Dry Ice vs Liquid Nitrogen - which costs less in the long run

JTknives

JTknives

Blade Heat Treating www.jarodtodd.com
Knifemaker / Craftsman / Service Provider
Joined
Jun 11, 2006
Messages
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I have been playing around with a few numbers hoping to really see if there is an advantage of one over the other. Now this is not taking into account that LN is much colder then DI. I'm just assuming if your considering DI then the -100° is ok and you don't need the -300° of LN. My therie has been that even though LN is more money and requires a dewer it's more cost affective in the long run. So let's start crunching some numbers. From here on out dry ice will be shortened to DI and liquid nitrogen will be LN.

First up we need to start by looking at the amount of heat energy each can absorb befor turning into a gas.
DI = 246 BTU/lb (Latent heat of sublimation)
LN = 856 BTU/lb (Latent heat of vaporisation)

Next up let's look at the density of both of them.
DI = 97.6 lb/ft^3
LN = 50.2 lb/ft^3

Let's compare these numbers to what a 20L dewar holds.
20 Litters = .7062933 ft^3
20L of DI = 68.934 lbs
20L of LN = 35.456 lbs

Now we can convert those numbers to actual Latent heat
20L DI = 16,957,76 BTUs
20L LN = 30,350.27 BTUs

Next up is price, I will use the prices local to me.
LN = $3.26L and DI = $1lb
Which means 20L of both costs
68.934lbs DI = $68.934
20L LN = $65.2

Now for fun let's figure out cost per 1000btu of latent heat
20L DI = 16,957,76 BTUs / 1000 = 16.95776kbtu
20L LN = 30,350.27 BTUs / 1000 = 30.35027kbtu
DI Cost is $68.934 / 16.95776kbtu = $4.065kbtu
LN cost is $65.2 / 30.35027 = $2.148kbtu

Now is where it gets a little fuzzy as the math would be complicated so we can just do some guessing. Let's say it takes 1000btus of energy absorbing to cool a knife. That means it cost you $4.065 for DI or $2.148 for LN. But I think the numbers are further apart then that because LN is used as is where DI is usually mixed with a liquid. It uses dry ice to cool the liquid as well as the blade where the LN just has to cool the blade.

Let's dig deeper if we can and see what we uncover. 1kbtu of DI is 4.065lbs which has a volume of
97.6 lb/ft^3 / 4.065 = .04164959ft^3 x 28.317L/ft^3= 1.18L
So if I assume a 50/50 mix of DI and say denatured alcohol which has a heat capacity of around .58 BTU/lb f.
1.18L of alcohol weighs 2.05 Lbs
Which means going from say 68° to -109° would require 177° drop for 2.05 Lbs at .58 BTU/lb F which is 210btus. Now you have wasted 210BTUs just to cool the alcohol and your now left with 790btus for your blade(s) which means you need more DI to make up for the loss. You now need 26.6% more DI to hit your 1000btu requirement.

Now if we look at the cost for 1000btus
1000btu of DI = $5.146
1000btu of LN = $2.148
So by looking at the math it seams like LN is much cheeper then DI by almost 2.5x

Now up to this point we have just ignored the fact that you buy DI as needed and LN is usually stored until needed. So let's look at that and see how the numbers look. My dewar is very efficient at a published evaporation rate of .09L/day. This means each month (30 days) it costs me 2.7L x $3.26 = $8.802 to have it on hand.
So if we used say 1000btus once a month for one knife batch then the cost goes up. Let's re do the numbers
DI = $5.146 month
LN = $10.95 month

But where I think LN has an advantage is in only using what you need. Where DI is used up no mater how much was actually needed. Another thing is trips to town to get the DI. Say I only did one batch a month, for me to drive and pick up DI it's a 50mile round trip and about 1.5hr time. My truck gets 13mpg which is 3.85gal @ $2.70gal is $10.395 added cost plus my time. I fill my LN dewar once and it lasts for a very very long time but let's do the math while we are at it.
LN = 1,517.5btu/L but we only use 1000btu each month. This means each month we use .659L plus the evaporation amount of 2.7L. Each month I would use a total of 3.359L. My dewar holds 20L so that's 5.95 months of use before needing to be refilled. So take the trip cost and divide it up to see what the trip costs per month for LN. $10.395 / 5.95 = $1.747
Let's redo the math now
DI = $15.541 month
LN = $12.697 month

So as you can see the cost is neck and neck but LN is cheeper. If you only do cold treatment once in a great while then dry ice is your baby. But you can see if you do any kind of volume then LN has the edge plus the huge time saver of always having it on hand and being able to cryo any time you want. But this also brings into light some other questions like maybe using a tiny dewar for the guys that only use it once in a blue moon. This way you save the storage waste but you still have the trip to town waste for each fill up. But then what if you put dry ice and alcohol in a dewar, oh man my brain is going to explode.
 
