Author Topic: Understanding Pressure in The Coolant System.  (Read 1308 times)

Offline Spool

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Understanding Pressure in The Coolant System.
« on: February 11, 2021, 08:49:17 PM »
Hey guys.
73 c20 here with a stock 3row radiator.

I've never had an overflow tank and just purchased an aftermarket stainless steel one.
Yet, I realized I don't fully understand the vacuum within my coolant system.
I was debating if I needed to mount the overflow tank above or below the radiator, etc...

My most specific question is, does a vacuum form once the coolant spills into the overflow tank?
How does the coolant return to the radiator if I break the vacuum when I remove the rad cap?

I found this video online to help me understand, but it's not quite clicking in my head for me yet.
https://www.youtube.com/watch?v=QhLUwsCdM08

Could anyone please explain it to me differently?

Offline bd

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Re: Understanding Pressure in The Coolant System.
« Reply #1 on: February 12, 2021, 01:30:57 AM »
An automotive "closed cooling system" relies on a fixed volume capacity and consists of a mechanism for absorbing waste heat (engine water jackets flooded with liquid coolant), a heat exchanger for rejecting the waste heat (radiator), a pressure regulator (radiator pressure cap), a mechanism for efficiently circulating coolant between the engine and radiator (water pump), and a coolant level buffer that ensures the cooling system is always full of liquid coolant (recovery bottle, aka, overflow tank, aka, coolant reservoir, and the interconnecting hose). 

As the coolant heats, it expands.  Since the cooling system has a fixed volume capacity, the cooling system pressure increases above the ambient pressure (to the limit of the radiator cap calibration - typically ~16 PSI) at which point any excess coolant forces past the radiator cap into the recovery tank where it is stored.  Subsequently, as the coolant cools, it contracts.  The cooling system pressure decreases until it drops below the ambient pressure, which creates a vacuum that draws stored coolant from the recovery tank back through the radiator cap into the radiator.  The normal heat cycling of the coolant between the radiator and the recovery tank ensures that the cooling system remains completely flooded with liquid coolant and devoid of air pockets.  So it is the expansion and contraction of the coolant that determines whether the system manifests positive or negative pressure.

A neat property of a closed cooling system is that it is self-purging of air as long as the recovery tank and radiator are suitably filled with coolant initially.  Momentarily removing the radiator pressure cap from a cold cooling system in order to verify proper coolant level will not disrupt normal system function.  NEVER remove a pressure cap from a hot cooling system lest you risk severe scalding and personal injury!

The 1/4" hose that runs between the radiator neck bibb at the base of the pressure cap over to the recovery tank should connect to the base of the tank so that the hose port is always submerged in coolant. 
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline VileZambonie

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Re: Understanding Pressure in The Coolant System.
« Reply #2 on: February 12, 2021, 06:29:36 AM »
Well said bd

Just like your hot water heater or boiler also needs an expansion tank as liquids expand when heated.
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Offline Spool

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Re: Understanding Pressure in The Coolant System.
« Reply #3 on: February 12, 2021, 12:43:39 PM »
BD, thank you for taking the time to write such a thorough description. I truly appreciate that.

Two questions:

1. How does the vacuum not break when I remove the rad cap on a cold system? Wouldn't the air break suction with the overflow tank below it?
(Unless the coolant is filled all the way to the radiator neck bibb, correct?)

2. When installing my overflow tank, how much coolant should I pour in?
Or better yet, what is the best process in ensuring my overflow tank is adequately filled?

Offline bd

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Re: Understanding Pressure in The Coolant System.
« Reply #4 on: February 12, 2021, 01:55:53 PM »
Answer #1 - Cooling system pressure will instantly normalize to atmospheric pressure as soon as the pressure cap is removed.  The point is, it doesn't matter.  The system is self-bleeding as long as the radiator is full, there are no leaks, and the recovery tank is filled and maintained at the "Full" mark on the recovery tank.

Answer #2 - Fill the recovery tank to its "Full" mark if it has one - otherwise, arbitrarily, ~60% of overflowing the tank filler cap.  Incidentally, a closed recovery system needs to have a recovery tank filler cap that is vented to atmosphere so the coolant can migrate freely between the radiator and the tank.  It is atmospheric pressure working against cooling system pressure (driven by coolant temperature fluctuations) that allows the recovery system to function correctly.  After the cooling system has completed the initial heat cycle, recheck the recovery tank coolant level and add coolant as needed.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Spool

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Re: Understanding Pressure in The Coolant System.
« Reply #5 on: February 28, 2021, 09:01:40 PM »
Thank you, BD.
It was this sentence that I needed to make it click: "It is atmospheric pressure working against cooling system pressure (driven by coolant temperature fluctuations) that allows the recovery system to function correctly."

Really appreciate the help.