Reciprocating Compressors 3

[Jyotish123]

Hi All,

It is a great forum to discuss engineering. I have a question related to reciprocating compressors, as in can the compressors be designed without piston rod for example in an IC engine there is no piston rod. Any thoughts on this, please share.

Best Regards,
Jyotish

 

[TugboatEng]

The need for a piston rod depends on the ratio of bore to stroke. If the bore is small and the stroke is long, the connecting rod will contact the cylinder.

 

[LittleInch]

Yes they can.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MintJulep]

Engine without piston connecting rods. Do you mean this?

https://www.thetruthaboutcars.com/2013/07/the-new-waissi-engine-pistons-but-no-connecting-rods/

https://www.hagerty.com/media/maint...ing-a-new-ice-in-an-ev-age-the-waissi-engine/

 

[TugboatEng]

OP is referring to a trunk piston vs cross-head arrangement.

 

[BEMPE16524]

yes and you need to consider few things in your design i.e. input/output power, size (space constraint), material, heat, cost, etc...



R.Efendy

 

[BrianPetersen]

The innards of every reciprocating air compressor that I've seen, look like the innards of a normal piston engine except with a simpler cylinder head (check valves). Single acting piston with piston rings, a normal piston pin across the piston connecting the piston to a normal-looking connecting rod to a normal-looking crankshaft.

I've never seen a reciprocating air compressor that used a "piston rod" (presumably this means a "piston rod" as a normal air cylinder would use - i.e. on-axis with the travel of the piston but of much smaller diameter and passing through a linear seal such that the bottom of the piston forms another air chamber, i.e. potentially a double-acting piston). If the original poster means something else by using this term, please advise.

I see no sense in using double-acting cylinders in this application. It's easier (and smaller and lighter) to just use two single-acting cylinders. Or one bigger single-acting cylinder.

There's no sense in using insanely long stroke relative to the bore such that it would warrant using a cross-head. Why? Just make the bore bigger and the stroke shorter so that you don't need the cross-head! As your bonus, the whole thing will now be smaller and lighter.

 

[TugboatEng]

Crosshead type compressors are ubiquitous in natural gas compression applications because the piston rod allows for a secondary seal beyond just piston rings to keep flammable gasses out of the crankcase.

You'll also see crosshead arrangements in high volume high pressure applications where the pressure limits the bore and the volume requires a long stroke.

Do note that the crosshead arrangement also exists in diesel engines. The slow speed diesel is designed for very high BMEP and expansion ratios so it has a very long stroke with a relatively small bore.

 

[MintJulep]

For anyone else like me who doesn't know these terms.

 

[Jyotish123]

I am asking this from the point of view of horizontally balanced opposed industrial reciprocating compressors, that are manufactured worldwide. Can they be re-designed without piston rod?

 

[BrianPetersen]

How about specifically identifying the application?

What's it compressing, what's the inlet and outlet pressure, what's the volume flow rate, give us a specific application?

My post referenced air compressors because that's what I've seen. Tugboat identified natural gas compressors and compressors for very high operating pressures which are different for very good reasons and explained why.

Chances are, the piece of equipment that you are looking at, was designed that way for good reason, but in order to establish that, you're going to have to provide more information.

 

[BrianPetersen]

Also note that your original question was basically "Can a compressor be designed without a crosshead" and given that the 2 horsepower 120 psi air compressor in my shop does not have one, the answer is "Yes".

You did not ask whether any and all compressors regardless of application no matter the size nor working fluid can be designed that way!

 

[Doug Hunter]

Yes, theoretically. You need to consider:
1. How will the cam (or other forces) move the piston (in both directions)?
2. Will the mechanism produce the correct function of piston position vs. shaft rotation that you want?

 

[Jyotish123]

I'm talking about reciprocating compressors used in oil and gas industry. For example, the Inlet presssure is 1 kg/cm2. Suction Temperature is 60 degrees.
Discharge pressure : 8 kg/cm2 . Discharge temperature : 111 degrees
Application : LP Gas

 

[BrianPetersen]

... and what mass or volume flow rate or horsepower? And how long is it expected to remain in service?

Your inlet pressure in proper SI units is about 1 bar (1 atmosphere) and the discharge pressure around 8 bar ... nothing special. My 2 horsepower shop air compressor (ordinary piston and crank, no crosshead) will do that pressure ratio. But, it isn't designed for handling flammable/explosive gas, and will probably explode. And it won't survive in continuous service for decades.

These details matter, and will strongly affect the design of the equipment.

"Can" something be done a certain way is a very different question from "should" something be done a certain way.

 

[Orange_kun]

I think TugBoatEng already explained this one. Sounds like a seal longevity and fluid compatibility/explosion constraint that really benefits from the additional piston rod seal.