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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January 16, 2009

Kinematic Viscosity of air Vs Temperature

This is similar to my previous post on the property correlation of a fluid to use it in calculations making results better & life simpler.

This time I am putting the formula for calculating the Kinematic Viscosity of air vs temperature.

The correlation is as below.

kv = -1.1555 x 10^(-14) x T^3 9.5728 x 10^(-11) x T^2 + 3.7604 x 10^(-8) x T - 3.4484 x 10^(-6)


kv is kinematic viscosity in M2/sec
T is in K

The above equation is valid from T = 100 to 1600 K.

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January 08, 2009

Thermal Conductivity of Air Vs Temperature correlation

Generally whenever you prepare some calculation model in excel for evaluating different scenarios or for simulating some process conditions, it is very frustrating that each time you have to input some or the other fluid property manually.

The basic purpose of developing an excel model is defeated & it can not give the flexibility & options you need for identifying all possible scenarios. Speed of work is also slow....What to do?

You need to develop some correlation for that relevant property so that a formula can be used which can give you all results at just a click of mouse.

Here is one such property i.e. thermal conductivity of air vs its correlation with temperature. The equation is...

k = 1.5207 x 10^(-11) x T^3 - 4.8574 x 10^(-8) x T^2 + 1.0184 x 10^(-4) x T - 0.00039333

Where k is in W/m/K
and T is in K

The range of use is T = 100 to 1600 K

Now you can use it in any calculation for example if you designing any air heater, you can use it to find out Nusselt no of air side & accordingly your U will vary in the Excel sheet automatically. Thus you will get better results.

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