Design of Simple -40 Freezer System with LN2 and Styrofoam
Design of Simple -40 Freezer System with LN2 and Styrofoam
(OP)
The goal is to design an LN2 Freezer system that is portable and inexpensive as possible. I want to use Styrofoam (polystyrene) walls with a certain R-value to be built around a unit with a certain heat gain. The idea is to purchase components such as a tank, hose, (pump?), solenoid, and nozzle, to inject LN2 into the system. I want to use existing controller boards (that I have), use thermocouples as input and the solenoid control as the controller output. I have not worked with LN2 before, nor am I particularly savvy on the subject of thermodynamics.
Given a chamber volume, heat resistance of chamber walls (R-value), and heat gain (in watts) of the unit under test;
how can I calculate the necessary flow of LN2 to set the chamber at a certain temperature?
I'm assuming LN2 tanks can be manually filled, pressurized, and LN2 flow can be controlled, all with commercial products, which I can (hopefully) buy separately. Manufacturers often include a controller with a stock LN2 injection setup, but in the interest of saving money and customization, I want to use existing controllers.
Any suggestions?
Given a chamber volume, heat resistance of chamber walls (R-value), and heat gain (in watts) of the unit under test;
how can I calculate the necessary flow of LN2 to set the chamber at a certain temperature?
I'm assuming LN2 tanks can be manually filled, pressurized, and LN2 flow can be controlled, all with commercial products, which I can (hopefully) buy separately. Manufacturers often include a controller with a stock LN2 injection setup, but in the interest of saving money and customization, I want to use existing controllers.
Any suggestions?





RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
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RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
LN2 can be dangerous:
>> all LN2 materials need to be insulated to prevent freeze burns to personnel/operators
>> ventilation needs to be considered to ensure adequate breathing air
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RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
I've never worked with LN2. My first thought to how this system should work would be to control the flow of LN2 through pipes or coils in the Styrofoam box. With a fixed surface area of the pipes, I could control flow of LN2, and while mathematically determining the approximate heat loss for the flow I'm providing with the gains from the unit and outside the box, use thermocouples to determine whether to increase or decrease the flow rate.
I was led to believe that we could simply inject a quantity of LN2 into the box, and some constant evaporation would facilitate the venting of the nitrogen gas. The determined rate of injection and evaporation could keep the box at an approximate temperature.
RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
However, if your goal is to maintain a given temperature in the enclosure, and you plan to have a closed-loop control system, why do you think you need to calculate the necessary flow in real-time? At any give instant you need to maintain the flow constant, increase it, or decrease it. That seems fully sufficient for a control system.
RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
Don't mount a LN2 dewar on a shaker head, if you shatter the vessel and spill the contents, you're gonna have problems. Modulating (pulsing) slugs of LN2 directly onto your device is probably going to cause a lot of frost buildup too, unless you take care to work in very low humidity.
Have you considered using dry ice? Pack a bunch of dry ice chips into a styro box, and monitor hardware temp. as it cools down, start testing when the temp. gets to -40F (remove the box and blow off excess chips at that point). Concerns for frost still apply; melting frost dripping down into the shaker table may not be a great idea.
RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
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RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
For instance: The surface temperature of your "evaporator" will be below the dewpoint of anything likely to be your ambient atmosphere. So it will very quickly become a block of ice.
RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
Yes, dewpoint is an issue...but I have seen it work okay (LN2 chilling) in very dry, cold, desert air, at high altitude. But, we designed all of the exposed surfaces to be easily scraped prior to functional (not vibration) testing, and had drip trays and gutters to contain and channel the inevitable runoff of melting ice/frost. LN2 slugs were directed into a steel block heat exchanger that was tucked under a insulation blanket along with the hardware to be tested.
RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
Why not use a Vortec Tube from Vortec or Exair that can achieve your required temperature. I've worked with both companies and gotten very good support.
I would also check with the low temperature freeze manufactures like Scientemp. I know some of them build small cabinets, around 1 cubic foot, that go to -85C.
http://www.vortec.com/solutions_cooling.php
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http://www.scientemp.com/freezers.htm
RE: Design of Simple -40 Freezer System with LN2 and Styrofoam
Any recommended suppliers in North America, specifically Canada, for LN2 tanks, hoses, and solenoid valves?