Cooling a body of oil with dry ice 2

[expo4213]

I need to figure out how much dry ice is needed to cool roughly 80 gallons of transformer oil (mineral oil) to -20 C and hold it for an hour. Does anyone have an equation to calculate this?

 

[MortenA]

A gazillion tons )assumting initial tempetarute of 56745618ºC

Serioulsy, your definition lacks substance BUT:

here a link to a lot of thermodynamics for co2:

http://www.union.dk/media/CO2-GB.pdf

Use the isenthalpic curves to determine the energy released when vaporizing the CO2. Remember that the final temperature of the CO2 changes so unless you cheat a little and assume that its all one big mixture at equlibrium its a tvist that will require some thinking

Use the heat capacity of the oil to calculate how much energy you need to remove

Do the math yourself....

For maintaining the temperature: Here it will depend on the insulation, the ambient temperature mainly - other factors such as the shape of the container, wind etc could also influence (short: Thats actually the tricky part - and cant be done with the info you provide)

 

[racookpe1978]

Homework?

From any practical standpoint, this makes no sense at all.

And, you can't "maintain" mineral oil at -20 degrees C with dry ice because while dry ice is condensed out from gas into vapor at -78.2 C, but the actual received temperature of your ice could be anything less than -78.2 C. Dry ice is STORED at much lower temeprature than -78.2 C.

" from the web: At temperatures below 195 K (-78.2 °C, -108.7 °F) carbon dioxide condenses into a white solid called dry ice.

Keep in mind that it can become much colder once the solid is formed. Materials that are not changing phase do not have one set temperature as the question implies. "

Now, your mineral oil will cool very rapidly (too rapidly ?) but only in very limited areas of your container because dry ice immediately around the oil will be boiling away and splashing the oil around; but then the mineral oil will keep cooling down until it (possibly) reaches the received temperature of the dry ice. So your temperature "could" go much further down past -20 C. Or maybe not - see comments above

Regardless, your mineral oil won't "stop" cooling down until all the dry ice is gone. At that time, your oil will immediately begin warming up. Until you dump in more dry ice. Atw hich time it will begin rapidly cooling again. Rinse, wash, repeat indefinitely ?

 

[MortenA]

@expo 4213: Commenting on my own first post that in hindsight is a bit hard to read:

- You dont mention the initial temperature of the oil - without this you cant complete the calc
- Calculate the amount of heat that shall be removed by: Q=Cp*M*(Tini- (-20ºC))
- Use my reference to find the amount of "cold" released when evaporint the co2 pr kg. CO2 X. The amount af CO2 needed will then be Q/X. In reality its a bit less since i disregard that the co2 will also be heated to oil temp. However, as with water the heat ov evaporation is much higher than the heat capacity pr ºC (ok unit are screwed up but i hope you get what i mean). So its an OK approx.

Keing the fluid cold is much more difficult!

 

[chicopee]

If you do estimate an amount of dry ice, you may want to wrap the transformer with thermal blankets.

 

[Hawxxy]

what you want do do is attach a pump to your oil container. fire it up so that the the oil is circulating with the in and out end as far away as possible from eachother in the container. then get an insulated box of any kind, the bigger the better. put an equal amount of hose and dry ice in the box. replace the dry ice as it evaporates. use a thermometer to check the temperature of the oil going into the pump. when it reaches -20C adjust the amount of dry ice in the box at any given moment so that the temperature remains more or less the same. this requires a pump that can take some intense temperatures.

 

[expo4213]

Thanks for your help. Sorry it has taken me a while to respond, I was out of town on work. Basically, I need to do a cold temperature test with a mechanism in transformer oil (mineral oil) and it needs to be at
-20C for the test. I will only have to maintain the -20C for a few moments until I can 20 operations on the mechanism, so keeping at -20C for long will not be my problem. The initial temperature will be room temperature and the amount of oil will be 87 gallons. But thanks MortenA for your help that was exactly want I needed to calculate the amount. Thanks everyone for your help!

 

[Montemayor]

Just to clear up a few facts:

CO[sub]2[/sub] liquid solidifies into "fluffy", solid CO[sub]2[/sub] snow. It DOES NOT CONDENSE into a white solid called dry ice.

In order to make dry ice, the CO2 snow must be compressed into solid blocks or pellets. Otherwise, it will scatter all over the place.

Additionally, the triple point for CO[sub]2[/sub] is: -70 [sup]o[/sup]F and 75 psia. The triple point is the conditions where the solid, liquid, and gas phases are in equilibrium. The solid CO[sub]2[/sub] phase is only possible below the triple point. The solid CO[sub]2[/sub] phase (or dry ice) at atmospheric pressure has a temperature of -109.5 [sup]o[/sup]F. Therefore the claim that "At temperatures below 195 K (-78.2 °C, -108.7 °F) carbon dioxide condenses into a white solid called dry ice" is totally false, erroneous, or a typo. But is is not correct and probably a total misunderstanding.

 

[MortenA]

whenever i re-read an old post that i wrote i get the feeling that i must have worn boxing gloves when i typed it...