LOWERING THE PRESSURE OF THE BOILER 1

[gladkenn]

Can anybody help me on this!
The production of our plant only uses around 4000 kg/hr( or less) steam for its heat exchanger. Currently, our CB600-800-150 Cleaver&Brookes firetube boiler is running at 100 psig and a firing rate of around 30 to 50%. There is no PRV at the point of steam usage. Only TCV. If I lower the boiler pressure to less than 100psig, can we have some saving regarding fuel consumption?

 

[yogibear1]

You will save some fuel, though you may or may not be as efficient, it depends upon your particular unit. Also as you turn down the burner the flame stability, atomization, and excess air should be observed; it all depends upon the burner turn down ratio.

 

[davefitz]

The Cleaver Brooks boiler is probably a firetube boiler that produces saturated steam. At 100 psig, the sat steam enthalpy is about 1190 btu/lb, while at 50 psig the enthalpy is about 1180 btu/lb. So,if the pressure setpoint is lowered to 50 psig, then a slightly lower firing rate ( about 9 % lower) can be expected.

To avoid liquid carryover to the steam, you would not be able to lower the presure lower than the corresponding firing rate, ie, at 50% firing rate you would need to maintain at least 50% of rated pressure.

Other issues that could arise:
a) more pressure drop across the feedwater cotnrol valve might lead to problems with drum level control
b)some activatged feeedwater additive might not be sufficiently active at the lower saturation temperature associated with the lower pressure.

 

[rmw]

gladkenn,

You did not state what your dead band is. How far below 100 psig do you go before you re start and fire again. Also, is your firing on/off or modulated or high fire/low fire/off.

If it is on/off, in either form mentioned above, every time you go through a purge/light off cycle, you are "pumping" cold air through the boiler, which transfers heat out of the boiler, and then requires some additional fuel to make up for that loss.

If it is modulating, it is pretty obvious that your boiler capacity is over running your Hx capacity. The optimum situation would be to have a boiler that fired at a high firing rate (because of efficiency, flame stability issues etc. already mentioned above) and whose output matched the requirement of your Hx.

Speaking of the Hx, will it perform with the lower pressure steam? I would check that closely.

Bottom line is that you are in a BTU in / BTU out situation, where all your boiler is doing is converting fuel BTU's to a form usable by the Hx. You may trade some inefficiencies in your system for others by reducing the operating pressure.

It is, however, worth trying, and won't hurt things. Some boilers just like yours have dual pressure controls so that with the 'flick of a switch' they can change from 100 psi operation to 15 psi operation with no ill effects. Firetubes are very forgiving.

rmw

 

[gladkenn]

Dvefitz,I am not so sure about your mentioning the saturated enthalpy. I suppose lowering the pressure of the boiler ( or anything) would have higher enthalpy of evaporation than for a higher pressure.


Anyway, thank you all for the valuable information. I am still trying to figure out what exactly the suitable pressure for the system. Thanks guys!

 

[Joesteam]

Please consider the steam piping. With a lower pressure you may have increase the line size to the heat exchanger due to specific volume at the lower pressure (Also, control valves and steam traps may need changing)