Progettare un fornoSulla 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 lunghezz
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
Furnace geometries
Crucible furnace
Top Loading Bottom loading
Gas furnace
Vista interna della camera di combustione
Forno a crogiuolo ribaltabile per colaggio di materiali fusi
http://www.fossati.com/
Tungsten furnace
Induction furnace for Czochralski Technique
Graphite furnace geometries
Heat Zone
Graphite furnace and accessories
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
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 barrier
A = area
T = temperature
d = thickness of barrier
Stefan-Boltzmann Law
The Law of Dulong e Petit
Convezione
300:12000 Acqua convenzione forzata 30:300 Aria convenzione forzata 6:30 Aria convenzione naturale h Mezzo 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
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.
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
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
Pyromethers operating principle
The Wiens’ law:
(max) ~ 2900/T
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.
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)
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