January 26, 2009

Understand the Boiler Efficiency

Energy Savings do not always mean efficiency improvement for Boilers.

This is with reference to my previous post long back on How much do you know your boiler efficiency?

Understand the formula
The general efficiency equation is as below

Efficiency = Absorbed heat / Energy Input

But it doesn’t say anything about useful heat. For example blow down is absorbing heat but it is not “useful”. Thus, if you increase the blow down the heat absorption will increase but your efficiency goes down.

In other words, energy in the blow down is not useful energy or is not utilized for useful work i.e. steam generation in case of boilers and hence we should revise the above formula as below

Efficiency = Useful energy / Energy Input
= 1 - Energy wasted / Energy Input

Now even with this formula, should I consider the blow down as a factor / component in energy efficiency calculation.

Factors affecting efficiency
First we have to define the system. Boiler is only furnace & water tube part. If you consider convection section preheater, fans, motors etc. they do not affect the boiler efficiency. They are the part of steam generation system & affect the overall effectiveness or efficiency of “Steam Generation System”.

Most of the people do vice versa even some standards do not considering blow down, steam pressure, steam temperature, condensate return temperature etc as the part of boiler efficiency. In fact they consider combustion air temperature as part of factors affecting efficiency. This is totally illogical from my side.

Let us understand them one by one.

Combustion air
Combustion air temperature entering into the furnace do not affect the Boiler efficiency (It affects the fuel savings or overall steam generation system efficiency but not the boiler part) as its temperature is very low (~200°C max) compared to furnace temperature of ~1200-1400°C. therefore, the changes in the temperature potential (see it equivalent to LMTD) for heat transfer across water tubes is not at all significantly affected.

There is no increase in boiler efficiency even if you preheat combustion air in the economizer in convection section. It is rather heat recovery from the system & not the increase in boiler efficiency.

Condensate return & its temperature
This also does not affect boiler efficiency. The temperature of return condensate is usually very low ~80°C or less compared to steam drum temperature of >200°C. therefore, practically there is no change in temperature potential again.

Similarly, the quantity of return condensate does not alter the total water flow to the boiler drum, which is usually preheated in economizers, & hence practically there is no visible change in heat transfer in the radiation zone of the furnace.

Blow down
Generally, we can assume 2% blow down in the large boilers. This can reduce the drum temperature by ~1.5% of the boiling temperature i.e. if drum temperature is 200°C it will reduce to ~197°C. it will positively affect the heat transfer in the furnace but overall system efficiency goes down. However, it is not considered in the efficiency due to the fact that most of the make up boiler feed water goes to the drum through preheater and therefore, practically drum temperature will not get affected.

Boiler efficiency
So what is the boiler efficiency? Is it really 75-92% depending on the fuel fired & boiler design? Or is it system efficiency for steam generation system?

In fact, what we are generally reading in the books or practically following in the plants is neither boiler (furnace) efficiency nor it is the efficiency of the steam generation system because……….

  • We need to consider only furnace part for efficiency calculation of boilers which should be in the range of ~55-60%, as it is usually the case with fired heaters. Therefore, fuel consumption varies based on this efficiency and not based on 80 or 90%. This is the reason that practical savings are always either very less or very high compared to prediction based on 80-90% efficiency.

  • We are considering heat recovery in the convection section when we consider final stack gas temperature of 150-200°C without giving credit for other streams being preheated. (Streams not used for boilers e.g. DM water, Air etc. used for other purpose in the plant)

  • We are not considering the total station approach e.g. the power used for different motors like ID fan or FD fan, efficiency of other equipments like pumps etc to call it the efficiency of “Steam Generation System” (SGS).

  • What we are doing / calculating is the efficiency of package unit supplied by the vendor as “BOILER”.


The objective of this paper is to make it clear in simple language to the young engineers & plant personnel what they are doing and what are the consequences of it. They are not doing anything wrong but one should be clear in what they are doing.

This will explain you the difference in the efficiency from 75-80% on coal to ~85-90% on gas in the same boiler. How it is possible? Is it just because of heat content of ash / fly ash or something else?

Invite your comments if you want to learn more.

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14 comments:

shahzad ahmed said...

dear BLOG ADMIN

WRITE MORE ABOUT THE TOPIC, AS YOU HAVE SEEN THAT NO BODY IS PARTICIPATING IN DISCUSSION (YOU INITIATED).
BUT FOR VIEWERES TOPIC IS INCOMPLETE.
SHAHZAD

Mano said...

Hi,

You've got a nice blog with beneficial content. Thanks for being so informative about steam boilers.

shahzad1166 said...

THANKS FOR RESPONSE.
VERY FRUITFUL INFORMATION.
TELL ME ABOUT THE PROBLEMS ASSOCIATED WITH COAL FIRED BOILERS. I AM INTENDED TO PURCHASE A BUBBLING FLUIDIZED COAL FIRD BOILER.

speedfit pipe said...

Great insights,These are great resource especially knowing the equation formula enable to get the efficiency of boilers.This are good reference materials in case we will have a cases like these.

plumbing said...

The information that you have shares are very helpful. Your insights and ideas should be considered before buying a boiler.

James said...

Hi,

You've got a nice blog with beneficial content. Thanks for being so informative about gas valves

PEX tubing said...

Your information is very helpful for me because I try to DIY boiler installation.

Steam Boiler Chemicals said...

Steam Boiler Chemicals are used to control scale, remove oxygen, and to control pH. Proper Steam Boiler Chemical selection will extend your boiler life and improve your overall operating efficiency.

Qasim said...

HI
I want to increase the efficiency of boiler by using the heat recovered from air compressor in other sense i want to feed the boiler with pre-heated water. My question is that is there any formula to calculate how much money can b saved by this method or how much full cost will b reduced..???

profmaster said...

Qasim
It will not increase efficiency of boiler & this is what I was trying to say in this post. Yes it may increase the efficiency of entire steam generation system.

See if you are preheating your water, finally your flue gas outlet temp will increase but this all depends on the design & operation of current system. If you have sufficiently large size of your boiler & heat recovery area it may help, but if it is already limiting you may not find any gain.

So you need to evaluate your entire system in new conditions.

Polina said...

Dear Blog Owner,

I am a graduate engineer and I have been assigned to do resesrch and problem solving on why Boiler effeciency is constantly low. Your blog has helped me find out what does not affect boiler efficiency, but I am more interested in what does affect the efficiency. You mentioned something about that in the blog but it wasn't really clear. would appreciate very much if you could share in more detail the factors that "do" affect boiler efficiency.

Thank you,

Polina

Polina said...

Dear Blog owner,

I am a graduate engineer and have been given the task of research and problem solving of low boiler efficiency.
your blog has helped me to find out what efficiency does not depend on but I would like to know what does the efficiency depend on.
would appreciate very much if you could share what factors do affect boiler efficiency.

Thank you
Polina

profmaster said...

Polina
Please be specific in your requirement. Currently I am not able to pay too much time for the blog as I am passing through rough times.

aduanrakyat said...

i am looking for boilers to heat the water that contained salt water. the problem is how to.avoid the scaling phenomena n get the best.efficiency.

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