What are the limitations of environmental chambers in achieving LOW temperatures?
When we hear from folks searching for a test chamber for the first time, they are sometimes surprised that we can get temperatures down to -94° Fahrenheit (-70° Celsius). To help you understand the different methods of cooling a chamber and how cold they can go, take a look below: It's a quick review of each plateau of limitation in cooling a test chamber, from room temperature, to as low as -170°C.
This lower limit is common for chambers that don't have any refrigeration. Because there is heat generated by the fan recirculating the air, we cannot guarantee temperatures close to the room ambient. So even if you just want a stable "ambient" temperature, you will need refrigeration. This low temperature is usually specified for ovens.
This is about as low as we can go with a standard refrigeration using R-134A refrigerant. This is the same refrigerant that is currently used in new air conditioning systems and refrigerators. This refrigerant is not widely used in the test chamber industry due to its limited range. To learn more about refrigeration, see this webpage.
This is the lowest range that chambers can achieve with R-404A or R-507 refrigerants. The type of refrigeration system they use is called "single stage", which basically means that there is one compressor. Chambers with this range are often only used to –20 or –25°C when cycling temperatures between hot and cold. Going to lower temperatures becomes a slow process. Higher performance systems may even limit the lower temperature more strictly in order to maximize the system to meet a desired test range in a specified time.
Chambers that can get this low use "cascade" refrigeration. By using a "high stage" refrigeration circuit to cool the "low stage" circuit, these cold temperatures can be achieved. The low stage refrigerants (R-23 or R-508) would create too much pressure (which would blow-out the piping) if they were used alone, which is why the high stage is added. The high stage ensures the low stage doesnt' reach too high a pressure by cooling it.
Cascade cooling is less efficient ... even though there are two compressors, you really only have the cooling power of one. Plus, there are losses due to inefficiencies of heat transfer between the stages.
To get this cold, a triple cascade refrigeration system is used. This is rare in test chambers, although it is common for ultra-cool freezers.
Chambers that can get this low use liquid nitrogen for cooling. Nitrogen is a cheap method for cooling a chamber, however long term use of liquid nitrogen can get expensive. It is best suited for: