Now there is a different method for defrosting or de-icing the icemaker's evaporator in one to two seconds with almost negligible energy and an immediate restart of the ice growth cycle. The technology was created by Victor Petrenko, a professor at the Dartmouth College Thayer School of Engineering. Pulse Electro Thermal De-icing (PETD) removes ice from an evaporator by introducing a very short-duration, high-power electric pulse to a very thin metal foil heater located at the interface of the ice and the evaporator's surface. A very thin dielectric film separates the evaporator from the metal foil heat film. This dielectric film separates the heater film from the evaporator. A very thin layer of ice, located next to the metal foil heater, is now heated above its melting point. This creates a thin liquid film between the solid ice slab and the metal foil heater. The ice will now slide and fall off the evaporator by gravity or some other external force (Fig. 1).
Due to the short time duration of the pulse, heat transfer is limited to a short distance into the ice and evaporator surface. Very little heat is introduced into the evaporator, ice-making grid, or ice. Instead, the heat is concentrated to a very thin layer of the ice itself, creating this thin liquid film for a quick release of the ice.
The metal foil or metal film thickness can range from 0.5 micrometers to 1 millimeter. Conductive paint, conductive polymer film, carbon fiber composite material, or carbon nano tube composite materials can be used as the heater film.
This thin dielectric film that isolates the heater foil (film) from the evaporator. It can be made from ceramics, glass, rubber, polymers, composite material, thermally conductive adhesive tape, or other dielectrics. These layers have very small heat capacities and use small amounts of energy to be heated.
The PETD WayPETD is designed to shorten the ice harvest time to just a few seconds, which will increase the ice production rate while simultaneously reducing electrical energy consumption. When the ice has grown to a preset thickness, short electrical heating pulses release the ice. Instant ice releases, the absence of moving parts at defrost, low energy consumption, increased ice production, almost uninterrupted ice growth, increased compressor life, and reliability make PETD very attractive for the ice maker industry.
The power required for PETD in small commercial ice makers varies proportionally with the area of the evaporator to be heated. The larger the evaporator, the larger the power requirements. A small commercial ice maker with an evaporator area of about 300 sq. cm to be de-iced will typically require a 1-kW pulse. If 120 volts from a wall outlet receptacle is used, it will require that the pulsing be disabled when the icemaker compartment is opened. Another alternative is to use an electronic transformer and supply 6 volts at 170 amperes, which would be safe to touch during de-icing. Both AC or DC power can be used, and the frequency may be selected according to the application.
Other ApplicationsPETD is successfully deployed or is being developed to be used in other de-icing applications such as bridge structures and cables, airplane wings, commercial refrigeration (fine and tube) evaporators, power lines, ships, large antennas, wind turbines, cooling towers, and buildings.
John Tomczyk is a professor of HVACR at Ferris State University, Big Rapids, Mich. He can be reached by e-mail at email@example.com.
Publication date: 11/07/2005