Latent Heat
ri...@dm2.deskmedia.com
Date: Wed, 11 Apr 2001
Reply From
On or about 11 Apr 2001 20:16:21 GMT, ri...@dm2.deskmedia.com () did
apparently write:
>
>Anyone interested in a discussion about latent heat..?
>
>From ice to water and from water to ice the latent heat
>required is 144 BTU's the temperature of the ice water
>never changes as stays at 32 degrees F.
>
> Why does the temperature of the water not change..?
>
> Hmmmmmm, neat stuff.. eh..!
I see where this is leading! But let's be 'sensible' :-)
One "Ton" of refrigeration is based lossless freezing of one ton
(2000 pounds) of water per day. So we have 12,000 BTU/hr as the basic
"ton". (2000 lbs * 24 hrs) / (144 BTU per lb) = 12,000 BTU per hour
per Ton of refrigeration.
In practice, there's about a 15 degree split in the evaporator, cold
plate, and condenser. So we lose about 500 BTU per ton in cooling
the ice to about 25 degrees after freezing, and about another 88,000
BTU per ton in chilling the water to the freezing point. Then there's
the sensible heat gains and losses in the evaporator, condenser, and
cooling towers, and any heat exchangers and/or clarifiers.
Compression is less than perfect, there is finite clearance, and the
process is neither adiabatic nor isentropic. Volumetric efficiency
therefore suffers, as well. The figure about a 20% in heat gains and
losses in the piping, and then the piping losses themselves.
After all is said and done, a "Ton" of refrigeration might be about
60% efficient. Then there are the fans, pumps, controls, heaters,
etc ... they knock it down to 40% or less. With a COP of around 2.5,
that leaves us with just about an honest "Ton" of cooling for every
4.71 Hp-hr of energy input, so a "Ton" is still a "Ton". It goes
to show, nothing's Freon this planet anymore. <g>
P.S. : Please pardon my half wit - ammonia novice at it.
--
-john
~~~~~~~~
"The time to repair the roof is when the sun is shining"
- JFK
~~~~~~~~
ri...@dm2.deskmedia.com
Thanks for the great reply, but I guess I should have been more specific.
I was looking for the reason a class of ice water stays at 32 degree F. and
the thing about hidden heat and the latent heat of melding etc. etc.
Also some of your numbers when it refers to NH3 refrigeration versus HVAC
do not apply, but thats true when comparing the two types of systems. Ammonia
is so much more efficient energy wise than freon approximately 8:1 compared to
R12. That's the biggest reason we use it in large commercial applications it's
cheap and works much harder per pound "Latent Capabilities of one pound" at
any compared temperature. Freon will do the same work it just takes more of it.
So I guess it was my fault for not being more detailed in what I was asking but
I do appreciate the response, keep up the good work..
Sincerely Richard M. Dumais
ri...@dm2.deskmedia.com
From: "Johnny" <neon@lightstrip,com>
Date: Thu, Apr 12 2001
Subject: RE: Latent Heat
On or about 12 Apr 2001 19:11:19 GMT, ri...@dm2.deskmedia.com () did
apparently write:
>I was looking for the reason a class of ice water stays at 32 degree F. and
>the thing about hidden heat and the latent heat of melding etc. etc.
Here's the way I lool at it (my quantam theory):
Not all molecules of a given mass contain the same kinetic energy.
The energy of any specific molecule is random - some move faster than
others. When a certain energy level is exceeded, a molecule will
break free of it's crystalline arrangement as dissociate itself into a
fluid state. Remember, ice is a crystalline solid. At 32 F, solid
H2O can dissociate into both liquid and vapor states (but not gas).
Since a specific energy level is required to dissociate any specific
molecule, the faster moving molecules will break free from the
crystal first. This tends to reduce the average kinetic energy of the
solid state, lowering its temperature. Now, vapor molecules near
the surface of the crystal must supply any heat energy to the solid
state to break loose any additional molecules. At the triple point,
equilibrium is established between all three phases, simultaneously.
For each molecule leaving the lower state, a molecule from a higher
state replaces it, and net energy transfer is zero between teh two
states. Adding energy to any substance in an equilibrium state will
simply move some of the molecules from a lower state to a higher
state. Since the two states have different kinetic energy levels for
the same molecular velocity, the energy (enthalpy) is increased, but
more of it is contained in the higher state than before, and less of
it in the lower state, therefore the average kinetic energy per
molecule is the same, as all of the additional heat input is used up
in maintaining an equilibrium between the energy levels.
When no more molecules exist in the lower (solid) state, any further
added energy will have no where to go except to contribute to an
overall average increase in the molecular energy of the higher
(liquid) state. The temperature will then rise, until the boiling
point is reached (saturation).
The reverse happens when water freezes. When there is no more liquid
left to absorb the energy, all the energy must come from the crystal
lattice alone. All the molecules now lose energy to the ambient,
reducing their average level, and the solid cools.
--
-john
ri...@dm2.deskmedia.com
OK here is a real challenge:
Who can explain latent heat in lay terms..?
Use the KISS method and explain to our readers what latent heat is
and how it affects refrigeration..?
Such as it's a hidden heat, that it's not measurable with a thermometer,
that ammonia has the highest latent capabilities, and do it without a
pressure ethalpy chart.. just explain it.. in simple terms.
good luck..!