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# Velocity Pressure Calculation Help

## Velocity Pressure Calculation Help

(OP)
Hello all, first time poster here

I am currently designing a wet coal scrubber for a longwall mining application.  Right now I am doing some pressure calculations to get the total pressure in the ducts, but am a little confused.

I already know the static pressure, but running into problems when trying to get the velocity pressure, the variable in question is the density factor of the air.  The air coming into the scrubber I know is going to full of coal dust, then once it enters the scrubber, the coal dust in the air will be soaked with 20 conical water sprays spraying at 35 psi.

The book I am reading about Industrial Ventilation is telling me to get the overall density factor, I need to calculate the elevation, duct pressure, temperature, and moisture density factors. I can calculate all of these except for the moisture density factor.  The equation they give is (lbm H20)/(lbm dry air).  How can I calculate these values?  Will I be able to knowing the velocity of the air stream and the amount of water entering the air stream? Also, will I need to take into account the amount of dust in the air stream for an additional density factor?

One last question also.  I am assuming that when I am calcuating the velocity pressure, I will be using a different density factor for the duct segments with only coal dust in the air stream than I will be using for the scrubber box that has air and coal in it.  Am I correct in assuming this?

Any help or insight you can offer will be greatly appreciated.  This is my first project out of school where I am designing something like this and it is a little frustrating.

Thanks again

### RE: Velocity Pressure Calculation Help

(OP)
After doing a little more reading and looking around on the web, I believe that I can disregard the amount of dust particulate in the air.  To calculate the amount of moisture content in the air, I think I can just use the ratio of the volume of water being sprayed to the volume of air space inside of the scrubber.  Does this sound correct to anyone?

### RE: Velocity Pressure Calculation Help

Do a search for Humidity calculations.

### RE: Velocity Pressure Calculation Help

For the first pass you can also ignore the excess water.  Just consider it saturated air.  This will give you a starting point.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

### RE: Velocity Pressure Calculation Help

If you are only using the humidity as the air entrained contaminant, you can use the psychometric chart to figure out the amount of water.  The two variables you need to know is the temperature of the air going thru the duct which is known as dry air temperature on the chart and the relative humidity.  The intersection of these two variables will give you the amount of moisture in lbm or grains per lbm of dry air.
How do you know that you can disregard coal dust as an entrained contaminant?

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