Generator exhaust pipe length

[[email protected]]

Dang! Lost Nick Pine's post on exhaust gas heat recovery for domestic
hot water.

In it he suggested running the exhaust through a 50' coil of soft
copper in a wash tub of water. (Hope I got that right.)

Exhaust back-pressure consideratons dictate that the tube be fairly
large diameter. Resonance in this tube will affect the engine's
operation. (Remember the farting sound of a school bus' exhaust at
a particular engine speed?)

I was concerned about resonance effects when extending the exhaust
line of my Onan 2 cylinder 1800 RPM generator
upward. The 1.5" OD of an electrical conduit fit inside some
flexible pipe from a NAPA auto parts store, so I could adjust the
length to tune the exhaust pipe. At 11 ft. there was a distinct
change in the exhaust sound (sharper), closely matching the length a
physicist friend had calculated.


Tom Willmon
near Mountainair, (mid) New Mexico, USA

Net-Tamer V 1.12.0 - Registered

 

[[email protected]]

Dang! Lost Nick Pine's post on exhaust gas heat recovery for domestic
hot water.

In it he suggested running the exhaust through a 50' coil of soft
copper in a wash tub of water. (Hope I got that right.)

Or a steel drum, with lots of holes in the coil, a few inches underwater.

Exhaust back-pressure consideratons dictate that the tube be fairly
large diameter.

The muffler pipe on a 2500 W Honda is only about 1/2" in diameter.

Nick

 

[SQLit]

Dang! Lost Nick Pine's post on exhaust gas heat recovery for domestic
hot water.

In it he suggested running the exhaust through a 50' coil of soft
copper in a wash tub of water. (Hope I got that right.)

Exhaust back-pressure consideratons dictate that the tube be fairly
large diameter. Resonance in this tube will affect the engine's
operation. (Remember the farting sound of a school bus' exhaust at
a particular engine speed?)

I was concerned about resonance effects when extending the exhaust
line of my Onan 2 cylinder 1800 RPM generator
upward. The 1.5" OD of an electrical conduit fit inside some
flexible pipe from a NAPA auto parts store, so I could adjust the
length to tune the exhaust pipe. At 11 ft. there was a distinct
change in the exhaust sound (sharper), closely matching the length a
physicist friend had calculated.


Tom Willmon
near Mountainair, (mid) New Mexico, USA

Net-Tamer V 1.12.0 - Registered

EMT is not rated for potable water. The galvanizing will flake off.

 

[Ecnerwal]

The muffler pipe on a 2500 W Honda is only about 1/2" in diameter.

Nick

Yes Nick, but it's also very short. Thus, the backpressure is quite low.
Put 50 feet of 1/2" pipe on there and the backpressure will be quite
significant.

 

[[email protected]]

Yes Nick, but it's also very short. Thus, the backpressure is quite low.

How low?

Put 50 feet of 1/2" pipe on there and the backpressure
will be quite significant.

How much? How much is acceptable? As I recall, a mechanic friend says
an extra 1/2 psi is OK.

Nick

 

[Mike Wilcox]

How low?




How much? How much is acceptable? As I recall, a mechanic friend says
an extra 1/2 psi is OK.

Nick

I know from playing with tuned exhaust systems on motor cycles ( and
some women) a couple of extra inches can greatly effect performamnce ;~)
Joking aside, a pipe too long will result in poor performance as the
exhaust gases don't scavenge properly. That's what the term "Tuned
Exhaust" means, getting the pipe to the length the exhaust pulse
clears the pie most effectively.

 

[daestrom]

How low?


How much? How much is acceptable? As I recall, a mechanic friend says
an extra 1/2 psi is OK.

First question is what is the volumetric flow rate of a typical gen-set?
1800 RPM, with what cylinder displacement? Two-stroke? Would be an
interesting calc. Hazard an estimate on the temperature leaving the engine
and how much heat could be recovered this way?

Of course, any backpressure rise will rob the unit of some amount of hp, so
keeping it to a minimum is best. And for heat recovery, maybe a larger
diameter pipe would be better (slow the exhaust flow so it leaves the end
cooler, and more surface area).

daestrom

 

[m II]

In it he suggested running the exhaust through a 50' coil of soft
copper in a wash tub of water. (Hope I got that right.)

Exhaust back-pressure consideratons dictate that the tube be fairly
large diameter. Resonance in this tube will affect the engine's
operation. (Remember the farting sound of a school bus' exhaust at
a particular engine speed?)

The exhaust exit diameter doesn't have to be any larger that the
diameter of the exhaust valve. The trick is to get the gas to cool down
BEFORE putting it through this 'small' hole.

The use of an expansion cannister before the long exhaust really helps.
I'm guessing here, but a tank capacity of eight or ten times the motor
displacement should be fine. If it's submerged in water, less will do.

For tuning purposes, calculate the length of exhaust as normal from the
valve To the cannister only. That's the effective length for resonance
purposes.

The long pipe venting this cannister is almost just window dressing.
It's important to think of any condensation forming in this set up. It
would be smart to get the long pipe to also act as a drain..as in 'Don't
exit it at the TOP of the cannister'.




mike

 

[[email protected]]

...what is the volumetric flow rate of a typical gen-set?

I seem to recall estimating 10-20 cfm for a 4 stroke Honda 2500 generator.
And about 900 F, based on gas consumption minus electrical output.

Nick

 

[Bruce in Alaska]

I was concerned about resonance effects when extending the exhaust
line of my Onan 2 cylinder 1800 RPM generator
upward. The 1.5" OD of an electrical conduit fit inside some
flexible pipe from a NAPA auto parts store, so I could adjust the
length to tune the exhaust pipe. At 11 ft. there was a distinct
change in the exhaust sound (sharper), closely matching the length a
physicist friend had calculated.

Is your Onan, air or water Cooled? If you have the air cooled version,
why not just get the Marine Waterjacket Exhaust Manifold for it and
use that to preheat your water? Most of the Parts for the DJx (Air
cooled) and MDJx (Water cooled) series Onans are interchangeable.

Bruce in alaska who owns both a DJA and a MDJA and love them

 

[daestrom]

I seem to recall estimating 10-20 cfm for a 4 stroke Honda 2500 generator.
And about 900 F, based on gas consumption minus electrical output.

Was that 10-20 cfm on the intake or exhaust?? The difference would be a
factor of about (900+460)/(70+460) in mass flow rate

daestrom