Sounds like you need a more fuel-efficient vehicle to pick up your DI! ;)

It sounds like operational efficiency is the tipping point. What does it cost to buy/rent the LN container? Did I miss that number?

Thanks for risking head-detonation to share this!
 
What about steels that don't respond to sub zero and LN is required?

Chuck
 
I know you considered it, but you didn't mention the "fire hazard" of the DI mix.

For the total heat moved from the metal by the coolant, I believe its Q(t) = mass x specific heat x differential temp. (you seem to like the calculations so I thought I'd share it) :)

Interesting that you put this much thought into it.
 
I think you are reaching a bit... Thinking about cost of gas to go to town? Really? Because you're not going to do anything else in town right? Just a special trip to pick up your LN. If you are like me and you live an hour from town, the trip is combined with everything else under the sun. Groceries, chiropractor, lunch, movies etc.
anyway, it's cheaper to buy and use DI. There's no setup cost and you don't need any special equipment to transport it.

If you are using a steel that requires lower temps than DI can achieve, then get LN and suck up the cost. Make the best product you can and cry about the cost to your customers. [emoji16]


Sent from my iPhone using Tapatalk
 
As best as I can tell, assuming that availability is not an issue for either of these products, the disadvantage of LN is mostly in the cost of the "container". I am using neither presently as I have sent out what little AEB-L and #V I have used, but for me, I have two welding gas stores within a block of my shop and the closet dry ice is a any of the local Publix supermarkets. One thing that has delayed my adoption of "cryo" is the shortage of cheap "surplus" dewars these days on eBay. When i first started thinking about cryo, I could find ones for fairly cheap. it seems that the market has caught on, because I don't see so many of those for sale today. Another issue is that you rarely see the larger "big mouth' variants for sale used and I would need one of those for kitchen knives.
The one thing that I have read about LN is that you don't need to put the blade into thelqiuid and you may not eat to if you can avoid it, so even when you LN levels get low, as longs you can get to that magic -120F or so number, you should be good as far as eliminated RA, right?
 
In a dewar I have read papers saying that the liquid sits at -321° and above the liquid all the way up to the foam plug it's -300°. That's how the cryo shippers work, there is no actual liquid sloshing around, the wall inside the dewar absorbs the liquid and the inside stays at -300°. So as long as you can get you blade in the dewar and put the foam plug back in your good to go even if there is just a tiny about of liquid nitrogen left in the bottom.

The reasion I included all the various costs is because well thy are costs. I was just trying to brake it down as far as I could in steps. But even if you remove the driving cost or just Wright it off as your normal trip to town LN still has the edge I think. In my case it costs me $8.80 a month to have LN on hand at all times. I don't know about you but I hate having to wait and not complete a project because I don't have somthing I need. When I'm in the knife zone I like to just keep going through the whole process in one smooth motion. But if you only do blades once every month or two then yeah DI is cheeper but if your like me and try and do at least one blade a week then DI becomes much more expensive.
 
I did not include the dewar because it's "value added" to your shop just like all your other tools. And you can always resell it if you want and it does not get used up over time like DI or LN.
 
ron_m80 said:
I know you considered it, but you didn't mention the "fire hazard" of the DI mix.

For the total heat moved from the metal by the coolant, I believe its Q(t) = mass x specific heat x differential temp. (you seem to like the calculations so I thought I'd share it) :)

Interesting that you put this much thought into it.
Click to expand...

The equation is correct but if I remember correctly, the specific heat of iron drops as temperature drops. You need less and less as it continuously cools.
 
Not everyone will agree, but buying dry ice on a regular basis could be a security risk too. Some police are interested in large volume dry ice purchases due to nefarious uses by other people, and it could get you attention you don't want.
 
About the thin neck LN containers. You could pour LN temporarily it into a wider Styrofoam vessel. After the cryo is done you'd pour it back.
Yes the evaporative loss will be greater during that time but you don't need to buy a wide mouth dewar.
 
My dewar has a opening of 2.1875 but that takes care of 99% of what I normally do or plan to do. But like was said you can pour it into something larger. Me personally would find an actual LN flask if I wanted to do something larger.
 
I suspect you'll find pouring LN from one container to another, then back again is going to use more LN than you planned to use. That stuff is going to evaporate pretty quickly in the pouring, and cooling of the 2nd container.
 
Ken H> said:
I suspect you'll find pouring LN from one container to another, then back again is going to use more LN than you planned to use. That stuff is going to evaporate pretty quickly in the pouring, and cooling of the 2nd container.
Click to expand...
I've done it and you are right it's just a work around which isn't ideal but might be cheaper than replacing an existing vessel for a one off bigger blade.
 
LN because you need to get the most from the treatment because it is so much colder.

DI just does not get it there.
 
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