Dual Compressor Refrigeration: The Technical Design Behind the Cold Bath's Ability to Reach -70 ℃

The minimum temperature of HZBD-1177's cold bath reaches -70 ℃, far below the usage range of most engine coolants (usually below -45 ℃ is already an extreme requirement). The implementation of this indicator relies on the combination design of dual compressor refrigeration technology and anhydrous ethanol as the cooling medium.

Why do we need a dual compressor

The refrigeration limit of a single-stage compressor is usually around -40 ℃, which is determined by the thermodynamic properties of the refrigerant - when the compression ratio is too high, the efficiency drops sharply, and a single compressor can no longer economically and reliably maintain lower temperatures. To achieve -70 ℃, it is necessary to use cascade refrigeration (two compressors connected in series, each responsible for different temperature zones).

The 800W refrigeration power of HZBD-1177, combined with dual machine cascade, enables the cold bath to maintain a stable temperature of -70 ℃, covering all the requirements for measuring the freezing point of the coolant and leaving sufficient margin for the determination of extreme low temperatures (such as special coolant with freezing points below -60 ℃).

Cold bath solution: selection of anhydrous ethanol

Anhydrous ethanol (alcohol) is used as the cooling medium in the cold bath of HZBD-1177, instead of water or glycerol. The reason is very direct: the freezing point of anhydrous ethanol is below -115 ℃, and it maintains complete fluidity at a working temperature of -70 ℃; Good thermal conductivity and fluidity ensure uniform temperature throughout the bath; Good compatibility with instrument materials (stainless steel, sealing materials).

If water is used, it will freeze below 0 ℃; Glycerol has extremely high low-temperature viscosity and can hardly flow below -50 ℃, affecting temperature uniformity.

Alcohol Level Management: The Most Important Maintenance Point

The first preparation item in the instruction manual is to check the alcohol level in the cold bath before starting up, ensuring that the alcohol level exceeds the liquid level of the refrigeration coil and sample in the cold bath.

Why is it so important: If the refrigeration coil is exposed to air, it means that there is no liquid on the surface of the evaporator to take away heat, and the temperature of the refrigerant inside the tube will abnormally rise, causing damage to the compressor. Meanwhile, if the sample position is higher than the alcohol level, there is no heat transfer medium between the test tube and the bath, resulting in extremely low cooling efficiency of the sample and inaccurate measurement results.

The first use must be supplemented with alcohol, and when alcohol is insufficient, it must be replenished before conducting the test - this requirement is not exceptional.