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re: exhaust tube/pipe for eu1000i?
15 aug 2000
db tech   wrote:

>>>i would not use a box that would depend on intake air for box purging,
>>>with the exhaust vented outside, as engine overheating will result.

>>not if the air surrounding the engine is cool enough, say 130 f,
>>with a fan blowing over it. we might measure the fin temp under
>>full load on a hot day, and make sure it's the same or less
>>with the fan in the box.

for instance, if a honda 1800emxk1 loses 50% of its heat through 500 f
cylinder fins at full load, ie 13k btu/h into still outdoor air on a
100 f day, they have a thermal conductance of 13kbtu/f/(500f-100f) = 32
btu/h-f. raising the air velocity over the fins to 4 mph would likely
double fin conductance, making them 130+13k/64 = 333 f inside the 130 f
box with a fan, so the engine would run cooler than it would outdoors.

we might use grainger's $65 4c847 550 cfm 61 watt fan with a 158 f upper
temp spec, with a thermostat to stop the engine if the fan fails. the
engine might warm the 550 cfm airstream by 13k/550 = 24 f.

how can we get heat out of the box while keeping the noise and smells
inside, and control the house temperature? the engine might be inside
an inner sound-deadening box, with the fan blowing air from the lower
part of the outer box through the engine box onto 4" pvc water pipes
under the ceiling of the outer box, which could thermosyphon warm water 
into some pipes above them, outside of the box vapor barrier. another
fan with a room heating thermostat could blow some room air over those
outer pipes as needed.

or we might just have a large single box, and start the engine and
wait when the house gets cool, and open the house windows for sudden
warm spells. jeff fowler says he prefers heating systems with fast
response times to russian fireplaces... 

we could duct the room air that flows into the box over the generator
first, to cool it. a 1500 w 90% efficient generator losing 150 w (512
btu/h) would raise a 70 f 100 cfm airstream to about 70+512/100 = 75 f. 
 
>i believe we have a miscommunication here, as i read your term "intake
>air," as meaning combustion air for the engine, and this in itself
>would not provide enough air through the box for engine cooling.

it wasn't my intention to suggest that the intake air cool the engine.
i had in mind a small cogen system that would lose heat through the box
walls by conduction, and also store heat in some mass inside the box
(see the numbers in a previous posting.) the generator would only run
for a few hours a day, as required for house heating.

i suggested using the intake air (about 100 cfm) to slightly depressurize
vs cool the box, in order to avoid foul oily smells, in the same way that
hospitals depressurize rooms containing people with infectious diseases.
if room air only flows into the box, the smells stay inside. jeff fowler
talks about these smells in his personal pv history book, and says these
indoor smells happen even when the exhaust is vented outside.

one of his other generator complaints was the expense of calling someone
to service a generator on site, and the problem of transporting something
that weighs 600 pounds to a service person. hence my interest in hondas
that weigh less than 100 pounds and can be lifted easily into a car trunk
or pickup and transported for service. these are also inexpensive enough
(eg $580 for a 70 pound en2500) to enable keeping a spare on site.

nick




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