Gas consumption

[dianad]

Hi,

I have an economizer wich saves aprox. 200kW of heat power.
I want to estimate the gas savings.
I have LHV of the gas = 37900 kJ/kg, and the aprox. gas consumption = 312 kg/h of the boiler. This leads me to: LHV*MassFlow=11824800 kJ/h = 3284.7 kW.

How can i relate the economizer power to this burner power, so that i can estimate the gas savings?

Thanks!

 

[Feric]

Hi there:

I do believe that you would need to do some kind of exergy type analysis -- maximum availability -- in order to figure out what kind of energy quality you are dealing with. Otherwise, you might end up making wrong conclusions when comparing different types of energy ...

Thanks,

G. Feric, PE

http://engware.i-dentity.com

 

[IRstuff]

Your statements are inconsistent. Obviously, you can't have something that saves more than what the process puts out in the first place. Are you stating what the economizer IS doing? or what it is SPEC'd to do? If so, you should also have what the conditions are that the supplier is claiming this savings, and what the efficiency would be for a much lower input.

TTFN

FAQ731-376

 

[dianad]

Thanks both!

Feric,
I understood perfectly what you just said. My point of view is that in most cases, most people don't use any exergy evaluation, because is too much complex. In most cases is used a value: "Burner time operation", presented in %, wich represents, the percentage of time that the burner stills ON.
This way, we can estimate, for example in a 24h per day operation time, what time did the burner stayed ON.

IRstuff,
Ok, forget the gas mass flowrate. The boiler power is 6500 kW and the efficiency is of 87,8%.
The 200kW is what's expected to achieve. Now, i'm not thinking in efficiencies, because that will be calculated later. This is only a simulation.

Thank you very much!

 

[quark]

I am not very sure what you are asking for. If you want to know economiser savings then you already know the answer, it is 200kW (or [200*3600/37900]kgs/hr * [no. of hours boiler is operated in a day]*cost of fule per kg in your monetary units). This gives you savings per day.

 

[dianad]

quark,

Thanks for your reply. I understood perfectly your point and i will take note.
My problem is related with efficiency, because as we all know, efficiency=heat output / heat input, so, this 200 kW do not represent 200 kW of power input by the natural gas, to achieve the 200 kW of heat transfered in the economiser by the flue gases.

This is what i would like to clarify!

Thanks!

 

[quark]

Economiser is the device that is used to extract heat from the flue gases, otherwise wasted. So, combustion efficiency of entire furnace has to be considered in this case.

The economiser, in classical sense, is a heat exchanger and heat released by hot fluid is heat gained by cold fluid. Efficiency is a wrong term in case of a heat exchanger, though it is 100%. So, a conservative energy saving calculation should only consider natural gas flowrate equivalent to 200kW.

If, at all, you want to glorify the savings then use the overall efficiency(of the manufacturer) of the furnace based on LHV. 90% of furnace efficiency based upon LHV gives you a NG flowrate of (200/0.9)*3600/37900 = 21kgs/hr, but then you are misleading.

 

[dianad]

quark,

Thanks for the reply. I think i will use the overall efficiency, because if we think what happens if we have an economiser installed and for example we need to remove it for repair, the power you have to introduce to overcome this, is the economiser power increased by the efficiency, because of the boiler losses.

Thank you all!