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Quotes - Engineering - SalesWaste streams
Recovering heat from water nobody wants to touch
Wastewater carries heat worth recovering and solids that make recovering it difficult. A digester needs its sludge held near 95–100°F and the heat usually comes from engine jacket water through a sludge heater; a food or textile plant discharges effluent at 100–140°F that can preheat incoming process water; a chemical or pharmaceutical plant cools a waste stream before it goes to treatment. In each case the dirty side carries rags, grit, hair, fiber, grease and biological solids, and the exchanger has to pass all of it without being opened every week.
The tube-in-tube unit is the standard answer because the sludge or effluent runs in a 2 to 4 in. inner tube with nothing in the path to catch a rag, the velocity can be held high enough to keep grit moving, and the tubes can be rodded from the end covers when something does lodge. The stainless question is sharper than most people assume. Treated effluent sounds benign and often is not: chlorides of 100–300 ppm are common in municipal flows and run to 1,000 ppm or more wherever water softeners, brine discharges or coastal infiltration contribute; sulfide from anaerobic digestion attacks stainless under deposits; and the temperature of a sludge heater loop sits right where the problems accelerate.
This page is about choosing between 316L and duplex for a waste stream.
Municipal sludge and mild effluent run in 316L; industrial waste with 1,000 ppm chloride or more, sulfide and heat go to 2205 duplex, and 304 is not offered on this duty. Grade limits are set out on Material & Construction.
Treated effluent is not clean water for a stainless tube
Municipal flows carry 100–300 ppm chloride, often more, with sulfide under the deposits, and industrial waste runs higher; 316L serves the mild end in a 2 to 4 in. full-bore tube, and 2205 duplex takes the chloride-plus-heat cases where 316L pits in a year.
What a waste stream does to an exchanger
Sludge at 2–6% solids is a non-Newtonian paste at rest and a fluid once moving, which is why sludge heaters run at 4–6 ft/s and never idle with sludge standing in them. Rags and hair bridge any opening smaller than the tube; grit erodes the bottom of a horizontal run; grease coats a wall colder than about 90°F and insulates it. Fouling is biological and builds in days, so the exchanger is designed on a heavy fouling allowance and on the ability to rod the tube rather than on a clean coefficient.
Chemically the dirty side is a mix: chloride from the water supply and from brine discharges, sulfide and ammonia from digestion, organic acids from fermentation, and whatever the upstream plant puts down the drain. Under a deposit the chemistry at the wall is more concentrated and lower in oxygen than the bulk, which is where pitting and crevice attack start on 316L long before the bulk chloride figure says they should.
The stainless unit for it
For municipal sludge heating and most food-plant effluent the inner tube is 316L, 2 to 4 in. bore in a single tube, smooth rather than corrugated so that rags slide and rodding is possible, with removable return bends and end covers at every pass. Nozzles are flanged, 150 psig at 350°F on both sides, and the annulus carries jacket water, hot water or the clean process stream being heated. A 304L annulus is acceptable when that clean stream is low in chloride.
For industrial effluent above about 1,000 ppm chloride, for any stream carrying sulfide at temperature, and for coastal plants with brackish infiltration, the inner tube goes to 2205 duplex, which resists pitting to roughly 3,000–5,000 ppm chloride and is highly resistant to chloride stress corrosion cracking at loop temperatures where 316L can crack. Its yield strength, about twice that of 316L, also helps where grit erosion thins a tube. Above that chloride range the next step is 6-moly, or titanium for seawater-diluted effluent.
What to send for a wastewater exchanger
A water analysis is the single most useful document on this duty, because it decides the grade. The solids description decides the bore and the fouling allowance.
- Stream type: digester sludge, primary or secondary effluent, process waste, with solids percentage
- Water analysis: chloride, sulfide, pH, ammonia and anything the upstream process adds
- Flow rate and the temperatures in and out, and the clean-side stream being heated or cooled
- Largest solid likely to reach the exchanger: screen size, rag load, grit
- Operating temperature of the loop and whether it ever runs above 140°F
- Access for rodding and bend removal, and the cleaning interval the plant can accept
- Required codes and connections, flanged sizes and any CRN or ASME stamp
Call and talk it through with an engineer: 1-805-484-2992
Questions on sludge and effluent
Carbon steel is used on some digester loops and corrodes steadily in the sulfide and the organic acids, so its life is measured in years and its wall is thick to buy them. A 316L or duplex tube costs more and typically outlasts carbon steel by a wide margin, holds a smoother surface that fouls more slowly, and does not shed rust into the sludge. For a plant that would rather not replace tubes in five years the stainless unit is the cheaper one.
We do not. A sample sent to a laboratory for chloride, sulfide and pH is inexpensive against the cost of a tube that pits in a year, and the result often surprises the plant: municipal flows that were assumed to be under 200 ppm chloride come back at 800. If the analysis cannot be obtained before the quote, we quote duplex as the safe grade and 316L as the alternate.
Bore and velocity. A 3 or 4 in. inner tube at 4–6 ft/s carries rags through without giving them a place to catch, provided every fitting, valve and bend is at the same bore. The failures come from a reducer, a thermowell projecting into the stream or a butterfly valve left half open. Where the plant has fine screens upstream the bore can come down; where it does not, we size for what a coarse screen passes.
Yes. Sulfide and hydrogen sulfide from anaerobic digestion attack stainless under deposits and at crevices, and they lower the chloride level at which 316L begins to pit. A stream with measurable sulfide and more than a few hundred ppm chloride, running above about 100°F, is a duplex duty. Sulfide also rules out copper and nickel brazed constructions on the clean side if there is any chance of carry-over.
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