When the discussion of convective cooling comes up, people always discuss the issues they have outdoors. Substation inspections, building envelope studies in windy locations, those sorts of things. I always like to break out a tale I have about an anomaly I almost missed due to convective cooling in a high-rise building. Imagine the looks I get, and the questions. “Umm, what kind of high-rise were you in that allowed the wind to blow through it?” That’s not what happened. This is a cool story (punny, huh!?), you’re going to like it.
If you’re ever watching a Braves game and you see them pan around and show you the Atlanta skyline, I've been in most of those buildings at one time or another with a thermal imager. Commercial high-rise buildings are a neat place to do thermography because of the type of equipment you see. Usually there’s redundant equipment on each floor for tenant loads, but there are also pumps, bus duct, switchgear, all sorts of electrical apparatus. Some bigger high-rises even have medium voltage equipment in them, depending on the design. High rise buildings are a great environment to get exposed to electrical inspections, and that’s where I really learned a lot early on in my thermographic career.
So, one day a number of years ago I was in one of the buildings you see to your left, and I got off of the freight elevator on the 24th floor, and headed to the air handler room. An air handler room is one of my favorite spots in a high rise building. It’s where all of the air exchange for HVAC takes place. In the summer months it’s usually about 64°F in this room with a lot of air movement. If you’re stomping around in full FR PPE, sweating like crazy, a cool breezy room is your best friend. So, I walk into the air handler room, and I open up the combination starter that runs the air handler motor. I immediately detect a thermal anomaly. It was on the Phase B switch contacts in the main disconnect for the starter. I started adjusting my level and span to optimize my image when the camera died. My low battery light had been flashing for several floors; luckily I had one with me so I swapped it out.
The camera I was using back then took a good 3-4 minutes to start up from a dead stop. As it was going through its boot up, I was standing there enjoying the cool air. Once the camera made its distinctive “thunk” when it was finished booting, I brought it back up to eye level and viewed the starter again. My anomaly was gone! The load hadn't changed, because the air handler was still running. What in the world happned? Ahh, I then realized the cool air currents had come in contact with the warm surface of the switch contacts because I opened the starter door. It only took a few minutes for convective cooling to take my anomaly closer to the temperature of the normal phases, and I couldn't detect it.
Air movement across a surface can cause cooling as long as there is a difference in temperature between the air and the surface. Convective cooling happens all the time, sometimes to lesser or greater degrees than others, and it happens indoors as well as outside in the wind. Fifteen years ago it was common practice to open a room full of electrical panels all at once and then scan them. Convective cooling takes place then as well. If I open 25 enclosures and then scan them one at a time by the time I get to number 25, it’s been uncovered a little while, and air has been moving across surfaces that usually are covered up. Anomalies can be cooled to a point that they’re undetectable to my imager.
The moral of this little story is, when you’re inspecting indoors, don’t discount the impact of moving air, even if it’s only slightly moving. Convective cooling is an all weather, indoor and outdoor phenomenon. Don’t let it fool you into not Thinking Thermally!