Thread: measuring cooling rate/capacity

I'm building a custom water cooling system for my PC. I need to determine the cooling rate & capacity of this system which should aid the design.

Can anyone help on this issue - i've got lots of ideas and hints but am a little unsure about how to put this all together. I have varying pipe diameters etc and carn't find anything about how to determine the rate of flow in relation to the changing density of the water as a result of it picking up heat & how much heat the water can pick up & carry away.................... bla bla

can anyone offer any guidance on this?

thanks!!!

update - found this equation on the net which seems to help answer my question.


The equation for convection can be expressed as:

q = k A dT (1)
where
q = heat transferred per unit time (W)
A = heat transfer area of the surface (mo)
k = convective heat transfer coefficient of the process (W/m2K or W/m2oC)
dT = temperature difference between the surface and the bulk fluid (K or oC)

however assuming a recirculating system you are dealing with conduction not cnvection, need more info on this.

You can pretty much ignore the change in density unless you're thinking of very large temperature changes. The heat picked up by the water is m.Cp.Dt where m is the mass flow rate, cp the specific heat and Dt the rise in temperature (all in consistent units of course). For PC surely you'll only need 6mm tubing for the sort of flowrate you'll be using.

For a start what's the total heat you need to remove, in Watts?

Hang on, when you mention change in density, are you envisaging a thermosyphon, i.e. no pump required?

Originally Posted by Bunbury

You can pretty much ignore the change in density unless you're thinking of very large temperature changes. The heat picked up by the water is m.Cp.Dt where m is the mass flow rate, cp the specific heat and Dt the rise in temperature (all in consistent units of course). For PC surely you'll only need 6mm tubing for the sort of flowrate you'll be using.

For a start what's the total heat you need to remove, in Watts?

Hang on, when you mention change in density, are you envisaging a thermosyphon, i.e. no pump required?

thats exacly why i'm doing it - wanna do some maths behind it too so i put a half decent one together with some trial and error here and there.

Dunno bout the wattage yet but am looking at a fair amount, my chip says it can put out a max of 80W but with a video card and northbridge in there too i'm prob looking at at least 150W.

I'm having a go at thermosyphon as its silent, increases the thermal efficiency of my pc & lowers power consumption (even if by a fraction!!!).

you seem to be right about the change in density - been doing some calculations and for a 1 ltr system i think i'm currently looking at an expansion of around 4ml at a change in temperature of 20 deg. C !!!

now i need to find the volumetric expansion of air but carn't find the figure anywhere!!!

what i also need to find out is how the heat will effect the flow rate..................

Have you considered a heat pipe? It's like a self-contained thermosyphon and you can poke one end in almost anywhere and put fins on the other end for air cooling.

http://blog.makezine.com/archive/200...heat_pipe.html

Originally Posted by Bunbury

Have you considered a heat pipe? It's like a self-contained thermosyphon and you can poke one end in almost anywhere and put fins on the other end for air cooling.

http://blog.makezine.com/archive/200...heat_pipe.html

i know of heat pipes but i'm also experimenting with the cooling system i'm building - some of the things i'm doing will hopefully get a better flow rate & cooling then a heat pipe would be capable of - i do agree that heat pipes are cheaper, lighter and easier to construct and use though!!!

anyone know how to calculate convective flow rate relative to temperature increase. Like said above it is a closed system and could be more conductive then convective - i'm a bit stuck on how to calculate how the heat will affect the flow rate apart from building the thing and putting a little meter on it................

You calculate the driving force that will circulate your liquid as the difference in head between two columns of the same height with different densities. This is H.(rhocold - rhohot).g where rho is the density at the hot and cold temps and H is the height of your column. The trouble is for a liquid system the density difference will be a failrly small number so you will likely need a tall column to get it to circulate enough for your purposes. In a heat pipe the up flow is part vapor and the density is much lower than the downflowing liquid so it circulates at a very high rate. Sometimes there is also a wick to assist the downflow.

Once you have got your equation for the driving force you have to balance that against the frictional resistance. You can use the Fanning equation for this (look it up). The frictional resistance is roughly proportional to the velocity squared. You need the equivalent length of your pipe and this has to include allowances for all the bends in it.

You then need to find the flow rate at which the frictional resistance equals the driving force available. An easy way to do this is to set up the equations in Excel and use the Goalseek function to converge the results. Otherwise it's trial and error.

(But heat pipe is the way to go.)

thnaks a lot for your input - will do some more number crunching and see if i can make sense of it all...............

believe it or not I am currently doing something similar, already have a tall column so i'm with you on that one, i'm also trying to add some further hydraulic principles to try and increase the flow a little, hoping it will work, need to get some figures together first though to compare..........

i can see and appreciate the efficencies of the heat pipe system, bearing in mind though that there is a better working fluid in these items then good old water. the industry seems to be going in this direction and is doing so for a good reason i would like to think..........might give it a go for the next project!!!

one thing, in your equation you have 'g' at the end - is this meant to be there??

If you include g your answer will be in Newtons/m2 and if you ignore it it will be in kilograms/m2 so it's your choice, as long as you are consistent in all related calcs.

Originally Posted by Bunbury

If you include g your answer will be in Newtons/m2 and if you ignore it it will be in kilograms/m2 so it's your choice, as long as you are consistent in all related calcs.

thanks - to clarify by 'g' did you mean gravitational constant?

No, that would be G.

g is the accelration due to gravity (on Earth) and is 9.81 m/s^2 or 32.2 ft/s^2.