Open System: Yay or Nay??

[Ecomike]

efficiency of the coolant may not care if the system if open or closed but the pressure of the closed system raises the boiling point of the coolant allowing it to transfer more heat away from the cylinders without boiling over than an open system, which is why most renix rigs sit between 180 and 210 and all HOs are happy at 210. the bottom line is a closed pressurized system will always run cooler (if operation properly obviously) than an open vented system.

Well, on that I totally disagree. The pressure in the Renix closed system increases as the coolant temperature rises, thus continuously suppressing the coolant from boiling. Granted if the coolant flow is too slow, the higher open system pressure might have a temporary advantage until the poor flow boils it over anyway.

The Renix system pressure will hit 16 PSI at about 265 F (IIRC) at sea level, with a 50/50 mix of regular green ethylene glycol based coolant and DI (or distilled water). The Renix closed system pressure rises as the temperature rises, until it reaches equilibrium.

The open system opens the vent cap at about 160 F or lower since the coolant is not compressible. Every time the cap vents liquid volume, which happens routinely as the coolant volume increases with temperature, is a point in time when the coolant pressure drops for a moment and coolant can boil in the block or head!!! Ever see a temperature gauge jump and spike? That happens when the open system cap opens and the coolant instantly turns to gas at the temp sensor (boils) because the pressure fell in a surge or wave towards the cap.

 

[Ecomike]

The main reason I favor the closed system is because it runs at about 8-9 lbs of operating pressure at 200 F, versus 16 lbs cold or hot in the open systems.

That extra 7-8 lbs of pressure is not needed (if you have proper coolant flow), and causes a lot of extra stress on the hoses and especially on the radiators. In fact with all the thin wall plastic tank radiator crap in use now days, that is probably why most of them went to closed systems the last 20 years.

Now if you have lime scale in areas in the block, and poor flow for any reason, a closed system might take a little longer to boil over but I would not count on it as the cap opening might just speed up the boil over BOOM! Meanwhile the Renix closed system, has a built in air cushion buffer in the bottle.

Also, My kids have a 1996 Ford Taurus, and a 2001 Saturn, that are both closed systems like Renix, that run all day at 212-217 F (E-fan cycles the temp), that work just fine with out running at 16 PSI. They run about 10-11 PSI.

 

[Boss Cherokee]

Either way I left it alone for now but replaced water pump and thermostat at 195*. The radiator was replaced in 03 when I got it but have put 3-4000 miles on it since then. It boiled over once in B.B. running 36's locked in and 8 psi driving back to camp driving for about 20-25 minutes. I don't know if it was low tire air pressure, one broken axle, high altitude, or something else but I hate to see any motor run at 235* 240*even if it's for 5 minutes. The trans cooler has helped but has not solved it but I will monitor it closely. I did find a hood grill at PickaPart and will be cutting the hood wider than the 13" I have now.

 

[5-90]

Actual efficiency of the cooling system will depend on two primary factors:

- Surfactants present in the cooling liquid. These break down surface tension, making the water "wetter," and allowing for more heat transfer (both acceptance and rejection.)

- Pressure within the cooling system. For every 1psig increase over atmospheric, the boiling point of water will increase by 3*F. 10psig increase? Boiling point goes from 212*F to 242*F.

This will reduce the potential for "steam voids" in the cooling jacket, since water vapour is far less efficient for heat transfer than liquid water (not to mention the increased heat retained in the system by the presence of the steam void in the first place.)

This applies whether you're using an "open" or "closed" system.

RENIX is often modified to run cooler because it can run cooler without incident. I don't like my engines being forced to run at 210-220*F when I know I can pull that down to 180-185*F without negative effect. This helps with oil longevity, tribology, parts longevity, and does not have a negative effect on tailpipe emissions (as some people claim.) "Before-and-after" tests showed a net increase in tailpipe emissions of ~1.5% - and, considering that the period between tests was the standard biennial interval, it would be foolish to attribute the increase strictly to the change in operating temperature (at the time, I was somewhere well north of 200Kmiles on the odo anyhow.)

OBD will pitch a fit if you pull the operating temperature down, particularly OBD-II (which tends to throw a "failure to warm up" code, or whatever OBD calls it. Yes, it's a "hard" code.)

[SARCHASM]I just can't wait for OBD-III to roll out...[/SARCHASM]

(No, I didn't misspell those tags. Chew on it for a bit...)

 

[Ecomike]

Actual efficiency of the cooling system will depend on two primary factors:

- Pressure within the cooling system. For every 1psig increase over atmospheric, the boiling point of water will increase by 3*F. 10psig increase? Boiling point goes from 212*F to 242*F.

This will reduce the potential for "steam voids" in the cooling jacket, since water vapour is far less efficient for heat transfer than liquid water (not to mention the increased heat retained in the system by the presence of the steam void in the first place.)

This applies whether you're using an "open" or "closed" system.

I am not sure how you got the idea those are the two major issues for efficiency?
In my world the % water would be the number one efficiency issue! Anyway,

Unless you are running 99-100% water that boiling point data is the wrong data.

As you know, most people run a 50/50 blend of winter/summer antifreeze/coolant, which drops the cooling efficiency based in calories per lb of coolant by about 15% ( a huge number). In return they get many other benefits, including higher boiling points......

http://hellafunctional.com/?p=629

A 50/50 mix has an initial boiling point of 225 F at atmospheric pressure. At the 7-8 psi, my Renix reaches at about 180 F, the boiling point of my 50/50 blend is already 248 F at that 7-8 psi!!!!

http://www.engineeringtoolbox.com/ethylene-glycol-d_146.html

Assuming this guys data is right:

for 50% EG by weight, here are the boiling points at different pressures

PSIG DEG F
0 224
1 228
2 231
3 234
4 237
5 240
6 243
8 248
10 253
12 258
14 262
16 266

http://www.eng-tips.com/viewthread.cfm?qid=175078

By the time a Renix system reaches 220 F, the system pressure is about 11 psi, raising the boiling point to 255 F, in other words as the coolant gets hotter, the pressure does rise in a closed system, and rises more than enough to keep the hottest spots in the block from boiling.

I actually measured the operating system pressure on mine 2 years ago when I was experimenting with my improvements, I used an inline filler neck and pressure reading cap tool, and discovered it was about 8 PSI at 185 F. Then it dawned on me that as the temperatures rise the partial pressures of the water and EG rise, raising the system pressure, instead of starting at 16 lbs and venting liquid as the temp rises and volume grows, the Closed system pressure rises slowly using the air cushion in the turtle bottle, but it does rise fast enough to keep the system from ever getting close to boiling in the hottest parts of the block, unless there is a mechanical failure, in which case either will boil over and exceed maximum system pressures. It dawned on me that air in the bottle being compressible, keeps the operating pressure lower at lower operating temperatures. And the pressure slowly rises as the coolant gets hotter.