Boiler feedwater is comprised of two very important, and increasingly expensive resources, steam condensate return and city (or well) makeup water. The condensate is valuable due to low electrical conductance, purity and high temperature. The makeup water is valuable because it is being overused and under conserved in many parts of the country, making the cost of water rise in almost all parts of the United States.
Historically, when it came to increasing the quality of boiler feedwater, there have been few alternates; either soften or demineralize the makeup or increase the condensate return. In recent years, the efficiency of Reverse Osmosis (RO) has improved significantly in quality of water produced, quantity of water throughput, and cost of operating the system. The economics of using reverse osmosis can make sense for many boiler installations, just as softener systems did many decades ago.
Although the use of a reverse osmosis system should be individually considered, there are many economic and operational advantages to using this technology. Some examples include:
- Substantial increase in boiler cycles of concentration (reduced blowdown) and reduced heat loss
- Lower makeup water use
- Removal of over 98% of dissolved solids in the makeup – including scale forming salts that lower heat transfer
- Increased boiler efficiency through minimized boiler deposits
- Reduction of feedwater alkalinity – with the direct result of lower neutralizing amine use
- Fuel cost reduction, less makeup water to heat and cleaner heat transfer surfaces
- If a cation exchange softener is used ahead of the RO unit, the concentrate output from the RO may be of high enough quality to use as make up for an evaporative cooling tower system or other process water systems
- Initial cost of the equipment
- RO membranes periodically need to be cleaned and/or replaced – average life for a well maintained commercial thin film composite membrane is approximately 3 years
- Dynamic flow demand could affect permeate quality – boilers that operate with large load swings may not obtain optimized output from the RO unit
- Depending on the quality of the RO feedwater, pre-treatment may be required – highly chlorinated or waters with high levels of scale forming salts may need to be pre-softened or treated with sulfite and/or acid
- Electrical power for the high pressures involved in pumping the RO feedwater is an added cost of operation
- If the concentrated reject RO water cannot be used for another purpose approximately 20-25% will be sent to drain
Estimates on savings:
- 50-60% reduction in chemical use
- 4-5% reduction in boiler blowdown
- 3-4% reduction in boiler fuel cost
- Steam pressure – Higher pressure requires higher quality feedwater
- Makeup water quality – higher dissolved solids can increase fuel, water and chemical consumption increasing the benefit of RO for makeup
- Variable steam loading – RO works more effectively at steady flow rates
- Condensate return quantity – high volume (+80%), high quality condensate return may cancel the benefit of RO
- Can the RO concentrate (reject) water be used for other utility water systems?
- What will be the energy and chemical cost of operation?
The operation and maintenance of a boiler system is one of the most expensive and labor intensive line items for any facility budget. It is important for facilty managers to keep informed on advances in boiler pre-treatment technologies. Water is abundant, but high quality water is becoming an increasingly expensive and limited resource. Being proactive and creative in how water is managed through a facility can have significant cost savings attached to it and depending on design concerns, the case for reverse osmosis is a solution worth considering.