Measuring Conductivity (TDS) and Flue Gas Oxygen
Two measurements let a boiler control its own losses. A conductivity probe in the boiler water lets blowdown follow TDS exactly. An oxygen analyzer in the flue lets the burner trim its air continuously. Both pay for themselves in fuel.
You'll learn
- Explain how conductivity measures TDS
- Describe automatic TDS blowdown control
- Describe in-situ oxygen analyzers and O₂ trim
- Maintain both so they stay accurate
Before this: TDS control and blowdown, Flue gas analysis
Two automatic savings
Blowdown done by hand is usually too much or too little, because the attendant cannot see TDS between tests. A conductivity probe can: it reads continuously and opens the blowdown valve only when TDS reaches its set point.
Burner air set once at commissioning drifts with fuel quality, air temperature and wear. An oxygen analyzer in the flue sees the drift and lets the control system correct the air continuously, keeping excess air near its best value.
How they work
- Conductivity: dissolved salts let water conduct electricity, and conductivity rises with their concentration. A probe in the boiler measures it; a controller compensates for temperature and converts to an approximate TDS.
- Automatic TDS control: when conductivity reaches the set point, a blowdown valve opens until it falls back, holding TDS just under the limit.
- Zirconia oxygen analyzer: a heated zirconia cell in the flue duct produces a signal related to the oxygen difference between flue gas and a reference air.
- O₂ trim: the measured oxygen adjusts the burner’s air damper or fan speed around its basic fuel–air curve.
Keeping them honest
- Conductivity probes foul and scale; clean them and check their reading against a sample tested by hand.
- Temperature compensation must be right: conductivity changes strongly with temperature.
- Oxygen analyzers drift; check them against calibration gases on schedule.
- Mount the oxygen probe where the gas is representative and where air leaks cannot dilute it, or the trim will chase a false reading.
- Trim limits: O₂ trim adjusts around a safe fuel–air curve within limits, so a failed analyzer cannot drive the burner rich.
Key takeaways
- Conductivity measures TDS continuously; automatic control opens blowdown only when needed.
- Zirconia analyzers measure flue O₂; O₂ trim corrects burner air continuously.
- Both need cleaning, calibration and representative mounting.
- Trim acts within safe limits around a fixed fuel–air curve.