Boiler Efficiency 2

[Murdul]

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

 

[TBP]

Think of the boiler as the engine in your car. Many things affect the overall efficiency of your car besides the engine, right? The transmission, the aerodynamics of the body, even the tire pressure. The condensate being returned or not, will have NO effect on boiler efficiency. It WILL affect how much fuel you have to put into the boiler, though. You need to make the distinction between overall plant efficiency, and boiler efficiency. They are NOT the same thing. Do not unfairly blame the boiler for problems elsewhere in the plant.

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.

 

[davefitz]

sounds like you need an economizer.

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.

 

[athomas236]

What are the O2 levels in flue gas and boiler flue gas exit temperatures at both 6000 and 20000lb/hour.

Regards,

athomas236

 

[Murdul]

Some more info:
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.

 

[TBP]

My money is on oversized steam meters that don't have pressure compensation.

 

[Murdul]

I cannot find any info for my steam meter. So I have also looked into water usage by the boilers. We have a totalizer on this meter as well. Assuming 1 pound of liquid water will make 1 pound of steam, I am still burning far to much Natural Gas. Actually I am still at my fuel to steam efficiency of 60%. I have double checked all my calculations with the SSAT software and I get the same results.

Will a low load on the boiler decrease its efficiency? I assume it does.

 

[davefitz]

6% O2 at the stack can be improved upon- shop around for a better burner that can run down to lowest O2 ( 1-2%) on natural gas at full load- this would require reliable O2 monitors in the stack and other modifications suchas a controllable combustion air flow, etc. If you are in the US, the newer burners have better NOx perforamcne and you can realize an environmental bonus - helps with air permit renewals.

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.

 

[ApC2Kp]

Murdul,

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.)

 

[chicopee]

Firetube boilers without economizers, airpreheaters, and superheaters are not that efficient. In older FT boilers,if you got 60% it was consider normal; with modern ones 70 to 75% would be normal.

 

[TBP]

If you can't hit 80% boiler efficiency, there's something wrong.

The effect of economizers, air preheaters, etc must be discounted when calculating BOILER efficiency. Boilers cannot count any "running start".

 

[rmw]

I agree with davefitz except I would use the word "downsize" in his sentence recommending 'upgrade'. You have a boiler that is probably capable of operating at or near 80% efficiency at 20,000 lb/hr when the firing temperatures (radiation heat transfer in the fire tube) and gas flows (goes to reynolds numbers and heat transfer coefficients) in the tube banks are at their design conditions. You are operating the boiler well below design conditions and the part load efficiencies are never as good as full load efficiency for this type of equipment. Your 02 reading indicates this but it may be impossible to get the burner tuned to produce optimum O2 levels at reduced operation.

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

 

[davefitz]

maybe your efficiency calc is off. Some simplifed efficiency calcs for small fire tube boilers assume the inlet fluid is water at 212 F- in your case it is at 60 F. Try using the asme ptc 4.0 calcs, if your using a simplified calc.

 

[rmw]

Excellent point by davefitz. I have never seen a firetube boiler that wasn't rated "from and at" 212F (100C). But that said, the O2 reading is very revealing as well. If you have the capability to measure CO, you might check that too. Returning to my diatribe about the burner above, the type of burner used on firetube boilers depend on air velocity through the burner throat for proper mixing of the fuel with the combustion air and at reduced loads the air velocity isn't high enough to get good mixing and good combustion. So while you have high O2, it is still possible to have some CO which would indicate what I just said.

rmw

 

[Murdul]

Thank you for all your replies. I will investigate my options for burners.

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%.

 

[TBP]

I dunno, Murdul - I've seen lots of work done on lots of boilers in the name of improving efficiency. A whole lot of it was wasted because everybody was flying blind. There were no reliable steam flow/totalizer readings. Having boilers run without accurate steam meters is like having a fleet of vehicles with no functioning odometers.

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.

 

[rmw]

Assuming an infinite bank account so that you can go out and buy sophisticated instrumentation as TBP recommends, to answer your question, downsizing the burner will just make your burner more efficient, minimizing your losses there. That said, your boiler was designed and the components such as furnace tubes and boiler tubes wer sized to operate at its design capacity. At that condition, the fire tube is full of fire and a certain amount of radiation (not negligible) as well as convection heat transfer occurs there. As the gasses exit the furnace tube and make their first pass, at design conditions there is a specific gas flow and temperature and the heat transfer in the tube bank passes is calculated on the predication that the mass flow of gas, (determines tube velocities) and temperatures at the various points along the flow path are present. Any heat transfer in the tubesheets is considered negligable, but some does occur there in real life.

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

 

[macmet]

Could you be burning lean because of an incorrect reading from your O2 sensor? I was just on a site where the O2 sensor wasn't calibrated properly for a solid fuel fired system.

Not sure that would cause such a significant efficiency drop though.

 

[macmet]

Sorry, burning rich... not lean.

 

[TBP]

rmw - An "infinite bank account" for a 3" or 4" vortex shedding meter, with pressure compensation? C'mon...