Thermal Technology

Progettare un forno

Sulla base di alcuni dati di input imposti dal cliente: dimensioni della zona di lavoro, temperatura massima di esercizio, velocità di raffreddamento e riscaldamento, Potenza massima richiesta ambiente gassoso e pressione

L’ingegnere è chiamato a dimensionare il forno determinando i materiali costitutivi: Coibentazione, elementi riscaldanti, Termocoppie elementi termostrutturali

e le specifiche costruttive: Potenza necessaria, dimensioni esterne, geometria e lunghezza degli

elementi resistivi riscaldanti, elettronica di controllo

Thermal Technology

Furnace geometries

                                      

                                   

Thermal Technology

Crucible furnace

Top Loading Bottom loading

                              

Thermal Technology

Gas furnace

Forno a crogiuolo ribaltabile per colaggio di materiali fusi

http://www.fossati.com/

Vista interna della camera di combustione

Thermal Technology

Tungsten furnace

Thermal Technology

Induction furnace for Czochralski Technique

Thermal Technology

Graphite furnace geometries

                 

                       

                   

                         

               

                      

Heat Zon

e

                                

                           

                              

                           

Thermal Technology

Graphite furnace and accessories

Thermal Technology

Calcolo della potenza

Un forno richiede energia per riscaldare un materiale e per conservarlo ad una certa temperatura

La potenza totale PT è data da

PT = PM + Pr + Pi + PB + Pc

PM= potenza per riscaldare la massa termica interna

Pr = potenza persa per perdite radiative

Pi = calore trasmesso attraverso la coibentazione

PB = calore perso per i ponti termici

Pc = calore perso per convenzione

La potenza di mantenimento PH

PH = Pr + Pi + PB + Pc

Thermal Technology

Heat Conduction

Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. Energy is transferred down the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones.

For heat transfer between two plane surfaces, such as heat loss through the wall of a house, the rate of conduction heat transfer is:

Q = heat transferred in time =k = thermal conductivity of the barrierA = areaT = temperatured = thickness of barrier

Thermal Technology

Stefan-Boltzmann Law

The energy radiated by a blackbody radiator per second per unit area is proportional to the fourth power of the absolute temperature and is given by

For hot objects other than ideal radiators, the law is expressed in the form:

where e is the emissivity of the object (e = 1 for ideal radiator). If the hot object is radiating energy to its cooler surroundings, the net radiation loss rate takes the form

The Stefan-Boltzmann relationship is also related to the energy density in the radiation in a given volume of space.

Thermal Technology

The Law of Dulong e Petit

From the translational degrees of freedom you get 3kT/2 of energy per atom.

Energy added to solids takes the form of atomic vibrations and that contributes three additional degrees of freedom and a total energy per atom of 3kT.

The specific heat at constant volume should be just the rate of change with temperature (temperature derivative) of that energy.

The specific heats of copper and lead are quite similar on molar basys but different on weight:

Thermal Technology

Convezione

La potenza termica scambiata per convezione tra una superficie a temperatura T2 e un fluido a T1 è

Pc = hS(T2 - T1)

h è il coefficiente di scambio termico per convenzione (W/m2K), dipende dalla geometria della suerficie dalla velocità e dalle proprietà fisiche del fluido

Mezzo h

Aria convenzione naturale

6:30

Aria convenzione forzata

30:300

Acqua convenzione forzata

300:12000

Thermal Technology

What is Temperature?

In a qualitative manner, we can describe the temperature of an object as that which determines the sensation of warmth or coldness felt from contact with it.

When two objects are put in contact the object with the higher temperature cools while the cooler object becomes warmer until a point is reached after which no more change occurs.

When the thermal changes have stopped, we say that the two objects (physicists define them more rigorously as systems) are in thermal equilibrium.

We can then define the temperature of the system by saying that the temperature is that quantity which is the same for both systems when they are in thermal equilibrium.

Thermal Technology

Absolute temperature

From statistical mechanics T characterize the internal energy of a system of N identical indistinguishable particles (Maxwell Boltzman distribution).

N = n1 + n2 + n3 + …….

ni = Ngie- Ei

The partition function of a system in statistical equilibrium is defined as:

Z = gie- Ei

The internal energy is calculated from the average energy

U = NEaverage

E average = -d(lnZ)/d

= kT

Thermal Technology

Temperature sensors

Contact Sensors Contact temperature sensors measure their

own temperature. One infers the temperature of the object to which the sensor is in contact by assuming or knowing that the two are in thermal equilibrium, that is, there is no heat flow between them.

