Effect of Vacuum on Engine Exhaust 1

[zdas04]

I'm working on an "Evaporation Pond" project. The goal of the project is to dispose of about 15 gpm of water produced in conjunction with natural-gas production.

The higher the temperature of the stream, the more effective the evaporation will be.

The transfer pump is powered by a natuarally-aspirated 90 Hp industrial engine. I can install an "eductor" to let the water I'm pumping suck on the engine exhaust-stack and create a 10-15 inHg vacuum on the engine exhaust (and thereby raising the temperature of the water 10-15F).

My question (finally) is: Will there be any adverse affects of pulling a medium vacuum on the engine?

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[ivymike]

depends on your valve timing. You run the risk of having a great deal of flow-thru (charge bypassing the cylinder) during overlap. If you have the option of grinding a new cam, or if unburned fuel and efficiency aren't problems, it might be worth looking at.

 

[ivymike]

I forgot to mention one other thing- you might start sucking a bit of oil past the exhaust valve seals.

Of course, before posting either comment, I should have asked whether the engine is 2 or 4 stroke, and whether it runs gasoline or natural gas as fuel...

 

[zdas04]

Four stroke, natural gas.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[SBBlue]

They may actually be some benefits, since the engine won't have to work as hard as an air pump. 15 inch Hg vacuum is about 1/2 atmosphere. Decreasing the exhaust manifold pressure one-half atmosphere will increase engine efficiency about one percent, which will translate into about a 3% improvement in fuel mileage.

With a 90 hp natural gas engine your waste coolant heat and exhaust heat will be about the same -- close to 4000 BTU/min. 15 gallons of water a minute is equal to about 120 lbs of water. 4000 BTU a minute will raise the temperature of this amount of water about twenty deg F (assuming a 100% efficient heat exchanger. You might want to consider a liquid/liquid intecooler to recover the coolant system heat, since this would essentially double your recovery to 8000 BTU/min.

I would recommend getting a custom cam that minimizes the exhaust and intake valve overlap. Otherwise the exhaust manifold suction will be sucking some of the fuel/air mixture when both the intake and exhaust valves are open.

 

[ivymike]

how about sucking down the c-case pressure too? That'll cut pumping&windage losses.

 

[zdas04]

Great ideas. If I can get 40F temp increase on the entire stream (probably closer to 30F with real-world effeciencies), it'll almost double the evaporation rate of the system and cut the size of the evaporation pond by a bunch. Fuel savings are just an added bonus.

David