Every boiler feedwater and steam condensate analyser on the market — including the Pyxis Guardian IK-2000 series — specifies that it must be installed downstream of a sample cooler. This requirement is stated prominently in the installation documentation, with operating temperature limits of 40–49°C at the sensor inlet.
In practice, sample coolers are frequently omitted from boiler monitoring installations — particularly in industrial sites where cost is a primary concern, or where the monitoring system is being retrofitted and the sample cooler was not included in the original specification. The result is not just poor sensor performance. It is sensor failure, measurement error, safety risk, and a monitoring programme that provides false confidence while the boiler continues to operate without real protection.
What a sample cooler does
Boiler feedwater and steam samples are hot — often between 100°C and 200°C depending on the boiler pressure and where the sample is taken from. Steam samples directly from a high-pressure boiler are hotter still. The sample cooler is a small heat exchanger that reduces this temperature to a level the sensor can measure accurately and safely — typically below 49°C for most inline sensor configurations.
The cooling is achieved by passing the hot sample through a coil or tube bundle immersed in cooling water. The Lecol LC series sample coolers use a self-circulating cooling design that requires no external cooling water — the cooler operates on natural convection, cooling the sample passively without a separate cooling water supply connection. For installations where a cooling water supply is not conveniently available, this is a practical advantage.
The sample cooler also reduces the pressure of the sample from boiler operating pressure to near atmospheric, through a combination of the pressure drop across the cooler and a pressure reduction valve downstream. This makes the sample safe to handle and routes it through the sensor at a flow rate that the sensor can process accurately.
What happens to sensors without a sample cooler
The damage from eliminating the sample cooler is not limited to reduced sensor accuracy. It damages the sensors and often destroys them rapidly.
Dissolved oxygen sensors — the ST-774 optical DO sensor used in the Guardian series — are calibrated and designed to operate at temperatures up to 50°C. At higher temperatures, the optical sensing membrane degrades. The fluorescence-based measurement principle is affected by temperature in ways that the sensor’s internal compensation cannot fully correct above its rated operating range. Readings at elevated temperature are not just inaccurate — they are meaningless, and the sensor life is measured in days rather than years.
pH sensors lose stability rapidly above 50°C. The reference junction, which is the most thermally sensitive part of a pH sensor, allows increased ion exchange between the reference electrolyte and the sample at high temperatures, causing junction potential drift that shifts the pH reading progressively away from the true value. A pH reading taken from an uncooled high-temperature boiler sample can be wrong by a full pH unit or more — which completely defeats the purpose of monitoring pH to control corrosion.
Conductivity sensors are more thermally robust than pH or DO sensors, but all conductivity measurements require temperature compensation. If the sample temperature is significantly above the sensor’s rated range, the temperature compensation algorithm cannot function correctly, and the conductivity reading will be systematically high or low depending on the temperature difference.
The safety dimension
Beyond sensor damage, connecting an analyser panel directly to a boiler sample line without pressure reduction and temperature control is a safety issue.
Boiler feedwater at operating pressure — even a low-pressure 10 bar boiler — will flash to steam when the pressure drops at a connection point. If a sensor is connected directly to a sample line at operating pressure without a pressure reducer, any loosening of a fitting or connection failure releases a high-pressure, high-temperature jet of steam and water. This is a burn hazard.
Sample coolers incorporate pressure reduction as part of their function — the pressure drop across the cooler, combined with a downstream pressure reducer, brings the sample to near-atmospheric pressure before it reaches the sensor panel. This is not an optional safety feature. It is a fundamental requirement of any sample conditioning system for boiler monitoring.
The dissolved oxygen measurement problem
This is where the omission of a sample cooler is most damaging to the monitoring programme’s purpose.
Dissolved oxygen in boiler feedwater must be measured at a very low level — typically below 10 ppb. At elevated temperatures, dissolved oxygen solubility in water decreases dramatically. A sample at 80°C can hold significantly less dissolved oxygen than the same sample at 25°C. If a dissolved oxygen measurement is taken at elevated temperature, the reading reflects the oxygen that remains dissolved at that temperature — not the oxygen that was present in the feedwater under actual operating conditions.
More importantly, the sample degasses as it travels from the boiler sample point to the sensor. Oxygen that was dissolved under pressure at operating temperature evolves as gas bubbles when pressure drops. If the sample is not cooled to maintain the dissolved oxygen in solution until it reaches the sensor, a portion of the oxygen has already escaped as gas before measurement. The DO reading will be falsely low — the opposite of the safe-side error. The system will appear to have less dissolved oxygen than it actually does, leading the operator to conclude that the corrosion protection from oxygen scavenger is adequate when it may not be.
What a complete sample conditioning system looks like
The correct installation sequence for boiler feedwater monitoring is: sample tap from the boiler or deaerator storage tank → sample cooler to reduce temperature to below 49°C → pressure reducer to bring sample to near atmospheric → flow indicator to verify adequate sample flow → sensor panel with dissolved oxygen, pH, conductivity, and other required sensors → drain.
The sample should be taken from the deaerator storage vessel, not from the feedwater pump discharge. The Pyxis Guardian documentation is explicit on this point: feedwater pump discharge pressure can exceed the sensor panel’s rated inlet pressure of 0.2 MPa (30 psi), and the turbulence from the pump discharge affects dissolved oxygen measurement accuracy.
For steam sample monitoring — where the sample is taken directly from the steam line — the sample cooler must be sized to condense the steam and cool the condensed liquid to the sensor operating range. A cooler sized for liquid feedwater is not adequate for a steam sample application.
The cost argument does not hold up
The argument for omitting a sample cooler is almost always cost. The Lecol LC series sample coolers represent a fraction of the cost of the sensor panel they protect. A sensor panel without sample conditioning will fail within weeks or months of installation — at which point the cost of sensor replacement typically exceeds the cost of a sample cooler several times over.
More significantly, a monitoring programme without adequate sample conditioning provides false confidence. The readings exist, the alarms are configured, the log shows data — but the data does not reflect actual boiler water quality. The monitoring programme appears to be functioning while the boiler continues to operate without the protection that the programme was installed to provide.
A sample cooler is not a luxury addition to a boiler monitoring system. It is the component that makes the monitoring system work.
Autoflo Technology supplies complete boiler water monitoring systems including Lecol SWAS sample coolers and pressure reducers, paired with Pyxis Guardian boiler feedwater analysers. Contact us at info@autoflotechnology.com for more information.