Impurities in water add significantly to the cost of effectively operating a boiler. As steam is lost and make-up water is added to the boiler, the concentration of impurities in the water "cycles up," quickly moving beyond the concentration at which they can be adequately treated by chemicals. To prevent this, high-solids water must be bled from the boiler ("blowdown") to maintain a reasonable level of solids in the boiler water. Unfortunately, blowdown - in addition to releasing high solids - also releases money that has been invested in the water. Specifically, the heat energy that brought the water up to temperature and the chemicals required to treat the water are lost during the blowdown process.
In determining whether a water treatment system investment makes sense in a boiler-feed application, you must consider the following potential sources of value:
- Reduced blowdown of high temperature water
- Improved heat transfer efficiency within the boiler (difficult to quantify and often treated as a side benefit)
- Reduced blowdown volume causes reduction in make-up water volumes and discharge volumes
- Reduced concentration of chemicals within the boiler, corresponding to the reduction of solids within the water
- Reduced chemical additions, corresponding to the reduction in blowdown volume
- Clean water within the boiler eliminates scale formation, and the resulting maintenance steps required to remove scale build-up
The concentration of solids allowable in the boiler water is dependent on the operating pressure of the boiler. As a general guideline, The American Boiler Manufacturing Association publishes the following table:
| Boiler Pressure - psi | Total Solids - ppm |
| 0-300 | 3500 |
| 301-450 | 3000 |
| 451-600 | 2500 |
| 601-750 | 2000 |
| 751-900 | 1500 |
| 901-1000 | 1250 |
| 1001-1500 | 1000 |
Using these guidelines, the most cost-effective water treatment system for a low pressure boiler may be softening, while a high pressure boiler using the same raw water source may require softening, double-pass RO, and EDI to provide adequate water quality.
In terms of creating value toward generating an attractive ROI, the RO permeate in this example is responsible for several significant cost-saving changes in boiler operation. The solids within the boiler have been significantly reduced, even at 50 cycles of concentration, allowing for corresponding reductions in the amount of boiler treatment levels required to treat these solids. In addition, the blowdown volume has been reduced from 22.2 gpm to 4.1 gpm, reducing the amount of treatment chemicals that are released from the boiler. Taken together, the lower level of solids within the boiler and the reduced blowdown rate will typically result in chemical treatment savings of 80-90%. The reduction in volume of high-temperature blowdown also results in significant energy savings. In addition, the "cleaner" water within the boiler will show a much lower tendency to form scale, resulting in improved heat transfer efficiency.