Mine Ventilation Questions

[Redskinsdb21]

I am curious about Fan Characteristics of Mine Ventilation Fans. Typically when you get a fan it tells you :

Quantity = 400,000 cfm
Power = 20 watts
Water Gauge = 3 inches

They usually list several other parameters

My question is....what does the 3 inches of water gauge tell you? how is it useful?

Is this the statsic head of the fan? How is it measured and what does it tell ya?

 

[CanuckMiner]

Redskin,

The water gauge is the pressure of the fan at your airflow of 400,000 cfm. It might be static pressure, or it might be total pressure. This would be a relatively low pressure for a mine fan.

Note that your power of 20 watts isn't even enough to power a light bulb. The power for a fan pushing 400,000 cfm at 3 inches w.g. would be about 200 kW.

Cheers,
CanuckMiner

 

[Redskinsdb21]

Canuck,

You say its the pressure/head at the point where you have 400,000 cfm? How is this point determined? Is this water gage measure with u-tube manometers? barometers? pitot tubes? WHat good does it for you to know water gage pressure at 400,000 cfm?

 

[CanuckMiner]

As with any fluid circuit, you need to determine the flowrate. After you know your flowrate, you can calculate the resistance to flow (i.e. pressure). You then look for a fan (or pump, or compressor) that can provide the flow and pressure that you've calculated. There may be many choices of fans, but some will have characteristics that are more attractive (e.g. more efficient, quieter, smaller, longer lasting, cheaper, etc.).

Water gauge can be measured with several devices, including the ones you've mentioned. The term "inches of water" is just a common unit for air pressure. You could just as easily use psi, kPa, mbar, etc.

I only used the 400,000 cfm and 3 inches water gauge because those were the numbers you provided. Obviously, not every mine fan has to be able to move 400,000 cfm.

Cheers,
CanuckMiner

 

[Redskinsdb21]

CanuckMiner,

I used the 3 inches and 400k cfm b/c thats just something that popped in my head.

So you say you need the flow rate you mean velocity right? Then you can determine Heads....which your calling pressure coorrect?

Then use to get your fan correct?

One more thing ..... refering back to fan information in this conversation:

I see that 1" of Velocity head is approximately equal to 4000 fpm. Then say we have a mine entry 5' high by 20' wide and a 4000 fpm velocity....this yields a 400,000 cfm flow rate(Q)....4,000*5*20 = 400,000 cfm

So when you get a fan....it says you have the following characteristice:

QUantity 60,000 cfm
Water gage 2 in
Horsepower 300 hp
volts 480 3 Phase
amps 360
etc.

Is this water gage they give you here the velocity water gage? THus saying if you put the fan on a mine described above...5*20 entry...then this fan will supply ....60,000 cfm....does the cfm not depend on the area of the entry multiplied by velocity of air? I am a little confused on this characteristic stuff....

 

[CanuckMiner]

Power equation can be approximated as HP = Pressure (in.w.g.) * Flowrate (cfm) / (6356 * Efficiency)

Therefore, 60,000 * 2 / (6356 * 70%) = 27 HP

Your example of 300 HP motor looks like overkill by a factor of 10.

Regarding what I mean by flowrate, it is not velocity. It is a volume of air moving in a given amount of time. It could be 100,000 cfm (cubic feet per minute) or 47.2 cubic metres per second.

The imparts total pressure to the air stream. Total pressure is comprised of static pressure and velocity pressure. As the air travels through various sized openings in the mine, the static pressure is converted to velocity pressure (in smaller openings), and from VP to SP in larger openings.

I suggest that you download and read the pertinent sections of Subsurface Ventilation Engineering by Dr. McPherson at

http://www.mvsengineering.com/index.php?cPath=25

Cheers,
CanuckMiner

 

[Redskinsdb21]

canuck miner,

Thannks for the direction to the link..I actually found this link a couple days ago and the book is a good one.

I have a question though...I understand that total head is the sum of static heads and velocity heads, and one goes up the other goes down, etc. I want to know....if you have say 2-in water gage as in your power equation above...how do the fan manufactures measure this water gage? DO they stand away from the fan like 2 feet and measure it with a special piece of equipment? IS this gage that they give you the total head, meaning the sum of the static and velocity heads?


Thanks much,
Redskins

 

[CanuckMiner]

Redskin,

The fan manufacturer may give you the static pressure, or the total pressure. Their description will tell you which is being provided.

I suggest you google pitot tube. Also, check out AMCA 210. Better yet, talk to a fan manufacturer's rep.

Cheers,
CanuckMiner