Many potential measurement error sources exist from too many unverified assumptions. Temperatures of surfaces are especially tricky to measure by contact means and very difficult if the surface is moving.

Non-Contact Sensors Most commercial and scientific non-contact

temperature sensors measure the thermal radiant power of the Infrared or Optical radiation that they receive and one then infers the temperature of an object from which the radiant power is assumed to be emitted

Thermal Technology

Pyromethers operating principle

The Wiens’ law:

(max) ~ 2900/T

Thermal Technology

Contact sensors

Thermocouples Based on the Seebeck effect that occurs in electrical conductors that experience a temperature gradient along their length.

Thermistors Thermistors are tiny bits of inexpensive semiconductor materials with highly temperature sensitive electrical resistance.

Liquid-In-Glass Thermometers The thermometer that checked your fever when you were young was a specialized version of this oldest and most familiar temperature sensor.

Resistance Temperature Detectors (RTDs) RTDs are among the most precise temperature sensors commercially used. They are based on the positive temperature coefficient of electrical resistance.

Bimetallic Thermometers The simple mechanical sensor that works in most "old-fashioned" thermostats based on the fact that two metals expand at different rates as a function of temperature.

Thermal Technology

Thermocouples

Thermocouples are based on the principle that when two dissimilar metals are joined a predictable voltage will be generated that relates to the difference in temperature (Seebeck effect)

The AB connection is called the "junction". When the junction temperature, TJct, is different from the reference temperature, TRef, a low-level DC voltage, E , will be available at the +/- terminals.

The value of E depends on the materials A and B, on the reference temperature, and on the junction temperature.

E = ∫(Tjcs,Tref)(A - B)dT

A = thermopower of metal A In Chromel-Alumel (Type K)

(A - B) ~ 40 µV/°C (22 µV/°F)

Thermal Technology

Thermocouple classification

Thermocouple Type Names of Materials Useful Application Range

B

Platinum30% Rhodium (+)

Platinum 6% Rhodium (-)

2500 -3100F

1370-1700C

C

W5Re Tungsten 5% Rhenium (+)

W26Re Tungsten 26% Rhenium (-)

3000-4200F

1650-2315C

E

Chromel (+)

Constantan (-)

200-1650F

95-900C

J

Iron (+)

Constantan (-)

200-1400F

95-760C

K

Chromel (+)

Alumel (-)

200-2300F

95-1260C

N

Nicrosil (+)

Nisil (-)

1200-2300F

650-1260C

R

Platinum 13% Rhodium (+)

Platinum (-)

1600-2640F

870-1450C

S

Platinum 10% Rhodium (+)

Platinum (-)

1800-2640F

980-1450C

T

Copper (+)

Constantan (-)

-330-660F

-200-350C

Thermal Technology

Thermocouple Color Codes

Thermocouple wiring is color coded by thermocouple types. Different countries utilize different color coding.

United States ASTM:

British BS4937: Part 30: 1993:

Thermal Technology

Selecting a thermocouple

The selection of the optimum thermocouple type (metals used in their construction) is based on application temperature, atmosphere, required length of service, accuracy and cost

Wire Size of Thermocouple: When longer life is required for the higher temperatures, the

larger size wires should be chosen. When sensitivity is the prime concern, the smaller sizes should be used.

Length of Thermocouple Probe: Since the effect of conduction of heat from the hot end of the

thermocouple must be minimized, the thermocouple probe must have sufficient length. Unless there is sufficient immersion, readings will be low. It is suggested the thermocouple be immersed for a minimum distance equivalent to four times the outside diameter of a protection tube or well.

Location of Thermocouple: Thermocouples should always be in a position to have a

definite temperature relationship to the work load. Usually, the thermocouple should be located between the work load and the heat source and be located approximately 1/3 the distance from the work load to the heat source.

