With carbon dioxide (CO2) being used in more commercial applications, Bacharach's product manager of fixed instrumentation, Tom Burniston, explains why it is important to employ leak detections systems to enhance the safety of those working in refrigerated spaces.

While CO2 is less environmentally harmful than other gases, it can be dangerous in sufficient volumes so care should be taken in selecting an effective solution with an appropriate response time and operating temperature.

With gases such as R-404A beginning to be phased out, CO2 is becoming an attractive option for a wide range of applications. At the same time manufacturers continue to develop a growing range of ever more efficient and cost-effective CO2 refrigeration system options, enabling wider adoption.

Typical figures stipulated for CO2 exposure by OSHA are:
5,000 ppm – Occupational Exposure Limit (OEL)
40,000 ppm – Immediately Dangerous to Life or Health (IDLH)

Refrigerant safety standards, including EN 378-1:2016 and ASHRAE 34- 2013, include these exposure levels to determine the safety of a refrigerant system and the cases in which refrigerant leak detection is mandated.

These use-cases will typically include refrigerant detection in machinery rooms and other refrigerated spaces including cold rooms and walk-in freezers.

Where larger refrigerant charge sizes are used, the potential for mandated refrigerant leak detection in larger spaces, such as storage freezers, is increased. As with all refrigeration systems, over time there is the potential for leaks to occur.

Causes can include improper maintenance, mechanical wear, inadvertent damage or improper installation. With pressurised systems, these factors can contribute to a significantly higher risk of leaks.

Due to design requirements, CO2 refrigeration systems typically operate at high pressures; subcritical or transcritical system pressure ratings often operate in excess of 2,000 PSI.

These high pressure ranges mean that in the event of a major CO2 leak, the gas can escape into a space at very high leak rates.

Response time

CO2 is monitored in a wide variety of applications, including indoor air quality (IAQ), occupational workplaces and refrigeration applications. However, it should be noted that not all CO2 monitoring devices are suitable for leak detection in refrigeration applications.

Two key factors that should be considered when looking for a suitable CO2 leak detector relative to the applicable application are response time and operating temperature.

The rate at which CO2 can leak and create a dangerous environment makes response time a critical factor in delivering effective CO2 leak detection.

Instruments suited for monitoring of gradual changes in the atmospheric CO2 level, such as used in IAQ applications, may not be suitable for leak detection purposes where a fast response is needed to ensure that a dangerous working environment is not created and/or entered.

It should be advocated to review both the stated response time of the sensor being used in a leak detector and, most importantly, the response time of the instrument.

Different instrument designs can affect how quickly gas can diffuse into a CO2 sensor. For example, a sensor can be directly presented to the atmosphere being monitored, thereby helping deliver a fast response time, or the gas may need to diffuse down a capillary tube to enter the sensor which would significantly increase the response time of the instrument and negate the stated response time of the sensor itself.

It is imperative that the gas detector response time is appropriate for the designated application to provide the benefit of its intended use.

Operating temperature is also a key factor in selecting an appropriate CO2 leak detector for a refrigeration application. There are comparatively few CO2 sensors available on the market that are specified for use at temperatures lower than 0°C / 32°F with response times required for effective use in the refrigerant gas detection applications.

As the use of CO2 in refrigeration continues to grow, it becomes imperative to ensure that sensors appropriate for use in low temperature environments are used by instrument manufacturers whose products are intended for leak detection in refrigerated spaces.

Fundamentally, it can't be determined what behaviour a sensor would exhibit if run outside of its specified operating range without test data, as each sensor may behave differently. What can be stated is that any reading could be invalid and the device behaviour could be undefined.

At best, the sensor may be inoperable and a fault detected. At worst an invalid or inaccurate reading may be provided. It could be argued that a sensor operating outside of its designed specifications is less useful than none at all, as it may provide false confidence the gas level is being accurately monitored and its benefit of installation and use is nullified, creating risk of exposure.

Bacharach, Inc. has over 100 years' experience as a global leader in the design and manufacture of gas detection systems for HVAC-R applications, helping to make industries safer, cleaner, and more energy efficient.

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