Temps in loop will reach equilibrium

lowboy · 12-21-2005, 07:43 PM

A forum member is posting that a loop will reach equilibrium within a few minutes and that temps will be equal through out loop. This is saying that temp at radiator inlet and outlet would be the same. As I am a Mech.Eng. I have studied some thermodynamics when in school. And as I remember the Laws this has to be incorrect. This member is saying that this is being said on this forum. I am always willing to learn if any member can inform me or is this poster not know what he's talking about. Thanks for any help.

bigben2k · 12-21-2005, 09:23 PM

It's partly true.

The difference does exist, but it's very small, and can be hard to measure. The roughest measurements will not conclusively show a difference.

Here's an article by my colleague Martin, which includes the formulae (on page 2) for estimating that difference:
http://www.nordichardware.com/Articl...lse=347&page=1

bobo5195 · 12-22-2005, 04:08 AM

WC systems do indeed obey the first law. What you have to consider is that the enthalpy changes in the system due to heat addition and removal are small when compared with the overall temperature variation from ambient. I.e. water maybe on average 10c above ambient due to the thermal resistance of the system and may only expel 1c of heat in the rad. As such as an engineer you could say that heat change in the system is small and that it runs at an equilibrium temp.

Punching a bit of math and you should be easily able to prove this to yourself, with the c/w and power input numbers available around here.

gmat · 12-22-2005, 04:17 AM

this is also due to the high specific heat (kinda thermal capacity) of water. A 1°C of temp difference in water represents a lot of energy. That's why the temp across the system is considered 'uniform' (the temp difference is considered unsignificant)

BillA · 12-22-2005, 07:56 AM

no (stop Ben)

time to equilibrium in the system, assuming a 'stiff' environmental chamber, depends on the flow rates;
- at 0.3 gpm with a 'low' fan ~1.5 hrs are required if the temp resolution is 0.01°C
- at 3 gpm with a screamer 10 min is sufficient

a temperature difference in the thermal world is not called equal,
and if the difference represents the work being done it certainly IS significant
(what are all these really small numbers abut eh ?)

EDIT
lb
many things are posted here, many wrong and 'let slide' by others
I commented only after 3 others failed to identify the time constant of the system as the object, and made a hash of the temps

Joe · 12-22-2005, 08:53 AM

I was just about to reply but then I see bill and gmat said what was going to say (more or less)

lowboy · 12-22-2005, 10:13 AM

I understand the assumptions being made in the models. The terminology being used raised questions for me though. Thanks for the replys and it may be not be PC today but I wish everyone a Merry Christmas!

12-21-2005, 07:43 PM	#1
lowboy Cooling Neophyte Join Date: Dec 2005 Location: OK Posts: 3	Temps in loop will reach equilibrium A forum member is posting that a loop will reach equilibrium within a few minutes and that temps will be equal through out loop. This is saying that temp at radiator inlet and outlet would be the same. As I am a Mech.Eng. I have studied some thermodynamics when in school. And as I remember the Laws this has to be incorrect. This member is saying that this is being said on this forum. I am always willing to learn if any member can inform me or is this poster not know what he's talking about. Thanks for any help. Last edited by lowboy; 12-21-2005 at 09:04 PM.

12-21-2005, 09:23 PM	#2
bigben2k Responsible for 2% of all the posts here. Join Date: May 2002 Location: Texas, U.S.A. Posts: 8,302	Re: Temps in loop will reach equilibrium It's partly true. The difference does exist, but it's very small, and can be hard to measure. The roughest measurements will not conclusively show a difference. Here's an article by my colleague Martin, which includes the formulae (on page 2) for estimating that difference: http://www.nordichardware.com/Articl...lse=347&page=1 __________________ WBTA (Water Block Testing Alliance) Nordic Hardware WC 101

12-22-2005, 04:08 AM	#3
bobo5195 Cooling Savant Join Date: Aug 2005 Location: uk Posts: 400	Re: Temps in loop will reach equilibrium WC systems do indeed obey the first law. What you have to consider is that the enthalpy changes in the system due to heat addition and removal are small when compared with the overall temperature variation from ambient. I.e. water maybe on average 10c above ambient due to the thermal resistance of the system and may only expel 1c of heat in the rad. As such as an engineer you could say that heat change in the system is small and that it runs at an equilibrium temp. Punching a bit of math and you should be easily able to prove this to yourself, with the c/w and power input numbers available around here.

12-22-2005, 04:17 AM	#4
gmat Thermophile Join Date: Mar 2001 Location: France Posts: 1,221	Re: Temps in loop will reach equilibrium this is also due to the high specific heat (kinda thermal capacity) of water. A 1°C of temp difference in water represents a lot of energy. That's why the temp across the system is considered 'uniform' (the temp difference is considered unsignificant)

12-22-2005, 07:56 AM	#5
BillA CoolingWorks Tech Guy Formerly "Unregistered" Join Date: Dec 2000 Location: Posts: 2,371.493,106 Posts: 4,440	Re: Temps in loop will reach equilibrium no (stop Ben) time to equilibrium in the system, assuming a 'stiff' environmental chamber, depends on the flow rates; - at 0.3 gpm with a 'low' fan ~1.5 hrs are required if the temp resolution is 0.01°C - at 3 gpm with a screamer 10 min is sufficient a temperature difference in the thermal world is not called equal, and if the difference represents the work being done it certainly IS significant (what are all these really small numbers abut eh ?) EDIT lb many things are posted here, many wrong and 'let slide' by others I commented only after 3 others failed to identify the time constant of the system as the object, and made a hash of the temps Last edited by BillA; 12-22-2005 at 08:17 AM.

12-22-2005, 08:53 AM	#6
Joe The Pro/Life Support System Join Date: Dec 1999 Location: Denver, CO Posts: 4,041	Re: Temps in loop will reach equilibrium I was just about to reply but then I see bill and gmat said what was going to say (more or less) __________________ Joe - I only take this hat off for one thing... ProCooling archive curator and dusty skeleton.

12-22-2005, 10:13 AM	#7
lowboy Cooling Neophyte Join Date: Dec 2005 Location: OK Posts: 3	Re: Temps in loop will reach equilibrium I understand the assumptions being made in the models. The terminology being used raised questions for me though. Thanks for the replys and it may be not be PC today but I wish everyone a Merry Christmas!

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