sample cooler on the outlet so the condensate can be tested.

• Culinary: softened or RO water, blowdown for conductivity
• Pure steam: PW or WFI feed, condensate tested to WFI
• Feedwater pump and level control sized for the swing
• Sample cooler on the clean steam outlet

Separation, dryness and carryover

Steam leaving a boiling surface carries droplets. A vertical separator section above the bundle, with a mesh pad or a cyclonic inlet, drops those back into the shell so the delivered steam is dry, typically better than 95 percent dryness at the outlet. The separator is sized on vapor velocity at the lowest operating pressure, since that is where specific volume and velocity are highest. Level control matters for the same reason: a high level floods the separator and carries water over, a low level uncovers tubes and scales them. A gauge glass, level transmitter and high and low switches are part of the package.

Drainage, cleaning and documentation

The shell drains through a sanitary valve at the bottom, the separator drains back to the shell, and the clean steam line is pitched to drain toward the generator so condensate never pools. Passivation after fabrication and a periodic acid wash for culinary units on softened water keep the boiling surface clean; pure steam units on WFI generally need no chemical cleaning but are inspected on a schedule. The package ships with the U-stamp, material certifications, weld maps, surface finish readings and, for pure steam, the documentation a validation protocol expects.

Clean steam is a well-understood piece of equipment when the feedwater, the separator and the level control are specified together. Call and talk it through with an engineer: 1-805-484-2992 Condensing the clean steam once it has done its work is covered on our clean steam condenser pages.