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Quotes - Engineering - SalesWater with nothing in it, and what that does to steel
Reverse osmosis strips 95 to 99 percent of dissolved ions and leaves permeate at 5 to 20 microsiemens per centimeter. Deionization by ion exchange takes it below 1 and demineralization by mixed bed and polishing takes it to 0.055, the theoretical limit. What is left is water that is far from equilibrium with everything it touches. It dissolves carbon dioxide from the air and turns mildly acidic, it pulls iron and chromium out of a poorly passivated surface, and any chloride that reaches it, from a fitting, a gasket, a cleaning chemical or the feed, has no competing ions to dilute its attack on the passive film.
The heat exchanger sees this water hot, which multiplies every effect. An RO feed heater lifts raw feed to 77 F for membrane flux; a DI loop cooler holds a laboratory or electronics loop at 68 to 72 F; a demineralized water heater feeds a boiler or a still; a rinse water temperer supplies 100 to 140 F water for equipment washing. In each case the metal in contact with the water is 316L at minimum, welded with matching filler and passivated so the chromium oxide film is complete before the water arrives. Where the water is hot and low-conductivity for long periods, or the chloride at the feed exceeds a few hundred ppm, a duplex stainless or a higher alloy is the honest choice.
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316L is the floor; hot or chloride-heavy water steps up to duplex
At 0.055 microsiemens the water has no competing ions, so any chloride attacks joints, and a duty safe at 70°F can pit at 160°F. The 2 to 3 percent molybdenum in 316L doubles chloride tolerance; feed over 200 ppm at temperature means 2205 duplex.
RO feed and permeate
- Feed heater to 77 F for design flux, 3 percent flux per degree
- Permeate cooler where downstream tanks must stay below 80 F
- Feed chloride and hardness known before material is chosen
- 316L plates or tubes; duplex on brackish or high-chloride feed
- Gaskets rated for the RO cleaning chemistry
DI and demineralized loops
- Loop cooler removes pump heat and holds 68 to 72 F
- Heater to 140 F for hot rinse or still feed
- Fully drainable so the loop can be sanitized
- Double tubesheet where the utility is plant steam or tower water
- Electropolish where the loop feeds WFI or semiconductor rinse
Recommended construction by water purity and duty
The purity and temperature pick the metal; the flow and the utility pick the geometry. Nothing here is exotic, but nothing here is carbon steel or 304 either.
| DUTY / CONDITION | RECOMMENDED CONSTRUCTION | WHY |
| RO feed heating to 77 F, large flow, low temperature rise | Gasketed Plate | Cheapest area for a large water-to-water or steam-heated-water duty; 316L plates, opens for membrane-cleaning chemical inspection |
| DI or demineralized loop cooling on tower or chilled water | Straight Tube | Fully drainable, brushable, 316L tubes welded and passivated; small approach, long life |
| Demineralized water heating on plant steam for still feed or hot rinse | U-Tube | Steam on the tube side, water on the shell, free expansion; bundle pulls for inspection of the water side |
| High-purity water against plant steam or tower water where a leak cannot be tolerated | Double Tubesheet | Vented gap between the two fluids; the standard for any loop that feeds pharmaceutical or semiconductor use |
| Hot low-conductivity water above 140 F, or feed chloride above 200 ppm | Straight Tube | Same geometry in 2205 duplex or a higher alloy; HeatX prices the alloy step on the same quote |
| Small laboratory or pilot DI loop, compact trim cooling | Double-Wall Plate | Leak visibility between plates on a small duty; not for hot service or SIP |
Material selection for RO, DI and demineralized water
The corrosion of stainless steel in high-purity water is not general wasting; it is pitting and crevice attack at the places where the passive chromium oxide film is thin or broken: tube-to-tubesheet joints, gasket grooves, weld heat-affected zones, and any spot where a chloride ion can concentrate. Low-conductivity water makes the attack worse because there are no other ions to compete for the surface, and it makes the water a poor conductor so that a pit, once started, is not repassivated by the surrounding metal. Temperature accelerates all of it: a duty that is safe at 70 F can pit at 160 F on the same water.
316L is the floor. Its 2 to 3 percent molybdenum roughly doubles the chloride tolerance of 304, its low carbon keeps the weld heat-affected zone from sensitizing, and it electropolishes and passivates well. On RO permeate and ambient DI loops it lasts indefinitely if the passivation was done and the surface is not damaged in fabrication. HeatX passivates every high-purity water exchanger to ASTM A967 after fabrication and ships the certificate.
The step past 316L is taken on three triggers: hot low-conductivity water for long periods, which is why still feed heaters and hot rinse heaters on DI water are candidates for 2205 duplex; chloride in the feed above about 200 ppm at elevated temperature, which is common on brackish RO feed and puts the feed heater into duplex; and any crevice the design cannot avoid, such as a rolled tube joint on a unit that must have a removable bundle. Duplex costs 30 to 50 percent more than 316L in a shell-and-tube and is the right price for a unit that will otherwise be replaced in five years.
Conductivity, chloride and temperature decide the metal
RO feed heaters and the flux gain
Membrane permeate flux rises about 3 percent per degree Fahrenheit, so a plant with 50 F winter feed gains roughly 80 percent capacity by heating the feed to 77 F. The heater is a plate or shell-and-tube on hot water or steam, sized on the feed flow and the coldest feed of the year, in 316L for the feed side because the feed carries whatever chloride the source water has and has been concentrated by nothing yet. Where the source is brackish, duplex is the safer plate material.
Loop coolers and pump heat
A recirculating DI loop picks up the pump's heat continuously; a 20 hp pump on a 200 gpm loop adds 1 to 2 degrees per pass, and without a cooler the loop drifts to 100 F in a day. The loop cooler is a small duty with a small approach on chilled water and runs forever; the trap is a cooler sized on the duty with a utility approach so tight that tower water in August cannot hold it. Size it on the warmest utility.
Semiconductor and laboratory ultrapure loops
Ultrapure water at 18 megohm for wafer rinse or analytical use is handled the same way with a stricter finish: 316L electropolished, passivated, hydrogen-peroxide sanitizable, and sometimes with a polymer-lined or titanium alternative where the site's metals specification excludes stainless in contact with the water. HeatX quotes 316L and titanium tube options on the same request when the site standard asks.
What to send for a quote
Water type and conductivity, feed chloride if RO, flow, inlet and outlet temperatures, utility available, whether the unit must drain fully and be brushable, the finish the site expects, and any metals restriction. With those a 316L quote and its alloy alternative come back together. Call and talk it through with an engineer: 1-805-484-2992
Common FAQs
On a cold, low-flow, non-critical loop it will survive for a while, but the cost difference to 316L on an exchanger is small and the failure mode is pitting at welds that shows as iron in the water. HeatX quotes 316L on all high-purity water product sides.
Hot low-conductivity water above about 140 F for long periods, feed chloride above roughly 200 ppm at elevated temperature, or a crevice the design cannot avoid. 2205 duplex is the usual step; higher alloys are rare on water duties.
If the permeate feeds a pharmaceutical or semiconductor system, or the utility side is plant steam or tower water and a leak would contaminate a loop, yes. On a plain RO feed heater with hot water on the utility side a single tubesheet is normal.
It removes free iron left by fabrication and thickens the chromium oxide film so the surface starts life protected. Without it a new 316L exchanger will shed iron into high-purity water for weeks and may pit where a grinding mark left the film thin. HeatX passivates to ASTM A967 and certifies it.
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