Europe // Technology
USING AIR AS A REFRIGERANT German cryogenic operator Coolinn is launching a sauna that uses Mirai’s COLD air-cycle technology to achieve -110°C (-166°F) temperatures. — by Tine Stausholm Mirai COLD 10 air-cycle refrigeration unit
October 12, German cryotherapy operator Coolinn will introduce its first cryogenic sauna with an air-cycle refrigeration unit, which boasts lower power consumption and reduced maintenance costs as compared to similar cascade vapor compression systems. A cryogenic sauna is essentially a reverse sauna − a very cold room without the humidity of a conventional sauna. Cryogenic saunas are used by athletes for “cryotherapy,” which can improve physical performance and aid recovery from injury. It can also be helpful in treating certain diseases like rheumatism, fibromyalgia and inflammatory ailments, according to Coolinn’s website (https://coolinn.de). The refrigeration technology driving the new cryogenic sauna, called Mirai COLD, is provided by European engineering and manufacturing company Mirai Intex. Founded in 2015, Mirai presented its first Mirai COLD unit at the Chillventa exhibition in Nuremberg, Germany, in 2016.
Accelerate Magazine // October 2019
Mirai COLD units use only air as a refrigerant to achieve very low temperatures, from -40°C (-40°F) to -110°C (-166°F), in a one-stage system. Traditional low-temperature equipment typically use threestage cascade systems charged with R404A, R23 and R14 – all of which have extremely high GWPs of 3,922, 14,800 and 5,700, respectively. Mirai COLD currently comes in three versions, with power capacities of 10kW, 15kW, and 22kW and cooling capacities of up to 5kw/1.4TR, up to 8kW/2.3TR and up to 12kW/3.4TR, respectively); each unit can be tailored to serve a particular temperature. A new model, called the Mirai Cryo, uses nitrogen instead of air and can achieve temperatures from -130°C (-202°F) to -160°C (-256°F), with a cooling capacity of up to 3kW (0.9TR). Mirai Cryo is not commercially available yet, though pre-orders can already be placed. Mirai is not the only company developing air-cycle low-temperature technology. Japanese manufacturers Mayekawa and
Mitsubishi have also developed air-cycle machines, mainly for rapid freezing of tuna.
The turbo module The core of Mirai’s low-temperature products is the “turbo module.” It consists of a centrifugal compressor and an expander mounted on opposite ends of the same drive shaft, which is driven by a speed-controlled electric motor. The mounting of the compressor and the expander on the same shaft makes the system up to 30% more energy efficient than similar systems, according to Mirai. In the Mirai COLD system, air is heated in the compressor. The high-pressure warm air is sent through a series of heat exchangers where it is cooled, before finally being expanded in the turbo expander to reach very low temperatures and low pressure. This type of air-cycle technology is also known as the BellColeman cycle and has been known for over 100 years.