Benefits of Liquid Nitrogen Cooled Chambers

The environment testing industry has long been accustomed to using thermal chambers, which are cooled with mechanical refrigerated systems. These systems have been viewed as simple and self contained. However, more and more companies are realizing the troubles associated with mechanical refrigerated systems. Recently, there has been a significant trend to use liquid nitrogen to cool the chambers. Utilizing liquid nitrogen yields many performance advantages and significant reductions in maintenance time and cost. The following illustrates some of the advantages to using liquid nitrogen to cool a test chamber.

• Faster Thermal Cycles
Liquid nitrogen (LN2) is capable of changing temperature many times faster than a mechanical refrigerated cooling system. Faster temperature change decreases total test time thereby increasing testing efficiency and saving money. Temperature cycling tests that would take hours with mechanical refrigerated cooling systems can be completed much more quickly with an LN2 system. Since product stress increases with higher temperature change rate, LN2 cooled chambers will quickly propagate failures in the products being tested.

In addition, the faster temperature change rates are capable of producing stresses which will induce failures typically not found in mechanically refrigerated chambers. The value in finding these additional failure modes is even greater than the previously mentioned value of quicker test times.

• Lower Temperature Limits
The lower temperature limits of mechanical refrigeration are around -35°C with a single stage compressor and -65°C with a cascade system. As the cooling process approaches its limits, it becomes less and less efficient. With a boiling point of -195°C liquid nitrogen can cool to -150°C efficiently whereas even a cascade style mechanically cooled system is only efficient to around -50°C. Even though a mechanically refrigerated system may be capable of achieving a temperature of -50°C, it will take considerably longer to reach those temperatures.

• Higher Temperature Limits
The refrigerant used in mechanical cooling systems will degrade at around 200°C. Since mechanical systems are closed loop, the refrigerant is always exposed to chamber temperatures thereby limiting the temperature range of the chamber. Liquid nitrogen only enters the chamber when needed so extreme chamber temperatures will not effect the cooling process in any way.

• Minimal Maintenance Costs
Once a liquid nitrogen cooled chamber is installed there are very few moving parts that can fail or require maintenance. Mechanical cooling requires a great deal of maintenance on the compressor and condenser, which is very costly. These costs tend to rise as the size and number of compressors increase. As these chambers get faster change rates and colder achievable temperatures, the required compressor maintenance will likely also go up.

• Smaller Equipment
Liquid nitrogen chambers require considerably less space than a mechanically refrigerated chamber because there’s no need for large compressors and condensers. With the high cost of lab space, LN2 cooled chambers can save considerable amounts of money.

• Quiet Lab
Mechanically refrigerated cooling produces a great deal of noise from the compressor whereas liquid nitrogen is no louder to use than the valve that controls it. Compressors can produce up to 90 decibels, requiring the use of ear plugs for everyone in the lab. Nitrogen valves are pneumatically actuated and are very quiet.

• Lower Electricity Usage
The compressor systems on mechanically refrigerated chambers require a significant amount of power. As the size of the chamber/compressor increases, the power requirement goes up as well. Nitrogen cooled systems do not consume any extra power during the cooling stage. There can be significant operating cost savings from the lower power requirements.

• Environmentally Friendly
Unlike refrigerants, nitrogen is common in the air we breathe every day. There are no adverse environmental effects from discharging the nitrogen gas into our environment. Mechanical cooling refrigerants contain CFCs that are proven to contribute heavily to ozone layer depletion.

• Operating Heat
Mechanical refrigeration exchanges heat with the air to provide cooling which can increase the temperature in a lab and strain building cooling systems. A properly installed liquid nitrogen system, in contrast, will have no effect on building temperature.

• Cooling Water System
Larger chambers cooled by mechanical refrigeration often need a cooling water system to help keep the temperature of the compressors down. This cooling water system requires more money in its installation, cost of space to place it, power to operate, and maintenance costs. An LN2 cooled chamber does not need such a cooling system, as it has no compressors to generate heat.