Boiler Feed Water: Softener, RO or DM Plant?
Which boiler feed water treatment you need, by boiler pressure and feed water quality: when a softener is enough, when RO pays for itself and when DM is required.
By Nishu Enterprises engineering team
Published · 7 min read
Boiler feed water treatment has two jobs: stop scale forming on the heating surfaces, and keep dissolved solids low enough that the boiler does not need constant blowdown. How far you need to go depends mainly on the boiler's operating pressure and on how much dissolved solids your raw water carries.
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The three options in one line each
- Softener: removes hardness (calcium and magnesium), which is what forms scale. Leaves total dissolved solids (TDS) almost unchanged.
- RO plant: removes most dissolved solids, hardness included. Cuts blowdown sharply. Usually needs a softener or antiscalant in front of it.
- DM plant: removes practically all dissolved ions, including silica. Produces water of very low conductivity for high-pressure boilers.
Choosing by boiler pressure
The bands below are a rule of thumb for Indian industrial boilers. The actual limits come from your boiler maker's feed water specification and the relevant Indian Standard, so check them before you buy.
| Boiler pressure | Usual feed water treatment | Why |
|---|---|---|
| Low, up to about 20 kg/cm² | Softener, with deaeration and chemical dosing | Hardness is the main risk. Add RO if raw water TDS is high, to cut blowdown. |
| Medium, about 20 to 40 kg/cm² | Softener plus RO, or a DM plant | Dissolved solids and silica start to matter. |
| High, above about 40 kg/cm² | DM plant with mixed bed, or RO followed by mixed bed | Very low conductivity and silica are needed to protect tubes and turbines. |
Why TDS decides the blowdown
As a boiler makes steam, dissolved solids stay behind and concentrate. To keep them below the boiler's limit, some water is blown down to drain, and with it the heat and treatment chemicals it contains. The blowdown needed is roughly the feed water TDS divided by the boiler water TDS limit.
| Example (boiler water limit 3,500 ppm, assumed) | Feed TDS | Blowdown as share of feed |
|---|---|---|
| Softened borewell water | 500 ppm | About 14% |
| RO-treated water | 50 ppm | About 1.4% |
Every percent of blowdown is hot, treated water sent to drain. On a boiler running round the clock, cutting blowdown from 14% to under 2% saves fuel, water and chemicals every day. That saving is often what pays for an RO plant ahead of a low-pressure boiler.
A softener alone stops scale but does nothing for blowdown. If your raw water TDS is high, ask for the blowdown with and without RO before deciding.
Do not forget corrosion
Scale is only half the problem. Dissolved oxygen and carbon dioxide in feed water corrode boiler tubes and condensate lines. Deaeration and correctly dosed oxygen scavenger and pH control chemicals are needed whatever treatment plant is chosen. Returning as much clean condensate as possible also reduces make-up water and treatment load.
What to send for a proposal
- Boiler make, type, capacity in tonnes per hour of steam and operating pressure.
- The boiler maker's feed water and boiler water specification.
- A raw water analysis including TDS, hardness, alkalinity and silica.
- How much condensate returns, and the make-up water needed per hour.
- Current problems: scale, tube failures, high blowdown or high fuel use.
With those numbers, the choice between softener, RO and DM becomes a calculation rather than a guess, and the running-cost difference between them can be shown before you buy.
Next step
Need this specified for your site?
Send us your water analysis and daily requirement and we will size the plant before quoting.
