Doublepass Radiators Over Rated?

[Nismo280zEd]

ok so i was about to buy a doulbe pass radiator from rondavisracing, but as usual i wanted to look up what double pass actual did to the radiator... found some interesting results... i think i might just stay with the single pass.

 

http://www.stewartcomponents.com/html/tech_support/techtip5.asp

Double pass radiators require 16x more pressure to flow the same volume of coolant through them, as compared to a single pass radiator. Triple pass radiators require 64x more pressure to maintain the same volume. Automotive water pumps are a centrifugal design, not positive displacement, so with a double pass radiator, the pressure is doubled and flow is reduced by approximately 33%. Modern radiator designs, using wide/thin cross sections tubes, seldom benefit from multiple pass configurations. The decrease in flow caused by multiple passes offsets any benefits of a high-flow water pump.

 

What do you guys think? Makes sense.. and i'd glady like to save an extra 50 bucks plus the pain of re-routing the hoses.

-Ed

[innerware]

Stay with what is known will work well right? I am putting in the JTR radiator this next week. Doing it at Mike Knells houes so I get to meet the man behind the book finally. He's taking before and after pics for new parts. He's going to be selling brackets and hoses for the chevy radiator to be put in to a stock l28. So anyway I am stiking with something that is known to work. Why bother with anything that might have problems right?

[Scottie-GNZ]

Not saying they are overrated but I have yet to see the need for one in my car. Take a look at my engine compartment and you see a FMIC that is wider than the radiator plus an oil cooler and a tranny cooler. The tranny cooler (black) in this pic has since been replaced by a bigger one that leaves no gap between the oil cooler and the end of the radiator. Approximately 500hp at the wheels and never a concern about running hot in FL. HOWEVER, there are certain things you have to do to accomplish this. You must have a shrouded fan that is properly wired and moves a lot of air. You must have a way to get the hot air OUT of the engine compartment. Couple of other things I do is run a 160*F thermostat, have the ECM control the fan at 168*F, run the minimum amount of coolant in the mixture and added Waterwetter and RMI-25. The low-$ 24x19 Griffin gets the job done for me following those guidelines.

 

[Guest Nic-Rebel450CA]

Here is another site with some good info:

http://www.geocities.com/provamo72/Cooling_3.html

 

 

Here's a scary thought!

A problem with this design is that it was common for the baffles to break loose allowing the coolant to bypass the radiator, running from the in pipe directly to the out pipe.

 

Here is probably the most important point:

Given the same number and size of tubes (same total cross section) the time the water is in the radiator is the same for either type, single pass or double pass, for a given water flow rate.

 

If you want a superior radiator, buy a honeycomb.

[johnc]

Well... I'm not a fluid dynamics engineer just a monkey-see, monkey-do engineer. I agree that a double pass radiator is not needed for a street car. But, when I go to SCCA, ALMS, NASCAR, USTCC, and other sedan racing series events I see 90% of the vehicles running double pass radiators. The advantage is that you can run a smaller, lighter radiator and get the same cooling capacity (maintain correct engine temperatures).

 

I currently run a C&R/Visteon double pass aluminum radiator with a built in Niagra heat exchanger. It keeps coolant temps between 200 and 210 degrees even when racing at Willow Springs in July. It also weighs 15.45 lbs empty including heat exchanger, radiator, and 4 -10 AN fittings, and is smaller then a stock 240Z radiator.

[SleeperZ]

I don't understand how they can cool any better at all, unless, as a result of the design, manage to get more fin area in the airflow. Seems to me, if the thickness is the same, and the fin area is the same, all you will get from a double pass is 16x the pressure drop with the same flow. The 16x comes from halving the flow area, then doubling the length. You can't dissipate more heat without increasing the fin area, or running the engine and radiator hotter.

 

Take a look at Scottie's intercooler - same surface area to dissipate heat, but an end-to-end design will have more pressure drop due to less intake air flow area.

[JMortensen]

This may be a stupid question, but how much coolant pressure is really needed? The pressure at the cap obviously comes from the coolant heating up, not the water pump pressure. I would think that as long as you have enough flow to keep it circulating well, pressure in a cooling system is a moot point.

 

Or am I missing something?

 

Jon

[johnc]

I don't know if that 16x number is correct. I suspect its wrong but I don't have the calculations to back up that opinion.

