5 replies to this topic

[raghav]

we need to calculate the boiler capacity based on F&A rating.

Our consumption is 2500 kg/hr steam at 17kg/cm2(g)

We provide feed water at 100degC to the boiler drum.

please suggest, how to arrive at F&A rating.

Thanks in Advance.


Regards,
raghav.

[Art Montemayor]

This has to be one of the laziest of all the lazy student questions we get on the Forum.

In less than 30 seconds, I went from this webpage to the Google Search Engine and then to:

http://www.forbesmarshall-inc.com/spirax/sgfaq1.asp

Where it is written:

"F& A means From & At rating of a boiler and it is defined as follows:

F&A rating * 540 = actual rating * (hg - Tf)

hg = the total enthalpy in each kg of steam, it is the sum of the liquid enthalpy and the enthalpy of evaporation
Tf = the temperature of the feedwater in ^oC

The energy from the steam is the actual evaporation, which is defined as:

Actual evaporation = (boiler capacity * 540) / (hg - Tf)"

Why couldn't you do the same? My intent is not to put you down; rather, it is to show you that when you are lazy, your classmates will soon overcome you in the classroom and out in industry. Don't let that happen to you.

[raghav]

Dear Montemayor,

Thanks a lot!

But still iam in confusion.

i know only the consumption as 2500kg/hr.

But as per your first equation, how to arrive at the actual rating to determin F&A rating?

is that the 2500kg/hr is the actual rating or not if else, please explain.

Thanks in advance.

regards,
raghav

[Senthil]

F&A rating:
The boiler is rated to work at a certain pressure and at that pressure it can generate a defined quantity of
steam. This can also be written as the F & A rating of the boiler.
This is used as a measure of the boiler ability to produce steam. It gives us the amount of steam (in
kgs) that a boiler will produce if supplied with water at 100ºC. e.g., a 2 ton boiler gives us 2,000 kgs of
steam per hour. It is written as 2 TPH F&A 100.

This means that when the water inside the boiler is at 100ºC and the steam take-off is also at 100ºC, the
boiler will gives us 2 tons/hour of steam.

F&A Rating X 540 = Actual Rating X ( hg – hfFW)
where,
hg = enthalpy of steam at generation pressure
hfFW = Feed water enthalpy

Example 6.1. Take a 10 TPH boiler, @ 10.54 barg, 75°C
= 9152 kg/hr nett
So, a 10TPH boiler will never give you 10,000 kg/hr of steam.

This disparity is because the feed is not at 100 deg C. Water at 100ºC has 100 kcal /kg latent heat. The
F&A rating assumes an ideal condition:
+ 540 kcal/kg
100 kcal/k ----------------------------> 640 kcal/kg
(water at 100°C) (+heat) (steam's latent heat)
But actually, the feedwater is at ambient temperature. So we need more heat to first raise the temperature
to saturation (100°C). This is what gives rise to the difference in boiler output.
+ 610 kcal/kg
30 kcal/k ----------------------------> 640 kcal/kg
(water at 30°C) (+heat) (steam's latent heat)

Thanks & regards
SENTHIL KUMAR,
Design engineer,
Engg design office - Mechanical,
Sembawang shipyard Pte Ltd, Singapore.
Ph no
Off : 6750 1737 Fax : 6750 8765
Email : ssenthil@sembship.com

[kkala]

+
"we need to calculate the boiler capacity based on F&A rating.
Our consumption is 2500 kg/hr steam at 17kg/cm2(g)
We provide feed water at 100degC to the boiler drum.
please suggest, how to arrive at F&A rating".

After the contributions so far, helpful indeed, let us try to calculate F&A rating (to clarify any doubt), subject to comments from others.

Actual rating: 2500 kg/h
Pressure of steam out: 17 kg/cm2 g ~ 18 kg/cm2 abs
Temperature of steam out: 220 deg C (assumed 14 deg C over saturation just to avoid condensate in transfer lines, but adapt temperature to real conditions).
Enthalpy of steam out: 677 kcal/kg
Enthalpy of water in : 100 kcal/kg
Hg-Hfw=677-100 kcal/kg=577 kcal/kg
Q=Actual rating x (Hg-Hfw)=2500 x 577 kcal/h = 1442500 kcal/h
F&A rating: 1442500 / 540 = 2671 kg/h (by definition).

Remarks: Lw0=540 kcal/kg is (very close to) heat of vaporization of water at 0 kg/cm2 g = 1 Atm abs.
So F&A rating is: actual rating x (Hg-Hfw)/Lw0
- The term "F&A rating" may not be widely used. It is the first time I see it, although at work we have made Process Duty Specs for several boilers.
- Blow down does not seem to be involved in F&A rating.
- A complaint to Perry 7th ed : I have not found steam tables in the book, despite it is a handbook. Besides several of its references are valuable, but hard to find outside of USA or UK, which does not help international readers.

Edited by kkala, 25 December 2009 - 03:43 AM.

[S.AHMAD]

we need to calculate the boiler capacity based on F&A rating.

Our consumption is 2500 kg/hr steam at 17kg/cm2(g)

We provide feed water at 100degC to the boiler drum.

please suggest, how to arrive at F&A rating.

Thanks in Advance.


Regards,
raghav.

Hi Reghav

The others have provided you with the equation for determining F&A rating. Let me explain the concept to enhance your understanding.

If we examine the equation, we will notice that the RHS is the energy required to generate the amount of steam at the pressure and specific BFW temperature.

The LHS term is the quantity of steam generated at 1 atm pressure and 100 C BFW, if the same amount of energy is used.

In other words, F&A Rating is the equivalent quantity of steam generated at 1 atm and 100 C water inlet.

If we look into the steam table, we will find that the number 540 is the heat of evaporation of water at 1 atm and 100 C in kcal/kg. So be careful when using the equation. The unit must be consistent. If we are using kJ/kg as unit of enthalpy then replace that 540 kcal/kg with 540 x 4.184 kJ/kg.

Hope the above explanation enhance your understanding and hence be able to determine F&A Rating of boiler.

Best regards

Edited by S.AHMAD, 04 January 2010 - 09:44 PM.