Air as a refrigerant is for free, safe and free from rules.
In 2019, we realized two senssations with the air cycle refrigeration systems from Mirai Intex (see references) and have been asked a lot about the new technology ever since. On this page we will set up information material on this cold air technology.
Basic video with Professor Michael Kauffeld
The cold air cooling technology from Mirai Intex works oil-free with only air as the refrigerant. For this purpose, Mirai Intex has developed a unique turbocompressor expander. This is air-borne and the compressor and expansion wheel are on a shaft, so that the energy of relaxation is fed back to the compression. The pressures are a maximum of 4 barg, which is comparable to a bicycle tire.
This makes the technology safe for people and the environment, long-lasting, reliable, quiet and maintenance friendly.
Air from the refrigerator or the heat exchanger is suked and compressed (1 → 2). The pressure and temperature increase. The existing pressure remains constant until relaxation. The process temperature in the gas cooler is cooled by ambient air (2 → 3). Energy is recovered in the internal heat exchanger, the so-called recuperator, the warmer air from point 3 heats the intake air from point 1. In reverse, the air cools further from 3 → 4. This is followed by relaxation in a turboexpander. The work done during relaxation is manifested in a reduction in pressure and temperature. In contrast to the steam cooling process, the energy intake does not function through the phase change, but through the warming of the air (5 → 6). Air is continuously suked from the consumer and cools the air in the recuperator that is already in the circuit (6 → 1). Then the cycle starts again (1).
In cold air machines, the expander is mounted on a shaft with the frequency-controlled electric motor and the compressor. With this arrangement, the work gained in the work-performing expansion is supplied to the compression. The shaft is air-borne, which enables completely lubricant-free operation.
How Mirai Intex machines work
The systems are essentially available in two versions.
as a closed machine - with an integrated heat exchanger
as an open machine - with automatic defrosting
The main difference is that the open machine sucks in air from a cold room and then cools it down. One therefore speaks of an open cycle. In contrast, in the closed machine the air is in a closed circuit. In this case, the consumer is a heat exchanger.
No evaporator with the
In an open circuit, air is drawn in directly from the cold room. Due to the low temperatures, all water in the air has frozen out to ice crystals. These crystals are caught by redundant filters when they are sucked in. When a filter is full, the ice is shaken off by a short pressure surge. The ice crystals fall into a channel. In the channel, the ice is mechanically transported out of the cold room without additional heat input. Outside the cold room, the ice crystals can thaw in the environment without having to introduce thermal energy into the cold room. Since there is no need for an evaporator, there is also no temperature difference for heat transport and fans. This reduces the cold store load by up to 30%. This has to be considered when designing and considering energy efficiency.
Areas of application for the open machine
The open machine works optimally in cold rooms. By eliminating the heat exchangers and the defrosting system, the cooling load of the entire cold room is reduced. Continuous operation is guaranteed. Due to the safe pressure level of maximum 3 barg, handling the chiller is wasy and safe.
Due to the dry air that prevails in the cold room, your refrigerated goods are stored reliably and with a reduced formation of an ice sheet.
With good control technology, the accuracy of the temperature can be set to within ± 0.5 K. The temperature can be set anywhere from ambient temperature to -160°C. In addition, a particularly homogeneous temperature distribution can be achieved.
When using the open machines in cold saunas, the deicing system can show what it can do. Continuous operation with permanently entering and leaving people works without any problems.
Areas of application for the closed machine
Provision of cooling brine
Due to the good part-load behavior of the turboexpander, the provision of cold for the process is always guaranteed. In addition, the system does not fail when there are rapid load changes.
Gastrap / Gasliquefaction
Due to the temperature spread and low temperatures, the machines are perfect for gas traps. In addition, the machines can be operated with pure nitrogen and then also used for explosive media.
Batch processes can be decisively optimized with the combination of cold storage and cold air cycle machines. The dynamic load profile during freeze drying can be carried out in this combination without peak loads in consuming electricity or cooling water.
Large loads can be run in a very short time thanks to a cold storage and the cold air machines.