"Nonexistence of Heat Momentum" - Page 6

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
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civ

#51: Post by civ » Nov 07, 2019, 3:00 pm

Hello:
another_jim wrote: ... not about exothermic or endothermic, since neither concept applies. It is about heat being input at one place and time, and affecting the beans at another place and time. Everything else is blather.
+1
rmongiovi wrote: You can't say a change took energy to happen or released energy while happening if there isn't any change.
+1

Cheers,
CIV

AZRich

#52: Post by AZRich » Nov 07, 2019, 5:48 pm

Forgive me if I am repeating someone else's observation. As I see it, you have moisture in the bean which undergoes a phase change into a gas. That water phase change is the whole crux of the discussion. What is the energy implication of this, and is the bean just an innocent vessel which contains and then releases this water/energy? If the bean started with no water content then there would be nothing to argue about, correct? Is this too simplistic?

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drgary
Team HB

#53: Post by drgary » Nov 07, 2019, 6:57 pm

From what I gather heat is entering the bean from the outside. This causes 1C, but it is not the bean generating heat via its own, independent chemical reaction.
Gary
LMWDP#308

What I WOULD do for a good cup of coffee!

rmongiovi

#54: Post by rmongiovi » Nov 07, 2019, 8:52 pm

AZRich wrote:Forgive me if I am repeating someone else's observation. As I see it, you have moisture in the bean which undergoes a phase change into a gas. That water phase change is the whole crux of the discussion. What is the energy implication of this, and is the bean just an innocent vessel which contains and then releases this water/energy? If the bean started with no water content then there would be nothing to argue about, correct? Is this too simplistic?
When you're boiling a pot of water, there's a phase change from liquid to gas. Since it is the hottest water molecules that make the change from liquid to gas escaping steam carries away the hottest water and leave the cooler molecules in the pot until they become hot enough to escape as steam. This is what limits the temperature of the remaining water to whatever the boiling point is at the current atmospheric pressure.

Water inside a coffee bean is contained in the same way that water in a pressure cooker is contained. Since it cannot escape to carry off the hottest molecules it is not limited in maximum temperature the same way water in a pot is. When 1C occurs the fracturing of the bean allows that steam to escape but it may already be hotter than the boiling point of water and I presume is at the same temperature as the solid parts of the bean that it had been in contact with prior to 1C.

Rao explains the drop in temperature as evaporative cooling. Evaporative cooling occurs when liquid water evaporates from the surface of an object because that liquid water requires heat to make the phase change from liquid to gas. With 1C I have a difficult time believing this is the case because prior to 1C the water was under pressure inside the bean and has already been given enough energy to convert to steam. This is what causes 1C in the first place. There is no need for liquid water to steal heat from the bean to become steam as it already has plenty. I think the drop in temperature we see after 1C is more likely a result of measuring air temperature + bean temperature + steam temperature when bean temperature is less than air temperature.

Prior to 1C, the BT thermocouple is in contact with the air inside the roaster and momentary contact with the beans. Since the beans are still heating up I would expect the air temperature to be higher than the bean temperature so I would expect BT to report a temperature lower than the ET reading but higher than the actual bean temperature (since contact with the beans is momentary). When 1C occurs, the BT probe is in contact with air, steam which is at the actual bean temperature, and the beans themselves. I would expect this to drop the reported temperature since the probe is spending more time in contact with stuff that is at the actual bean temperature.

HoldTheOnions

#55: Post by HoldTheOnions » Nov 07, 2019, 10:42 pm

Man, this thread is just painful.

rmongiovi

#56: Post by rmongiovi » replying to HoldTheOnions » Nov 07, 2019, 10:51 pm

Perhaps you should have a talk with the person who forced you to read it and see if you can be excused?

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LBIespresso

#57: Post by LBIespresso » Nov 08, 2019, 12:18 am

HoldTheOnions wrote:Man, this thread is just painful.
Agreed. But it is awesome too. Most I have ever enjoyed reading about phase changes etc...
LMWDP #580

crunchybean

#58: Post by crunchybean » Nov 08, 2019, 12:55 am

rmongiovi wrote:
Rao explains the drop in temperature as evaporative cooling. Evaporative cooling occurs when liquid water evaporates from the surface of an object because that liquid water requires heat to make the phase change from liquid to gas. With 1C I have a difficult time believing this is the case because prior to 1C the water was under pressure inside the bean and has already been given enough energy to convert to steam. This is what causes 1C in the first place. There is no need for liquid water to steal heat from the bean to become steam as it already has plenty. I think the drop in temperature we see after 1C is more likely a result of measuring air temperature + bean temperature + steam temperature when bean temperature is less than air temperature.
Actually, I explained that. But who's counting*.




*correctly

rmongiovi

#59: Post by rmongiovi » Nov 08, 2019, 1:16 am

crunchybean wrote:Actually, I explained that. But who's counting*.

*correctly
Mea maxima culpa but I didn't see any mention of evaporative cooling anywhere. You contemplate whether the steam escaping at 1C could recondense on the bean. I hit 1C at about 380F BT and my ET is even higher. This is so far above the boiling point of water that I can't imagine the steam release being endothermic. It already has much more thermal energy than required to be in the gaseous state. I don't see how it could be possible for the escaping steam to condense unless it comes in contact with a surface that is below the boiling point. But even if it does, wouldn't the heat released by the condensation equal the heat required by the re-vaporization?

HoldTheOnions

#60: Post by HoldTheOnions » Nov 08, 2019, 1:53 am

rmongiovi wrote:Perhaps you should have a talk with the person who forced you to read it and see if you can be excused?
Cuz I like the pain :P