Boiler Efficiency
Boiler Efficiency
(OP)
Our boiler is burning very inefficiently. I have modelled our system with SSAT software for Excel and made my own calculations and I get the same results.
We have a 20,000lb/hr fire tube boiler. We burn Natural Gas. On average our the facility only draws 6,000lb/hr steam at 140psi. I have calculated our fuel to steam efficiency at ~60%. I assumed 1030BTU/CF of N.G.
Would the low load account for the low efficiency of the boiler?
Is there any other way to increase our efficiency other than buying a smaller boiler?
I have checked the traps, looked for leaks.
Currently we do not have condensate return, actually you'll love this we pump our condensate into our cooling water system.
I would like to hear some suggestions we could make to increase efficiency.
Thank you
We have a 20,000lb/hr fire tube boiler. We burn Natural Gas. On average our the facility only draws 6,000lb/hr steam at 140psi. I have calculated our fuel to steam efficiency at ~60%. I assumed 1030BTU/CF of N.G.
Would the low load account for the low efficiency of the boiler?
Is there any other way to increase our efficiency other than buying a smaller boiler?
I have checked the traps, looked for leaks.
Currently we do not have condensate return, actually you'll love this we pump our condensate into our cooling water system.
I would like to hear some suggestions we could make to increase efficiency.
Thank you





RE: Boiler Efficiency
Remember that the boiler cannot count a "running start" from anything. The biggest player is normally feedwater temperature. Discount the heat in the feedwater from the heat in the steam leaving the boiler. The next thing to be considered is the steam meter on the boiler. Averages don't matter. You need an accurate measurement of the steam produced over (typically) 24 hours, counted against the fuel input to the boiler over the same time period. Your gas meter does JUST the boiler, right? Make sure. Your steam meter on the boiler is correctly sized, and PRESSURE COMPENSATED, right? Make sure. (I'll bet it's NOT - very few are.) How's the water treatment? Do you have a nasty stack temperature, like 700*F? Maybe your boiler is scaled-up.
RE: Boiler Efficiency
reducing final end gas temperature to the stack will increase the boiler efficiency. Also, installing a better burner that can run at a lower excess iar would also improve efficiency. Finally , installing an air heater would also do the trick. In other words, you have to spend money in order to save money.
RE: Boiler Efficiency
Regards,
athomas236
RE: Boiler Efficiency
I will find out if our flow meter is pressure compensated, it is the original flow meter installed in 1997 with the boilers and no one has info on it here.
Also my calculations are not based on a single data point I have used several months of cumulative readings to calculate these figures. Both the Steam flow meter and the Natural Gas flow meter have totalizers.
Feed water temperature is ~60F. We are in the PNW. Ambient temperature is above 40F even in the winter.
We do have an economizer and the final stack temp. is ~290F. Our economizer is used to heat the feedwater before it enters the deaerator
The water is de-min and well treated, we open the boilers every year for an inspection and there is very little to no buildup.
O2% is 6.7% as tested May 2008 during an average load. I cannot provide O2% at max load because we never get there. The highest we have ever seen is 14,000lb/hr draw and it was a one time spike.
Also in case this is important I was told our boilers are 15,000lb/hr production steam with a 20,100lb/hr max process heating. They are Cleaver Brooks CBLE boilers. We have 2 and run 1 at a time.
Thank you for the help.
RE: Boiler Efficiency
RE: Boiler Efficiency
Will a low load on the boiler decrease its efficiency? I assume it does.
RE: Boiler Efficiency
efficiency drops a bit as load drops because (a) radiation loss as a fraction of heat input will increase (b) higher excess air ( or higher O2) due to limited turndown of the burner or fd fan. The best way to reduce this part load loss is to upgrade the burners and combustion air control.
RE: Boiler Efficiency
Do you also have data for blow down and blow off flows?
You have demin make-up water, but for others using the 'once-thru' water with more solids content of the boiler feedwater, some boilers have as much as 25% blowdown. It could be a significant part of the reduction of your plant efficiency, even more during low demand flow. (Assumed timer fixed cycling of bottom blow off.)
RE: Boiler Efficiency
RE: Boiler Efficiency
The effect of economizers, air preheaters, etc must be discounted when calculating BOILER efficiency. Boilers cannot count any "running start".
RE: Boiler Efficiency
A burner rated for some safe margin above, but still near to your 6000 lb/hr, will operate at its sweet point and will be able to produce optimal O2 levels at your steaming conditions. Do some calculations. Your efficiency gain may have the ROI needed to justify the change.
rmw
RE: Boiler Efficiency
RE: Boiler Efficiency
rmw
RE: Boiler Efficiency
Is the option of installing a new burner a viable alternative to buying a new boiler? Meaning we can turn a 20,000lb/hr boiler into a 10,000lb/hr boiler simply by changing burners. Wouldn't the size of the boiler affect performance?
As far as blowdown we blowdown 2%.
RE: Boiler Efficiency
Can you downfire your boilers? Sure. Will it change the surface area? No. Can you actually calculate a payback? No. You can't. You've got no "odometer" reading that means anything. Without an accurate steam meter reading, you're missing half of the values necessary to figure boiler efficiency. And that's just a cold hard fact.
Good luck. I think you're going to need it.
RE: Boiler Efficiency
If you downsize the burner, you will be converting all the gas (or whatever the fuel) to energy with minimal combustion losses, so you will have done all you can do there, but the overall mass flow will naturally be reduced, reducing the gas velocities in the tubes and who knows what the temperature profiles will look like. There will be less radiation heat transfer in the furnace tube as well.
But, you will be about the best you can be. You will have optimum combustion, but less than optimal heat transfer rates in the tube sections. The only to conquer that is to go find some load to put on it.
That is about all I can tell you about it. I have had to do what you are faced with and I just made the best of it with what I had to work with.
rmw
RE: Boiler Efficiency
Not sure that would cause such a significant efficiency drop though.
RE: Boiler Efficiency
RE: Boiler Efficiency
RE: Boiler Efficiency
I recommend the meter and the calibration of the current instrumentation, e.g. O2 meter, as well as other improvements, but I can just picture the reaction of those that control the purse strings.
My point was to move to the fixes and the limiting parameters whether or not they turned loose of the funds to buy the needed instrumentation.
rmw
RE: Boiler Efficiency
RE: Boiler Efficiency
Steaming rates: 5 to 7 lbs/hr per ft^2 of heating surface
: should not exceed the 7 lbs/hr-ft^2
Efficiency :75% to 80%
(steam generation to fuel input to boiler)
(w/o econmizer,superheater and air preheater)
You can easily determine the burner max firing rate by using the above stats.