Engine exhaust gas pressure

[21121956]

Hello everybody:

In our Diesel power plant it is planned to use the engine exhaust gases to generate energy by cogeneration.

For medium speed (514 rpm) twin turbocharged Diesel engines, delivering 17.000 kW, it is required to know the exhaust gas pressure.

As we all know, once the exhaust gases exit the cylinders, they end up in the exhaust manifold, from here the gases flow into the turbines of the turbochargers and then, they flow out to the heat recovery boiler (if any) or directly to the stack (if no boiler is installed).

It is in that zone, between the outlet of the turbocharger and the entrance to the boiler that the pressure of the gases is required.

Is there, besides the direct measurement of course, any method to ascertain/calculate this pressure? In that point of the stack there is no pressure gage.

Your opinions or comments are thanked in advance

 

[nickelkid]

Have you tried to obtain the turbo's performance map?

 

[Bribyk]

Your pressure there is going to be the backpressure created by the downstream equipment after the turbocharger (boiler, piping, rain cap, etc...). If the gases went straight to atmosphere after the turbine, your pressure would be zero. There's not much point in measuring it either if you're going to change the downstream equipment because this is going to change your backpressure (unless you want to model the current system, use your measurement to check your model and then model the proposed system similarily). The pressure is whatever you force it to be because of flow restriction, you can raise of lower it to suit your needs, just watch the you don't exceed the manufacturer's maximum recommended backpressure.

 

[ivymike]

you can get the exhaust gas mass flow data and stack temperature from the manufacturer (many publish this info in spec sheets).

The pressure at the point you describe (downstream of the turbo) will be determined by the mass flow rate, the temperature (giving density and thus volume flow rate), and the restrictiveness of the downstream exhaust ducting. Backpressure includes restrictions due to pipe size, silencer, system configuration, boiler, rain cap, and other exhaust-related components.

Old engine application guide calculations:

P(kPa) = (L*S*Q^2*3.6E6)/D^5 + P.s + P.b
or
P (in. H2O) = (L*S*Q^2)/(187*D^5) + P.S + P.b

Where:
P = Back pressure (kPa), (in. H2O)
psi = 0.0361 x in. water column
kPa = 0.00981 x mm water column
L = Total Equivalent Length of pipe (m) (ft)
Q = Exhaust gas flow (m3/min),(cfm)
D = Inside diameter of pipe (mm),(in.)
S = Density of gas (kg/m3), (lb/ft3)
P.s = Pressure drop of silencer/raincap (kPa),(in. H2O)
P.b = Pressure drop of boiler (kPa),(in. H2O)

 

[ivymike]

darn simultaneous postings

 

[SMOKEY44211]

Difficult to predict. As you pull heat out of the exhaust volume is reduced along with pressure. The Cowanda affect of the fluid flowing through the conduit tends to insulate the dynamic fluid from the surface. That along with the residence time the exhaust spends in the system make it difficult to recover more then 50% of the heat available. I think in the end you are going to end up with cut and try method to get accurate data.------Phil

 

[21121956]

Hello everybody:

Thanks to all for your advise. I have found this site:

http://www.bombaszeda.com/_upload/Departamento/2/Es_19Escape.pdf

 

[21121956]

Hello everybody:

In order to get the pressure, I wonder if it is technically acceptable to install a standard pressure gage?, or we have to take into account some particular points besides the temperature of the exhaust gases of around 700º F?

Your comments are appreciated. Thanks.

 

[dgallup]

You can install either a flush pressure tap to get the pressure at the wall or install a tap with a right angle facing upstream to get total (stagnation) pressure. Usually there is enough distance between the tap & the gage that you don't have to worry about the temperature, there is no flow. Since the pressure is pretty low, manometers are commonly used.

 

[21121956]

Hello everybody:

Thanks dgallup for your support.

 

[Bribyk]

I wouldn't install the gauge directly into the piping, use a foot or two of steel or copper hose/tubing to keep the gauge cool. We use a portable manometer as well for these types of measurements.

 

[Fabrico]

We measure this constantly on all of our on-road Diesels that have exhaust particulate traps. There is a high temp 1/4" ID braided line about 30" long that runs from the exhaust pipe to a small, chassis mounted pressure switch manifold.

 

[MikeHalloran]

You'd better drill the stack and make a direct measurement of what the backpressure actually is now, before inserting a lot of junk into the flow. Record the power output level at the same time, of course.

The backpressure _limit_ on engines of that size is usually less than a foot of water, so you need a really sensitive pressure gage, like a Magnehelic, or an actual water manometer.

As mentioned, a couple of feet of metallic tubing will radiate enough heat to keep from melting the gage.



Mike Halloran
Pembroke Pines, FL, USA