Forced Air Diesel Fuel Burner

[Toymaker]

When I first began considering how to design my water tube boiler I made the incorrect assumption that the working fluid would quickly begin to boil, creating vapor and greatly expanding; I assumed, incorrectly, the boiling would take place within the first few meters of boiler tubing. This false belief is why I thought it a good idea to increase the tube volume by splitting a single tube into two parallel tubes, thereby doubling the area inside the tubing.
However, I neglected to take into account that the feed pump would be delivering the working fluid into the boiler tubing at 500+ psi, and at that pressure, the R123 would not boil until it reached 180C. The pressure-enthalpy chart clearly shows the working fluid wont be changing into a vapor until it's very nearly ready to exit the boiler and enter the turbine.
This means that a single monotube is the better approach.

Studying the diagram, it's clear that if I heat the fluid to 190C (blue lines at #3), instead of 180C as planned (green line from #3 down to #4), then the vapor in the turbine will remain a vapor as it looses heat passing through the turbine blades. The green line from #3 down to #4 falls within the saturated vapor region where most, but not all, of the working fluid is a vapor, but a few droplets of liquid could be present; liquid droplets erode turbine blades so liquids suspended in vapor must be avoided.

BTW, Steamchick, I'm starting to warm up to the value of using pressure-enthalpy vs temperature-entropy diagrams. It's obvious in the P-Enthalpy diagram below that R123 remains in the liquid phase the majority of time it's being heated, from #1 to #2. This important aspect isn't at all clear in a Temperature-Entropy diagram.

 

[HMEL]

Trying to understand - - - -

If I want a steam boiler to produce say 1000 psi - - - does that mean my feedwater pump must also be rated at 1000 psi - - - - or not?

Yes, actually that pressure and a little more to account for what is called piping loss. The horse power required to do this is part of the parasitic draw on the cycle. Technically you would also add to the flow about 10% over the steaming rate to account for blowdown. The parasitic draw also includes condensate pumps and fans. These numbers are usually included to get a unit heat rate which is the kw per btu supplied to the cycle. A bit different than boiler efficiency.

 

[ajoeiam]

Yes, actually that pressure and a little more to account for what is called piping loss. The horse power required to do this is part of the parasitic draw on the cycle. Technically you would also add to the flow about 10% over the steaming rate to account for blowdown. The parasitic draw also includes condensate pumps and fans. These numbers are usually included to get a unit heat rate which is the kw per btu supplied to the cycle. A bit different than boiler efficiency.

Interesting - - - for me, at least, these power draws would be part of what is involved in 'boiler efficiency'.

Can also see why more moderate steam pressures are more prevalent.

Thanks

 

[Steamchick]

Hi All,
One simple "error" that I find many people make, is that you cannot get the exhaust gases cooler then the "boiler temperature" - unless you have a post-boiler "economiser".
I some cases, there is insufficient heat in the flue gases to heat the boiler all the way through the various passages, so it (flue gas) reaches the boiler temp quite rapidly, and there is NO HEAT remaining for a post boiler superheater/drier. In fact, there are many loco boilers where, at "below maximum" firing, there is no superheat, as any heat gained when the superheater tubes are at the firebox ends of tubes is given back to the cooling flue gases passing along the superheater tubes, and then passed into the "cooler" boiler.... And when only part fired, any smoke-box superheater doesn't do anything, as the flue gases are at the same temperature as the boiler water. So, with conventional steam boilers I advocate a superheater that takes boiler (wet) steam and passes it through the firebox to the engine, so it leaves the hottest part of the system before passing to the engine.
Just as this mono-tube boiler is doing.
But that was an aside, not really of benefit to this boiler.
K2

 

[Toymaker]

Hey everyone, I'm moving this discussion, which started out on Burners, over to a new thread in the Boilers forum:

Monotube Flash Boiler Design