Date:         Sat, 4 Aug 2007 21:40:53 -0700
Reply-To:     Alistair Bell <albell@UVIC.CA>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Alistair Bell <albell@UVIC.CA>
Subject:      Re: Comments wanted on this portable fridge
Comments: To: Pensioner <al_knoll@PACBELL.NET>
In-Reply-To:  <HHEAJIOMDPBGGCKHACGJMEAFCNAA.al_knoll@pacbell.net>
Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed

back of envelope calculations to original question...

On 8/4/07, Mark Drillock <mdrillock@cox.net> wrote:

>
> OK, students on the list, snap quiz time. How much mass in one cubic
> foot of air at sea level and 50% humidity?  How much energy does it
> take
> to raise or lower that mass by 40 degrees F?
>
> Mark
> too old to remember how to figure this out
>
>

air weighs approx .0755 pounds per cubic foot at sea level at 60 or
so degrees F.

at same temp, 50% saturation would be approx 7 grams or water  per kg
of air.



converting to same unit, 0.755 pounds is about 0.034 kg, so at 50%
sat, one cubic foot of water holds (very approx) (0.034 * 7.0 g), or,
0.238 g of water.



1 calorie is amount of energy needed to raise one gram of water by on
degree C. to raise 1 gram 40 degrees F (very approx 22 C) would take
22 Cal. To raise 0.238 g water same amount would take (0.238*22) = 5
cal.

all this is VERY approx estimations, forgive me if i have dropped any
decimal points. Bottom line is that there is not much water in t he 1
cubic foot of air at 50% sat, really inconsequential compared to the
amount of beer in the fridge.



Alistair







On 4-Aug-07, at 8:36 PM, Pensioner wrote:

Normalizing to 1cu ft is left as an exercise for the interested.
Curves for
the specific heat of one cc of air at various temperatures  and RH
values
exist.  So to figure out how many btus it takes to change the mass of
air
one degree either way (same value) becomes a plug and play exercise.
If it
takes n BTU to raise one cc of air at 50% rh and  50f one degree f,
determined from the curve values then some multiplication to scale up
to one
cubic foot will yield the number N btus required to raise that mass one
degree from 50F. A similar scaling not likely linear will allow a
calculation of the heat required to be removed or added from the thermal
mass to change it's temperature T degrees F.  QED?  No, you can't
easily add
with a Post Versalog but you can do the rest to within 1 RCH or so
for three
significant digits worth.  The adding will require the rest of your
significant digits.