A quick tagline about ohmic heating
C-Tech Innovation has more than 30 years experience in design and build of specialised Ohmic Heating equipment for a wide variety of applications. Ohmic heating, developed by C-Tech Innovation, is an energy efficient and high quality method of heating either a continuous flow or batch quantity of wet or moist material.
The mechanisms by which heat is generated in the product varies with the operating frequency of the electric field: at low frequencies (less than 104 Hz) and radio frequencies (104 to 108 Hz) the effects are primarily due to resistive heating caused by the movement of ions / electrons in the electric field. At higher frequencies (e.g. microwave heating (108 to 1011 Hz) molecular dipole rotation is the main heating mechanism, whereas at higher frequencies still (e.g. infra-red (IR)) the dominant mechanism is one of molecular vibration, and at visible frequencies is by the elevation of electrons to higher energy levels. As the frequency of the electromagnetic field increases so the energy penetrates less deeply into the product. At IR and visible frequencies, for most food products, the energy will be absorbed very rapidly as it enters the product resulting in essentially surface heating; though for other materials, e.g. some polymers, IR heating can provide volumetric heating.
Typical volumetric heating technologies of use in the food processing industries include microwave, radio frequency and Ohmic heating. Common to these volumetric heating methods is the fact that they do not require hot surfaces, or rely on conductive or convective heat transfer through a product.
Ohmic heating is a proven technology for processing high quality fluid food products. It can be used to pasteurize, sterilize or cook foodstuffs ranging from juice and dairy products to soups and stews. With Ohmic heating, as for all volumetric heating techniques, the absence of hot surfaces in contact with the product leads to reduced thermal degradation and thus improved product quality. It is also capable of extremely rapid heating and is a very efficient heating technique. This makes Ohmic heaters a good choice in areas where traditional heat exchangers can cause problems, such as fouling and product quality reduction from overheating, and where products are difficult to heat because of a large solid content or the product contains delicate particulates. Ohmic heating involves passing an alternating (AC) current (typically 50 Hz to 100 kHz) through the product being processed. Due to the electrical resistance of the product, this current flow then causes ‘Ohmic’ heating throughout, giving a uniform temperature profile. Any conductive and pumpable product can be successfully heated including most food products. The rapid and uniform heating made possible with the technology lends itself to high temperature short time (HTST) processes with potential benefits in terms of product taste, colour and texture.
‘Ohmic Heating’ (also known as a Joule heating, and electric resistance heating) is direct volumetric heating of a product by conduction of electric current through it.
Conventional heating using heat exchangers or jacketed vessels is dependent on heat transfer to and from the heated surface followed by heat transfer into the product by conduction and convection and often requiring assistance by mixing. Ohmic heating heats the whole of the product internally including particulates without the drawback of the hot heat transfer surface. The heating can be very fast (1000 °C / min) very uniform and very accurately controlled which helps specificity of reactions, yield and conversion efficiency.
Ohmic heating brings therefore;
• Direct volumetric heat generation within the product
• No external heat transfer required
• No hot surfaces involved
• Very rapid start-up and shutdown and rapid heating
• Very precise temperature control
• High efficiency >95%
• Can handle slurries with high solid contents up to 80% solids
• Can process streams with large particulates
• Can heat viscous products e.g. to 100000 cP
• Low pressure drop, minimal shear, with large open flow channel
• Low heater product volume with in-line heating
• Reduced fouling and thermal degradation
• Not limited by heat transfer area or low heat transfer coefficients
Ohmic heating can be used for heating chemicals and reactants, for recirculation heating, for boost heating, for in line temperature recovery/trimming, for very precise temperature control, for electrical conductivity modification. The method can be combined with hold sections where time at temperature is required. In line mixing can be combined or introduced between heating zones or post heating.
The method enables a shift away from batch processing with slow heat transfer in vessels, long heating times reliant on mixing for uniformity of heating, poor accuracy of time at target temperature.
In contrast very fast heat generation within all of the product, high accuracy of time at target temperature and a continuous inline process is provided. Almost any pumpable product/fluid with an electrical conductivity can be heated.
The heating method is very efficient with typically 95% of the supplied electrical energy dissipated in the product. The wholly electrical nature of the technology also means that the process is highly controllable, convenient for services, instantaneously controlled. The voltage and current flow into the product and can be measured very accurately so precise energy delivery into the product can be realised coupled with continuous mass flow measurement if desired. When the electrical process cost is compared to the overall cost and efficiency and infrastructure requirements for a steam process the economics and convenience are very favourable. The average steam heating system using heat exchangers only achieves an energy efficiency of 56% from fossil fuel into the product.
Bespoke designs can be tailored to specific processes and products and we have many years experience building bespoke heaters and plants. Bespoke units can incorporate;
• Innovative and varied electrode arrangements, collinear, transverse plates, transverse rods, open channel heaters, meander electrodes
• Cooled or temperature controlled electrodes
• Pipe/flow-channel surfaces can be heated or cooled if this is desired to influence the temperature in the boundary layer
• Some non-thermal electroporation effects can be introduced if desired for extractions
• Designs and operation to include pigging can be implemented
• Ultra-high temperatures can be configured, e.g. 300°C, 100 Bar
• Glass or ceramic lined sections can be specified
• Bespoke heaters with in line mixing or turbulent flow or subsequent agitation
• Ohmic heating can also be configured in batch applicators with agitation if desired
• Heaters can be combined with or onto a vessels if desired
End on view of OCP Heater
Pilot Scale Ohmic Heater
Ohmic test units at C-Tech include:-
• Batch Ohmic heaters, 0.3 litres to 10 litres.
• Pressured batch Ohmic heaters to 150°C and 180°C, 1 to 2 litres.
• Continuous pilot Ohmic heater, 50 to 300kg/hr, to 150°C
C-Tech Innovation have installed and commissioned Ohmic heating equipment around the world.