On Wed, 12 Sep 2007 00:36:28 GMT, Jere Lull wrote:

TW, though I don't believe a small engine such as ours could
practically boil much water --that last degree to flash to steam is a
killer-- it seems a good idea to distill water drawn from the hot water
tank, gaining a good bit of the required BTUs for free.

A BTU is a British Thermal Unit. It is the ammount of heat that it
takes to raise one pound of water by one degree F. It takes about 1073
BTUs to evaporate a pound of water. Roughly 140 BTUs to raise the
water from room temperature to the boiling point. If you are heating
the water in the hot water tank for free, it would help, but not by
much.
There is not really a last degree. The phase change from liquid to gas
takes place at a constant temperature, the boiling point, and and the
last degree of heating below that point, is the same as any other
degree, one BTU per pound. In round figures, seven eights of the
energy goes into the evaporation, one eighth to temperature change.

Boiling point varies greatly with pressure, but I assume that we are
all talking about ordinary sea level, 14.7 psi type air.[give or take
changes with the weather]

Casady

Richard Casady wrote:
snip

The phase change from liquid to gas
takes place at a constant temperature, the boiling point,

Which isn't a constant temperature, as you address below..

and and the
last degree of heating below that point, is the same as any other
degree, one BTU per pound.

This is true from a physics perspective, but not, unfortunately from an
applications engineering perspective. Since the effectiveness of
whatever heat exchange mechanism you use is proportional to the delta-T
between the process and the exchange medium, each degree of process rise
requires more heat input into the system than the previous one. Not into
the 'process', but into the 'system'.

This, IMO, is the crux of the issue of trying to use engine heat for
evaporation (i.e. distillation), versus just preheating. For an
efficient process, the engine-to-transfer medium exchanger needs to run
with a significant delta-t, and so to does the transfer
medium-to-process exchanger. This two-step cascade would likely require
much higher engine operating temperatures than normal, with all the
attendant maintenance and longevity issues.

In round figures, seven eights of the
energy goes into the evaporation, one eighth to temperature change.

Boiling point varies greatly with pressure, but I assume that we are
all talking about ordinary sea level, 14.7 psi type air.[give or take
changes with the weather]

Casady

Keith Hughes

On Wed, 12 Sep 2007 23:17:40 -0700, Keith Hughes
wrote:

This, IMO, is the crux of the issue of trying to use engine heat for
evaporation (i.e. distillation), versus just preheating. For an
efficient process, the engine-to-transfer medium exchanger needs to run
with a significant delta-t, and so to does the transfer
medium-to-process exchanger. This two-step cascade would likely require
much higher engine operating temperatures than normal, with all the
attendant maintenance and longevity issues.

There is no escaping the simple fact that equipment for using the
waste heat from an engine for distillation was around for decades. Off
the shelf. It was intended for boats, of all things. RO may have
killed them off, however. Why do you insist that proven, available off
the shelf [ at one time, at least,] equipment cannot work?
Under load, the exhaust headers on my car run yellow hot, with a
ninety MPH breeze cooling them Enough temperature difference?
Something like a quarter of the fuel goes to a hot exhaust. Three
quarters of the fuel burned in a gas engine goes to waste heat.
Diesels do a bit better, and get maybe one third as shaft work.

Casady

On Sep 13, 2:39 am, (Richard Casady)
wrote:
On Wed, 12 Sep 2007 23:17:40 -0700, Keith Hughes

wrote:
This, IMO, is the crux of the issue of trying to use engine heat for
evaporation (i.e. distillation), versus just preheating. For an
efficient process, the engine-to-transfer medium exchanger needs to run
with a significant delta-t, and so to does the transfer
medium-to-process exchanger. This two-step cascade would likely require
much higher engine operating temperatures than normal, with all the
attendant maintenance and longevity issues.

There is no escaping the simple fact that equipment for using the
waste heat from an engine for distillation was around for decades. Off
the shelf. It was intended for boats, of all things. RO may have
killed them off, however. Why do you insist that proven, available off
the shelf [ at one time, at least,] equipment cannot work?
Under load, the exhaust headers on my car run yellow hot, with a
ninety MPH breeze cooling them Enough temperature difference?
Something like a quarter of the fuel goes to a hot exhaust. Three
quarters of the fuel burned in a gas engine goes to waste heat.
Diesels do a bit better, and get maybe one third as shaft work.

Casady