building a radiator
 

I plan on making a raidator type of device with 2 120mm fans on it. How big should my radiator be.

Well bigger is always better. At bare minimum you want to have a fin area no smaller than 240mm x 120mm.

more help please

what kind of help are you looking for?

It depends on a lot of factors. First, what kind of fans are you planning to use with it? You'll need to consider the fin spacing (lower fpi for less airflow), core depth (thinner for less airflow), height and width (does it need to fit inside the case or an external radiator box? Larger for lower airflow), and spacing between tubes, to name a few things. I'm assuming you're trying to make a tube and fin radiator, though I don't know how you would manage that easily. This thread should be fairly helpful if you're looking for a low-noise design.

Since you're designing this for a very specific use, and probably not for mass production, you should strongly consider finding fans that exactly meet your requirements, then build the radiator to best match those fans.

In most cases you are better off buying some old heater core from a car. Making a rad is alot of work if you want it to be efficient. =/

If you're serious about making your own radiator, I have a design I think will perform darn well. However, it will be labourious and expensive compared with just ripping out a heater core or buying something professional, and there's a risk I am wrong and it will perform crap.

We'll probably need to make two or three and learn from mistakes.

Materials are predicted to cost two to three times what a heater core would cost.

If you're serious, PM me.

I plan on having the radiator in and external box made of wood. In the box will be the pump, resivor, radiator, and 120mm fans.

How long should I leave the liquid cooling system on after I shut down my PC?

please help answer my questions

IMHO there is no reason to do so.(Because of huge thermal capacity of block + liquid)

Try look at this one.
Simple but expensive. No fans required. Silence is everything :) .
//edited link

I hate to discourage a hobbyist, but you would be better off buying a BIP 2 and building a shroud for 2 12cm fans. You would be hard pressed to build a radiator that is SUBSTANTIALLY more effective.

Truth.

Now I am using quite different DIY no-fan design and it has dT=1,5 K at 100W load. Not so great performance as 2-fan solution but sufficient for me. :cool:

I like watercooling because of its low-cost, silence and acceptable performance. I do not like moving parts either :) .
Thats all, good luck!