Convenction, conduction radiation --describe this process

[SaintJude]

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Experiment 2:
Sample 2 undergoes a process known as freeze drying. The sample is frozen and placed in an identical chamber at -20 °C. The pressure in the chamber is then reduced to 0.005 atm. With chamber pressure held constant, the chamber temperature is raised to –5 °C. The sample is allowed to sublime until only a granular solid remains.

q: Regulation of the chamber temperature is achieved by increasing or decreasing the flow of liquid nitrogen (–200 °C) or steam (150 °C) through pipes about the chamber's metal walls. Which of the following could describe the flow of heat to the sample in Experiment 2?

A Conduction then radiation
B. Radiation then conduction
C. Convection then conduction then radiation
D. Convection then radiation then conduction

Can someone please carefully (slowly explain) b/c I don't understand the answer.

 

[Buttafuoco]

I believe it's C unless their is some intricacy of design that isn't readily apparent. For sure the first step is convection- you have a bulk liquid at a certain temperature moving to regulate temp. Then the material of the walls collide to pass the energy through the wall, and then I assume that since the sample is in a very low presure environment, there is very little gas in the chamber that could pass energy directly through collisions to the sample, so the energy is radiated as IR photons to the sample where it raises the energy of individual particles until they gain enough energy to sublime.

 

[EnginrTheFuture]

here's my take:

Convection is the first step because they seem to be unnecessarily mentioning that the steam/liq nitrogen are fluids being passed through a pipe (they are pointing this out for a reason). That narrows it down to C and D. They then mention that the chamber walls are metal, so it is likely that the pipe is passing on this heat through conduction (solid to solid). But then, the heat must get from the chamber walls to the particles of the sample inside the chamber... I go with C because there isn't much gas in there to pass the heat through.

radiation --> conduction doesn't seem to make too much sense to me for this set-up.


Is this correct?

 

[SaintJude]

Yeah, it's C. I narrowed it down to C as well but not with confidence. And I'm still confused by the phrasing...

What are the saying when they say "increasing or decreasing the flow of liquid nitrogen (–200 °C) or steam (150 °C) through pipes about the walls?"

Should I just ignore the details and focus on the fact that they are talking about the flow of liquid or gas = convection?

I knew that the end process was radiation, b/c the low pressure environment of the chamber means that the chamber resembles vacuum conditions. And radiation is the transfer of heat via electromagnetic waves which travel and can occur through vacuums.

Can radiation only occur in vacuums? If the pressure was at 1 atm, would radiation be "impossible?"

 

[EnginrTheFuture]

So the fluid (which can be gas or liquid) is being circulated through the pipes to keep it at a specific temp. Think convection oven moving fluid, i.e. air, about an oven to keep it uniformly heated. The oven is the equivalent of the pipe in this case. Anytime you hear fluid motion and heat transfer, as you seem to know, assume convection unless something crazy is at play.

The "about" the walls of the chamber means the pipe is around the chamber walls, not in them. It implies that the pipes are essentially touching and the heat is moving from one material (the pipe) to another (the chamber metal)... that they are not one in the same. They are trying to say that they are having solid-solid heat transfer without telling you it outright.

Radiation can happen in non-vacuum. The heat of the metal can be radiated off or conducted/convected into the air particles. Some of both will occur but it is much more likely that it will be conducted/convected as opposed to radiated if there are a lot of particles bumping into the metal.

 

[Buttafuoco]

Radiation can occur through any or no medium. Plants make glucose by using radiation and that black shirt makes you hot in the summer because of radiation. I think they key here is that they're telling you this sample is contained in basically a vacuum and therefore the final energy transfer can only occur through radiation. Of course the solid sample is sitting on the floor of the unit IRL but whateveskis, they probably want you to not assume that it is sitting directly on the metal floor of the container and able to pick up energy though conduction.

I definitely agree that the wording "about" is confusing and doesn't make sense. Who knows if they write crap like that to force you to go through weird reasoning or what?

 

[SaintJude]

Ah, thank you. One final question regarding Experiment 2. How does the darn sample just magically sublime? Are we asked to infer that the temperature is then cranked up?

 

[Buttafuoco]

I don't understand your question. It tells you in what you posted that the pressure is lowered and that the temperature increases and that it sublimes.

 

[SaintJude]

Hmm, I see. I didn't realize that something can sublime at -5 degrees.

 

[Buttafuoco]

I would guess that there are wildly varying temperatures of sublimation for different chemicals. It happens at 1 atm in your freezer to some extent at -20 degrees C, and if you ever did an orgo lab where you used a cold finger, I think that was some decently low pressure and somewhere near room temp. It's just one of those things where you you need to see the phase change diagram methinks.