Calculating Nitrogen consumption

[mihaivoivod]

Hi guys,

I'm new to this and the forum so please bare with me. I have been given the mission to calculate the nitrogen consumption by our oven.

The system I'm dealing with is:

Nitrogen enters the oven so we get better solder joints, the oven temperature varies. Nitrogen flows from the tank to various machines in the shop. I need to calculate the nitrogen cost per board.

This is what I have so far:

Flow rate into the oven is set at 820 SCFH
Oven is running with Nitrogen for 17.90667 minutes / unit
Nitrogen cost is $929.43 for 45437 FT^3

What I calculated:

-I divided time (17.90667) by 60 to get the results in hours. (0.298444 hour/unit)
- I multiplied the flow rate by duration (820*0.298444)=244.7244444

Then I did the following:

244.724444 costs X dollars, 45437 FT^3 costs $929.43. x=(244.7244444*929.43)/45437 this results in a grand total of $5.005926 of Nitrogen per board.

Did I calculate this right? Am I thinking correctly about this?

[mihaivoivod]

I guess my question is if I need to correct the results to pressure and temperature.

[gubment_cheez]

You are thinking correctly about this, but your answer is wrong, because you're using SCFH, and you should be using ACFH.

SCFH is only valid for the "standard" condition for pressure as variously defined as an absolute pressure of 101,325 pascals (Atmospheric pressure), 1.0 bar (i.e., 100,000 pascals), 14.73 psia, or 14.696 psia and the "standard" temperature is variously defined as 68 °F, 60 °F, 0 °C, 15 °C, 20 °C, or 25 °C.

You can solve this by determining the standard your unit uses, and then converting to ACFH by resolving for ACFH in SCFH = ACFH * (^Pa/_Ps) * (^Ts/_Ta) where ^P and ^T are the parameters Pressure and Temperature, and _a and _s are the Actual and Standard values for the aforementioned parameters, and then using ACFH in place of your SCFH from the oven.

[mihaivoivod]

Thank you gubment_cheez for your insight.

[mihaivoivod]

Do you think the cost could change significantly? or do you think I have a decent enough of an estimate?

[gubment_cheez]

I don't know what standard your SCFH is based on so I used the SCFH temperature standard of 0°C. Using these values I calculated a flow of 895 ACFH at a temperature of 25°C. To avoid having to divide by zero (Here be Black Hole Dragons) I used Kelvin to calculate ACFH
The difference of 75 CFH may or may not be significant, it's an additional 22 CF per board as you described it, about 45 cents I believe.

A higher temperature standard will decrease this difference (Or more specifically a temperature standard that is closer to your environmental conditions). But you cannot pick and choose which standard you use. You have to use the standard conditions that are indicated by the manufacturer.

Pa = 1	Pa/Ps
Ps = 1	1
Ts (Kelvin) = 273	Ts/Ta	Pa/Ps*Ts/Ta
Ta (Kelvin) = 298	0.916107382550335	0.916107382550335
SCFH		ACFH (SCFH* Pa/Ps*Ts/Ta)
820		895.091575091575