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Should I Choose an Air-cooled or Water-cooled Thermal Imaging Camera for my Furnace?

Our engineering team has always been quite cautious about the cooling flows we state in our specifications on instruments designed for furnace imaging (for example for instruments like the water-cooled NIR-B-640-EX), both in terms of volumes and quality of air and water.

The reason comes down to reliability; we can control the manufacturing quality to tight tolerances, but we cannot control the cooling media over the total lifetime of our instruments. For that reason, we prefer to state slightly higher flows and qualities than what is strictly required. 

James Cross - August 2022 BlogIt also comes down to the lifetime of the instruments. Depending on the lifetime and application, we like to aim for a minimum of 10 years in operation – this in turn helps achieve a lower total cost of ownership versus less reliable technology. We are not aiming only for the instruments to survive their warranty period.

When selecting which cooling media to use, there are a few considerations. The first and most obvious step is to ascertain what air and water volumes/pressures/qualities are available at the camera (remembering to check the height of the installation and subsequent pressure drops as height increases). Then compare those to the specified cooling jacket’s minimum requirements. We generally state a minimum flow or pressure and recommend instrument air (versus plant air) and cooling water with a pH level within 6.5 to 8.5 pH and hardness must be less than 150 mg/l (CaCO3).

The second is the ability to allow air into the furnace. Most air-cooled designs require comparatively high volumes and pressures of air versus water, due to the huge difference in thermal conductivity between the two media. Water conducts heat more than twenty times as well as air. This means that we cannot circulate the air around the tube in the same way that we can with water – the air is allowed to vent into the furnace flue gas atmosphere, whereas the cooling water is returned as the water-cooled jacket is a closed loop system.

When cooling with water or air, both jackets do still require a specified volume of purge air to prevent contamination of the lens, especially in furnaces with high concentrations of soot or refractory dust in the flue gas.

James Cross - August 2022 Blog
Figure 1: The water-cooled jacket flow diagram shows cold water entering through one inlet, and warm water exiting another.

James Cross - August 2022 Blog
Figure 2: The air-cooled jacket flow diagram shows the key difference, there are no outlets except the end of the borescope tube, which ended looking more like a burner nozzle than an advanced thermal imaging camera.

In summary, a borescope camera cooled by water may be lower cost to run based on its higher thermal conductivity, but air may be more practical based on availability of clean, dry instrument air. When air is the best option, the volume of air being inlet into the furnace should be considered so that combustion or flue gas distribution is not affected.On large furnaces with large volumes of combustion air, this is rarely an issue even with 10+ cameras, but this maybe become a factor on smaller furnaces with multiple cameras.

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