6 replies to this topic

[Rahimzadeh]

What is best type protection of screw compressor againest low flow?

[fallah]

Mr. Rahimzadeh,

 

Recycle control is normally used to manage flow changes, also low flow, in mentioned positive displacement compressors...

[Zauberberg]

Screws also work quite well in variable speed arrangements - turndowns to 20% are possible. Below the minimum/turndown capacity, some sort of recycle flow control would be required, as noted by Fallah.

[Bodhisatya]

Dear Zauberberg:

 

I am in the process of finalizing a VFD scheme for a screw  Air compressor, while it is understandable for Centrifugal varying speed gives different head & flow, I can't figure out how it figures out a positive displacement machine like a screw compressor.  Screw Compressors are constant volume and variable pressure machines, how do VFD results in less flow via those screws.  A detailed explanation or sharing a literature on the same would be highly appreciated.

 

Bodhisatya 

[Zauberberg]

https://www.bitzer.d.../sv-0402-gb.pdf

http://www09.abb.com...e/18-23m184.pdf

http://docs.lib.purd...47&context=icec

http://www.petroassi...Compressors.pdf

[Art Montemayor]

Bodhisatya:

 

Everything Zauber has stated is correct.  Nothing he has written points or indicates that a screw compressor is a “constant volume and variable pressure machine”.  Please identify what you mean by “VFD”.  Do you mean "Variable Frequency Device"?

 

I believe you are confusing yourself because you don’t have a complete grasp or understanding of how the positive displacement machines - particularly the screw type - operate.  No compressor - whether it be a centrifugal or a positive displacement type - deliver a “pressure”.  All compressors respond to the downstream pressure resistance imposed on their discharge with volumetric delivery capacities.  They do not “generate” a discharge pressure; they respond to the discharge resistance.  The positive displacement types deliver what is essentially a constant mass flow rate of gas.  The centrifugals deliver a variable amount of gas - depending on the total head imposed on them.

 

What Zauber has stated is that you can vary the capacity of a screw compressor by varying its speed - down to a certain minimum rpm.  Below that minimum, you have to resort to a recycle valve that recirculates a portion of the discharge back to the suction of the machine (a power-inefficient method).  This is 100% correct.  However, there is another way to vary a screw’s capacity: use a built-in slide valve device that reduces the displacement of the screws.  This device also has its minimum limitations and can vary from manufacturer to manufacturer.

 

Refer to the attached literature.

 

 Rotary Screw Compression Process.pdf   122.52KB   83 downloads

[gegio1960]

bodhi,

you can imagine the screw compressor like something that, while rotating, traps a volume of gas.

the initial volume progressively reduces during the rotation and this fact causes the compression of the trapped material.

at the end of each rotation the material is delivered to the outlet.

you can easily understand that a decreased velocity, ie less cycles per unit of time, will result in the delivery of less material, ie reduced flow.

you can also think the slide valve as a device that reduces the trapped volume.

this is only an extreme simplifaction; of course, the words and the papers of art and zaub, explains these facts in a professional way :-)

good luck!

Edited by gegio1960, 27 May 2015 - 02:12 AM.