Steam Humidifier

[lownox]

My company makes steam humidifers. One design is that of a tank with submerged steam pipes. I would like to get some help with my math please.

Assuming the tank is full and boiling (minimal back pressure) and steam is being fed at 5 psi, how can I calculate the amount of output steam I will be producing. I think I know just enough to be dangerous. hehehe. The unit is already designed, but we don't sell many and those who actually designed it are no longer here, so I am left to figure it out. My background is combustion and control, so heat transfer is not one of my strengths.

Any help is appreciated. Thanks.

 

[imok2]

The amount of energy required to produce 1 pound of steam per hour at a pressure and temperature of 5 Psig and 227 degrees F is 960 btu/lb so if you know how many lbs of water or gallons you are using then devide by 960 btu/hr to get lbs of steam. PS 1 gallon of water weighs 8.33 lbs

 

[Joesteam]

I think you are using 5 psig steam in the tubes to make atmospheric steam for your humidifier. the 5psig steam will condense and give you about 960 btu/lb, while the atmospheric steam will need 970 btu/lb to evaporate. the approximately 15 deg F difference in temperature will drive the heat exchange.

I believe all of the energy of the condensing 5 psig steam will enter your boiling water and produce the atmospheric steam. You will get some heat loss from the vessel itself and the piping.

So, without going too wild with calculations, your ratio of steam in to steam out will approach 1:1, and if you put on like a 10-20% safety factor to account for heat loss, this would probably be ok.....

 

[lownox]

Thank you for the replies, imok2 and Joesteam. You are correct, Joesteam, about my application. I guess another aspect to my ponderance would be compensating for the degredation of heat transfer if the boiler steam pressure were increased.

The unit has ratings at 5 psi steam, 10 psi, and 15 psi... so if I used 15 psi there would be more steam flowing through the heat exchanger so I would expect less than 1:1, is this the correct way of thinking about it.

There is a condensate trap that send the water back to the boiler, but does it also send un-condensed steam back to the boiler?

I appreciate all the comments.

 

[danberry]

The 1:1 ratio is about correct but it will depend on make-up temperature in the tank.

Using 5 PSI steam to heat atmospheric water leaves little delta T for the coil to work with; 15F in fact. Power will be low with little steam produced. Why not use the steam directly instead of installing a reboiler.

The steam trap will not send steam to the boiler. If it leaks, the steam will be vented in the condensate tank.

 

[lownox]

Reboiler is required for "clean steam" i.e. no boiler chemicals.

I was thinking about it this way... once the tank were heated (approx. 212), the heat requirement is purely latent? Then at 5 psi inlet steam, output would be X. With 10 psi steam, output would be Y. I guess I am trying to determine (in my mind at this point) if the efficiency degrades as I inject more steam pressure. I figure I will get more steam output with more steam pressure in, but the overall efficiency of the HE would be less the more steam I put in. Right way to think about it?

Sorry if this seems elementary.

 

[danberry]

Thought so.

No, you will get the same efficiency with the high pressure steam. (Not strictly true because the condensate will be rejected at a higher temperature, but close enough)

 

[Joesteam]

For higher pressure steam inlet you can get more steam outlet, however you will use more steam because the latent heat of the high pressure steam is lower, and the sensible heat is higher so you will drain hotter condensate. In general, the lower the pressure the higher the efficiency.
Another factor is the steam velocity, higher pressure takes up less space and you can fit more steam in the pipe....

 

[KiwiMace]

When you boil water in a pot on the stove, the steam doesn't survive it's short journey into the world.
This "atmospheric pressure" steam you are making is in fact water vapour, or near saturated wet steam of very low quality.

How are you using it? This will condense on practically everything and be a nightmare. Are you sure this is right?

 

[Joesteam]

Many 'steam to steam' humidifiers work this way, with very little pressure. It is not strictly atmospheric as the steam must pass through nozzles in the delivery tube, but it is close.
The delivery tubes themselves are often steam jacketed, so that the initial heat up condensation does not end up in the air duct. Once everything is hot the steam is indeed atmospheric and not very wet, ready to add humidity to the air stream.

 

[lownox]

One more question... For a given heat exchanger in this application, it might be rated at 40 lbs/hr output steam (atmospheric for this purpose) with 5 psi inlet steam pressure. The heat exchanger is 316 SS... the rating at 10 psi inlet steam is 80 lbs/hr.

I understand that the steam density/temperature goes up and the latent heat goes down some, but I don't see how the output rate doubles. Any insight? You probably guessed that I am not really experienced with this stuff.

Thanks.

 

[Joesteam]

There are two factors here. It is the differential temperature that is the driving force of the heat transfer. With higher pressure steam, the steam temperature is increased and this gives you more heat exchange.
Then, the denser higher pressure steam has more chances of condensing on the walls of the tubes. however, it is more of the temperature factor....