efficiency calculations for oil fired boilers

[grnblr]

Can anyone guide me to any website where I can get the complete details including formula for calculating the efficiency (by indirect method) of furnace oil fired steam boilers.

Thankx in advance

 

[jmw]

Try

http://www.steemesteam.com

as a start point for all sorts of useful information on boiler operation.

This is a link to a fuel specific energy calculator:

http://www.dan-bunkering.dk/Valuecalc.htm

However, you may never get the full energy benefit from your fuel due to a variety of operational factors. For example, the problem of fuel heater control, as the spray pattern is very much incluenced by fuel viscosity.

Fuel viscosity is vary variable due to quality variability, and most steam plant or boilers use temperature to control the heaters. This then requires excess oxygen, frequent sampling to determine the temperature set points and routine flame inspections. Modern viscometers can improve the situation for you if you get one that works.

This can be a critical issue if the EPA get after you as not only does the incomplete or inefficient combustion cost money in excess fuel consumption but environmental issues come to the fore.

PREPA (Puerto Rico Electric Power Association, or something similar) had a rather innocuous comment on their web-site about upgrading their power stations which included a reference to new viscometers. A search of EPA records shows thhat they had many problems and this was just one of the actions they were required to take. If I can find the link, I will post it.

JMW

http://www.viscoanalyser.com

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[davefitz]

the complete formula is as per ASME PTC 4 ( power test code) or is also presented in the book "STEAM" by B+W.

 

[TBP]

Do the boilers you're looking at have accurate steam flow meters, and individual fuel metering? Without those, you've to nowhere to start.

 

[davefitz]

TBP
You do not need to measure any fuel or steam flow at all. Instead, you measure only the water inlet flowrate, all required P+T's, and the exhasut gas O2 content. No steam or fuel flow mesurements are required at all.

 

[TBP]

davefitz - How do you account for blowdown & blowoff? Depending upon the application and how much condensate gets returned, I've seen steam boilers with a CBD rate approaching 10%.

 

[grnblr]

Actually there is a direct method of calculating efficiency based on heat output divided by heat input.

The heat output is calculated by the formula .. steam flow x (steam enthalpy - feedwater enthalpy)

The heat input = fuel firing rate x gross calorific value.

but I am interested in the indirect method, which is 100% minus (the losses due to dry flue gases+ lossed due to moisture in the fuel etc etc). This cal is more complex and requires the fuel analysis to be done.

I was trying to get at any website having the indirect method

regards

 

[TBP]

The ASHRAE folks likely have what you're looking for.

I realize that there are times the indirect method has to be used, but I dislike it. It's complexity often leads to errors, or worse. I'm not for a moment suggesting that this is your intention, but I've seen it manipulated by some, to provide the value they want to be found, as opposed to the correct efficiency. It's the old "figures don't lie, but liars can figure" thing. Much harder to do with a nice short formula.

 

[quark]

Actually I observed furnace oil specifications varying from lot to lot (even the density by a little bit) and is often difficult to consolidate the efficiency from one single equation. i.e for all fixed parameters, you may not get constant CO and CO2.

It is better if you go for a flue gas analyzer. Forbes Marshall has one analyzer which costs around Rs. 60,000 and Ace Instruments, Mumbai has cheaper and good analyzers(around Rs. 40000). These are extremely useful for making excess air corrections and daily efficiency analysis.

Come back if you still wish to know the equations.

Regards,

 

[jmw]

Quark,
fuel oil quality varies enough even for commercial grades but I discovered that in refineries and petrochemical plant "commercial grade" was a euphemism for "burning the mistakes"; the fuel oil tanks often became the recepticle into which to pour unrecoverable production errors.

The main problem with using viscosity to control the heaters was the fuel quality which often coated or even blocked the capillary visometers normally used, though other types fared equally badly. Modern digital viscometers (with special coating) have no such problems and allow the engineer to rely on the viscometer to control heating; this in turn allows the excess to be reduced somewhat.

JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[grnblr]

Quark

No doubt we will be using the forbes o2 analyser. I still need the various equations for dry flue gas loss etc. I have a doubt in the excess air qty equation.Pl forward me the equations if it is not a bother. My Id is grn_blr@yahoo.co.in

thanks in advance

grnblr

 

[davefitz]

TBP:
The usual practice is to close all blowdowns , blowoffs and drains during a 1 hr ASME PTC test. If it is essential to have a blowdown during the test, the blowdown flowrate could be measured, using a flow element upstream of the blowdwon valve. IN that case, the blowdown valve should be located at least 40 feet below the drum NWL.

Another newer practice is to measure in the field the casing emmissivity and temperature so as to obtain a more accurate indication of radiation loss. This has significance for small boilers and for fluidixed bed bilers ( which have large surface areas of uncooled refractory).

 

[quark]

Grnblr,

For a general reference and comments, I thought it is better to put them in the forums.

1. %Heat loss in dry flue gas = Wx24(T[sub]2[/sub]-T[sub]1[/sub])/GCV

W = weight of dry flue gas/lb of fuel

W = [(11CO[sub]2[/sub]+7CO+7N[sub]2[/sub]+8O[sub]2[/sub])/(3(CO[sub]2[/sub]+CO))]*(C[sub]g[/sub]+S/183)

CO[sub]2[/sub], O[sub]2[/sub], N[sub]2[/sub] and CO are % of these gases by vol. in dry fuel gas.

T[sub]2[/sub] = Flue gas temperature in deg.F
T[sub]1[/sub] = Air inlet temperature in deg.F
C[sub]g[/sub] = Weight of carbon in flue gas per lb. of fuel fired.
S = % of sulphur in fuel

2. %Heat lost in moisture in flue gas = [[212-T[sub]1[/sub]+970+0.5(T[sub]2[/sub]-212)](9H+M)]/GCV

M is % moisture in fuel
H is % hydrogen in fuel.

3. %Heat lost by combustibles in ashes or grits = (Ash as % of fuel fired x Ca x 145.5)/GCV

Ca is %combustible in ashes or grits.

All these calculations require careful analysis of the fuel and flue gas as well.

For radiation losses, you can refer either Spirax Marshall or Thermax Databooks. Both are free if you have a good contact with those people.

JMW,

Viscometers can be a good option but mine is a world of small boilers (not more than 10TPH). So far I am doing only manual adjustments with the excess air flow based on O[sub]2[/sub] in the flue gases. The analyzers I mentioned are in a price range of 800 to 1200 USD at the present rate of conversion.

What is the rough cost of an inline viscoanalyzer? Any specifics about them will be much helpful to me.

PS: Sorry for the delayed reply. I thought to have two birds at one shot

Regards,

 

[TBP]

davefitz - I realize that there are several ways to skin the cat, depending on the size of the boiler, and how much instrumentation is available. Boilers in central stations are extensively instrumented. Boilers in general industry are typically much more basic in the information available. I very much prefer simple over complicated - if there's a dedicated fuel meter for the boiler, and proper pressure compensted steam meter, then the "big picture" efficiency calculation is simple.

 

[jmw]

Quark,
No simple answer as HFO viscosity to burners was a problematic application, and still is for some manufacturers.

Conventional viscometers for heavy fuel oil:
[ul square][li] to engines have a good reputation and are available at around $2-3000 (OEM prices)[/li]
[li] those same viscometers mostly have a history of failures on heavy fuel oil to burners and were/are offered at $12-13k[/li][/ul]

Today, more technologies are availale.
Viscometers proven on both burners and engines (and on bitumens and asphalts) are available at nearer to the engine price ($2-3k).

Viscometers for lube oils to trucks, that I would consider appropriate (with minor mods) could bring the price down very significantly for burners. Probably to <$1000 for HFO.

These are all behavioural applications (viscosity at the operating temperature) so no great accuracy is required.

For many such applications dynamic viscosity is often considered acceptable. It is far easier to keep track of density variation in the lab than viscosity, and it varies far less. Applying a lab based density correction to the dynamic viscosity readings may be considered by some as a good trade for a competitive price.

However, while there are many manufacturers offering viscometers for fuel oil, not all can demonstrate satisfactory operation for burners (a reason why the market price for burner applications is so high); so be sure to follow up references (and be wary of being given references to engine based power generation instead of burner operations; these may be the same HFO but believe me, the conditions are different).




JMW

http://www.viscoanalyser.com

Eng-Tips: Pro bono publico, by engineers, for engineers.

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.