Why is stainless steel better than aluminum in cryogenics?
In the field of cryogenics, most heat exchangers are made of aluminum. However, stainless steel offers much more significant advantages and remains less well known.
This article provides a comparison between aluminum and stainless steel heat exchangers (Ambient temperature: 25°C).
Thermal conductivity
If the temperature difference between the two flows is very small, axial convection exceeds radial convection: a fin effect is generated, which requires a longer heat exchanger.
Consequences :
Additionally, it is possible to perform electrolytic polishing of stainless steel to reduce the emissivity on the external surface of the heat exchanger. The environment will therefore have a negligible effect on the exchanger.
(This further reduces the emissivity of a stainless steel exchanger, which is already lower than that of an aluminum exchanger due to their difference in size.)
Consequences :
- No more pressure losses
- Less compact
- Higher cost
Additionally, it is possible to perform electrolytic polishing of stainless steel to reduce the emissivity on the external surface of the heat exchanger. The environment will therefore have a negligible effect on the exchanger.
(This further reduces the emissivity of a stainless steel exchanger, which is already lower than that of an aluminum exchanger due to their difference in size.)
Aluminum | 150 à 220 W.m⁻¹.K⁻¹ |
|---|---|
Stainless steel | 14 W.m⁻¹.K⁻¹ |
Mechanical resistance (Elastic limit)
The thickness of the aluminum exchanger must be greater to compensate for its lower mechanical resistance.
Aluminum | 50 at 150 MPa |
|---|---|
Stainless steel | 220 at 270 MPa |
Density
Aluminum is less dense, but as its thickness and volume must be increased to ensure sufficient mechanical strength, this advantage becomes insignificant, if not non-existent.
Aluminum | 2,7 t/m³ |
|---|---|
Stainless steel | 8 t/m³ |
Manufacturing (Welding process)
Brazing requires a special oven, which entails a significant investment and can pose a problem if the exchanger is large.
Only stainless steel, combined with TIG welding, makes it possible to achieve a very low leak rate: up to 10⁻⁹ mb.l.s⁻¹.
Another advantage: A TIG welded exchanger is easier to repair than a brazed exchanger.
Only stainless steel, combined with TIG welding, makes it possible to achieve a very low leak rate: up to 10⁻⁹ mb.l.s⁻¹.
Another advantage: A TIG welded exchanger is easier to repair than a brazed exchanger.
Aluminum | Brazing |
|---|---|
Stainless steel | Brazing / TIG welding (D.A.T.E.) |
Pipe interface welding
Welding aluminum with stainless steel is difficult and has a shorter lifespan.
At cryogenic temperature, the very different expansion coefficients of these materials will cause leaks.
At cryogenic temperature, the very different expansion coefficients of these materials will cause leaks.
Aluminum | Aluminum to Stainless Steel |
|---|---|
Stainless Steel | Stainless Steel to Stainless Steel |
Additional benefit:
The cooling time of a TIG welded stainless steel exchanger allows it to withstand greater temperature gradients compared to aluminum and brazed stainless steel exchangers.
This translates to:
Faster cooling
Better resistance to thermal fatigue (more thermal cycles possible)
This translates to:
Faster cooling
Better resistance to thermal fatigue (more thermal cycles possible)
Usage range (depends on design)
Temperature range | 1,5K – 750K |
|---|---|
Pressure range | Up to 50 bar |
Mass flow | Generally less than 500 g/s, up to several kg/s in special cases |
Pressure drop | Can go down to a few pascals |
