Need help with BTU input calculation 1

[ProjectEng]

I want to calculate how much gas we would need to burn to heat the water in the package boiler up from ambient to 375F.

Can I just assume that I'm only heating the water at the boiler efficiency provided by the manufacturer? Or do I need to take into account the heating of the tubes, drum steel, membrane, etc?

My gut feeling is that heating the steel is negligible compared to heating all the water...

 

[TBP]

Most industrial steam boilers will give you about 80% eff, under real-world conditions. (Based on net BTU out divided by BTU in as fuel, for the same time period.) Get the BTU/lb off the steam tables for the operating pressure of the boiler (remembering that most steam tables are in absolute, not gauge pressure), and subtract the BTU/lb in the feedwater. This will tell you how many BTU/lb the boiler needs to add to the water to generate one lb of steam. Multipy that value by the lbs per hour the boiler will generate. This will give you the BTU/hr out of the boiler. Divide that number by 0.8, and that will be very close to the BTU/hr you need to supply to the burner. Flip the BTU into MCF or whatever units you buy gas in, and there you are.

 

[ProjectEng]

After I subtract initial enthalpy from final enthalpy, I also need to add in the heat of vaporization?

Thanks.

 

[TBP]

The nice thing about "boiler eff" is that it's the big picture. It captures all of the losses. And it's a nice, simple calculation: net heat output, divided by heat input for the same time period, multiplied by 100. The drawback is, is that it doesn't identify where the losses are.

Say the boiler runs at 125 PSIG, and the DA water temp is 227*F. The boiler can't count the "running start" from the feedwater temp. One lb of steam at 125 PSIG contains 1153 BTU. One lb of feedwater at 227*F has 195 BTU. 1153 - 195 = 958. The boiler needs to add 958 BTU to each lb of feedwater to make steam, in this case. At 80% eff, you'll need to get 1197.5 BTU of fuel to the burner for each lb of steam generated.