Electric Fan Sizing...

[richardm6994]

I'm having a slight overheating problem at low speeds (traffic jams / cruising through town).

 

I've put it down to the fact my electric fan is both old and cheap.

 

I plan to upgrade it to a better fan, but I'm wondering about the size?

 

At the moment, it is 14inch, but this is slightly bigger than the rad core and overlaps the radiator head & sump. When the fan his operating, you can actually feel the air blowing back through the fan around the head & sump because of he obstruction caused due to it being a solid area.

 

The photo below is the best I have illustrating the fan overlap at the head and sump. The fan is sealed to the rad so no air can escape outside the cowling.

 

My query is......do I got for the same size fan simply because it covers more area and has a high cfm or do I go for a 12 inch fan which will sit nicely on the core (no overlap with the head nd sump) and not have any blow-back issues / turbulance cause by the head & sump obstruction....obviously though the smaller fan covers less area and has a slightly lower cfm

 

I would appreciate your thoughts.....

 

[brumster]

Personally I would move to a smaller, but decent, 12" fan - something from Pacet, Kenlow or Spal should be more than capable of cooling your ali motor. I use Spal on my competition car and now on the Hood, after trying out a £20 ebay special (I figured there was only one way to find out if they were as crap as I suspected; I was correct ). They don't half shift some air, I doubt the difference between 12 and 14 is all that great....

 

http://www.spalautomotive.co.uk/acatalog/AXIAL_COOLING_FANS.html

 

12" - 1450, 1710, 2190 or 2860 m^3/hr

14" - 1620 m^3/hr

 

So 12" available that shifts more than the 14" anyway!

[richardm6994]

That was my thinking brum. My gut feeling is that 12" of fan providing an unobstructed air flow would be better than the 14 inch fan than is suffering bad turbulance at the blades due to the radiator head and sump solid obstruction.

 

The fan on it now was a £20 eBay special!!!

[richardm6994]

The fan I'm looking at is the kenlowe. A 12inch provides 3126m3/h

 

http://www.burtonpower.com/parts-by-category/cooling-system/electric-fans/kenlowe-electric-fan-only-12-dia-heavy-duty-kendf12hp.html

 

14 inch is only about an extra 150m3/h

[Guest Tim Norman]

Jet boost second speed!

Make sure that the radiator is "cowled " so the air cant get past it.

And if you can find the space take on board some of

[brumster]

One word of caution on the vid; what he doesn't talk about is effective ducting of the air into that angled radiator. By his rationale, tilting the rad 90 degrees would make for all the surface area for cooling but with minimal drag because the rad is horizontal! But of course, that's daft, because the air coming in wouldn't be directed onto the square face of the rad. I pretty sure some analysis would show that as you tilt the radiator from the vertical, at some point the cooling efficiency seriously drops off IF you're not forcing air to go through it. So by that I mean, if you've got gaps above and around your rad, the air will flow OVER the surface of the angled rad and over the top of rad, rather than through it.

 

The ducting idea goes for all rads really, even vertical ones, but just be aware tilting one in your nosecone but leaving ma-hoo-sive gaps around the side won't do it any favours. Probably stating the obvious, but you never know ....

[fry61]

Is it not a general rule of thumb that petrol engines give 33% power to fly-wheel -- loose 33% out the exhaust -- & need to dissipate 33% in the cooling system which includes the engine/head/sump. So if Richard is making 200HP at the fly he needs cooling capacity of 150kw when at fullchat. Sounds an awful lot of rad (sorry convector core) needed Richard.

[richardm6994]

To give you more details what I mean by overheating...fistly I'm reading the temp from the electric water pump digital controller. The controller varies the speed of the water pump according to temp from 3volts to 12volts

 

On the motorway...temp sits solid at 85degrees regardless of how much right foot I give it and occasionally temp drops to 80 degrees when coasting / going down hill.

In this condition, the electric water pump is running at about 3-6volts.