 

You also have to consider the amount of time the fluid is in contact with the fin area. Heat transfer takes time and the more time the coolant is in contact with the fins the more heat can be transferred. In single pass radiators the fluid only passes theough the fin area once. In double pass the fuild travels through the fin area twice. Now, that doesn't mean it cools twice as fast but it does increase heat transfer by some substantial number.

 

On my L6 I'm runnig the stock water pump and have no problems with coolant flow, etc. We've even put a restrictor in the system to slow down flow through the radiator. I do need to increase system pressure to eliminate some local boiling in the parts of the head, but that's system pessure, not flow pressure.

[gramercyjam]

At one time in my ME career, a long time ago, lead a project to develop software for power plant designers called Heat Exchanger Workstation. Most of that knowledge has been archived in those hard to access places in my head, but something I may remember correctly is that heat is not measured by just the temperature. It is a function of temperature and pressure. Heat energy stored in the fluid, and a fluid at higher pressure will store more energy than one a the same temperature and a lower pressure. So if you are just measuring temperature, you aren't seeing the whole picture.

 

That is one reason why you need to do the actual heat transfer calc's to see if a two pass heat exchanger is going to remove more heat than a single pass. It may or may not. I'm way to lazy anymore to do those calcs. I'm pretty sure you can find them in an SAE or ASME handbood or proceedings somewhere ...

[Drax240z]

Here are some basic numbers offered up by Visteon for FSAE comparing 1 pass and 2 pass radiators:

 

Core Dimensions Predicted performance*

number L[mm] H[mm] D[mm] FPDM 3m/s-core face air velocity 6m/s-core face air velocity

(Length) (Height) (Depth) (Fins/dm) Q[kW] dPair[Pa] dpH2O[kPa] Q[kW] dPair[Pa] dpH2O[kPa]

1 267 210 26 75 8.04 83 3.7 12.61 250 3.7

2 267 232 26 75 8.78 83 3.5 13.63 250 3.5

3 267 255 26 75 9.48 83 3.3 14.58 250 3.3

4 330 255 26 75 11.71 84 3.5 17.73 250 3.5

5 330 277 26 75 12.52 84 3.3 18.73 250 3.3

6 330 300 26 75 13.29 84 3.2 19.63 250 3.2

7 267 237 36 75 10.48 122 1.8 16.27 335 1.8

8 267 259 36 75 11.24 122 1.6 17.15 335 1.6

9 267 280 36 63 11.41 96 1.4 16.48 267 1.4

10 267 302 36 63 12.01 96 1.2 17.12 267 1.2

11 330 204 36 75 11.51 124 2.7 17.94 339 2.7

12 330 226 36 63 11.92 98 2.3 17.54 270 2.3

13 330 248 36 63 12.81 98 2.0 18.57 270 2.0

14 330 270 36 63 13.63 98 1.7 19.45 270 1.7

 

* Notes:

1. Radiator type: 1-pass parallel flow.

2. Predicted performance was calculated with HXCTM (1D model) for smooth tube, tall fin radiators.

3. Inlet air temperature: 40oC

4. Inlet water temperature: 100oC

5. Coolant flowrate: 20 l/min

 

 

Core Dimensions Predicted performance*

number L[mm] H[mm] D[mm] FPDM 3m/s-core face air velocity 6m/s-core face air velocity

(Length) (Height) (Depth) (Fins/dm) Q[kW] dPair[Pa] dpH2O[kPa] Q[kW] dPair[Pa] dpH2O[kPa]

1 267 210 26 75 8.24 83 10.9 13.19 250 10.9

2 267 232 26 75 9.08 83 9.4 14.50 250 9.4

3 267 255 26 75 9.91 83 8.3 15.74 250 8.3

4 330 255 26 75 12.27 84 9.5 19.22 252 9.5

5 330 277 26 75 13.24 84 8.5 20.61 251 8.5

6 330 300 26 75 14.19 84 7.6 21.93 251 7.7

7 267 237 36 75 10.98 122 10.4 17.90 338 10.5

8 267 259 36 75 11.91 123 8.9 19.24 338 8.9

9 267 280 36 63 12.25 97 7.7 18.72 270 7.7

10 267 302 36 63 13.09 97 6.7 19.85 270 6.7

11 330 204 36 75 11.93 125 16.6 19.33 342 16.6

12 330 226 36 63 12.49 98 13.7 19.11 272 13.8

13 330 248 36 63 13.57 98 11.6 20.60 272 11.6

14 330 270 36 63 14.63 98 10.0 22.01 271 10.0

 

* Notes:

 

1. Radiator type: 2-pass flow.

2. Predicted performance was calculated with HXCTM (1D model) for smooth tube, tall fin radiators.

3. Inlet air temperature: 40oC

4. Inlet water temperature: 100oC

5. Coolant flowrate: 20 l/min

 

If anyone would like I can send them the excel file that lists this in some sort of legible format.

