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Deicing
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De-icing is the process of removing frozen contaminant, snow, ice, slush,from a surface.
Anti-icing is the process of protecting against the formation of frozen contaminant, snow, ice, slush on a surface.
De-icing can be accomplished by mechanical methods (scraping, pushing); through the application of heat; by use of chemicals, known as de-icing fluids, designed to lower the freezing point of water (various salts, alcohols, glycols); or by a combination of these different techniques.
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Encyclopedia
De-icing is the process of removing frozen contaminant, snow, ice, slush,from a surface.
Anti-icing is the process of protecting against the formation of frozen contaminant, snow, ice, slush on a surface.
De-icing can be accomplished by mechanical methods (scraping, pushing); through the application of heat; by use of chemicals, known as de-icing fluids, designed to lower the freezing point of water (various salts, alcohols, glycols); or by a combination of these different techniques. De-icing fluids are always applied heated and diluted.
Anti-icing is accomplished by applying a protective layer, using a viscous fluid called anti-ice fluid, over a surface to absorb the contaminate. All anti-ice fluids offer only limited protection, dependent upon frozen contaminant type and precipitation rate. A fluid has failed when it no longer can absorb the contaminant and it essentially becomes a contaminant itself. If it fails it must be washed from the surface using a de-icing fluid.
Aircraft
When there are freezing conditions and precipitation, de-icing an aircraft is crucial. Frozen contaminants cause critical control surfaces to be rough and uneven disrupting smooth air flow and greatly degrading the ability of the wing to generate lift (force) and increasing drag. This situation can cause a crash. If large pieces of ice separate when the aircraft is in motion, they can be ingested in engines or hit propellers and cause catastrophic failure. Frozen contaminants can jam control surfaces, preventing them from moving properly. Because of this potentially severe consequence, de-icing is performed at airports where temperatures are likely to drop below the freezing point.
De-icing techniques are also employed to ensure that engine inlets and various sensors on the outside of the aircraft are clear of ice or snow.
De-icing on the ground is usually done by spraying aircraft with a de-icing fluid such as Propylene glycol, similar to ethylene glycol antifreeze used in some automobile engine coolants. Ethylene Glycol (EG) is still in use for aircraft de-icing in some parts of the world because it has a lower operational use temperature (LOUT) than PG and is more versatile in application, but Propylene Glycol (PG) is more common because it is classified as non-toxic, unlike Ethylene Glycol.
Nevertheless, it still must be used with a containment system to capture the used liquid, so that it cannot seep into the ground and streams. Even though classified as non-toxic, it has negative effects in nature, as it uses oxygen during breakdown, causing life to suffocate. (In one case, a significant snow in Atlanta in early January 2002 caused an overflow of such a system, briefly contaminating the Flint River downstream of the Atlanta airport.) Many airports recycle used de-icing fluid, separating water and solid contaminants, enabling reuse of the fluid in other applications.
There are several formulations of de-icing fluid, falling into two basic categories: Heated glycol diluted with water for de-icing and snow/frost removal, also referred to as "Newtonian fluids", and unheated, undiluted glycol that has been thickened (imagine half-set gelatin), also referred to as "Non-Newtonian fluids", applied to retard the future development of ice or to prevent falling snow or sleet from accumulating. In some cases both types of fluid are applied, first the heated glycol/water mixture to remove contaminants, followed by the unheated thickened fluid to keep ice from reforming before the aircraft takes off. This is called "a two-step procedure".
Inflight ice buildups are most frequent on the leading edges of the wings, tail and engines (including the propellers or fan blades). Lower speed aircraft frequently use pneumatic boots on the leading edges of wings and tail for inflight de-icing. The rubber coverings are periodically inflated, causing ice to crack and flake off. Once the system is activated by the pilot, the inflation/deflation cycle is automatically controlled. In the past, it was thought such systems can be defeated if they are inflated prematurely; that the pilot must allow a fairly thick layer of ice to form before inflating the boots. Recent research shows “bridging” does not occur with modern boots.
