Managing water quality in a boiler is essential to ensure its efficiency, durability, and to avoid operational issues. A key parameter to monitor is conductivity, which reflects the concentration of total dissolved solids (TDS). Excessive conductivity can lead to several problems:
- Scaling of heat exchange surfaces: Reduced energy efficiency and risk of tube overheating.
- Corrosion: Accelerated in the presence of aggressive ions such as chlorides and sulfates.
- Carrying of water droplets in the steam (priming and foaming): Deterioration of steam quality and potential risks for downstream equipment.
Blow Down : The Only Method to Control Conductivity
There are two types of blow down:
Surface blow down (continuous): Removes water with a high TDS concentration from the boiler’s surface.
Bottom blow down (intermittent): Evacuates sludge and solids accumulated at the bottom of the boiler.
Optimal management of these purges helps control conductivity while minimizing water and energy losses.
Factors Influencing the Blow Down Calculation
Makeup Water Conductivity
Makeup water is the fresh water added to compensate for losses due to evaporation, purging, and leaks. Its conductivity depends on its source and the treatments applied (reverse osmosis, demineralization).
Condensate Return Percentage and Steam Quality
Condensate return plays a key role in reducing the blow down rate. Its conductivity level mainly depends on the quality of the produced steam:
- The drier and purer the steam, the less liquid water it carries, resulting in lower condensate conductivity.
- A high condensate return limits the introduction of new dissolved solids into the boiler, thereby reducing the need for purging and chemical consumption.
Steam Injection at the Deaerator
Thermal deaeration involves injecting steam into a deaertor or condensate tank to remove dissolved gases (oxygen and CO₂) through thermal effects. Since the injected steam contains no dissolved solids, it dilutes the conductivity of the boiler feed water.
Composition of Boiler Feed Water
The conductivity of the boiler feed water is determined by the mixture of the following streams:
- Makeup water (containing TDS)
- Condensate return (generally low in TDS)
- Steam injected at the deaerator (without TDS)
It is calculated based on the principle of conservation of the mass of dissolved solids in the tank. It can be approximated by the following formula
Boiler feed water conductivity (ppm) ≈ 0.8 × (1 – Condensate Return (%)) × Makeup Water Conductivity (ppm)
Maximum Tolerated Boiler Conductivity
Each boiler has a conductivity limit defined by its operating pressure and design. It is essential to refer to the manufacturer’s manual for the specific value.
For example, a boiler operating at 300 psi typically has a maximum allowable conductivity of about 3,500 ppm.
Since the outgoing steam contains very little TDS, the solids remain and accumulate in the boiler. Purging is therefore essential to remove these solids and maintain balance.
Blow Down Rate Calculations
The purge rate is based on the conservation of the mass of dissolved solids in the boiler. For simplicity, it is approximated by the following formula:
Blow Down Rate (%) ≈ Feed Water Conductivity (ppm) / Boiler Water Conductivity (ppm)
Important Note
- Boiler manufacturers use a purge rate of 3% in their performance calculations without considering the actual rate.
- They often neglect the impact of the condensate return rate.
Which can lead to an underestimated purge rate in the design and an effective steam production capacity lower than expected.
Energy Impact of a High Blow Down Rate
An excessively high purge rate results in significant losses:
- Loss of hot water: The purged water contains a large amount of thermal energy.
- Increased makeup water consumption: Water is billed based on the volume consumed.
- Excessive chemical consumption: Each liter of makeup water requires treatment.
- Overproduction of CO₂: The loss of energy results in higher fuel consumption.
Factors to Reduce the Blow Down Rate and Minimize Energy Impact
- Maximize condensate return.
- Optimize makeup water treatment by incorporating a reverse osmosis system.
- Use automatic blow down control to adjust purges in real time.
- Recover heat from blow downs to preheat the makeup water.
Resources:
- Excel file for purge calculations (Coming soon for download. Contact us to receive a copy by email.)