1-805-484-2992
Quotes - Engineering - SalesBoth sides of the membrane
The membrane sets the feed temperature ceiling and the permeate sets the metal
A reverse osmosis system pushes feed water through a semipermeable membrane at 150 to 300 psi and rejects 95 to 99 percent of dissolved salts, metal ions and microbes. Two exchangers sit around it. The feed heater lifts cold source water toward 77 F and must never send it above the membrane limit of 113 F. The permeate exchanger handles what comes out: water at 5 to 20 microsiemens, slightly acidic because carbon dioxide passes the membrane, carrying the few ppm of chloride the membrane let through.
This page covers the two duties, the membrane limits, and why a plate that is fine on the feed may be the wrong metal on the permeate.
Feed heaters and permeate exchangers in practice
The feed heater and the membrane limit
Polyamide membranes are rated to 113 F continuous and the warranty usually stops at 95 to 100 F. A feed heater on steam can pass that at the wall while the mixed outlet reads 77 F, because a tube touching 300 F steam has a film temperature far above the bulk. HeatX heats RO feed with hot water at 140 to 180 F, or with steam only through a hot water loop, and puts a high-limit switch on the outlet that closes the utility valve at 95 F.
The feed is raw water with everything in it, so the feed heater is not a high-purity exchanger. A gasketed plate in 316L with EPDM is the normal choice; it opens for inspection and its plates are cheap to replace if the source water scales.
What the permeate carries with it
Permeate is not empty. The membrane rejects chloride at about 98 percent, so a 500 ppm chloride feed makes 10 ppm chloride permeate, and a brackish 2,000 ppm feed makes 40. Carbon dioxide passes the membrane untouched, so the permeate leaves at pH 5.5 to 6.5 as weak carbonic acid with no alkalinity to buffer it. Add heat and that water pits 304 at welds and crevices within a year.
The permeate side is therefore 316L at minimum, welded and passivated, with the alloy step to 2205 duplex taken when the permeate is heated above 140 F or when the feed chloride is high enough that the permeate itself passes 20 ppm.
- No copper-brazed plates anywhere on the permeate; copper dissolves into low-conductivity water
- No brass or bronze fittings, gauges or valves on the permeate side
- Gaskets on a permeate plate: EPDM for peracetic sanitizing, not nitrile
Permeate coolers and hot rinse heaters
The high-pressure pump adds 1 to 2 degrees per pass and a concentrate recycle adds more, so permeate leaves warmer than the feed, and a storage tank that must stay below 80 F needs a small permeate cooler on chilled water. Where permeate is heated for hot rinse or still feed, a 316L U-tube with hot water in the tubes and permeate on the shell expands without a bellows and drains through a low-point port so the permeate never sits.
Portable and skid-mounted RO units carry the same exchangers scaled down.
The common mistake
Sizing the feed heater on the average feed temperature. The plant needs 77 F in February when the source is 40 F, a 37 degree rise, not the 15 the average implies. The heater runs out of area, the operator opens the steam valve, and the feed sees 110 F at the wall on the first warm day. Size on the coldest feed, heat with hot water, high limit on the outlet. Call and talk it through with an engineer: 1-805-484-2992
RO water process conditions
Reverse osmosis water is aggressive and the rows record what the exchanger is built to handle, including the chloride limit. Compare them with your system and send your water analysis and flow with the request.
| Condition | Detail |
| Fluid | Feed: softened or filtered raw water, 200 to 2,000 ppm TDS. Permeate: 5 to 20 microsiemens, pH 5.5 to 6.5 |
| Flow range | 20 to 1,500 gpm feed; 15 to 1,000 gpm permeate at 65 to 75 percent recovery |
| Temperatures in / out | Feed 40 to 60 F in, 77 F out, never above 100 F; permeate cooled to 70 F for storage or heated to 140 F for hot rinse |
| Hold | None |
| Utility | Hot water at 140 to 180 F on the feed heater; steam only through a hot water loop; chilled or tower water on the permeate cooler |
| Approach | 10 degrees on the feed heater; 5 to 8 on a permeate cooler |
| Construction | Gasketed plate on the feed; straight tube or plate in 316L on the permeate; double tubesheet where permeate feeds a purified water system |
| Finish / class | Feed side 304 or 316L mill finish; permeate side 316L, 32 Ra or better, passivated |
| Chloride | Feed chloride known; permeate chloride taken as 2 percent of feed |
| CIP / SIP | Feed heater cleaned with the membrane cleaning chemistry; permeate side sanitized with peracetic acid or hot water |
Common FAQs
Flux rises about 3 percent per degree, so 40 F winter feed makes roughly half the permeate that 77 F feed does through the same membranes. The heater is cheaper than doubling the membrane array.
Only through a hot water loop or with a plate designed and interlocked so the wall never exceeds the membrane limit. Direct steam in a shell-and-tube on the feed will find 113 F at the tube wall on a low-flow day.
Mildly acidic from carbon dioxide, low in conductivity and carrying about 2 percent of the feed chloride. Cold, it is fine in 316L. Hot, it pits 304 and argues for duplex above 140 F or above 20 ppm chloride.
Where the permeate is a product ingredient or feeds a purified water system in a food or dairy plant, the permeate side is built to 3-A practice. On a boiler feed or a rinse duty, sanitary construction without the symbol is usual.
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