Liquid Nitrogen Usage
Liquid Nitrogen is the chief cost of cryogenic processing. The more you use, the more it costs. So let's take a look at how much it is necessary to use. Note that the only reasonable way to compare our machines with our competition's machines is to compare like cycles with the same weight of parts in the machine. One excellent way of comparing machines is to know how much nitrogen it takes to run the machine through a particular cycle with nothing in the machine. That is why we give you the following information.
How is nitrogen used?
Cooling the Load
First, you need to cool the parts that you are treating. The most commonly treated material is steel and cast iron. It takes a quarter of a liter of LN2 to cool one pound of steel or cast iron down to -300oF. So for every 100 pounds of steel you need to process, you need 25 liters of LN2 just to cool the parts.
Cooling the Machine
The inside of the machine also has to be cooled to -300F. The interior of cryoprocessors are generally made of steel. So if you have a machine that has an interior made of 200 pounds of steel, you need 50 liters of liquid nitrogen to cool that interior.
Machine Insulation Efficiency
Heat always makes its way past the insulation and into the machine from the outside atmosphere. There are many paths for heat to take. The overall efficiency of a machine's insulation can be expressed as the number of liters of liquid nitrogen it takes to maintain the machine at -300F. We publish these numbers along with the weight of the internal structure so that you can calculate the amount of nitrogen it will take to run any particular load.
We've calculated the nitrogen usage for each of our standard machines with no load and a hypothetical cycle. In order to do this, we assumed the cycle to be one of:
Cryo Processing Cycle Used
Ramp Down 8 Hours
Hold 20 Hours
The calculation of the Nitrogen usage is done as follows:
1. First calculate the nitrogen used to cool the interior of the machine or "dead weight" as it is sometimes called. To do this multiply the interior weight in pounds x 0.25 liters/pound.
For a Cp-200vi, this is 120 pounds times 0.25 liters/pound=30 liters
2. Next, calculate the nitrogen used to make up for insulation losses during the ramp down part of the cycle. The rate of heat flow into the machine is directly proportional to the difference between the temperature outside of the machine and inside the machine. All this means is the overall heat flow into the machine during the ramp down is one half the total heat flow into the machine for the same period of time when the machine is down at -300F.
So in the case of a CP-200vi and an 8 hour ramp down, the nitrogen used is .5(5 liters/hour x 8 hours)=20 liters
3. Calculate the amount of nitrogen used to maintain -300F for the 20 hour hold part of the cycle.
For the Cp-200vi, this is 5 liters/hour X 20 hours=100 liters
You can see that it will take about 150 liters to run this program in an empty CP-200vi. If you add the nitrogen needed to cool the load, you can tell how much LN2 will be needed for any particular load.
Different metals need different amounts of LN2 to cool them to -300F. For purposes of estimating LN2 usage, the following values will get you fairly close.
Nitrogen to Cool 1 Pound of Material Down to -300F
Steel 0.25 L
Aluminum 0.58 L
Copper 0.23 L
Tungsten 0.08 L
Magnesium 0.325 to 0.625 L
Carbide 0.3 to .47 L
You can also use this means of calculation to estimate what our competitors' machines would cost to use on a similar cycle, if you can get them to tell you the interior weight of the machine and the nitrogen usage per hour.