KW freezers: Simply cool

Freezers are ideal for storing medicines or reagents, as well as for preserving blood, plasma, serum or vaccines. KW freezers offer innovative, durable cooling technologies and reliable temperature control.

Laboratory freezers are in everyday use keeping samples cool and stopping unwanted biological processes by eliminating the risks posed by warming temperatures. But how does this cooling actually work?

The basic principle behind almost every freezer is evaporation. A refrigerant which normally occurs in a gas state is pressurised inside the refrigerator and thus remains in a liquid form. This refrigerant liquid is then fed into an evaporator where it partially changes from a liquid to a gaseous state. The gas extracts heat from the environment – and thus becomes colder. Many people experience this same effect every day when using a spray deodorant. In physics, one speaks of this as a property of thermodynamics: A gas is initially compressed, and when the pressure is released, the gas then expands and becomes colder. More precisely, it absorbs a specific amount of heat, the enthalpy (latent heat) of evaporation.

Because a refrigerator does not cool down just once, but is expected to maintain a permanently cool environment, a cooling circuit is created inside a refrigerator. After the refrigerant has expanded with the help of the evaporator and has "generated cold", a compressor then pumps the gaseous refrigerant back into a condenser. And by increasing the pressure, the gas is then converted back into a liquid state. Heat is discharged during this process and released into the environment. This liquid refrigerant is now fed back into the evaporator, and evaporated once more to form a gas, and the cooling cycle starts all over again.

With good insulation, the temperature inside a refrigerator can be kept stable over a long period of time. Its compressor only becomes active when the temperature rises – for example when the doors are opened. This allows warmer air from the outside to penetrate the refrigerator, which increases the temperature and triggers the cooling process.

The clever minds at KW use a cascade system to cool the temperature from room temperature down to -86°C. If there are large differences between the condensing and evaporation pressures (as for example in a freezer), the final compression temperature rises rapidly. This is known to have particularly negative effects on the durability of the compressor. Therefore KW simply combines two vapour compression cycles into a two-stage cascade cooling system. The use of dual compressors results in a significantly better cooling performance, and above all, ensures any cooling device will offer a longer service life.

Despite its excellent cooling performance, a freezer must still be opened several times a day. And every time its doors are opened, cold air escapes and warm air flows into the interior. So, as well as providing a safe, stable cooling system, a freezer must be capable of fast re-cooling to facilitate efficient work practices. Standard refrigerating devices use what is known as jacket cooling, where the cooling circuit is built into the refrigerator casing. However, KW freezers offer the option of direct cooling via the shelves of the unit, which means the evaporation process takes place directly on the shelves. So, any samples placed there will be immediately washed with cold gas. Direct cooling can be achieved much faster than jacket cooling, which also means very little interior heating can take place when a refrigerator door is opened.

In addition, KW freezers have a triple insulation layer, which ensures a longer temperature retention time. The inner layer forms a protective barrier down to -86°C, while the middle layer acts as a protective barrier for the outer layer, which it is of course assumed will be at room temperature. The target temperature is also secured by the use of 140mm-thick polyurethane resin insulation within the cooling jacket.

In the unlikely event of a power failure, KW's freezers can still maintain a temperature of -56°C with the door closed for more than 5 hours. This gives users enough time to transfer the samples to another refrigerator. But for additional sample protection, CO2 and N2 backup cooling can also be installed. In the event of a power outage, this system directs gas into the freezer to maintain a low temperature for even longer.

And for particularly delicate samples, KW offers maximum security with their Biological Bank system. These innovative freezers offer two self-sufficient cooling systems which each work entirely independently. Four compressors and two evaporators work alternately, extending the shelf life of a freezer by some 15 to 20 years. In addition, these biological banks offer unbeatably fast cooling times: With four compressors it’s possible to cool an environment from room temperature down to -86°C in just 1.5 hours. But the best thing about the biological bank system is its temperature security. If one of the two systems fails (either the fluid-dynamic-thermal system or the electrical system), an internal control mechanism registers that heating has occurred and automatically shuts down the defective system. This facilitates a simple repair which may not necessarily be required immediately, because the internal temperature will be maintained by those system elements which are still active. 

The principle of the Biological Bank with two independent cooling systems that work independently of each other:

The biological bank is a perfect example of KW’s core competences – namely innovative, durable cooling technologies which offer reliable temperature control.