By George Melchior
In the June issue of Snow Business, I noted the three forms of heat transfer: conduction, convection and radiation and that they all affect surface temperature. That article (read here
) focused on conduction: the transfer of heat through matter, which makes the rate of conduction dependent on material properties. In this article, we’ll look at heat transfer via convection, and the implications of convective heat transfer on ice management.
Convection is the transfer of heat from a material surface and a moving fluid. When we address convection as it pertains to ice management, the material surface is the walking or driving surface, and the moving fluid is the air. When we labor to prevent or treat the formation of ice on a surface, we are most concerned about surface temperature.
In the winter, surface temperatures frequently fluctuate above and below the freezing point of water. As we learned with conduction, surface temperatures do not instantly equilibrate with the atmosphere when the air temperature decreases. Rather, the time it takes for the surface to equilibrate with the atmosphere is a function of the surface material’s ability to release heat from its mass. With convection, we shift our focus from the material to the air, and, specifically, the capacity for the air to receive the heat and carry it away. Convection and ice management
Recall from my last article the difference between heat and hot air: Hot air rises, but heat flows from hot to cold, regardless of direction. With convection, because air is our fluid, we are concerned about the transfer of heat and the movement of hot air. Heat transfer through convection is described by Newton’s Law of Cooling, which states that the rate of heat transfer is a function of the difference in temperature between the surface and the air; the area of the surface; and the convective properties of the air. Much like conduction, the first two factors are easy to understand. The higher the temperature difference, the faster an object warms or cools. Likewise, the more surface area that is exposed to the fluid, the more heat is released in a given time frame. The last variable, the convective property of air, has the highest influence on the rate of heat transfer from a surface, and is the most important factor for ice managers to understand.
The convective property of air is a function of its velocity, or speed across a surface. When we discuss air speed, it’s important to identify the type of convection that the surface is experiencing. There are two types of convection: natural and forced (Case study graphic, below).
Natural convection is a natural movement of air as it gets warmer. As air absorbs heat, it becomes less dense, which causes the air to rise. The air that rises is replaced by cooler, denser air that sinks to the surface. An example of natural convection is when we light a fire in our stove on the first floor of our house and find later that the second floor is much warmer than the first, because the hot air rose. What’s important to remember about natural convection is that air moves slowly, so the temperature difference between the surface and the layer of air right above the surface, called the boundary layer, is very small.
Forced convection occurs when the air is forced or propelled across the surface at a velocity that is independent of the air’s change in temperature and density, like turning on a fan in the summer to cool off. For ice managers, think of wind as forced convection. Wind is caused by a difference in atmospheric pressure. Air travels from high pressure systems to low pressure systems across the Earth’s surface to create wind. Contrary to natural convection, forced convection constantly refreshes the boundary layer with new air, which prevents the temperature of the boundary layer from increasing as it absorbs heat from the surface. So, with forced convection, the temperature difference between the air right above the surface and the surface temperature remains relatively high. Why convection matters
Why do we care? Because, as stated previously, temperature differential influences the time it takes for heat to transfer and for surfaces to equilibrate with the air.
Imagine a parking lot that was warmed in the afternoon sun to 35˚F, and the air temperature was around freezing. Then, after the sun went down, the air temperature dropped to 25˚F. If there was no wind, the parking lot may be transferring about 500 btu/hr of heat into the atmosphere, which could take four hours or more for the surface to fall below freezing. In four hours or more after sunset, people have left work for the day, the peak hour for dining out has come and gone, etc. So, if moisture from snowmelt (roof, snow piles, etc.) ran across the lot surface, it would freeze after the majority of people were done using the commercial property for the evening.
However, if we had the same exact conditions, only we added a 10 mph wind to the mix, then the rate of heat transfer would be closer to 6,000 btu/hr. That’s a rate of heat loss 12 times faster than if there was no wind. At that rate, the lot may only take 20 minutes to fall below freezing. And, if there was moisture on that lot from earlier in the day, then it has quickly frozen to form black ice, jeopardizing the safety of pedestrians using those commercial properties.
Case study / natural and forced convection Natural convection is a natural movement of air as it gets warmer. As air absorbs heat, it becomes less dense, which causes air to rise. Forced convection is when the air is propelled across the surface at a velocity that is independent of the air’s change in temperature and density.
A woman slipped and fell on untreated black ice in a parking lot of an office building as she was departing work for the evening. It was a day in mid-January where the region experienced light precipitation (~1” of snow) in the morning, followed by partly sunny skies in the afternoon, and a high air temperature of 40˚F. By sunset (4:12 p.m.), the air temperature dropped to 28˚F with winds at about 12 mph. The office building had concrete walkways and an asphalt parking lot. Quitting time was typically 5 p.m.
The snow and ice management contractor and the property owner testified that they were aware of the weather conditions that day. They further testified that they knew the majority of the snow melted throughout the day, and that refreeze would be an issue overnight. Their intention was to wait until all employees left the building for the day and the parking lot was empty, and then apply chemicals for anti-icing. The contractor even testified that there was no need to inspect the property prior to quitting time at the building, because the time it takes for the surface temperature to fall below freezing after a mildly warm day is usually five or six hours.
As a result of forced convection from the wind, the surface fell below freezing in less than an hour after sunset. Numerous witness statements confirmed that by quitting time the parking lot and walkways were completely covered in a thin sheet of ice. The injured party left work unaware of the ice, slipped and fell on the walkway, and suffered a serious injury. The case settled for approximately $38,000. Had the property owner and contractor been aware of the effects of wind and forced convection on the rate of surface cooling, they would have known to respond to the property earlier and treated accordingly.