How does an ultra low freezer work


Ultra low freezer: how does it work?

What is an Ultra freezer?

The Ultra freezer is one of the main tools used in research laboratories. The ULT freezer safeguards biological samples important for research being carried out in laboratories, by keeping them at very low temperatures. The operating temperature can range from -45° C to -89° C.

Two versions are generally available: vertical cabinet or horizontal unit with access from above. The most common one is the vertical version, as laboratories are increasingly looking to save space to make it easier to move around. The internal storage volume of the low temperature freezer can also differ. It can start from an internal capacity of 200 litres up to a maximum capacity of 800 litres.

The Ultra freezer has shelves on the inside where research samples are placed. Each shelf is closed by an internal door so as to maintain the temperature as uniform as possible on the shelves where no sample is removed or placed.

In order to keep the cold stored inside the cabinet, between the external wall and the internal one, there is a pollutant free polyurethane insulation with high insulation coefficient, which keeps the temperature stable for several hours.

Another key element to maintaining the freezing status is the seal which is positioned around the entire perimeter of the main access door. The most efficient ones are silicone quadruple-step seals, which guarantee even greater temperature stability.

The main entry door, also insulated with polyurethane, guarantees the highest degree of coldness and is blocked by an external closure generally fitted with a safety key. The most practical laboratory freezers are equipped with pivoting wheels at the bottom, in order to make it more practical to move within the laboratory.

The most modern freezers are equipped with electronic PLC controllers, and a display that indicates freezing parameters. The entire control system is generally equipped with a back-up battery, which ensures the digital temperature display works even when there is a power failure.

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How does the refrigerator circuit of an ultra freezer work?

The refrigeration systems can be a single high-power compressor hermetically sealed or two cascade compressors. In this second case, there are two refrigeration circuits connected so that the evaporator of one cools the condenser of the other, facilitating the condensation of the compressed gas in the first circuit.

Air-cooled condensers are generally used in laboratory refrigeration systems, consisting of tubular batteries (copper or copper-aluminium) arranged to provide as much surface heat transfer as possible. In Ultra freezers, the circulation of the cooling air is forced by an engine driven fan and the expansion of the refrigerant fluids is obtained by capillary tubes.

Evaporation takes place through steel plate heat exchangers, located inside the chamber, or by means of a coil. In horizontal freezers, the copper or copper-aluminium coil evaporator is placed along the walls and bottom, while in vertical freezers it runs along the walls and at the top. The coil in the room eliminates the problems of efficiency in the heat exchange of freezers with the coil in the insulation cavity.


Coolant fluids used in the ultra low temperature freezers

Refrigerant fluids are selected from gaseous chemical compounds that can easily be liquefied and characterised by certain properties such as high critical temperature and low critical pressure, limited hazard and toxicity. They are indicated by the letter R followed by a number that refer to the molecule’s atoms.

Each refrigerant system must be sealed and contain eco-friendly refrigerant HFC gas, free from CFC and HCFC as well as being non-explosive, non-flammable and non-toxic. The most efficient, environmentally friendly refrigerant fluids used in ultra freezers are typically R170, R23 and R290.


Which accessories can Ultra Freezer have?

The accessories generally supplied include: a CO2 and / or LN2 emergency system (both with independent power supply, probe and battery). In addition, in order to track room temperature, freezers are optionally equipped with a weekly charted temperature recorder, which has almost completely been replaced by digital ones.

In line with current international standards, the recorder is compulsory for freezers at -45° C and -86° C for the storage of blood products. Other accessories include: various kinds of steel containers, drawers and racks for storage of cryoboxes, microplates, blood bags and test tubes designed for low and very low temperature storage. Handling goods inside the ultra freezers requires you to wear suitable protective headgear, leather gloves to protect your hands and overalls and smocks to protect your body.