Friction of a pipe and the effects on compressed air?

In summary: The Reynolds number is 6500. The relative roughness is 0.5. So the compressor would need to generate 550 horsepower to generate the same pressure drop as found in the original conversation.
  • #1
TheDarkChomp
6
0
Let's say a receiving device that requires compressed air is 1.5 miles away from the source and demands 600 cubic feet per minute. Let's say the pipes have a diameter of four inches and already contain air at 90psi.

If friction in the pipes would be ignored, it would require the same amount of energy to supply air regardless of pipe distance correct?

SO how would one understand the properties of pipe friction on the compressed air? How would I figure out the energy required to keep pushing the air to meet the demand of 600 cubic feet per minute through 1.5 miles of piping?

Any insight would be appreciated, thanks in advance
 
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  • #2
I think what you are asking is, what is the pressure drop in the pipe. To do this problem, you need to know the viscosity of the air at the operating temperature. Do you know the operating temperature?

The first thing to do is to assume that the air is incompressible, so that its density is constant. If the pressure drop you calculate is significant compared to your 90 psi, then you need to get more serious, and to redo the calculation taking into account the compressibility. Use the ideal gas law to calculate the density of the gas. Divide the volumetric throughput rate 600 cfm by the cross sectional area of the pipe to get the velocity of the air in the pipe. Use the velocity, density, viscosity, and pipe diameter to get the Reynolds number. Use the friction factor-Reynolds number relationship to get the friction factor at the wall. Use the calculated wall friction factor, gas density, and gas velocity to get the shear stress at the wall. Use the shear stress at the wall to calculate the pressure drop.
 
  • #3
Sorry if I'm not understanding correctly I'm pretty new to this stuff

I'm trying to determine total cost to run a compressor based of the energy required to push compressed air to the receiving device plus the cost of the compressed air being used at the receiver.

The actual of the compression of the air causes the temperature to heat up a lot so air is first compressed to 60 psi, cooled, compressed to 90 psi, cooled, and sent through a dryer and out through the pipes.

(I'm ignoring the dryer because the compressors are being replaced but not the dryers and to simplify the problem slightly)
Chestermiller said:
To do this problem, you need to know the viscosity of the air at the operating temperature. Do you know the operating temperature?

When the air is first sent through the pipes it is 70 degrees Fahrenheit (dynamic viscosity would be 0.018424 I think) How would this allow me to calculate pressure drop? Would I need the temperature at the end of the pipe also?

Chestermiller said:
The first thing to do is to assume that the air is incompressible, so that its density is constant. If the pressure drop you calculate is significant compared to your 90 psi, then you need to get more serious,
 
  • #4
TheDarkChomp said:
Sorry if I'm not understanding correctly I'm pretty new to this stuff

I'm trying to determine total cost to run a compressor based of the energy required to push compressed air to the receiving device plus the cost of the compressed air being used at the receiver.

The actual of the compression of the air causes the temperature to heat up a lot so air is first compressed to 60 psi, cooled, compressed to 90 psi, cooled, and sent through a dryer and out through the pipes.

(I'm ignoring the dryer because the compressors are being replaced but not the dryers and to simplify the problem slightly)


When the air is first sent through the pipes it is 70 degrees Fahrenheit (dynamic viscosity would be 0.018424 I think) How would this allow me to calculate pressure drop? Would I need the temperature at the end of the pipe also?

You need the viscosity to calculate the Reynolds number. Is the temperature constant at about 70F over the entire length of the pipe, or does the surrounding environment heat or cool the air from one end of the pipe to the other? If significant heating and cooling are involved, then you also have a coupled heat transfer problem that you are dealing with.
 
  • #5
Ok I think I've calculated everything...

Would converting this pressure drop to pounds per square foot then using that in
(pounds per square foot * cfpm)/33000 = theoretical horsepower allow me to quantify the energy needed to compensate for this drop?
 
  • #6
Disregard that last post...

I could convert the pressure drop to volume drop, and using the efficiency of the compressor find out how much energy it takes to generate that much volume correct?
 
  • #7
What pressure drop did you calculate?
 
  • #8
I found Reynolds number and relative roughness then used the moody diagram to find the friction factor, but not sure how to get to pressure drop from there. Why is shear stress needed?

(Have velocity, distance, diameter, viscosity, flow rate also)
 
  • #9
The shear stress is not required. You already have a friction factor from the Moody Diagram.
 
  • #10
Ok found the formula Δp = (v² × f × L × ρ)/2D

With the pressure drop how would I convert to volume drop or energy required to accomidate for that pressure drop?

(I tried ideal gas law but that definitely doesn't work)
 
  • #11
I think you're missing a factor of 4 in the equation. What number of psi do you calculate for the pressure drop?
 
