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Progettare un forno
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Sulla base di alcuni dati di input imposti dal cliente:
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L’ingegnere è chiamato a dimensionare il forno
determinando i materiali costitutivi:
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dimensioni della zona di lavoro,
temperatura massima di esercizio,
velocità di raffreddamento e riscaldamento,
Potenza massima richiesta
ambiente gassoso e pressione
Coibentazione,
elementi riscaldanti,
Termocoppie
elementi termostrutturali
e le specifiche costruttive:
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Potenza necessaria,
dimensioni esterne, geometria e lunghezza degli
elementi resistivi riscaldanti, elettronica di controllo
Thermal Technology
Furnace geometries
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Thermal Technology
Crucible furnace
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Top Loading
Bottom loading
Thermal Technology
Gas furnace
Vista interna
della camera di
combustione
Forno a crogiuolo ribaltabile
per colaggio di materiali fusi
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http://www.fossati.com/
Thermal Technology
Tungsten furnace
Thermal Technology
Induction furnace for Czochralski
Technique
Thermal Technology
Graphite furnace geometries
Hea
t
Zon
e
Thermal Technology
Graphite furnace and accessories
Thermal Technology
Calcolo della potenza
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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
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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
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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 barrier
A = area
T = temperature
d = thickness of barrier
Thermal Technology
Stefan-Boltzmann Law
Thermal Technology
The Law of Dulong e Petit
Thermal Technology
Convezione
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La potenza termica scambiata
per convezione tra una
superficie a temperatura T2 e
un fluido a T1 è
Pc = hS(T2 - T1)
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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?
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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
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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
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Contact Sensors
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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
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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
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The Wiens’ law:
 (max) ~ 2900/T
Thermal Technology
Contact sensors
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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 "oldfashioned" thermostats based on the fact that two
metals expand at different rates as a function of
temperature.
Thermal Technology
Thermocouples
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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
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A = thermopower of metal A
In Chromel-Alumel (Type K)
(A - B) ~ 40 µV/°C (22 µV/°F)
Thermal Technology
Thermocouple classification
Thermocouple Type
B
C
E
J
K
N
R
S
T
Names of Materials
Useful Application Range
Platinum30% Rhodium (+)
2500 -3100F
Platinum 6% Rhodium (-)
W5Re Tungsten 5%
Rhenium (+)
1370-1700C
W26Re Tungsten 26%
Rhenium (-)
Chromel (+)
1650-2315C
Constantan (-)
Iron (+)
95-900C
200-1400F
Constantan (-)
Chromel (+)
95-760C
200-2300F
Alumel (-)
Nicrosil (+)
95-1260C
1200-2300F
Nisil (-)
Platinum 13% Rhodium
(+)
650-1260C
Platinum (-)
Platinum 10% Rhodium
(+)
Platinum (-)
Copper (+)
Constantan (-)
3000-4200F
200-1650F
1600-2640F
870-1450C
1800-2640F
980-1450C
-330-660F
-200-350C
Thermal Technology
Thermocouple Color Codes
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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
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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:
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Length of Thermocouple Probe:
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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.
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:
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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
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Temperatura di classificazione
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Composizione chimica
Caratteristiche morfologiche
Proprietà misurate a temperatura ambiente:
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Temperatura limite massima e temperatura limite di
uso continuo
Densità geometrica,
Resistenza alla trazione e alla compressione
Calore specifico
Proprietà ad alta temperatura:
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Conducibilità termica
Ritiro lineare permanente
Thermal Technology
Thermal conductivity of insulating fibres
Thermal Technology
Caratteristiche principali degli elementi
riscaldanti
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Dati in input
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Coefficiente di resistenza
Resistività alla temperatura di esercizio
Potenza richiesta
Carico massimo raccomandato per unità di superficie
espresso in W/cm2
Dati in output
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Dimensioni (diametro o altro tipo di sezione)
Lunghezza
Geometria spirale hairpin
Tensione
Thermal Technology
Molybdenum disilicide heating elements
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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
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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% Silica
5% Other trace materials
Thermal Technology
Heating elements maximum temperature
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Maximum temperatures in Centigrade for
various resistors when exposed to different
atmospheres
Atmosphere Ferritic Alloys Silicon Carbide Molybdenum Disilicide
Kanthal A-1 Kanthal Crusilite
Kanthal Super 1800
Air
1400
1700
1800
Nitrogen
1050 - 1200
1400
1700
Dry Hydrogen
1400
1200
1400
Moist
1500
Hydrogen
Exogas
1150
1250 / 1400
1700
Endogas
1050
1250 / 1400
1450
Vacuum
1200
1100 - 1500
Thermal Technology
Progettazione degli elementi riscaldanti
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Calcolo della resistenza massima
RTmax = V2/PT
RTmax = Rc(1 + a*Tmax)
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Resistenza lineare Rl (ohm/m)
Rl = (resistività) / 
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Rc = resistenza misurata a bassa temperatura
a = coefficiente di resistenza
= resistività (quantità tabulata in microhm*cm)
 = sezione del conduttore
Rl (ohm/m) =  (microhm*cm)/100*(mm2)
Lunghezza del conduttore
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L = Rc/Rl
Thermal Technology
Progettazione degli elementi riscaldanti
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Potenza radiata per cm2 L (surface loading)
L = PT/S
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S = superficie totale dell’elemento riscaldante
Nel caso di un filo:
S = L* 
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Curva di riferimento per riscaldatori in MoSi2
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Thermal Technology
Temperature controllers
Thermal Technology
Regolatori a microprocessore 1
Thermal Technology
Regolatori a microprocessore 2
Thermal Technology
Power controllers
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Solid state relay
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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
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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
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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
Thermal Technology