Pumps vs. Compressors: Understanding the Differences and Interchangeability

[sayhaitopalani]

pumps r compressors?

a pump is used to increase the head of the fluid and a compressor is used to increase the pressure of the fluid...this pressure head is a form of energy and is transferable to other form [say increase in vertical height the fluid is raised]
so can i use a pump and a compressor interchangeably... i.e.can i use a compressor to pump a fluid and can i use a pump to compress a compressible fluid?

 

[russ_watters]

Theoretically, yes. In specific applications, no.

 

[sayhaitopalani]

why it is not possible to use it practically...what are the constraints that make this impossible?...

 

[Ivan Seeking]

Originally posted by sayhaitopalani
why it is not possible to use it practically...what are the constraints that make this impossible?...

Russ is the mechanical engineer so he can answer this best. I can add is that pumps are designed to operate at a peak efficiency along a specific curve that relates to pressure and volumetric flow. So, to use a compressor as a pump likely means less or much less efficiency than if the proper pump was chosen. To use a pump as a compressor would probably create overheating problems. Note the large cooling fins on your air compressor.

 

[russ_watters]

Originally posted by sayhaitopalani
why it is not possible to use it practically...what are the constraints that make this impossible?...

Pumps, compressors, fans, nozzles, pipes, ducts, anything that you use to move a fluid has a relatively specific range of performance characteristics, like I.S. said.

 

[sayhaitopalani]

i don't understand...can u give me a little detailed information please...

 

[Bystander]

Consider a piston/cylinder and two valves; now consider the dead volume to displacement ratio for this assembly; now consider the dead volume to diplacement ratios that can be attained in the real world.

 

[russ_watters]

Originally posted by sayhaitopalani
i don't understand...can u give me a little detailed information please...

Usuing the piston/cylinder example (aka reciprocating compressor):

The pistons in the compresor compress air and fill up a tank. A bicycle pump works exactly the same way. With a simple, inexpensive compressor you can easily get 150psi. If you need a little bit of air (5 cubic feet or so for something you might get at Home Depot) at reasonably high pressures (30-150psi), it works great. It works with a low volume of air at high pressure. But it doesn't work very well at all as a fan because it isn't capable of moving a high volume of air at low pressure.

A typical air handler (commercial air condtioner) generates a pressure of 2.5" of water or 0.09psi. Inches of water is used to make the numbers easier to work with since the pressure is so small. A room fan generates even less. So with a good air handler or fan you move hundreds or thousands of cubic feet of air per minute, but at a very small pressure. You can't use your air conditioner to pump up the tires on your car.

So the two devices, a compressor and a fan, do the same thing in a thermodynamic sense - they move and pressurize air - but as real components of real machines they are not interchangeable.

 

[Ivan Seeking]

Originally posted by sayhaitopalani
i don't understand...can u give me a little detailed information please...

I'm not sure what you're asking. Do you mean the exact design criteria that fixes the operational range of a device?

 

[Artman]

Compressors are used with gas, which can be compressed. Pumps are used with fluids, which basically are incompressable.

Direct displacement pumps are similar in design to compressors and can achieve high pressure with relatively low flow they are used for highly viscous fluids such as Heating oil.

With a compressor if the flow is stopped it continues to compress until it reaches the maximum pressure setting and shuts off. With a direct displacement pump flow is usually bypassed if the unit does not require it and relief valves are installed at the pump to keep the pressure from bursting the pump body or piping should the flow become stopped.

 

[Ivan Seeking]

Originally posted by Artman
Compressors are used with gas, which can be compressed. Pumps are used with fluids, which basically are incompressable.

Direct displacement pumps are similar in design to compressors and can achieve high pressure with relatively low flow they are used for highly viscous fluids such as Heating oil.

With a compressor if the flow is stopped it continues to compress until it reaches the maximum pressure setting and shuts off. With a direct displacement pump flow is usually bypassed if the unit does not require it and relief valves are installed at the pump to keep the pressure from bursting the pump body or piping should the flow become stopped.

Without meaning to detract from the quality of your post, I should point out that technically, gases AND liquids are fluids. On an even more extreme note, nearly all solids can be seen to display fluidic properties over very long periods of time. Like I said though, I don't mean to take away from your main points.

 

[Artman]

Oops. You are correct that gases are considered fluids (and solids also display fluid properties). I should have said liquids. Correct terminology is one of my (many) weak points.

In machinery, gas and liquid often cannot be treated interchangably. A slug of water in a steam turbine can shatter the impeller, steam forming in a centrifugal pump moving water is caviation and can damage the system.

 

[sayhaitopalani]

got the answer from Russ and Artman...thnx guys...