  • #12
Sorry I missed this thread before, but the discussion is waaaaay too complicated. Engineers don't figure this stuff out from scratch every time they design a system, they use tables and/or specialized calculators or slide rules. Just Google "compressed air pipe friction loss" and pick one.

For the energy, select an air compressor and see how much power it uses at the different pressures.
 
  • #13
It's good to understand what's going on and what factors come into play in these things, but I'm with Russ on this one. Pressure losses in pipelines is a pretty well established science.

Here's a calculator on Eng. Toolbox
 
  • #14
Travis_King said:
It's good to understand what's going on and what factors come into play in these things, but I'm with Russ on this one. Pressure losses in pipelines is a pretty well established science.

Here's a calculator on Eng. Toolbox

This is a great link Travis_King. So what do you get for the pressure drop when you plug the OPs numbers in? I assumed 600 scfm flow rate and a pipe ID of 1 ft, and, using the link, got a pressure drop of <1psi. The OP didn't tell us the pipe ID. What do you get. If the pressure drop truly is < 1psi, then this is what I was initially driving at when I speculated that it might be possible to treat the gas as incompressible. Obviously, if the diameter is a lot less than 1 ft, the pressure drop is going to be higher.

Chet
 
  • #15
Dia was four inches.
 
  • #16
Yea, Engineering Toolbox is great. And yep, OP said ID was 4 inches, it results in an approximate pressure drop in the viscinity of 5.4 psi. This of course doesn't include the losses from any valves, elbows, tees, flanges, etc.

Compressibility isn't really a huge issue here anyway. The difference in the density of air (at otherwise standard conditions) between 95 psi and 90 psi is only something like .026 lb per cubic foot. It's not going to alter the pressure drop too much, and since these calculations are only good as close approximations anyway, it most likely falls within the margin of error.

Edit:
The one issue I see is that the the airflow the calculator (indeed, most calculations) uses is the airflow at atmospheric conditions. So it really depends on whether the OP meant 600 ACFM or SCFM.
If the OP is expecting 600 ACFM (at the 90 psi condition), then that means that the actual, standard conditions, flow is in the realm of 3600 SCFM, making the pressure drop more significant, probably prohibitively so in a 4" ID pipe.
 
Last edited:
  • #17
Travis_King said:
Yea, Engineering Toolbox is great. And yep, OP said ID was 4 inches, it results in an approximate pressure drop in the viscinity of 5.4 psi. This of course doesn't include the losses from any valves, elbows, tees, flanges, etc.

Compressibility isn't really a huge issue here anyway. The difference in the density of air (at otherwise standard conditions) between 95 psi and 90 psi is only something like .026 lb per cubic foot. It's not going to alter the pressure drop too much, and since these calculations are only good as close approximations anyway, it most likely falls within the margin of error.

Edit:
The one issue I see is that the the airflow the calculator (indeed, most calculations) uses is the airflow at atmospheric conditions. So it really depends on whether the OP meant 600 ACFM or SCFM.
If the OP is expecting 600 ACFM (at the 90 psi condition), then that means that the actual, standard conditions, flow is in the realm of 3600 SCFM, making the pressure drop more significant, probably prohibitively so in a 4" ID pipe.

Nice analysis. The OP never told us whether the 600 cfm is scfm or acfm. However, for pipelines, throughput rates are usually reported as scfm. That means that your 5 psi would be about right. You got to figure also that the pipe diameter would have been specified such that not much pressure loss would ensue from the compressor to the delivery point. This would be consistent with 600 scfm rather than 600 acfm (as you alluded to).
 

FAQ: Friction of a pipe and the effects on compressed air?

1. What is friction of a pipe?

Friction of a pipe refers to the resistance or force that opposes the flow of air through a pipe. It is caused by the interaction between the air molecules and the surface of the pipe.

2. How does friction affect the flow of compressed air?

Friction in a pipe can cause a decrease in the flow rate of compressed air, as it requires more force and energy to overcome the resistance and maintain the same velocity. This can result in a decrease in the efficiency of the system and can also lead to pressure drops.

3. What factors influence friction in a pipe?

The surface roughness, diameter and length of the pipe, as well as the viscosity and density of the air, can all affect the amount of friction in a pipe. Additionally, the velocity of the air and the presence of any bends, fittings or obstructions in the pipe can also contribute to friction.

4. How can friction in a pipe be reduced?

Friction in a pipe can be reduced by using pipes with smooth surfaces, increasing the diameter of the pipe, and limiting the length and number of bends and obstructions in the system. Proper maintenance and cleaning of the pipes can also help to reduce friction and maintain efficient flow.

5. What are the consequences of high friction in a compressed air system?

High friction in a compressed air system can result in decreased efficiency, increased energy consumption, and higher operating costs. It can also lead to pressure drops, which can affect the performance of equipment and cause malfunctions. In extreme cases, high friction can even lead to damage to the pipes and other components of the system.

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