Dividing Load Among Boilers 2

[Herman2]

I just joined a food manufacturing company that has four(4) 200-hp boilers. The operator runs all four units at about 20 - 30% rated load. He says that we want to minimize flue gas temperature in order to maximize efficiency. Running at 25% keeps the flue gas temperature down.

Is this the most efficient way to dispatch the boilers? I think I would run one unit flat-out at 100% and start a second if necessary. But I'm just a skeptical electrical.

What do the experts say?

Thanks!

 

[imok2]

Herman
Boilers that are trimed correctly will run lower CO,CO2 and O2 at hi fire then at low fire and if flue gases get too low ..condensation will develop and eventually destroy the stack it seems to me the operator has it up side down.

 

[JKEngineer]

Generally losses from a low loaded boiler are high and its efficiency is low. Better plant efficiency should be obtained with a heavily loaded and a moderately loaded boiler. Swing capacity may be a factor, however. The boiler manufacturer should be able to supply efficiency curves for the boilers. Use them to calculate your overall plant efficiency at different load distributions. If they are not available, measurements of the efficiency can be made relatively easily. They include flue gas temperature as it leaves the boiler, but also include % excess air, skin losses, etc. Excess air tends to be higher at low loads.

I used to do a lot of this. I could be induced to do it for your specific case if you want help. ;-)

Jack

Jack M. Kleinfeld, P.E. Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

http://www.KleinfeldTechnical.com

 

[quark]

Both the replies are very good. Generally it is better not to run the boilers below 75% load.

Your stack temperature should not be too low to cause sulphur condensation as suggested by imok.

With less load you will have more radiation losses and more heat loss through exhaust. I would prefer JKEngineer's suggestion to check efficiency of the boiler at various loads and then choose your control scheme. Portable flue gas analyzers, though not very precise,will help you in deciding the loading.

It is not good to run the boiler at 100% load either. This will decrease the quality of steam you get.

Regards,

 

[hacksaw]

operationally, a cold start on a boiler can be a pain, so you keep a boiler online to pick up your swing loads with minimal delay.

sounds like your operator is telling you that the current system design is really bad, but that he has found a way to make the plant run smoothly inspite of the design.

you have to observe a certain amount of stack temperature to avoid acid condensation in high sulfur fuels and in some cases to keep the stack properly drafting.

take a look at your operational requirements in terms of your product delivery then identify if there is a better (engineered) solution for your boiler setup, and then sort out the most efficient configuration and system improvements. some boiler designs simply may not be suited to a food plant environment (loads) so you have to look at the whole system. good luck