Si-SiC Radiant Tubes

“ Faster, Cheaper, Better ”

What is a Si-SiC Composite?

50% Si + 50% SiC by volume, metallurgical grade Silicon metal with 80 mesh SiC particles

Rationale for  Si-SiC Radiant Tubes

• Conventional = Very Long Life
• Enabling = Very High Temp
• Productivity = Higher Throughput

Conventional Savings

• Excellent creep properties to 1343°C (2450°F) = No tube droop or distortion
• Excellent resistance to carburization = No tube corrosion or embrittlement
• Excellent thermal shock & low CTE = No fracture in heat-up or cool-down

18+ Years Continuous Carburizing Service

Compare Alloys to Si-SiC at 1343°C (2450°F) = ~8.5 KPSI Stress at <0.6% Strain

Si-SiC Composite vs. 600 Alloy (after just 1 hour)

No deformation for the INEX tube tested 360 hours at 1343°C (2450°F).

Metal Alloy Tubes after < 24 Months

Si-SiC Enables Processing > 982°C (1800°F)

• Stainless Steel Aging
• Powdered Metal Sintering
• Minerals Processing

What Limits Furnace Throughput ?

• Metallurgical objectives
• Mechanical constraints
• Radiant tube heat flux
• NOT refractory or burners

What is Heat Flux ?

Heat flux or thermal flux is the rate of heat energy transfer through a given surface,
per unit surface.
(BTU/hour)/inch²
kW/meter²    =   2.2 BTU/hr/in²

Design Criteria for Radiant Tubes

Metal Alloy Tubes at 982°C (1800°F) :

• Conservative = 50 BTU/hr/in²
• Nominal = 55 BTU/hr/in²
• Aggressive = 60 BTU/hr/in²

Max Service temperature depends on Alloy & Atmosphere

Si-SiC Composite Tubes @ 982°C (1800°F) :

Nominal = 110 BTU/hr/in²
Maximum Service Temp is 1343°C (2450°F)

Parasitic or Standing Losses average ~20%

• Wall Losses
• Opening Losses
• Cooling & Conveyor Losses
• Fixtures, Trays, Baskets
• Storage Losses (Batch Furnace)

FASTER Cycle Time

• Increases furnace throughput
• Reduces per unit standing (parasitic) losses
• Does NOT reduce process energy required
• Does NOT improve combustion  efficiency

Compare Alloys to Si-SiC at 1343°C (2450°F) = ~8.5 KPSI Stress @ <0.6% Strain

CHEAPER Furnace Operation

• 25% Increase in Throughput  = 25% More Load (Reducing Furnace Operating Hours by 20%)
• Process Energy Required Remains the Same i.e. Work on Load is Unchanged = 0.0
• Standing Energy Losses of 20% Eliminated  for the 20% of Furnace Hours Reduced  = +4.0%
• Offset Somewhat by Higher Exhaust Losses (1906°F > 1999°F) Available Heat   =  -2.0%

“SAME-WORK” SCENARIO

“Back of the Envelope” Savings

1200 more hours/year/furnace
5.2 hours/cycle
231 cycles/year
2,000 lbs/cycle
461,538 lbs/year
$1.10 Sales Value / lb
$507,692 Increased Sales
50% Variable Cost
$253,846 EBIT / furnace

Compare with:

• New Si-SiC Radiant Tubes <$25,000
• New EGR-type Burners, if Needed <$25,000
• Used or New Furnace

Why Are Si-SiC Tubes BETTER ?

• Proven Life >18 Years in Carburizing
• High Temp Processing to 2300°F
• 25% More Product Throughput
• < 12 Months Payback

FASTER, CHEAPER, BETTER @ 1400°F - 2300°F