Re: flashing of condensate return through the vent of condensate return tank.


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Posted by Raymond on July 03, 2001 at 10:07:51:

In Reply to: flashing of condensate return through the vent of condensate return tank. posted by zaib on June 28, 2001 at 03:03:01:

: we have a 5 ton/h steam capacity 25 kg/sq.cm.pressure water tube boiler. the steam from boiler goes to plant(our is a BOOP poly films plant) here it passes through coils heats up the plant & return to condensate tank(our is gravity return system).capacity of condensate return tank is about 4 tons, it is vertical shell type. It has a three inch vent pipe at top of shell.we mix zeolite softener treated water as make-up in this tank which is of about 20 centigrade.now this mixture of (condensate return & soft water)we call it boiler feed water is forwarded to overhead feed water tank by means of a pump and from here it is again feed to boiler by means of boiler feed water pump.the problem is that there is a heavy flashing of condensate return through the 3 inch vent pipe of condensate return tank which I think is causing many tons of condensate to evaporate & is causing a big loss of condensate.moreover if we reduce vent pipe dia it will cause increase in pressure in condensate tank which may effect the return of condensate & also it will cause increase in water temprature which our pumps may not be able to forward(the pumps are ksb single impeller centrifugal).to forward very hot water above 100 centigrade what kind of pumps we should use who may forward the water of more than 100 centigrade.
: I request the members of this forum that kindly they give me there precious suggestions to resolve this problem.
: thanks & regard zaib.

Zaib,

The operating conditions you mentioned are not sufficient to perform an heat balans to analyse the situation. Ill have to go by some assumptions.
1- I take it the 25 bar working pressure you mentioned are gauge readings. If the steam is only used for heating purposes no superheating is carried out. In this case the operating pressure is chosen to have a corresponding boiling point delivering a sufficient LMTD between the heating steam and the process conditions.
2- In this case the steam conditions at the boiler outlet is 5ton/hr; 220C; its enthalpy is 673 kcal/kg in total 3.365.000 kcal/hr.
3- If steam, by giving of its heat, is condensed with no sub cooling the condensate at the outlet of the process heaters (inlet condensate flash drum) is 5 tons/hr; 220C; its enthalpy 225 kcal/kg a total of 1.125.000kcals/hr.
4- The conditions of the flash steam leaving the flash pot to atmosphere is 100C its enthalpy 644kcal/kg; its amount x kg/hr meaning a total of 644xkcal/hr.
5- The make up water conditions to the flash pot balancing the flash steam are 20C its enthalpy 20kcal/kg its amount equal to the flash steam x kg/hr a total of 20x kcal/hr.
6- The conditions of the boiler feed water leaving the flash drum to the boiler feed water pump are 5 tons/hr 100C 100kcal/kg a total of 500.000kcals/hr

A heat balans of the flash drum based on the above figures

in = out

644x + 500.000 = 20x = 1.125.000

644x 20x = 1.125.000 500.000

624x = 625.000

x =1000 kg/hr

The overall steam efficiently of your system is 3.365.000 500.000 625.000/ 3.365.000 500.000 = 78.2%

The result of this simple heat balans exercise meets the figures you mentioned in the posit of the problem. It seems that during the design of the process this amount of flash steam has been chosen. Sub cooling of the process could improve the energy efficiency of your system, with less flash steam to atmosphere. If means are available to dispose the heat. However this should be carefully designed since a minimum of flash steam to atmosphere is needed to dearate the cold make up water.

I could look more in detail in to your system providing you gave me a flow sheet complete with temperature and pressure data. Contact me to my private email.

Raymond





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