Steam generator calculation

[MelloTango]

Hi,

I am trying to see if its possible to build a steam generator using waste heat from my furnace, via a heat exchanger which turns basically turns water into steam. Basically, I need help with calculations. Here are some of the parameters:

1) Steam required = 30 kg/hr
2) Flue gas temperature = 800 degrees
3) Flue gas flow rate = 118 m3/hr
4) Pressure = normal atmospheric pressure
5) Water temperature = 25 degrees celcius

Is it possible with the above parameters to generate the required 30kg of steam per hour? And if yes, how much water need to be added per hour?

Hope anyone can help and thx!

 

[25362]

The heat content of the flue gas being about 270 kcal/normal m³, depending on the % excess air.

If the 118 m³/h flue gas rate is measured at 800^oC, meaning about 30 normal m³/h, "normal" taken at 0^oC and 1 atm abs, the apparent answer is no, you cannot.

 

[MelloTango]

Thanks for pointing that out. Now if I may ask, under these circumstances, if not 30kg/hr of steam, what would be the max amount of kg/hr steam that can be achieved?

 

[25362]

The minimum theoretical heat required to convert 25^oC water into steam, barring inefficiencies (losses), is about 620 kcal/kg water, assuming a small pressure above atmospheric to enable using the steam elsewhere.

Now, also assuming an excellent counter-current heat transfer (with finned tubes), and a good draft, the gases would cool down to, say, 150^oC (no acid dew point envisaged?). At this temperature the gases' heat content is about 50 kcal/Nm³.

The available heat would then be: 30[×](270-50) [≈] 6600 kcal/h.

The maximum theoretical amount of steam would then be: 6600[÷]620 [≈] 10.6 kg/h.

 

[MelloTango]

Wow to those dazzling calculations! Overall 10kg/hr steam production is not bad, considering it will be generated from heat that would have gone to waste anyway. That's equivalent to 1/3 potential cost savings. For the other 20 kg/hr, options for me will include an electrical or oil fired steam generator. Appreciate the help as now we got come clear directions for our upcoming project. Thx again!

 

[25362]

When speaking of savings, don't forget to include capex and opex, which, in small installations, seem to override the economies of energy. Good luck.