Cryogenic (L-N2 or L-CO2) vs. Mechanical Cooling

 

Mechanical Refrigeration may be advantageous over expendable gas for cooling applications where expendable Liquid Nitrogen (L-N2) or Liquid Carbon Dioxide (L-CO2) is not readily available, for long periods of cooling, and where the time required to ramp down to temperature is not critical. Mechanical refrigeration may also be the only choice for clean environments and for areas where the spent L-CO2 or L-N2 exhaust gas could pose a risk to workers.

Cryogenic coolants pose certain health risks that are easily tolerated but important to consider. Neither L-CO2 nor L-N2 is poisonous to humans, but both have the potential to displace breathable oxygen in a closed room. While excessive quantities of L-CO2 will usually, but not always, show itself with signs of headache or dizziness, L-N2 is normally 78% of our atmosphere and excessive concentrations that displace oxygen may accumulate without warning. Any device that uses expendable L-CO2 or L-N2 should be vented to the outside atmosphere or should be used in a very well ventilated area with an oxygen monitor. To help you in minimizing this risk, all Sigma Systems cryogenic chambers and platforms are provided with exhaust ports which can be connected to external exhaust systems. If ventilation is a problem, consider using a mechanically refrigerated chamber or platform.

Mechanically refrigerated systems normally provide cooling by exchanging heat with the air. In a closed room, heat introduced by a refrigeration system will add to room heat and may tax the room air conditioning system. For smaller units, this will likely not be an issue, but users expecting to employ multiple and/or large mechanically refrigerated systems in a closed room need to consider this issue. A simple solution can often be using water cooled mechanical refrigeration. Water cooled refrigeration is an option for most Sigma Systems chambers and platforms. The heat from refrigeration can then be taken to a central water chiller system for dissipation away from the work area.

Cryogenic cooling is the better choice for applications that require fast downward temperature transitions or very low temperatures below the limits of mechanical refrigeration. If low temperatures are not held for long periods, cryogenic cooling can be very cost effective. Cryogenic cooling is simple to use and chambers and platforms with only cryogenic cooling are less expensive to buy. However, users are cautioned to consider the costs of not only the cryogenic coolant used, but the cost of managing and changing the coolant tanks in facilities that do not have built-in cryogenic liquid coolant delivery systems. For long tests, the possibility of having the coolant run out in the middle of a critical test, or of managing coolant supplies to prevent this problem, are also part of the cost formula when evaluating the cryogenic vs. mechanical refrigeration choice.

Mechanical refrigeration is more costly initially and requires more maintenance, but may ultimately be less expensive when considering long-term L-CO2 or L-N2 usage cost. All Sigma Systems mechanically refrigerated chambers and platforms use highly reliable industrial duty refrigeration units and non ozone depleting refrigerants. Other than periodic cleaning of the condenser coil to remove dust and lint, no periodic maintenance is required. The practical limits of mechanical refrigeration are as follows:

     Single stage refrigeration -30°C to -40°C
     Cascade refrigeration -55°C to -75°C

The lower limit of mechanical refrigeration is highly dependant upon the size of the chamber or platform and the size of the refrigeration unit and evaporator. As the amount of heat that can be removed is related to the difference between the current temperature of the chamber or platform and the evaporator, the performance of the chamber or platform will degrade substantially as the limits of the system are approached. Therefore, unless you have a lot of time for a ramp down, using a single stage refrigeration system to achieve -35°C in a middle size chamber or platform will be possible, but will prove tedious. It usually makes more sense to use the system that can achieve the goal with ease. In this case, a temperature of -35°C is easily achieved by a cascade system which would likely be a better choice.

Cryogenic coolants have lower limits than mechanical refrigeration, but the same discussion applies to approaching the limits of the coolants. The practical limits for cryogenic coolants used by Sigma Systems are as follows:

     Liquid Carbon Dioxide (L-CO2) -65°C
     Liquid Nitrogen (L-N2) -170°C

Using these coolants for lower temperatures will prove to be a poor decision. The sublimation temperature of L-CO2 is -69.9°C at 1 atm, but operation of a chamber or platform there results in puddling and dry ice formation that is both wasteful and a hazard to workers. L-N2 boils at -195.85°C but oxygen will liquefy and puddle at -182.95°C. Staying well away from this limit is imperative for safety. As with mechanical refrigeration, it makes sense and improves efficiency to not use a coolant near the limits of its performance unless absolutely necessary. L-CO2 is inefficient at -65°C, but L-N2 is well above its lower limit and is very efficient at this temperature.

The concept is simple. Unless you have a compelling reason to the contrary, buy a system that will operate to your needs well within the extreme limits of its capability. You don't need to carried away with excess, but operating within the middle 80% of the range range of a platform or chamber will improve performance and efficiency and reduce maintenance costs.

L-CO2/L-N2 Boost (for chambers only) - for applications where the chamber needs to ramp quickly to cold temperatures or to temperatures below the rating of the compressor, (typically –20 or –40 C) it may be advantageous to specify a chamber with mechanical refrigeration and expendable L-CO2 or L-N2 boost. The inexpensive addition of a boost solenoid/injector provides for rapid, near limitless live load dissipation capability. A chamber with expendable coolant boost retains most of the lower long-term operating cost advantage of mechanical refrigeration since the expendable coolant consumption automatically stops as soon as the chamber reaches its set point and the refrigeration system is removing sufficient heat to maintain temperature.

Your Sigma System sales engineer can provide guidance and assistance in evaluating these issues.