Water temp - techie question

[AlShack]

Excuse my ignorance, but could somebody please explain the theory behind cooling systems and the effect ambient air temp has on them?

 

To my simple mind, if there is a temperature differential between the water in a car radiator (assuming the thermostat is open) and the ambient air temperature, the radiator temp should decrease until such time that the thermostat closes.

 

For example: water temp 80 degrees, air temp 20 degrees, water rises to 90, thermostat opens, water cools to 80 because 20 is lower than 90 therefore bringing the temp down.

 

My temp is brought down nicely to 80 when the air temp is 15 or so, but when the air temp is 25 it stays at 85-90.

 

I guess it is not just a case of hot here, cold there so equilibrium prevails! Am I going to be bombarded with thermodynamics theories

 

Cheers

Al

[Shaun_E]

Any given radiator has a limit to how much heat it can dispel and that limit is dependent on the temperature differential between the coolant and the surrounding air. Air flow of course has an important effect but assuming the car moves at a fixed speed then with a greater temperature differential, more heat is exchanged. When the air temp is 15 there is a minimum temperature differential of 65 degrees and the radiator is able to lose enough heat to keep the water at 80. When the ambient temp is 25 degrees then the minimum differential, would only be 55 degrees and less heat can be lost by the radiator. The water temp therefore rises to 90 and the minumum differential is now 65 degrees again so the radiator once more can dump the heat at the same rate.

This is hugely simplified (and I'm no thermodynamicist) but I think is roughly correct.

Shaun

 

Yellow SL #32

[Henry]

is it true that an aluminium rad keeps things cooler?

Henry.

[Shaun_E]

Henry - worked for me. I went from the standard heavy black thing to a Radtec Ali radiator (mainly because I was fitting a Laminova oil coller) and this has kept temperatures much lower especially on track.

 

Yellow SL #32

[Henry]

ahhh....mine's not overheating, but is sitting a bit hotter post convertion.

how much did the new ali rad set you back?

easy to fit yourself?

H.

[Graham Sewell]

Also worth noting that the temp that you are seeing on the guage is at the top of the rad and is the entry temp. The driving force (temperature differential) decreases as you go down the rad as the water has already started to be cooled.

 

Low tech luddite - xflow and proud!

[Shaun_E]

Radtec radiator is about £200 (L7OC bulk buy to get that price). Alternatives are the Caterham ali road rad or their triple pass racing rad. Search on here for comments on these. I went the Radtec route because it seems to be more efficient than the Caterham road one and cheaper than the racing triple pass job. Some people have concerns over the flow rates achieved in the triple pass rad as the cores are smaller.

Fitting is a doddle - disconnect fan, disconnect lower radiator hose making sure you have a suitable reeceptacle for the coolant, disconnect upper radiator hoses, unbolt radiator (4 bolts), ubolt fan from radiator (4 bolts) then reverse the process and refill with coolant. Bleeding can be a pain but get the front of the car as high as possible to remove airlocks (search techtalk for more info on this).

 

Yellow SL #32

[Sheds Moderator]

Air has a lower specific heat capacity than water. This means that you need a lot of air to absorb the heat in a rad full of hot water. Don't forget also that the engine is generating heat as the rad is cooling it, so once the stat is open the temp will be a function of energy in (combustion) less energy out (airflow, size/efficiency of rad, air temp). Generally speaking rads are only slightly oversized and correctly designed cooling systems will only just cope with driving at full throttle up a steep hill at low speed and 40°C heat, with the cooling fan running flat out. (I wonder how many actually could stand this?). This is why race rads are bigger and thicker than road ones - the water stays in there longer and has longer to cool down.

[SLR No.77]

Switching to a Radtec over the standard will definately improve cooling, but so will any decent bigger capacity rad. I fitted a steel triple pass rad (ex-SLR) and temperatures have been rock-solid ever since, even with a Laminova fitted. These can be sourced s/h for around £50 if you want to keep costs down.

 

Stu.