Thermal Technology

Caratteristiche principali degli elementi coibentanti

Temperatura di classificazione Temperatura limite massima e temperatura limite di

uso continuo Composizione chimica Caratteristiche morfologiche Proprietà misurate a temperatura ambiente:

Densità geometrica, Resistenza alla trazione e alla compressione Calore specifico

Proprietà ad alta temperatura: Conducibilità termica Ritiro lineare permanente

                     

Thermal Technology

Thermal conductivity of insulating fibres

Thermal Technology

Caratteristiche principali degli elementi riscaldanti

Dati in input Coefficiente di resistenza Resistività alla temperatura di esercizio Potenza richiesta Carico massimo raccomandato per unità di superficie

espresso in W/cm2

Dati in output Dimensioni (diametro o altro tipo di sezione) Lunghezza Geometria spirale hairpin Tensione

                                  

Thermal Technology

Molybdenum disilicide heating elements

Moly-D heating elements are manufactured by powder metallurgy. They consist of molybdenum disilicide with additives that prevent recrystallization. Since Moly-D is completely stable up to 1800ºC (3270ºF), it surpasses other heating element types for high temperature performance.

The resistance of Moly-D elements to oxidation lies in the formation of an impermeable quartz, or glass-like protective layer which re-forms when heated if damaged in operation.

Moly-D elements become somewhat ductile at approximately 1200ºC (2190ºF).

Thermal Technology

Silicon carbide furnace

                           

Thermal Technology

SiC radiators

The SiCrad material (from Furnace Concepts) is a cast ceramic material, which is fired at elevated temperature. Because of its excellent thermal shock resistance and high thermal conductivity it is an excellent material for use as immersion protection and radiant tubes for use in many molten materials.

As thermocouple protection tubes the material offers excellent thermal conductivity, which allows the optimum temperature measurement. SiCrad also exhibits a resistance to abrasion and wetting by molten metals.

When used as a radiant tube for electric and gas heating the SiCrad offers the same characteristics as mentioned above coupled with the ability to uniformly distribute and efficiently dissipate the energy from the heat source to the molten bath. 

omposition Mechanical Properties 64% Silicon carbide 27% Alumina 4% Silica5% Other trace materials 

Thermal Technology

Heating elements maximum temperature

Maximum temperatures in Centigrade for various resistors when exposed to different atmospheres

Atmosphere Ferritic Alloys Kanthal A-1

Silicon Carbide Kanthal Crusilite

Molybdenum Disilicide Kanthal Super 1800

Air 1400 1700 1800

Nitrogen 1050 - 1200 1400 1700

Dry Hydrogen 1400 1200 1400

Moist Hydrogen

- - 1500

Exogas 1150 1250 / 1400 1700

Endogas 1050 1250 / 1400 1450

Vacuum 1200 - 1100 - 1500

Thermal Technology

Progettazione degli elementi riscaldanti

Calcolo della resistenza massima

RTmax = V2/PT

RTmax = Rc(1 + a*Tmax)

Rc = resistenza misurata a bassa temperatura a = coefficiente di resistenza

Resistenza lineare Rl (ohm/m)

Rl = (resistività) / = resistività (quantità tabulata in microhm*cm) = sezione del conduttore Rl (ohm/m) = (microhm*cm)/100*(mm2)

Lunghezza del conduttore L = Rc/Rl

Thermal Technology

Progettazione degli elementi riscaldanti

Potenza radiata per cm2 L (surface loading)

L = PT/S S = superficie totale dell’elemento riscaldante Nel caso di un filo: S = L*

Curva di riferimento per riscaldatori in MoSi2

Thermal Technology

Temperature controllers

             

                     

Thermal Technology

Regolatori a microprocessore 1

Thermal Technology

Regolatori a microprocessore 2

Thermal Technology

Power controllers

Solid state relay Solid state relays incorporate SCRs (or triacs) and

their isolation/control electronics in a convenient modular package. They are available in single phase and three phase versions with low voltage DC or line voltage AC control voltages.

Phase angle mode power controllers The control electronics turn on the SCRs over a

portion of the AC sine wave in proportion to the control input. The result is a continuously variable voltage.

Thermal Technology

Graphite furnace

Heating elements are high-density graphite. insulation is all graphite felt and carbon powder. A graphite radiation shield to isolate the insulation from

the hot zone and facilitate element replacement. The furnace shell is of double-wall water-cooled

stainless steel construction. Bulkheads are nickel-plated aluminum with integral

water cooling channels and O-ring seals. Temperature Sensors: Recommended are type C with

a tungsten-coated moly sheath thermocouple for temperatures to 2000°C, or a radiation pyrometer for temperatures above 2000°C.

Thermal Technology

Forni in MoSi2

                              

Thermal Technology

Crucibles for dental industry

Thermal Technology

Electrode thermal and electrical insulation