 

 

Blasting down country roads, temp sits solid at 85degrees again, but it will have the occasional rise to 90degrees for a few minutes before dropping back to 85degrees.

In this condition, the pump is running about 8-12 volts.

 

 

When sitting in traffic, or slow driving through town, the temp rises to 95-100degrees over about a 20 minute period.

Obviously the pump is running at 100% along with the electric fan.

 

I daren't take it above 100degrees for fear of warping the ali heads.

 

Also, I can turn the engine off, and leave the water pump and fan running. This brings the temp back down to 85degrees within 5mins.

 

 

The radiator is a cortina size but 4-core 70mm thick.

 

The fan is a £20 special off ebay which is why I'm looking at that as being my problem. The fan I'm looking at to replace it with is either the 14" (3300m3/h) or 12" (3126m3/h) kenlowe fan....

[sj-bradley]

What is NOT mentioned in the video and the obvious flaw in the reasoning is this.

 

The radiator in the top with a sloping front is now a custom made item and we assume that the cooling fins are still parallel to the direction of flow of air.

 

With a normal radiator, tilting this will mean that the flow of air NO LONGER passes straight through the core but has to be deflected and this will mean the air gains a little heat from the impact from the deflection.

 

It is much more complicated than this video describes as you would need to take into consideration fluid dynamics of flow through the radiator and you end up with a differential equation to solve the problem.

 

And, I have forgotten more maths than I would like So to solve this would take me longer than necessary.

 

The larger (ducted) fan with large flow capacity should do. But remember that this flow rate is for the fan ONLY and does not take into consideration the radiator behind it. (Manufacturers love to do this, it makes their products sound fantastic )

 

Simon.

[fry61]

Can't see where the prob. is ---- aren't petrol engines meant to run close to 100*C for optimum efficiency? Is that not why we have pressure caps -- to allow the coolant to work above it's normal boiling point?? I only start to be concerned if temp is 110*C & remains there.

[richardm6994]

the thing is bob, I'm almost certain it would go higher than 100degrees if I let it....I'm just playing it safe at the moment by turning it off before it gets there.

 

Ideally I would like it to stay below 100c in traffic / driving through town so I'm hoping that the better fan will help

[fry61]

As Simon says (Q record) fan air flow stated is usually calculated by the manufacturers as a product of swept area of blades x the angle & number of blades. As with fuel efficiency figures it's theoretical flow not actual. Cooling can only be increased (given same air mass & temp) by increasing air flow OR temp. differential between engine coolant & ambient air.

[sj-bradley]

The second half of the video still does not mention the fact that the air flow through the core must be parallel to the direction of incoming air to be effective.

 

With our tilted radiators, we can not do this (After all F1 can afford to get custom made radiators !) so the air flow has to slow down as it passes through our tilted radiators and will have longer to heat the air so we need to force as much though with the added fan as possible.

 

(Also F1 cars rarely stop in stop-go traffic so fans aren't required as they are constantly moving and a fan would obstruct more air than a free flowing radiator)

 

So, overall, putting the best fan on would improve your stationary traffic heat problem I think.

 

Simon.

[fry61]

With you there Richard, just need to aim for a point where the fan is able to overtake your use of right foot & get the temp down quicker than it can go up; & stuff the maths.

[richardm6994]

I'm hoping that the kenlow fan will increase the air flow. for a start it's a 265w motor whereas the fan Ive got at the moment is 80w.

 

kenlowe state that "airflow is measured at a standard radiator resistance of 3.75mm wg staic pressure"

 

 

The main question I had at the start of this is...

 

do I use a 14" fan which would have part of it's air flow obstructed by the radiator header & sump. With my current 14" fan, this is causing the air to hit the obstructions and blow back through the fan....could this blow back be disrupting the air flow to the rest of the fan when i'm sat in traffic?

 

or

 

do I use a 12" fan which does not have it's air flow obstructed by the radiator head & sump and as such will not suffer any blow-back through the fan blades.