What is the relationship between airflow and pressure in a compressor?

In summary: Thank you!A decrease in mass flow rate results in a higher pressure due to the increased energy applied to the flow.
  • #1
sam_nich
4
0
Does this below seem correct?

Considerations:
1) Airflow/Delivery Pressure chracteristic
2) Speed increased/decreased

Firstly, having drawn a graph of mass flow of air vs. pressure, I have noticed that there is a strong negative correlation. Hence, therefore, the greater mass flow of air passing through the compressor each second, the lower discharge pressure is acheived.

Secondly, when considering the speed of the compressor, an increase in speed will increase the mass flow of air per second, which will in turn decrease the delivery pressure?

Thanks!
 
Engineering news on Phys.org
  • #2
Here are some compressor performance curves -

http://www.burtoncorblin.com/BCTB202.pdf [Broken]
 
Last edited by a moderator:
  • #3
sam_nich said:
Does this below seem correct?

Considerations:
1) Airflow/Delivery Pressure chracteristic
2) Speed increased/decreased

Firstly, having drawn a graph of mass flow of air vs. pressure, I have noticed that there is a strong negative correlation. Hence, therefore, the greater mass flow of air passing through the compressor each second, the lower discharge pressure is acheived.

Secondly, when considering the speed of the compressor, an increase in speed will increase the mass flow of air per second, which will in turn decrease the delivery pressure?

Thanks!
Both statements are true (assuming a constant suction/inlet pressure).
 
  • #4
Hi there:

Check out eFunda.com at http://www.efunda.com where you can find some useful online caculators and information on the subject matter.

Your observations are pretty much correct.

Thanks,

Gordan
 
  • #5
Can someone help to explain why when the mass flow is decreased a higher pressure is achieved?

Thanks.
 
  • #6
First, what kind of compressor are you talking about?

Second, when you reference your mass flow vs. pressure, where is the pressure you are referring? Inlet, outlet...?
 
  • #7
Sorry...for the lack of detail.

I am referring to a Rotary Vane Compressor and the delivery pressure (is this the exit pressure by the way?).

Thanks.
 
  • #8
Yes this is confusing me too. I would also be very grateful if someone could explain why a lower mass flow rate gives a higher pressure rise (in a centrifugal compressor).

This is the way I see things: Total pressure rise in a centrifugal compressor is achieved by adding energy to the flow, by giving the initially (nearly) axial flow a tangential component. Once the flow has reached the tip blade (at the exit), it has received as much energy as it can, and now has the maximum possible total pressure. If you decrease the flow rate, the total pressure rise is still limited by this blade speed, which hasn't changed, so surely this can't be the physical mechanism at work? I then think about how losses. Could it be that at high flow rates, the air exit velocity achieves a smaller fraction of the blade tip speed (i.e. low slip factor) than at low flow rates, hence giving a lower pressure rise? This still doesn't seem likely though because if I look at exprimental results for low rotational speeds, and varying flow rates, the same trend is observable... that lower mass flow rates give higher pressure rises.

Could someone please tell me what physical mechanism is at work here?

Thank you very much!
 
  • #9
If anyone is interested here is my solution (right or wrong):

For a compressor, the volumetric flow rate is proportional to the impeller rotational speed: Q = kN. For low flow rates at a given engine speed, the gradient of the line, k is small.

For a simplified analysis of a fan, the change in total pressure (Delta P0) = rho * U^2 (where rho is air density and U is blade tip speed).

U is a linear function of N. Therefore Delta P0 = constant x N^2.

Take the initial relationship Q = kN. Therefore N = Q/k.

Substituting into Delta P0 = constant x N^2 gives:

Delta P0 = constant x (Q/k)^2.

Recall from earlier that low flow rates have a low k value. From the above equation, low k values give higher changes in total pressure, which answers mathematically (I think) my question.

I still don't have a physical explanation for this though so I can really understand it.
 

1. What is the relationship between airflow and pressure?

The relationship between airflow and pressure is referred to as the Bernoulli's principle. According to this principle, as the speed of airflow increases, the pressure decreases and vice versa. This means that when there is a high airflow rate, there will be low pressure and when there is low airflow rate, there will be high pressure. This relationship is crucial in understanding the behavior of fluids and gases in various systems.

2. How does airflow affect pressure in a closed system?

In a closed system, such as a pipe or duct, airflow has a direct impact on pressure. When there is a high airflow rate, the pressure decreases and when there is a low airflow rate, the pressure increases. This is because as the air moves faster, it creates a low-pressure area, causing the air to move from high-pressure areas to low-pressure areas. This is why fans and blowers are used to increase airflow and decrease pressure in closed systems.

3. What factors influence the airflow/pressure relationship?

Several factors can influence the airflow/pressure relationship, including the velocity, density, and viscosity of the fluid or gas, the shape and size of the duct or pipe, and the presence of obstacles or restrictions in the system. These factors can affect the speed and direction of airflow, ultimately impacting the pressure within the system.

4. How is the airflow/pressure relationship used in HVAC systems?

The airflow/pressure relationship is crucial in HVAC (heating, ventilation, and air conditioning) systems. By controlling the airflow rate and pressure, HVAC systems can ensure proper ventilation, temperature regulation, and air quality in buildings. For example, by adjusting the speed of fans and blowers, HVAC systems can increase or decrease the airflow rate and pressure to maintain a comfortable and healthy indoor environment.

5. What are some practical applications of the airflow/pressure relationship?

The airflow/pressure relationship has several practical applications in different fields. In addition to HVAC systems, it is used in aerodynamics for designing aircraft and automobiles, in fluid dynamics for predicting the behavior of fluids in industrial processes, and in wind tunnels for testing the aerodynamic properties of various objects. It is also crucial in understanding the respiratory system and how air moves in and out of the lungs during breathing.

Similar threads

  • Mechanical Engineering
Replies
3
Views
2K
  • Mechanical Engineering
Replies
5
Views
1K
  • Mechanical Engineering
Replies
0
Views
311
Replies
1
Views
1K
  • Mechanical Engineering
Replies
8
Views
2K
  • Mechanical Engineering
Replies
15
Views
651
  • Mechanical Engineering
Replies
6
Views
299
  • Mechanical Engineering
Replies
4
Views
2K
Replies
12
Views
2K
Replies
8
Views
3K
Back
Top