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Calculate Energy efficiency

Calculate Energy efficiency

(OP)
Hi

I don't know if this is in the correct forum so my apologies if Ive posted in the wrong place. My field is more on the electrical side and controls than on the mechanical side so I need a bit of help doing some calculations .

I have two boilers, one LPG and one oil, and i need to compare some reading on both. I have a control system set up which is feeding back live data to me from the meters installed. I need to calculate the following:

Gas into the LPG boiler per hour
Heat out of LPG boiler per hour
Gas Boiler Efficiency per hour (Heat Out/Gas In)
Gas Boiler Efficiency per day (Heat Out/Gas In)
Oil into oil boiler per hour
Heat out of oil boiler per hour
Oil Boiler Efficiency per hour (Heat Out/Gas In)
Oil Boiler Efficiency per day (Heat Out/Gas In)

I have the following readings;

Oil Boiler Meter on Input(m3)
Flow Temperature (C)
Return Temperature (C)
Volume Flow (m3/hr)

LPG Boiler Meter on Input (m3)
Flow Temperature (C)
Return Temperature (C)
Volume Flow (m3/hr)

Could anybody tell me what formulas to use to do the calculations?
Any help greatly appreciated


RE: Calculate Energy efficiency

I use skid-edge energy to compare technologies. You know the flow rate of fuel in both cases. You can find the net heating value of the fuel (e.g., methane contains 0.997 BTU/SCF). Multiply the flow rate over a set time period times the NHV to get BTU input. In this case the skid edge output is steam flow at a pressure and temperature. Flow rate times energy content of the steam gives you BTU output. Divide them. The number is usually well under 30%. The missing 70% went up the stack (and heated the space, etc).

You can also put the "skid edge" at the plant fence, this accounts for incidental losses and mechanical efficiency. If you are talking about an electrical generation facility, compare the energy content in the fuel to the plant gate energy output. Last time I did one of these on a conventional plant (no significant waste heat recovery) I got a number under 15%. I once did an energy balance on a co-gen natural gas processing facility (natural gas was used to run a gas turbine genset and the exhaust gas went to a boiler and all the pumps, heat exchangers, and compressors were steam powered). The input in that case was the mechanical energy in the suction gas and the fuel into the genset. The output was the mechanical energy in the discharge gas (actual volume after plant shrinkage and fuel) and the outlet electrical meter. This was a sweetening plant so there were no NGL's to account for. I had to avoid using the chemical energy of the process stream because it really masked what was happening since virtually all of the BTU in the process stream was passed through the plant (we got rid of 22% CO2, but there are no BTU/SCF in the CO2 and the fuel was less than 2% of the throughput). This plant calculation showed nearly 60% efficiency (compared to 8% on a similar sized electric-driven sweetening plant just up the road).

The question always comes up "my generator is 95% efficient, my pumps are over 90% efficient, why does your skid-edge number look so crappy?" The answer (obviously) is you get a different number when you compare output to theoretical output than if you compare input to output.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Calculate Energy efficiency

Boiler efficiency = power out/power in
Power in for a boiler is fuel massflow x heating value
Power out is steam massflow x (enthalpy steam out - enthalpy steam in)
steam massflow = volume flow x density@flowmeter

David. 30% is very low for a boiler - sounds more like a heat engine.

je suis charlie

RE: Calculate Energy efficiency

(OP)
Thanks gruntguru

Thats exactly the type of thing I'm looking for but I'm afraid my ignorance in terminology is still causing a problem. Boiler may be the wrong description, although that's what the manufacturers call them, as I am not sure does it produce steam or just hot water. The boilers in question are Buderus Logano GE515 and are used for a heating system in a school.

"Boiler efficiency = power out/power in"
This is perfect exactly what I needed

"Power in for a boiler is fuel massflow x heating value"
Fuel massflow is fine, I have that. Is the heating value the specific calorific value for the fuel?

"Power out is steam massflow x (enthalpy steam out - enthalpy steam in)
steam massflow = volume flow x density@flowmeter"

I'm struggling with these two formulas given what Ive said above. The readings I'm getting back from the installed meters on the return side are on the return water from the school.

RE: Calculate Energy efficiency

Fuel massflow is fine, I have that. Is the heating value the specific calorific value for the fuel?
YES

"Power out is steam massflow x (enthalpy steam out - enthalpy steam in)
steam massflow = volume flow x density@flowmeter"
I'm struggling with these two formulas given what I've said above. The readings I'm getting back from the installed meters on the return side are on the return water from the school.

I assumed steam was being supplied. Liquid water is easier. Enthalpy = Cp x T where Cp(water) = 4.2 kJ/kg/*K Note. *K = *C for the purpose of delta temps.

je suis charlie

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