Last week we focused on the affect radiational cooling and heating has on surface temperature. If you still have any doubts about this, just go outside in the sun and again on a clear night and observe the great variation of temperatures you find on your thermal images. Many of them are related to radiation.
When the wind is blowing (or inside when air is moving), we also see variations in temperature, often significant, related to convective cooling and heating. If we fail to see or understand these, we can end up making serious mistakes in our analysis. Most often moving air cools the areas we are looking at, a hot spot on a disconnect switch or the warm wall of a house, for example. Convective heating can also occur, however, especially around machinery and any sun-warmed surface. Convective heat transfer occurs in either direction as long as there is a temperature difference between the air and the surface and is most significant when that difference is greatest.
Accompanying the change to Fall weather are winds, often cool ones. The wind plays a huge role in cooling down warm surfaces toward ambient temperatures. A simple analysis suggests that in many cases a 10mph cooling wind can cut the temperature difference by 50% and a 15mph wind can cut it by 66%. While this may not always hold true, the influence is significant. As you watch for these influences, I think you’ll be surprised at the effect even a “light” wind or air current can have on surface temperature.
I clearly remember one of the first times I learned about the power of convective cooling. I was conducting an inspection of a large substation over the course of two days. On the first day the wind was blowing at nearly 20mph. I found very few problems. On the second day, with winds reduced to 5 mph, I immediately began to see a large number of problems. It was not obvious that on the first day problems existed but had been cooled to the point where I either missed them or underestimated their significance. I ended up going back over the area and found dozens of problems, both warm and very real, where none had appeared before.
Convective cooling can be very noticeable on buildings. The upwind exterior elevations may be cooled to uniformity while the downwind ones retain most of their normal patterns. On the inside of the building, air leakage, especially when enhanced by a blower door, is another instance of convective influence, both cooling and heating.
Machinery, inside or outside, is nearly always plagued with convection. Motors, hydraulic lines, transformers, steam traps and lines, heaters and many energized components are all sources of heat just waiting for an air current. The transfer is often obvious but that fact alone should cause you to always have your “thermal guard” up and ready to look more closely.
One trick you may employ to better understand the influence of convection is to try blocking it with a piece of cardboard. You must be careful, of course, and wait long enough for the change to show up but it can provide very good evidence. Practice this with the warm air rising off a cup of coffee if you want to see it in a simple instance.
What keeps things warm on a clear Fall night? Air! Air warmed by the heat of the day’s sunshine stored in the ground and more massive objects. At night as energy radiates to deep space, it is the air circulating around that keeps the temperature from really plummeting. Think about what happens on the moon where there is little atmosphere to drive convective heating.
While we usually think of air currents as cooling, there are many good examples of convective heating as well. Warm air can often be seen exfiltrating or leaking out of a building. The pressure differences across the thermal envelope often create situations where a positive pressure literally forces the air out. During the heating season we see the effect of that as the surfaces near the leakage site are warmed by the flow of air.
I also often see the heating effect of air conditioner condensing units or the exhaust from condensing furnaces on the exteriors of buildings. Up on a low-slope roof it is very common to see convective heating resulting from the larger exhaust vents and roof-mounted AC units. These can be particularly confusing as it is also not unusual to find a water leak around the flashing to these devices. Both result in a warmer thermal pattern but the one caused by convective heating is typically more uniform that those associated with water intrusion.
One potentially confusing aspect of convection is when both the velocity and the temperature of the fluid, most often air, change! Since the rate of convection depends on both, it can be a challenge to determine which is the greater influence. Many of the utility thermographers we’ve trained over the years have had to contend with this when monitoring a critical component, such as a transformer bushing, by tracking both wind speed and air temperature. The results can be challenging to understand without good data and a friendly engineer! My advice? Go slowly and work hard to understand all the variables. If you have a million dollar shutdown depending on your analysis, you don’t want to get it wrong.
Convection is very powerful because it can move heat quickly and in quantities large enough to quickly change temperatures. It is also relevant to so much of what we do because our work is surrounded by air, an efficient fluid with which convection works its magic. If we are to make sense of our thermal images and the temperatures we are measuring, we must understand what effects it has. Next week I’ll write more about several more influential factors we must consider when we are doing our work.
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner