Top Reasons People Use Liquid Nitrogen for Thermal Test Cooling

Thermal testing can be time consuming. Getting it done in a reliable, verifiable way using less time means less costs.

Thermal Platforms heat and cool with conduction which is inherently faster than with convection in a temperature chamber.

Coolant choices are detailed elsewhere in the TotalTemp Blogs but to make a long story short, The main prevailing cooling choices are – Refrigeration compressors and expendable refrigerants ie. Liquid Carbon Dioxide (CO₂) and Liquid Nitrogen(LN₂).

Liquid Nitrogen has the ability to provide the fastest ramp rates and achieve the coldest temperatures. Temperatures down to -100ºC or colder are achievable with ramp rates over 50ºC per minute.  Typically the full range is not needed but in the event that future testing requires yet colder temperatures it is good to have the capability built in and not have to replace equipment.

LN₂ systems are far simpler than refrigeration systems with many less ways they can fail.

LN₂ cooling uses very little power so no special electrical services or usage costs are involved.

In my experience, LN₂ delivered through a distribution system can have it’s issues but is often less troublesome than CO₂ distribution systems. CO₂ systems can sometimes have blockage issues revolving around 1) Water intrusion and 2) the physical property of CO₂‘s triple point.  Triple point is the temperature and pressure where CO₂ can exist as liquid gas and solid. For CO₂, this occurs at a relatively low pressure resulting in the possibility of coolant becoming solid (dry ice) in the delivery line or elsewhere besides the point of use.

Mechanical refrigeration systems are initially considerably more expensive than expendable coolants and in often in the long run are more expensive than expendable coolants when electricity usage and maintenance is included in the equation.  Mechanical refrigeration is also far more complicated and expensive when temperatures below -35º or -40ºC are required.  Modern refrigeration systems to achieve testing temperatures below -40 have evolved considerably but they are still extremely complicated more expensive and prone to requiring service.

I’d like to end on a positive note but first it would be fair to address the down side as I see it for using LN₂ as a coolant. When smaller scale usage is the case, portable tanks make more sense, changing tanks out can be a nuisance but if usage is greater, a distribution systems would be in order. Distribution systems are fairly expensive,  typically much more than $100/linear foot.  Since LN₂ is so incredibly cold, about -185ºC vacuum jacketed insulation is required to reduce losses.  There will always be some losses in the delivery line with small or large LN₂systems. Finally the biggest concerns with LN2 is the recommendation for an automated LN2 shut off valve to prevent run-away cooling in the event of a valve sticking open.

As an alternative to LN₂ distribution systems, there are distribution systems using liquid CO₂ that are designed with the ability to distribute the coolant through an uninsulated small diameter line thus reducing losses. Makers of CO₂ distribution systems claim significant economy over LN₂ delivery systems. However as a down side, a single refrigeration compressor must run to condense the coolant into a reservoir above the points of use.

So the the Top Reasons to use Liquid Nitrogen are 1) Speed of cooling, 2) Temperature range, 3) Low initial cost and 4) Favorable long term costs 5) Superior reliability.

As a manufacturer of thermal testing products, the majority of our requests are for LN₂ cooled systems. Equally we offer CO₂ cooled or mechanical refrigeration systems. We endeavor to help customers acquire the systems that best fill their testing requirements.

Call or email us now with any questions regarding your thermal testing

Thermal Testing Optimized

It is well known that test and evaluation of production items is a very important part of producing quality products.

The consequences of a lack of quality are unfortunately well known but covering the bases with appropriate testing can be quite time-consuming and expensive and easily run into the weeds of under testing, over-testing, or testing the wrong condition.  The tendency to an error on the side of safety is generally accepted but the flip side of thorough testing can be spending too much time and money testing or literally testing a product to death.

Equally important to understanding the electronic parameters and functions under various electrical conditions which need to be verified is at least a very basic understanding of thermal testing.

A high percentage of failures in electronics are ultimately due to thermal issues.

Subjects for other conversations relating to thermal failures include microscopic cracking, electromigration, the thermal runaway of parameters, or melting of materials.  Many electrolytic capacitors have been shown to dry out and fail more quickly at higher temperatures, critical connections can corrode or otherwise deteriorate enough to cause failure. General expansion and contraction along with the associated moisture cycling are often at the root of many failures.

Three ways

As we learned from an engineering background, the three ways temperature can be changed are by conduction, convection, and radiation.  In that order, they are generally most effective in use for the thermal tests.  Without a big dive into the math and logic behind the statement, conduction is the most effective heat transfer method. Surprisingly, convection, such as in employed in a temperature chamber is far more widely used. As I understand it the main reasons for that disparity are:

1) Heat transfer by conduction requires intimate contact between the heating and cooling equipment (thermal platform) and the device.  Fortunately, many device packages for microwave and other power equipment come in a package with a flat thermally conductive surface. Additionally, many other parts can readily have a machined fixture to provide the needed surface contact.

2) The acknowledgment and the ability to cope with the fact there is always some thermal gradient between the temperature driving the equipment and the furthest part from the driving point that needs to be conditioned.

3) Thinking “That is just the way it is done or one size fits all”. Truly, not all items are compatible with a thermal platform however, with a little prior planning work many parts can readily be more effectively tested on a thermal platform or in some cases even using two or more thermal platforms.

Getting the job done

Convection heat transfer can in cases rival conductive heat transfer effectiveness.  High airflow is important to heat transfer just with air, just as the clamping force is important to conductive heat transfer.  While good airflow is needed for heat transfer, it also comes at a cost. Air friction from high-volume blowers is a surprisingly large source of unintended heating and other losses.  It is often seen that temperature chambers designed to maximize heat transfer and minimize test time produce a surprising amount of heat from air friction.  For example, a high-performance chamber with strong airflow to maximize heat transfer can be seen to cause the chamber temperature to climb from ambient to over 100°C without any other heat added to the system.  Of course, if you are only going that hot, the additional heat from air friction can actually be a benefit.  Fighting that heating with a refrigeration compressor system definitely requires a careful recalculation of this heating contribution to achieve expected cooling results not to mention this also adds more heat to the room, eventually adding to the air conditioning load of the lab room.  For many applications, a temperature chamber with mechanical refrigeration is a very good choice but before purchasing, we suggest you take time to talk to the professionals at Totaltemp and consider understanding the choices before you buy.