HEAT PROCESSES

Electroheat

Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010

HEAT PROCESSESHP12

Electroheat. Direct ohmic heating, radiofrequency heating and microwave heating.

Direct Ohmic HeatingHP12

Wagner L.M.

Ohmic heating is one of these new technologies, which consists of the direct passage of electric current through the product. The permanent motion of electrical charges creates heat in the product in agreement with Joule's law

)U( = Q 2Gradient of electric potential [V/m]

Specific electrical conductivity [S/m]

Volumetric heat source [W/m3]

Continuous ohmic heater APV

Direct Ohmic HeatingHP12

Obr.1 Kontinuální ohřev APVAseptic cooler

Holding tube

Cooler

Aseptic tank

Pump

Electrical insulation

Electrode

Transformer

Waste

Product

Filling line

APV Baker Ltd. power 75 kW up to 300 kW, mass flowrates 750 kg/h up to 3000 kg/h. Pasterization of acidic products from 20°C to 90°C and sterilization of less acidic products from 65°C to 140°C (the whole line operates at overpressure 4 bars so that the boiling point temperature will be increased)

Direct ohmic heating of foods. Examples

Aseptic processing line for apricots and plums

Tomatoes

Direct Ohmic Heating ExampleHP12

Continuous direct ohmic heater for liquids in our laboratories

S. Jun, S. Sastry: Reusable pouch development for long term space missions: A 3D ohmic model for verification of sterilization efficacy. Journal of Food Engineering 80 (2007) 1199–1205

Direct Ohmic Heating ExampleHP12

Simulations suggest the presence of significant hot and cold zones, suggesting the need to further optimize pouch design for more uniform heating. In particular, the zones within the V-formed by metal foil electrodes, and the edge of the pouch, where current densities are lowered, are identified as points of concern

applications: space (Mars), earth-based in-package sterilization of foods, or for reheating of military rations

Direct Ohmic Heating-MilkHP12

Experimental setup of milk heater in our laboratory

Ohmic Heating-corrosionHP12

Electrode corrosion

Metal that is migrating into the medium can be oxidized and can start new secondary reactions. For example Fe2+ or Fe3+ can be responsible as catalyzers.The corrosion effects can be suppressed either by using a noble material like gold or platinum for the electrodes, or by using an increased frequency above 50 Hz.

Samaranayake, C.P., Sastry, S.K., & Zhang, Q.H. (2005). Pulsed ohmic heating A novel technique for minimization of electrochemical reactions during processing. Journal of Food Science, 70(8), 460-465.

Stainless steel electrodes

TiN coating electrodes

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0 500 1000 1500 2000 2500 3000 3500

t [s]

P[W

]

0,00

20,00

40,00

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100,00

3000 3200 3400 3600 3800 4000

t [s]

P [

W]

V=45,5 ml/s

Sel=30x40 mm

H=10 mm

Istart=3 A

Tvsádky=70 °C

V=43,5 ml/s

Sel=30x40 mm

H=10 mm

Istart=3 A

Tvsádky=70 °C

Ohmic Heating-corrosionHP12

See also: M., Ayadi et al. , Innovative l'ood Science and Emerying Technologies 5 (2004) 465-473

Ohmic Heating-Milk foulingHP12

Model of thermal milk fouling in continuous ohmic heater Jong (1996), Toyoda et al. (1994)

Rate of the denaturation and agglomeration of milk proteins

Denaturation of native protein -lactoglobulin

Agglomeration of denatured -lactoglobulin

Mesh of finite

elements T

CNativní CDenat. CAgglom

Ohmic Heating-Milk foulingHP12

Model of fouling in continuous ohmic heater using Finite Element Program FEMINA

temperature distribution

Ohmic Heating-SolidHP12

0)()(

y

U

yx

U

x

Laplace equation for electric potential distribution

F.K equation heat transfer with ohmic heating source

22

2

2

2

2

)()()(y

U

x

U

y

T

x

T

t

Tcp

Ohmic Heating-Solid in LiquidHP12

density of generated heat

[W/m3]2)( UQ

What is heated faster: liquid (conductivity l) or solid (S)?

Ohmic Heating-Solid in LiquidHP12

2 1sin 0

sin

U Ur

r r

cos,2

r

brarU f

ff

3

0 0

3cos , 1 cos .

2 2l l s

S Ll s l s

RU E r U E r

r

2 20

0 2

1

3 91, ,0 cos , sin ,0 .

2 2s s l l s

s s s s sl s s

U U EU E Q U U

r r

Laplace equation for voltage in spherical coordinate system

f=l,s

Continuity of voltage at the

sphere surface

Remarkable result: Intensity of heat production (Q) is constant

(independent of r,)!!!

sl

sl

s

l

Q

Q

9

2 2

.41 l

s

Conclusion: spherical particle will be heated faster than liquid if

kappa L=0.04 kappa S=0.1 potato heats faster than liquid

kappa L=0.04 kappa S=0.01 potato heats slower than liquid

Model FEMINA-Solid in LiquidHP12

+100V -100V

SL

30oC 30oC

The solution is more complicated for solid particles having form of cubes or platelets.

Heat generated inside the particles is not uniform and depends upon particle orientation.

Numerical solution using FEM is shown as an example

Minced meat sample between electrodes with adjustable contact pressure

1 2

4

5

6

7

3

8

Description:1 Moving electrode2 Fixed elecrode3 Carrier4 Frame 5 Roller6 Weight 7 Silon fibre 8 Stop

Ohmic Heating-Minced meatHP12

Temperature measurement by optical fibres

T1 T2 T3

¼ H

½H

H



Meat sample

Contact layer

Tmmm 10

nkkkk pT 210

Electrode

1, , ,

( )EF km

HR T H p R p T

T S

Contact pressure

Thickness of sample


Contact surface and contact pressure

B

Hz

F

]2

1)1(

2

1)1ln()1)[((8)( 222

H

z

H

z

H

zEBzF

0

10

20

30

40

50

60

70

80

90

100

110

0 50 100 150 200 250 300t[s]

T[°

C]

P [W]

T [°C](průměr T2-T3)

T1 T2 T3

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T[°

C]

T1exp

T2exp

T3exp

T1mod

T2mod

T3mod

Comparison of temperatures T1 -T2 -T3 for pressure p5=8,175kPa.

Results


Modular Heating systemHP12

Power source + control


• Kettle• Boiling pot• Toaster


Applicators

Boiling pot Kettle

Toaster combined heating

Control unit

Power control

Control unit

Filter Rectifier

Pulse generator

Microprocessor

Pulse control

Power measurementConnector

Direct ohmic heating

Indirect ohmic heating

Temperature/power control


Direct ohmic heating

Contact heating

power control (triacs)

rectifier

pulse contro

l

MOSFET


Toaster design (combined heating)

MicrowaveHP12

Veneziano

Microwave heating operates at higher frequencies of electromagnetic waves 900 MHz or 2500 MHz. Polar molecules are heated due to excitation of vibrational mode and viscous friction

Intensity of electric field [V/m]

Frequency f , Permeability

Volumetric heat source [W/m3]

2

0 "Q f E

MicrowaveHP12

K – cathode

A – anode

M - electromagnet

Magnetron

Waveguide

MicrowaveHP12

xkxkAfEfQ '20

2*0 2cos"2cosh"2"

E A e e A kxikx ikx* cos . 2

2 *2 *

20

d Ek E

dx

Maxwell equation for distribution of electromagnetic field results to Helmholtz equation for intensity of electric field.

Example: One dimensional Helmholt equation describing E in a plate of meat in microwave oven

Nonuniform distribution of absorbed energy inside the plate of meat (6 cm thick) at microwave oven with different frequencies

HEAT PROCESSES

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