Condensate Return Required?

[Newgeer]

Hi All,

I am working on a farm-scale ethanol plant project. One of the systems requires ethanol to be vaporized. The design calls for a 24kW boiler to feed saturated steam to a heat exchanger (vaporizer) at 50psig. The heat exchanger load is approximately 25,000BTU. There are no other loads. My question is, if the heat exchanger is situated some 10 feet above the boiler vessel, are the condensate return, cold water make-up, and feedwater pump necessary? Can the condensate simply drain back to the boiler through the steam outlet? There will be an air bleed at the top of the heat exchanger, and a vacuum break on the boiler.

 

[dcasto]

If you have a 2" steam line and a 1" condensate the system would thermal siphon, but you would need to make sure the pressure drop of the staem in the exchanger is very small, say less than .5 psi. Another concern is making sure there is a seal leg on the condensate, There needs to be an upside down u shaped piece of the 1" condensate to assur that the steam condenses and has to build up some liquid "seal" condensate in the exchanger.

You should not need a cold water makeup unless your supply pressure is below 50 psig. But you shouldn't need any make up once you get it lined out.

 

[Sharik]

Instead of using steam, why not flood the boiler and just have the hot water going from the boiler to the exchanger and the cool water from the flowing down to the bottom of the boiler? You would need a circulating pump but this would be a simpler system then using steam and condensate.

 

[Newgeer]

Is the 'seal leg' typical of installations? I would have thought the steam would condense wherever the load is. Also, I can mount the heat exchanger up to 18 feet above the boiler. Is higher better?

 

[dcasto]

Higher is better. Given a path of least resistance, steam vapor would shortcut and exit out the condensate line. The seal acts just like a steam trap, same design and principal. You'll only need to extra 90's and 2 6" nipples versus a trap.

 

[rzrbk]

Note that without a condensate return pump and control, you have no way to control heat input to the reboiler; full pressure steam is seeing the full surface area of the tubes at all times.

To do without the cold water make-up pump you would need a supply of water above 50 psig.

Also, raising the reboiler may have an affect the level of ethanol in the vessel feeding it.

 

[olmedo]

You will need to install a steam trap at the outlet of the heat exchanger.