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Within a ULT freezer, there are parts which support cooling passively (e.g. insulation or gaskets) as well as active cooling parts as the compressors. The compressors are literally the core of every freezer.
How can you create coldness?
The compressor creates coldness indirectly: Within the compressor, the cooling gas is compressed. The gas leaves the compressor while being pressurized and heated-up gas. After the compressor, the compressed gas reaches the condenser where the gas liquefies. The temperature of the liquid is still high. By passing a so-called expansion valve, the pressure of the liquid is drastically reduced within a very short timeframe. By this flash-like evaporation from liquid to gas phase, coldness is created. This coldness within the cooling pipe is transferred to the cooling loops which enclose the freezer chamber. Locally, the coldness is passed to the stainless-steel interior walls of the inner freezer chamber. The cooling loops are even visible at the beginning of the cooling process when you cool down the freezer.
Why are there 2 compressors?
The major suppliers of ULT freezers trust in 2-stage compressor systems. Within this 2-stage or 2-loop systems, there are two temperature levels. Simplified, the temperature of the first coolant is cooled down from room temperature to -45 °C by the first compressor. The heat exchanger transfers the coldness of -45°C from the first stage to the second stage. Afterwards, by using the compressor of the second stage, the coolant of the second stage is cooled down from -45 °C to -86 °C. This split into two steps enables a smooth and reliable running of the single compressor per step as they work in a kind of comfort zone. The used coolants work in a very efficient way as their pressure and temperature range are optimized for the first stage respective the second stage. When using just one compressor or even a Stirling engine to cool down from +20 °C room temperature to -86 °C, more than 100 °C must be spanned in one single step. Technically, this works but the load for the cooling system is much higher.
The 2-stage system is very durable approach, supported in well-known ULTs by high-value compressors from benchmark suppliers.
How can you extend the lifetime of your compressors?
Standard high-quality compressors have a long lifetime but there are still some aspects which might extend their lifetime: