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  #1  
Old 11-17-2010, 10:41 AM
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Default converting normal litres/min to g/s

hi there,

so, ive been given a value of air flow in normal litres per minute, which i need to convert to grams/sec. i've never encountered 'normal litres' before so ive been madly googling and ended up here

i know the conditions under which the flow is taking place (high temp & pressure), and i've also been given reference normal conditions for the air density (kg/Nm^3) (where T= 273.15K and P = 1.01325bar)

my question: is the correct way to do this: convert the flow rate and density to the values they will actually have [ie. NL * (1.01325/actual pressure) * (actual temp/273.15) and Ndensity* (1.01325/actual pressure) * (actual temp/273.15)]. then convert the litres to m^3 and multiply by the density to get kg/min which is easy to convert to g/s?

or can i just say: normal litres -> convert to normal m^3 ->times normal density to get 'normal' mass flow ->then multiply by (1.01325/pressure)*(actual temp/273.15)

i get two different answers if i do this, and im not sure which one is right...i would guess the first one?
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  #2  
Old 11-17-2010, 12:46 PM
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Default Re: converting normal litres/min to g/s

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Originally Posted by Unregistered View Post
hi there,

so, ive been given a value of air flow in normal litres per minute, which i need to convert to grams/sec. i've never encountered 'normal litres' before so ive been madly googling and ended up here

i know the conditions under which the flow is taking place (high temp & pressure), and i've also been given reference normal conditions for the air density (kg/Nm^3) (where T= 273.15K and P = 1.01325bar)

my question: is the correct way to do this: convert the flow rate and density to the values they will actually have [ie. NL * (1.01325/actual pressure) * (actual temp/273.15) and Ndensity* (1.01325/actual pressure) * (actual temp/273.15)]. then convert the litres to m^3 and multiply by the density to get kg/min which is easy to convert to g/s?

or can i just say: normal litres -> convert to normal m^3 ->times normal density to get 'normal' mass flow ->then multiply by (1.01325/pressure)*(actual temp/273.15)

i get two different answers if i do this, and im not sure which one is right...i would guess the first one?
Normal liters ignore actual temperature and pressure and calculate the volume the gas would have at "normal" conditions, the ones you stated. At normal, the molar volume of any gas, at least approximated by the Ideal Gas Law is 22.414 L. The accepted average molecular weight (and molar mass) for air is 28.964 g/mol. Hence 22.414 L is 28.964 g.
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Old 11-17-2010, 09:40 PM
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Default Re: converting normal litres/min to g/s

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Originally Posted by JohnS View Post
Normal liters ignore actual temperature and pressure and calculate the volume the gas would have at "normal" conditions, the ones you stated. At normal, the molar volume of any gas, at least approximated by the Ideal Gas Law is 22.414 L. The accepted average molecular weight (and molar mass) for air is 28.964 g/mol. Hence 22.414 L is 28.964 g.
so, if im understanding you right, its even simpler - just say NL*molecular weight/molar volume?

this gives me...yet another different answer. *sigh* when did units get so complicated!

thanks for the help!
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  #4  
Old 11-18-2010, 03:34 AM
JohnS JohnS is offline
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Default Re: converting normal litres/min to g/s

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Originally Posted by Unregistered View Post
so, if im understanding you right, its even simpler - just say NL*molecular weight/molar volume?

this gives me...yet another different answer. *sigh* when did units get so complicated!

thanks for the help!
If you know actual flow and actual density, you can just multiply

If you know normal flow and normal density, you can just multiply

A few cautions:
*Don't mix normal and actual
*In actuals, be sure you are using absolute pressure, not gauge (also absolute temperature)
*The molecular weight method I provided assumes dry air. The Handbook of Chemistry & Physics has some corrections for moist air if you know the moisture content. (Moist air is slightly lighter, because MW of water is 18).
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Old 11-18-2010, 03:38 AM
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Default Re: converting normal litres/min to g/s

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Originally Posted by JohnS View Post
If you know actual flow and actual density, you can just multiply

If you know normal flow and normal density, you can just multiply

A few cautions:
*Don't mix normal and actual
*In actuals, be sure you are using absolute pressure, not gauge (also absolute temperature)
*The molecular weight method I provided assumes dry air. The Handbook of Chemistry & Physics has some corrections for moist air if you know the moisture content. (Moist air is slightly lighter, because MW of water is 18).
okay. as always, i was overthinking things...thanks!
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  #6  
Old 11-22-2010, 11:53 PM
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Default Re: converting normal litres/min to g/s

Hello.

I have similar problem. I have flow given in normal liters per minute, i must convert to velocity. but i must use the value for simulation, which will be at 600K and 3 bar pressure.

what is the correct approach?
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  #7  
Old 11-23-2010, 03:10 AM
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Default Re: converting normal litres/min to g/s

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Originally Posted by Unregistered View Post
Hello.

I have similar problem. I have flow given in normal liters per minute, i must convert to velocity. but i must use the value for simulation, which will be at 600K and 3 bar pressure.

what is the correct approach?
"Normal" liters are measured at 273.15 K, 1.01325 bar, At your conditions, get actual flow as
normal liters/min x 1.01325 bar/3 bar x 600 K/273.15 K
(this assumes ideal gas law is good approximation and compressibility factor can be ignored)

Recognize that lier is also a cubic decimeter to convert to cubic volume flow. Divide by area to get average velocity.
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  #8  
Old 11-24-2010, 03:48 AM
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Default Re: converting normal litres/min to g/s

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Originally Posted by JohnS View Post
"Normal" liters are measured at 273.15 K, 1.01325 bar, At your conditions, get actual flow as
normal liters/min x 1.01325 bar/3 bar x 600 K/273.15 K
(this assumes ideal gas law is good approximation and compressibility factor can be ignored)

Recognize that lier is also a cubic decimeter to convert to cubic volume flow. Divide by area to get average velocity.
Is clear. Thank you.
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  #9  
Old 10-16-2011, 09:46 PM
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Default Re: converting normal litres/min to g/s

I have been given a fuel usage in a Diesel generator of 200 g per Kwh how much is that in litres per Kwh
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  #10  
Old 10-17-2011, 02:13 AM
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Default Re: converting normal litres/min to g/s

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I have been given a fuel usage in a Diesel generator of 200 g per Kwh how much is that in litres per Kwh
Depends on the density of your diesel, which varies. However, a typical value is 0.84 kg/L or 0.238 L per kWh.
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