Some aircraft may also use electrically heated resistive elements embedded in a rubber sheet cemented to the leading edges of wings and tail surfaces, propeller leading edges, and helicopter rotor blade leading edges. Such systems usually operate continuously. When ice is detected, they first function as de-icing systems, then as anti-icing systems for continued flight in icing conditions. Some aircraft use chemical de-icing systems which pump antifreeze such as alcohol or propylene glycol through small holes in the wing surfaces and at the roots of propeller blades, melting the ice, and making the surface inhospitable to ice formation. A fourth system, developed by the National Aeronautics and Space Administration, detects ice on the surface by sensing a change in resonance frequency. Once an electronic control module has determined that ice has formed, a large current spike is pumped into the transducers to generate a sharp mechanical shock, cracking the ice layer and causing it to be peeled off by the slipstream.
Many modern civil fixed-wing transport aircraft use anti-ice systems on the leading edge of wings, engine inlets and air data probes using warm air. This is bled from engines and is ducted into a cavity beneath the surface to be anti-iced. The warm air heats the surface up to a few degrees above zero, preventing ice from forming. The system may operate autonomously, switching on and off as the aircraft enters and leaves icing conditions.
Infrared de-icing
Infrared is the transmission of energy by means of electromagnetic waves or rays. Infrared is invisible and travels in straight lines from the heat source to surfaces and objects without significantly heating the space (air) it passes through. When infrared waves strike an object, they release their energy as heat. This heat is either absorbed or reflected by the cooler surface. Infrared energy is continually exchanged between "hot" and "cold" surfaces until all surfaces have reached the same temperature (equilibrium). The colder the surfaces, the more effective the infrared transfer from the emitter. This heat transfer mechanism is substantially faster than conventional heat transfer modes used by conventional deicing (convection and conduction) due to the cooling effect of the air on the deicing fluid spray.
Inflatable Wings
Some aircraft, such as the SP-PM510 made by lesser known Chinese aircraft builder SPP Heavy Industries, use inflatable sections of wing that are inflated via exhaust gases to increase the surface area of the wing and break up any ice formed on the area. These are affectionately known in the inner circle of airline enthusiasts as fun-time Susies or Pamelas for their exceptionally rapid inflation of each vessel.
Roads
De-icing of roads has traditionally been done with salt, spread by snowplows or dump trucks designed to spread it, along with sand and gravel, on slick roads. Sodium chloride (rock salt) is normally used, as it is inexpensive and readily available in large quantities. However, since salt water still freezes at -18°C or 0°F, it is of no help when the temperature falls below this point. It also has a strong tendency to cause corrosion, rusting the steel used in most vehicles and the rebar in concrete bridges. More recent snowmelters use other salts, such as calcium chloride and magnesium chloride, which not only depress the freezing point of water to a much lower temperature, but also produce an exothermic reaction. They are somewhat safer for concrete sidewalks, but excess should still be removed.
More recently, organic compounds have been developed that reduce the environmental issues connected with salts and have longer residual effects when spread on roadways, usually in conjunction with salt brines or solids. These compounds are generated as byproducts of agricultural operations such as sugar beet refining or the distillation process that produces ethanol.
Since the 1990s, use of liquid chemical melters has been increasing, sprayed on roads by nozzles instead of a spinning spreader. Liquid melters are more effective at preventing the ice from bonding to the surface than melting through existing ice.
In Nagano, Japan, relatively inexpensive hot water bubbles up through holes in the pavement to melt snow, though this solution is only practical within a city or town. Some individual buildings may melt snow and ice with electric heating elements buried in the pavement, or even on a roof to prevent ice dams under the shingles, or to keep massive chunks of snow and dangerous icicles from collapsing on anyone below. Small areas of pavement can be kept ice-free by circulating heated liquids in embedded piping systems.
See also
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