Air Velocity and Pressure

Dynamic Pressure - impossible - directly related to velocity

Static Pressure - already increases due to fundamental Bernoulli's equation.

Can you elaborate more on your situation? Air flowing in a duct? Wind pressure against a sign? ...........

The application is as follows: I have a 100m airline operating a water pump. Diameter is 20mm ID. As the pump is working to a high head (80m), I require high pressure from the compressor. From what I have read, resistance in the air line is less when the air velocity is slower. So I need to slow down the air velocity without lowering the pressure. I am not sure whether partially closing the tap on the compressor outlet is sufficient. Does this drop the pressure, slow the flow, or both? The pump canister has a capacity of 17 litres and cycles about twice a minute, so large volumes are not required.

Ok so lets see I'm I'm getting this, You have 100 meters of 20mm ID air line supplying pressure to run a air over water pump? Your water pump requires a minimum of 34 litres of air per minute to operate?

Knowns:

Flow rate: 35lpm or 1.2 ft³/sec or 7.48 Gal/minute... required to drive pump.

Line size: 314mm² or .487 in²

Velocity: 4.92 Ft/sec

Pressure: ??

CV:

SCFM:

Ok... it's getting late, sorry I can't spend the time to go through it, I was going to, but I'll just toss in a few snapshots of the pneumatics handbook. Without knowing your pressures, I cannot give you any values.

I apologize for the poor quality images.. I just scanned the "Pneumatic Application and Reference Hand Book", and tossed them up here for you to see. (I hope the copyright police don't knock my door down)

So far you still have not qualified your pressures...

What is the required pressure to operate the water pump?

What is the compressors operating pressure?

I'm off to the airport, hope the tables help somewhat... Goodnight.

What type of compressor is it? Reciprocating or centrifugal?

What is the air flow rating of the compressor?

And what are the pressures of the system (compressor and pump)?

And, probably most importantly - do you have the compressor performance curve?

..... (how well do you understand performance curves/heads/..?)

Have a look at the linked picture for what you are going to need to do .... (it may be that you require a different compressor set-up [impellor?])

http://yfrog.com/2rcompressorcurvesj

(by introducing more restriction into your discharge line - you will move from the line "Head Required by Process" #2 up to #1)

Your water pump requires a minimum of 34 litres of air per minute to operate?

I don't think the the calculation is as simple as that, depends on:

The pressure at which you are measuring the volume.
The Mark space ratio of the compressor cycle.
The "on" and "off" set points.

Assuming your assumption about air flow 34 lpm is right (looks reasonable, but airnut needs to confirm) as Randall points out, this is at the pressure, assumed 8 barg = water delivery pressure, but depends on pump design.

If all that is correct, I make friction loss in 100m pipe 20mm ID about 25 mbar, i.e. negligible relative to the 8 barg.

Cheers.........Codey

I am afraid that it is not obvious.It is the pressure drop that is linked to velocity.

ΔP = 4fLv²ρ/2g_cD is the Fannin's equation relating velocity and pressure drop.

You may have to use a flow control valve(FCV) and a large flow diameter to achieve what you seem to want. The downside is that the friction will apopear across the FCV.

for a constant flow of any fluid, the velocity is inversely proportional to the cross-section of the pipe. If you want a lower velocity, you need a larger-diameter pipe.

Please explain the application.... I have just started to understand the "rules" of this forum so as not to get flamed for being non specific in my questions...LOL As a pilot and aerospace type, this thread caught my eye....

The application is as follows: I have a 100m airline operating a water pump. Diameter is 20mm ID. As the pump is working to a high head (80m), I require high pressure from the compressor. From what I have read, resistance in the air line is less when the air velocity is slower. So I need to slow down the air velocity without lowering the pressure. I am not sure whether partially closing the tap on the compressor outlet is sufficient. Does this drop the pressure, slow the flow, or both? The pump canister has a capacity of 17 litres and cycles about twice a minute, so large volumes are not required.

Have you tried increasing the temperature? If this falls under the bounds of the ideal gas law then increasing temperature will increase pressure. You can then drop your velocity to drop the pressure back to where you want it.

Increasing the temperature is not an option as I also need to minimise the amount of water condensing in the line.

You will need to supply more detail, because on the face of it, this seems self-evident. Ordinarily, lowering the velocity will increase the pressure, per Bernoulli. In that case. anything you do to reduce velocity (e.g., slowing down the motor on whatever is moving the air) will reduce friction and will not lower the pressure. You will have achieved your goal, without having to do anything special at all.

Do you mean impact pressure? Free stream pressure? Air velocity relative to what? Is this a moving vehicle? A pipe? Wind passing across a building?

The application is as follows: I have a 100m airline operating a water pump. Diameter is 20mm ID. As the pump is working to a high head (80m), I require high pressure from the compressor. From what I have read, resistance in the air line is less when the air velocity is slower. So I need to slow down the air velocity without lowering the pressure. I am not sure whether partially closing the tap on the compressor outlet is sufficient. Does this drop the pressure, slow the flow, or both? The pump canister has a capacity of 17 litres and cycles about twice a minute, so large volumes are not required.

And. What's the problem?

Presumably the compressor has an "on" pressure and an "off" pressure. If you set the "on" pressure higher, then, the average pressure in the pipe will be higher, and, the actual volume of air carried by the pipe will be lowered. Of course the cycle frequency of the compressor will go up: you could lower this by increasing the size of the receiver (reservoir tank).

(I'm assuming that the "off" pressure is already set as high as acceptable.)

As you may have gather from other posts, if you are talking about pressure losses in a pipe, then slowing the linear flow speed (by using a larger pipe) will reduce these losses. RVZ717's excellent post #12 should give you all the info you need. As you can probably intuit from these tables, any fitting (especially turns) that you add to a piping run increases the restriction and pressure losses. Closing the tap on the compressor outlet would have the same effect, causing a pressure drop across the valve whenever there is flow in the line -- i.e when the pump cycles on. As a rule of thumb, for overall efficiency you want large pipes and few turns. Cost and practicality dictate against large pipes, however, so you strike a balance, based on what you are willing to live with in terms of pressure loss.

If the compressor is running at a pressure that is less than the relief valve setting, then adding a restriction between the compressor and load will increase the pressure at the compressor and decrease pressure at the load. Conversely, if the compressor feeds directly to atmosphere, then it will not develop any meaningful pressure.

Take a look around a large plant. The long air lines are apt to be 3 or 4 inches (75 -100 mm), and the shorter runs to individual machines (or groups of machines) are 25-50 mm, or even less. (All this depends upon how big the plant is, how much air the machines consume, etc., of course.)

Some questions to ask yourself: Is the pump performing correctly? Do you suspect that the pressure at the pump is inadequate? Is the compressor relief valve setting correct, and higher than the pump requirement? Is the compressor regulator set correctly (to a pressure that is higher than the pump requirement)? Does the compressor perform up to spec? Have you taken pressures at points along the path to the compressor to see where there are significant pressure drops. Have you measured the pressure and flow at the pump?