China
Africa
Explore chapters and articles related to this topic
*
Published in Clark W. Gellings, The Smart Grid: Enabling Energy Efficiency and Demand Response, 2020
Dielectric heating is accomplished with the application of electromagnetic fields. The material is placed between two electrodes that are connected to a high-frequency generator. The electromagnetic fields excite the molecular makeup of material, thereby generating heat within the material. Dielectric systems can be divided into two types: RF (radio frequency) and microwave. RF systems operate in the 1 to 100 MHz range, and microwave systems operate in the 100 to 10 000 MHz range. RF systems are less expensive and are capable of larger penetration depths because of their lower frequencies and longer wavelengths than microwave systems, but they are not as well suited for materials or products with irregular shapes. Both types of dielectric processes are good for applications in which the surface to volume ratio is small. In these cases, heating processes that rely on conductive, radiative and convective heat transfer are less efficient.
Nonthermal and Alternative Food Processing Technologies
Published in C. Anandharamakrishnan, S. Padma Ishwarya, Essentials and Applications of Food Engineering, 2019
C. Anandharamakrishnan, S. Padma Ishwarya
Dielectric heating includes the microwave and radiofrequency (RF) heating techniques, which are frequency specific. The penetration depth of microwaves and RF waves is directly dependent on their wave frequency. Dielectric heating of foods is governed by the dipolar nature of water and its ability to absorb the electromagnetic energy and convert it into heat. This occurs by two mechanisms: dipole rotation and ionic conduction (Figure 14.7a and b). The permanently polarized dipolar water molecules respond to the oscillating electric field incident on them by realigning in the direction of the electric field. Due to the high frequency of the applied electric field, this realignment of water molecules occurs at the rate of million times per second. This phenomenon is termed the dipole rotation which leads to internal friction between the molecules. Molecular friction resulting from the rotation of dipolar water molecules generates heat and results in the volumetric heating of the food product. The volumetric heating from within the food product leads to internal vapor generation. This results in the accumulation of an internal vapor pressure which drives the moisture out of the food product.
Fiber and Filament Dyeing
Published in Tom Cassidy, Parikshit Goswami, Textile and Clothing Design Technology, 2017
The radio frequency (RF) or dielectric energy and microwave have recently been used for drying of textiles industrially. Dielectric heating has a low frequency but high field strength, whereas microwave heating has a high frequency but low field strength. The advantages of these forms of heating are as follows: The interior of the fabric is heated at the same rate as the surface, so that the migration of dyes or resins toward the surface during drying is lowest.Drying times are considerably reduced.Over-drying cannot occur. The yarn quality, therefore, is not compromised during drying.
SARA fractions evaluation during microwave-assisted upgrading of an oil refinery vacuum residue: effects of operational conditions
Published in Petroleum Science and Technology, 2021
The powders of Ni and Fe metals (particles size less than 40 microns) are rapidly warmed up under microwaves (Chen, Gutmann, and Kappe 2012). Microwaves are a type of electromagnetic radiation consisting of electric and magnetic fields. There are two categories of materials heating under electromagnetic field: induction and dielectric heating. The materials with high electric conductivity are heated by induction heating. In this case, the magnetic field is dominant. On the other hand, materials with low electrical conductivity are heated by dielectric heating. In this case, the electric field is more considerable. Studies show when metals are exposed to microwave irradiation, the increasing temperature decreases with increasing heat capacity and density. However, it enhances with increasing power dissipation distribution and thermal conductivity (Bjorndalen, Mustafiz, and Islam 2003). Among the used catalysts, the Fe catalyst has the highest electrical resistance.
Iron ore pellet drying assisted by microwave: A kinetic evaluation
Published in Mineral Processing and Extractive Metallurgy Review, 2018
Maycon Athayde, Mauricio Cota, Maurício Covcevich
The drying process for pellets is traditionally carried out by convective heat transfer, based on up- and down-draft hot air. This process has kinetic limitations which are intensified in ore with high level of mineral hydration (goethite/martite usually with higher level of microporosity, higher slimes content and moisture generated during concentration process to be released in the furnace) and even larger pellet size (Ljung et al. 2011; Patisson et al. 1991; Thurlby and Batterham 1980). An alternative, in order to overcome this limitation in iron ore pellet drying is the use of microwave irradiation, which has been extensively studied in mining and extractive metallurgy (Haque 1999). The dielectric heating is the interaction between the electro-magnetic field of microwaves with matter, where dipoles align and flip around, as an alternate field is applied and the internal energy convert in heat due to friction. Industrial electromagnetic frequencies are typically 915 and 2450 MHz (Haque 1999; Menéndez et al. 2010).
Comparison of machining performance of microwave post-heated WC insert with dry, wet and MQL cutting in turning operation
Published in Journal of Microwave Power and Electromagnetic Energy, 2018
Durwesh Jhodkar, H. Chelladurai, Akhilesh Kumar Choudhary, J. Ramkumar
Microwave heating refers to dielectric heating using high-frequency (2.45 GHz) electromagnetic radiation on charged partials of material that interact with each other (Jia 1993). Interaction of the material takes place at the molecular level in the presence of microwave energy. This form of interaction is caused by two different types of effects. These types of effects are called dipolar polarization and Maxwell–Wagner effect polarization. In dipolar polarization, the charged ions and dipoles in the presence of an electromagnetic field start transitional and rotational motions (2450 million times/sec). This movement of molecules dissociates the heat energy generated due to inertial, elastic and frictional forces. This phenomenon of alignment of dipoles in the presence of an induced electromagnetic field is called dipolar polarization. This effect takes place in water and other kinds of fluids. The other one is the Maxwell–Wagner effect that takes places in solid absorbent materials known as dielectric solids, just like carbon in which the π-electron cannot couple with the change of phase of an electromagnetic field and starts energy dissipation in the form of heat (Menéndez et al. 2010; Santos et al. 2011; Chandrasekaran et al. 2013). The power absorbed per unit volume, P (W/m3) is expressed as where E (V/m) is the magnitude of the internal field, σ is the total effective conductivity (S/m), f is the frequency (GHz), ϵo is the permittivity of free space (ϵo = 8.86 10−12 F/m), ϵ′r is the relative dielectric constant and tan δ is the loss tangent. Equation (1) represents that the absorbed power varies linearly with the frequency. The penetration depth of microwaves (D) at which the incident power is reduced by one-half is expressed as (Clark et al. 2000)where λo is the incident of free-space wave length.