ESA Document - Emits - ESA

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HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 173 of 422
reported in the Apollo programme and affected the MIR station in 1997 (leaks in one of the core
loops, loss of the Kvant external loop). For external loops, fluids with a low freezing point are
required: silicone-based fluid, Freons or ammonia. The progressive elimination of chlorines
(CFC in 1992, HCFC by 2030) from the refrigerant industry raises a challenge in finding
alternatives that can satisfy both thermal and safety requirements.
Fluid Basic features Data
Water High freezing point => not suitable in external
loops
Low vapour pressure => not suitable in two
phase loops
Not compatible with aluminium
Ammonia High vapour pressure
No corrosion with aluminium
Highly toxic
Water - glycol mixture
(e.g. LZ-TK-5)
Needs proper inhibitor to prevent corrosion
High viscosity at low temperature
Slightly toxic, flammability concern
Specific heat: 4180 J/kg/K
Density: 998 kg/m3
Thermal conductivity: 0.604 W/m/K
latent heat: 2.2E6 J/kg (at 100C)
Freezing point: -78C at 1b; evap.: -33C at 1b
Specific heat: 4600 J/kg/K (at 0C)
Density: 642 kg/m3 (at 0C)
Latent heat: 1.27E6 J/kg (at 0C)
Freezing point: -18C at 1b
Specific heat: 3530 J/kg/K
Density: 1057 kg/m3
Thermal conductivity: 0.43 W/m/K
Silicone mixture
Very low freezing point but high viscosity and Freezing point: -134C at 1b
(e.g. Polymethylsiloxane)
low specific heat
Specific heat: 1800 J/kg/K
Corrosive to certain metals
Density: 912 kg/m3
Toxic, flammability concern
Thermal conductivity: 0.106 W/m/K
Flash point: 56C
Freon 114 Ozone depletion fluid (ODP) Freezing point: -94C at 1b
Specific heat: 998 J/kg/K
Density: 838 kg/m3
Thermal conductivity: 0.066 W/m/K
Table 3-25: Working fluids properties
Note that, if gases (CO2, air) could also be used over a wider temperature range, their poor
cooling effectiveness compared to liquid restricts their usage to very specific conditions.
Design selection
• For large systems such as a transfer vehicle, CPLs are not suitable as a primary loop
because of high heat loads and long line length (>10kW, >20 m), and a mechanical
pumped loop is preferred.
• Candidate fluids for a biphasic system include ammonia, propylene (excluding
freons), which tends to discard their use (at least in a extensive way) in a habited
module for toxicity reasons. Selection of a friendly fluid such as water in this
environment is preferred.
• A single-phase primary loop currently appears to be the best solution when
considering the robustness of such system, although at a certain cost for the system.
So far there is no evidence that a biphasic system, although higher performance,