Softening plant capacity 1

[SAK9]

What should be the capacity of a softening plant( in litres/hr) for a 3 MW boiler @1000 Kpa.All condensate is being returned back to the boiler.Any guidelines/ basis for working out the make up rate?

 

[quark]

I will try to do some calculation in the absence of clear data.

The basic assumption is that the boiler feed water (including the returned condensate) is at atmospheric pressure and at 100C. The enthalpy difference of saturated steam at 11 bara and feed water at 1 bara is 2363.14 kJ/kg. If your boiler capacity is 3 MW, then the steam output rate is (3000 kJ/s)/(2363.14 kJ/kg) = 1.26 kg/s or 4570 kg/hr.

Presuming 100% condensate is returned back and the feed water tank is at atmospheric pressure, you need to count the amount of flashing. Enthalpy of liquid at 11 bara is 781 kJ/kg and enthalpy of liquid at 1 bara is 417.5 kJ/kg. The difference is the driving force for flashing of condensate into steam. The enthalpy of steam at 1 bara is 2674.95 kJ/kg. So, the flash steam is (781-417.5)*100/(2674.95-417.5) = 16%

The loss of condensate, therefore, is 4570*16/100 = 731kg/hr or 731 L/hr. This is the minimum makeup flowrate required for the above assumptions.

Now check for the actual feed water tank pressures and then assume some steam leakage and get actual condensate flowdata if you have metering on the line.

 

[SAK9]

Thanks Quark for a good explanation!Do we need to add blow down rate to this figure? If yes what would be an acceptable figure?Thanks again.

 

[quark]

Yes, you must add blowdown rate to the above figure (infact, I forgot about it). The rate of blowdown may vary from 4 to 8% and can be anything if the water quality is bad.

Theoretically, you can calculate it by input water TDS and maximum permissible TDS limit in the boiler drum. ABMA recommends 3500 ppm for a boiler of 10 barg pressure (actually for the pressure range 51-300 psig). Suppose, if your feed water quality is 250 ppm TDS then the blowdown rate is 250/(3500-250) = 7.69%. This procedure assumes that dissolved solids are absent in the steam and are fully accumulated in the boiler drum.

 

[786392]

Dear,
As most usually 100% condensate return is not manageable,thus a safety factor of around 4%~6% be considered for this part.

Further to cater for possible problems with water quality an overall 30%~33% plus capacity should serve as Min. benchmark.

Hope this helps

Best Regards
Qalander(Chem)

 

[quark]

I think, other uncertainities can be mitigated by capacity of the day tank and Softner OBR. Per hour output is dependant upon the superficial velocity (and thus vessel diameter) and OBR is dependant upon the volume. You can study various softner sizes vis-a-vis the day tank volume and can come to an optimal solution.

For same resin volume, higher H/D ratio results in lower cost of the softner.