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Date: Thu, 15 Dec 2005 12:34:51 -0700
From: Bob White <rlwhite@comcast.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Radiator Expansion Tank
Message-Id: <20051215123451.12bdbb8d.rlwhite@comcast.net>
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Starting with the cap referenced to the atmosphere, it will relieve
itself at it's rated pressure (above the reference pressure).  Let's
say it's rated at 15 psi. At sea level, the absolute pressure would be
15 + 14.7 = 29.7 psi.  But A pressure gauge is typically referenced to
ambient and would read 15 psi.  This 15 psi is the reference for the
second cap. If it were a 15 psi cap, it would relieve itself at 15 psi
above it's reference pressure or 30 psi.  The absolute pressure at sea
level would be 30 + 14.7 = 44.7 psi.

The actual pressure in the system depends on the temperature of the
coolant (vapor pressure).  The vapor pressure of water at 230 F is
around 21 psi. That's absolute pressure, so a 22 psi pressure cap
should not open at 230 F even in outer space.

Bob W.


On Thu, 15 Dec 2005 10:06:58 -0800
"Joe Hull" <joeh@pilgrimtech.com> wrote:

> I think he's saying that it's 15lb rated pressure when there is 15lbs on one
> side and normal atmospheric pressure on the other side (14.7lbs/sq inch).
> If the cap has 22 lbs of pressure on the "atmosphere side" instead of the
> normal 14.7 then it would take 15lbs plus 7lbs (the difference between 14.7
> and 22) or a total of 22lbs to open it. 
> 
> I think the original math was off a little because it didn't subtract the
> expected atmospheric pressure from the cap rating. In other words I don't
> think the pressure rating is additive. 
> 
> Joe Hull
> 
> Cozy Mk-IV #991 (preping for DAR inspection - details, details) 
> 
> Redmond (Seattle), Washington
> 
>  
> 
>   _____  
> 
> From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
> Behalf Of Echo Lake Fishing Resort (Georges Boucher)
> Sent: Thursday, December 15, 2005 9:53 AM
> To: Rotary motors in aircraft
> Subject: [FlyRotary] Re: Radiator Expansion Tank
> 
>  
> 
> 
> Hi! Todd 
> 
> I'm not getting it, what stops the 15lb cap relief valve from  opening at
> its rated pressure? Even if it dumps into the 22lb reservoir the pressure
> still can't be greater than 22lbs. What am I missing?
> 
> Georges B.
> 
>  
> 
> -------Original Message-------
> 
>  
> 
> From: Todd <mailto:haywire@telus.net>  Bartrim
> 
> Date: 12/15/05 05:50:21
> 
> To: Rotary motors in aircraft <mailto:flyrotary@lancaironline.net> 
> 
> Subject: [FlyRotary] Re: Radiator Expansion Tank
> 
>  
> 
> 
> Todd
> 
> Can you explain how using two pressure caps (one 22lb & one 15 lb) add up to
> a possible total system pressure of 37lbs. I can only see it as a max of
> 22lbs,
> 
> Georges B. 
> 
>  
> 
> Hi Georges;
> 
>     The rad caps are differential meaning that they will hold back their
> rated pressure above the pressure seen at the outlet. So if the the first
> cap produces a 15 psi increase over atmospheric pressure, then the second
> cap which has it's outlet plumbed to the 15 psi reservoir will further
> increase this by it's rated pressure (22 psi in this case) to give a total
> of 37 psi.
> 
>     At least that's the way I see it.
> 
>     
> 
> Todd
> 
>  
> 
>  
> 
> 
> 
>  
> 
>  
> 
>  
> 
>  
> 
> 


-- 
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