Elevation of steam drum relative to evap coil

[Inspectorengineer]

We are designing a steam generator using a single-path evaporator coil through the convection section of a process heater. We are using forced recirculation from a steam drum mounted on a pipe rack. The current design has the drum mounted at a lower elevation than the steam coil. I've never seen a steam generator with the drum mounted lower than the coil. Is there a problem with this design?

 

[MJCronin]

Naaaaa.....

Steam is denser than water....ummmm..... isn't it ??

 

[Inspectorengineer]

MJCronin:
Mechanical Engineer, Maine PE.... 28 yrs experince in power plant and process chemical work....ex-New Englander....intolerant of bad spellers
_____________________________________
I don't think I spelled anything wrong, so I guess your reply was sincere.

Seriously,
I guess you have never seen a steam drum mounted lower than the evaporator either.

Maybe I can work my way through this...

In our case, the evaporator coil is designed withstand operation of the heater, even when dry. So you don't have to have the coil full of water at startup. However, say you started the circulation pump before there was enough heat in the coil to evaporate the water. The level control on the drum would allow more water into the drum until the coil and piping to and from the coil was full. Water in the
coil may even find its way down to the drum and fill the vapor space.

Now, I'm not sure if what I said makes sense, It would be great if you could comment.

PS All my misspellings are just typos.

Thanks

 

[davefitz]

sounds like a bad idea. If there is only a modest pressure drop thru the coil, then the the water may flash to 2 phase fluid prior to entering the coil distribution header- unless this header and feeder is specially designed to distribute 2 phase fluid equitably, you will get extreme unbalances in water distribution to the various tubes.

Those tubes that "dry out" prior to their heated outlet sections will accumulate a lot of dryout solids and eventually plug up or suffer corrosion.

It is theroretcially possible to design it to work, but unless you can assure yourself that no tube sill "dry out" , then you will need to maintain "holy water" ultrapure water quality, as with once thru steam generators.