Progettare un forno

1. 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

2. Furnace geometries Thermal Technology

3. Crucible furnace • Top Loading Bottom loading Thermal Technology

4. Gas furnace Forno a crogiuolo ribaltabile per colaggio di materiali fusi • http://www.fossati.com/ Vista interna della camera di combustione Thermal Technology

5. Tungsten furnace Thermal Technology

6. Induction furnace for Czochralski Technique Thermal Technology

7. Heat Zone Graphite furnace geometries Thermal Technology

8. Graphite furnace and accessories Thermal Technology

9. 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

10. 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 Thermal Technology

11. Stefan-Boltzmann Law Thermal Technology

12. The Law of Dulong e Petit Thermal Technology

13. 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 Thermal Technology

14. 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

15. 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

16. 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

17. Pyromethers operating principle • The Wiens’ law: (max) ~ 2900/T Thermal Technology

18. 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

19. 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

20. Thermocouple classification Thermal Technology

21. Thermocouple Color Codes • Thermocouple wiring is color coded by thermocouple types. Different countries utilize different color coding. Thermal Technology

22. 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 ThermocoupleProbe: • 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

23. 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

24. Thermal conductivity of insulating fibres Thermal Technology

25. 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

26. Molybdenum disilicide heating elements • Moly-D heating elements are manufactured by powder metallurgy. They consist of molybdenum disilicide withadditives that prevent recrystallization. Since Moly-D iscompletely stable up to 1800ºC (3270ºF), it surpassesother heating element types for high temperatureperformance. • The resistance of Moly-D elements to oxidation lies inthe formation of an impermeable quartz, or glass-likeprotective layer which re-forms when heated if damagedin operation. • Moly-D elements become somewhat ductile atapproximately 1200ºC (2190ºF). Thermal Technology

27. Silicon carbide furnace Thermal Technology

28. 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

29. Heating elements maximum temperature • Maximum temperatures in Centigrade for various resistors when exposed to different atmospheres Thermal Technology

30. 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

31. 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

32. Temperature controllers Thermal Technology

33. Regolatori a microprocessore 1 Thermal Technology

34. Regolatori a microprocessore 2 Thermal Technology

35. 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

36. Graphite furnace • Heating elements are high-density graphite. • insulation is all graphite feltand 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

37. Forni in MoSi2 Thermal Technology

38. Crucibles for dental industry Thermal Technology

39. Electrode thermal and electrical insulation Thermal Technology