Calculation of Boiler Efficiency

[xus]

It seems that every resource I can find does this a different way. In the case of the policy I am required to follow, I cannot find any resources to help with some issues that I'm running into. In this case, I'm trying to break down where certain figures are coming from, so that I can break it down into a spreadsheet for calculation.

In the policies example:

Feed Water Temperature: 220F
Steam Press.: 115PSIA
Stack Temp: 450F
Room Temp: 80F
Nat. Gas CO2%: 11%
Evaporated: 1,400,000 lbs ** What do they actually mean here? Would this be steam produced in the past 24 hours?
Fuel Consumed: 1,700,000 CF

Then they simply go on to explain that

E1 (Enthalpy of steam at operating pressure) = 1190BTU/lb at 115 PSIA boiler pressure
E2 (Enthalpy of feed water) = 188 BTU/lb at pressure feed

...and that's where I'm lost. Is this calculated? Is it from a chart? Thanks for the help.

 

[xus]

Well, I've got most of that figured out using a steam properties table, however when I input my actual readings, I'm showing 108% Thermal Efficiency. Is that even possible?

 

[macmet]

Xus, does that seem like it would be possible based on what efficiency means?

How did you get the 108%?

 

[xus]

This is how the policy states to calculate it:

Q1(E1-E2) / Q2*X

Q1 = Water evaporated in lbs
Q2 = Fuel consumed in Cubic Ft of Natural Gas
E1 = Enthalpy of steam at operating pressure
E2 = Enthalpy of feed water
C = BTU value of fuel (1031BTU/CF for Natural Gas)

We take reading of steam produced & fuel consumed at midnight, then again after 24 hours.

We produced 157,222lbs of Steam, using 141,366CF of fuel.

Enthalpy of steam at 115psi = 1192 BTU/lb
Enthalpy of feedwater at 220F = 187 BTU/lb

Steam Produced * (Enthalpy of Steam - Enthalpy of FW) / Fuel Used * Enthalpy of Natural Gas

= 1.08

--------------------
Not only is this nowhere close to the combustion efficiency of 82%, but it should be impossible. I either am messing something up in the numbers, or the way they are asking to calculate Thermal efficiency is wrong and I need to be able to tell them why so it can be changed.

 

[chicopee]

What is the story with the denominator Q2*X?

 

[xus]

My apologies, that's C. (Enthalpy of Natural Gas)

 

[chicopee]

1,400,000 lbs actually means 1,400,000 lbs/hr., which is the amount of steam that the unit is designed to operate continuously, safely.
C is not really an enthalpy value; C is known as the heating value of the natural gas--big difference in terminology.
Your policy formula is not correct in determining the boiler efficiency because it does not consider heat loss from boiler to ambient and heat loss thru the flue gas. There will be other minor gains and losses in energy,however, the ones mentioned are the big ones.

 

[LittleInch]

xus,

I can't see anything particularly wring with your calculation, so there must be some issue with the numbers you're using. It is a very basic efficiency calculation, but if tis the one specified then so be it.

My guesses are as follows:
1) The volume being given for the gas is not corrected to standard conditions, hence it appears as if less gas is being burnt.
2) Is the gas BTU value correct - 1031 appears to be correct for "export quality" Natural gas, but with a CO2 value of 11%(!!) I suspect you may be actually burning oil field or non standard gas which might have much higher heating value as it has much more C2,C3 and C4 components?? Do you have a gas composition or gas specification?
3) how is the steam production being measured? Is there room for error here or if this is water volume going in?? Is there some leakage somewhere else and hence not all the heat is being used to evaporate the water volume?
4) Check your units - if the capacity is 1.4 million lbs/ HR and you're quoting 155,000 lbs per DAY, there seems to be something wrong here. Even if it is 1.4 million per day this system would appear to be running at about 10% capacity? Does this have an impact on the figures??

Let us know what you find out.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[xus]

LittleInch, thanks for your insight. I'll do my best to provide correct answers.

1 & 3) I get the readings for the steam produced and the fuel consumed from the Engineer's daily log, which is recorded straight from the Hayes chart.
2) It's perfectly plausible that this could be different than 1031. While researching this problem and how to calculate this per specification, it just states in the specification/policy that the BTU value of Nat. Gas is 1031. I'll dig for more here.
4) In my first post I used the 1.4 mil figure because that was in the example. At that point in time I couldn't figure out where they got the Enthalpy values, etc, so that I could test the equation with my own numbers to see what I got. Once I got a table of Steam Properties, I was able to calculate using my own numbers which is when I came up with over 100% efficiency. My fourth post is my concern using my own numbers. How do I get steam produced? Subtract total steam output from the hayes chart at 12AM one day by the steam produced the day prior. Same for fuel. Maybe that's the wrong way?

 

[LittleInch]

Hmmmm, Not sure where to go but excuse my ignorance and tell me what a "hayes chart" is. Mr google doesn't seem to know, but it's clearly where you're getting your data. Can you scan a daily example and post it?

WRT the gas quality and volume I don't know if your CO2 of 11% was for this calculation, but if it is there is no way it is "standard" gas. Also the chart might record the volume but are you absolutely sure it is corrected to standard conditions? Your conditions just happen to mean 1scf of "gas" should produce 1lb of steam. At 82% efficiency this should be 191,000scf, not 141,000. That's a big difference.

Only other thing to check is the boiler pressure is 115psia.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SepiaTones]

You actually have more information than you need. There is no need for any steam data to calculate efficiency.

The best way to do it is the heat loss method (try googling). Basically, you figure out how much heat is being lost up the stack, based on the fuel analysis and excess air (stoichiometric ratio). For natural gas, the two biggest heat losses will be the dry sensible heat and the latent heat from the combustion of H2 in the CH4 (natural gas). Then there are assumed losses, such as as radiation, unaccounted, manufacturer's margin etc. For CH4 there should be very little unburned fuel loss. If you want to get fancy, you can included blowdown, but that is minimal.

The B&W Steam book has some good examples.

 

[alianapaul]

(Steam value per hour :kg)*(h2-h1)*100
Boiler Efficiency＝ ------------------------------------------------------------------------------ %
(Fuel consumption per hour :kg)* (Fuel low calorific heating value : kcal/kg)

h2： The ratio enthalpy of feed water （kcal/kg）
h1： The ratio enthalpy of steam （kcal/kg）

Calculate the Boiler Efficiency with the help of above formula.. AWH