What is an Ohmic Heater?
An ohmic heater also known as a joule heater is an electrical heating device that uses a liquid’s own electrical resistance to
generate the heat. Heat is produced directly within the fluid itself by Joule heating as electrical current passes through it
and is not transmitted to it by means of temperature gradients or hot surfaces. This has a number of benefits in
comparison to hot surface heaters for ‘difficult’ fluids such as slurries, highly viscous materials, liquids containing large
particles and materials susceptible to damage from hot surfaces.
Introduction
The most commonly used heating
techniques for liquids and slurries rely on
heat transfer from a hot surface. This
heat can be generated directly via an
electrical heating element or indirectly
from a hot medium (e.g. steam) via a
heat exchanger (e.g. shell and tube,
plate). These methods require a
temperature gradient to transfer heat to
the process liquid and as such the
surfaces are at a higher temperature than
the product. This can cause fouling of the
surfaces for certain products which
become burnt onto the hot surfaces
reducing heat transfer rates and
adversely affecting the product. A further
issue with heat transfer is found when
heating very viscous fluid and fluids with
particulates where effective, even heat
transfer is difficult to achieve.
Ohmic
heaters
address
the
aforementioned problems by removing
hot surfaces from the heating of the
fluids. The fluid is heated directly by
passing an electrical current (normally
AC) through the product, its own
electrical resistance causes heating
throughout giving a uniform temperature
rise. This effect is seen independently of
turbulence effects, so there is no loss in
performance for low flow rates or high
viscosity. Particulates in the stream will
also be heated evenly.
Any conductive and pumpable product
can be successfully heated including most
food products. Ohmic heaters are very
efficient with over 95% of the applied
electrical power converted to heat. They
have been shown to improve the colour
and vitamin retention of foods and
provide a longer shelf life for pasteurised
products when compared to hot surface
[1][2][3]
heaters
.
Ohmic Heater Design
Ohmic heaters use two or more
electrodes to impart the current upon
the fluid. They can be set up either as
static systems in container vessel or with
continuous flow through them.
Ohmic Kettle – example of a static Ohmic
[9]
Heater
In a flow through situation, the Ohmic
heater electrodes are typically arranged
in one of four different configurations
which optimise the operation:
A parallel plate flow through Ohmic
Heater (Joule Heater)
Flow Through Ohmic Heater
Flow-through ohmic heaters are the most
commonly type employed in industrial
applications. They use a simple open
geometry which prevents damage to
products, reduces the effects of fouling
[4]
and makes cleaning easier. The rapid
and uniform heating made possible with
the technology lends itself to high
temperature short time (HTST) processes
with potential benefits in particular to
food production in terms of product
taste, colour and texture.
The parallel plate is most suitable for
low conductivity fluids (<5S/m) and
also offers a benefit where there are
large solids due to the completely
unrestricted flow channel. The
electric field uniformity is optimised
in this geometry improving even
heating. The design can usually
operate at standard voltages (e.g.
[4][5]
240V or 415V).
The collinear design is a better option
for high conductivity offering wider
electrode spacing. The electrodes can
be position in the fluid stream or as
collars around a pipe which provides a
fully unrestricted flow channel. For
most applications this design requires
a higher voltage than the parallel
plate. Also the current distribution is
less even and areas of high current
density found at the leading edges of
the electrodes can produce localised
[7]
boiling and arcing.
The parallel rod design is typically
used where cost considerations are
paramount such as waste slurries. The
design is much less expensive to
construct than parallel plates or
Colinear designs, but provides less
even heating of the medium. As a
result the fluid often must be mixed
after heating to even out the
temperature, reducing its suitability
for heating solids without causing
[8]
damage to them.
be
used
without
[10][11]
frequency
.
alteration
in
The heat generated in the product will be
passed to the electrodes by conduction.
In some products which are very heat
sensitive this can cause burn on (e.g.
liquid egg). In such cases the electrodes
can have a cooling system built into them
to ensure they remain well below the
[6]
temperature of the product.
Chemical processing such as chemical
reactions involving the use of
heterogeneous catalysts, continuous
liquid-liquid extraction and general
laboratory scale research.
Water heating – small scale domestic
use such as kettle and shower.
Waste treatment; sterilisation of
animal wastes, sewage sludge and
compost leachate. Hydrolysis of
cellulosic waste. Heating of clay slip
and other slurries.
The staggered rod arrangement is
again a low cost option but can
provide more even heating than the
parallel rod design.
Ohmic Heater integrated with fruit juice
[12]
pasteurisation equipment
Static Ohmic Heaters
A static ohmic heater will have a number
electrodes inserted within a container.
They are most commonly employed in
small scale applications such as water
heaters and laboratory scale chemical
reactors.
Electrode Design
The electrodes can be made from most
conductive materials and are usually
selected based on price and corrosion
resistance. Where product quality is not
essential such as waste treatment, low
cost carbon electrodes are often
employed. For high product quality
applications, metals such as stainless
steel are preferred. However, the
frequency of the power supply must be
increased significantly to prevent
corrosion.
A coating of inert metals used by C-Tech
Innovation prevents corrosion at low
frequency meaning mains electricity can
Typical electrode arrangements in flow
through Ohmic heaters.
Applications
Food production is a major use for
Ohmic Heaters. Common foods which
use ohmic heaters are dairy products,
milk based foods (e.g. rice pudding),
tomato products, fruit pieces/whole
fruit, fruit juice, liquid egg, jams,
compotes,
mincemeat,
soups,
casseroles and ragouts. Often food
based processes integrate the ohmic
heater into pasteurisation lines
incorporating the heater, heat
recovery, cooling, storage and filling.
C-Tech Innovation Ltd,
Capenhurst Technology Park,
Capenhurst, Chester CH1 6EH
+44 (0) 151 347 2900
info@ctechinnovation.com
www.ctechinnovation.com
References
1. Anderson et al. (2008), OHMIC HEATING AS AN ALTERNATIVE FOOD PROCESSING TECHNOLOGY, Food Science
Institute, College of Agriculture, KANSAS STATE UNIVERSITY.
2. Castro et al. (November 2003), Ohmic heating of strawberry products: electrical conductivity measurements
and ascorbic acid degradation kinetics, aCentro de Engenharia Biolo´gica, Universidade do Minho, and Ohio
State University, Department of Food, Agricultural and Biological Engineering.
3. LEIZERSON et al. (2005), Stability and Sensory Shelf Life of Orange Juice Pasteurized by Continuous Ohmic
Heating, Department of Biotechnology and Food Engineering, TechnionsIsrael Institute of Technology,
4. Simpson, D.P. (January 1994). Internal resistance ohmic heating apparatus for fluids, UK Patent No.
GB2268671A.
5. Simpson, D.P. (November 1983). Apparatus for heating electrically conductive flowable media, United States
Patent 4,417,132.
6. Simpson, D.P. et al. (November 1983). Apparatus for heating electrically conductive flowable media, United
States Patent 4,434,537.
7. Stirling et al. (September 1990). Apparatus for heating electrically conductive flowable material flowing
through a pipeline, United States Patent 4,959,525.
8. Simpson, D.P. et al. (August 1995).Ohmic Heater Including electrodes arranged along a flow axis to reduce
leakage current, United States Patent 5,440,667.
9. Harrison (July 2005). Liquid heating apparatus and method, WO2005/066551 A1.
10. Stirling, R. (November 1990). Surface Fouling Resistant Materials, UK Patent No. GB2231761A.
11. Stirling (July 1993). Ohmic heating apparatus using electrodes formed of closed microperosity material, United
States Patent 5,226,106.
12. C-Tech Innovation Ohmic Heater Rig