[johnc]

If anyone would like I can send them the excel file that lists this in some sort of legible format.

 

Why would anyone need that?

[260DET]

On the efficiency of stock type aluminium crossflow radiators, the ones with the plastic tanks, I'm getting one which is slightly smaller in area than the OE '77 260Z one, thinner at 35mm, yet 20% more effective. And it is a lot lighter.

[rc's240z]

My single pass cross flow Ron Davis Radiator works great for cooling my high performance L6 2.8

[Guest penguin]

Seems to me, if the thickness is the same, and the fin area is the same, all you will get from a double pass is 16x the pressure drop with the same flow. The 16x comes from halving the flow area, then doubling the length. You can't dissipate more heat without increasing the fin area, or running the engine and radiator hotter.

 

ok, if there is a pressure drop, they could cool more due to the whole boyles law dealy.

[Nismo280zEd]

Well i've made up my mind... single pass Ron Davis radiator model 26163.

219 bucks.

 

Your car is designed with certain pressures in mind.... when you start changing them... things don't work like you think they will.... If you drop the pressure from stock... that means less water going through your pump with each turn which in time could wear out your pump faster from less lubrication.

[pparaska]

Based on Johnc's advise, I took a look at C&R Racing / Visteon for a radiator. The 84 Camaro radiator (used to be recommended by JTR) couldn't really cool well enough. I never overheated, but I couldn't rely on the temp staying in the range of the thermostat temp under load - it did creep up to 220 under extended high rpm running.

 

I talked with Chet at C&R. His opinion was that for a 24" wide radiator, I'd be better off with a double pass. His reasoning was that only ~20" of tube length with only 19" of core height was borderline for 500 hp that I'm hoping my 406 will get close to. He said the double pass would ensure that the coolant had sufficient overall tube length to let the heat transfer happen. But those guys don't know anything - because NASCAR and other pro racers buy from them doesn't mean a thing...

 

BTW, the C&R radiator is a very sturdy piece and the workmanship is beautiful.

[260DET]

Yes, I was talking with a radiator man who is himself a racer and he said he regularly fixed problem overheaters by modifying single pass radiators to double or triple pass. And that modification alone will usually fix the problem, if the radiator is big enough in the first place.

 

But he did say that the baffles so used must be very securely fixed in place.

[johnc]

Your car is designed with certain pressures in mind.... when you start changing them... things don't work like you think they will.... If you drop the pressure from stock... that means less water going through your pump with each turn which in time could wear out your pump faster from less lubrication.

 

I think there's some confusion regarding the "system pressure" (SP) in the cooling system and the force needed to move coolant through the radiator and engine. The SP in a cooling system is a result of heat, not any effort by the water pump to move coolant (although the work the water performs does add heat, but its insignificant). Adding flow length (and thus restriction) in the form of a double pass radiator does not affect SP. In fact, you can completely block off the radiator or add a dozen radiators in series and SP will still be unaffected, except for the amount of time it takes to reach the SP limit set by the radiator cap.

 

A perfect example of this is when someone removes the thermostat. The SP still reaches whatever pressure limit is set by the radiator cap even with the reduced restriction. Put the thermostat back in and the SP still reaches the pressure limit set by the radiator cap.

[johnc]

Pete,

 

I've got the dimensions for the C&R I had built for my car. I you want them, let me know. C&R should also have them on file under my name.

[pparaska]

Thanks, John. I ended up getting a regular, stocked radiator, as it's alot cheaper than a custom one. Still not cheap though, at $287. Here's the info:

 

PN 8200240075

Universal, Two Row, Double Pass Open, 24"x19", 1-3/4" Lower Outlet

 

PN 1030002050

Fitting, Universal, -20 AN Port to 1 1/2" O.D. Inlet

 

(Note: The newer design radiators they sell have the -20 AN port, so that the fitting is needed - this makes it easier for them to stock one radiator that fits several Inlet hose diameters.)

 

BTW, I had them weld a 1/4" bung to the radiator at the bottom for draining. That added $35 to the radiator.

 

If you know what tabs, etc. you want welded to the tanks for mounting, they'll do that also.