Compressor valve test

[RackMechanic]

I have an ongoing arguement with my boss, and I can't think of a simple way to prove myself. I know some of you guys can help me out and/or correct me if I'm wrong.

I say pumping down a reciprocating compressor to a vacuum, then shutting it off to see if it holds only confirms that one valve is good. He says; " if it holds, both the suction and discharge valves are good.
My understanding is this, A weak suction valve, or one with a small peice missing, may still have enough surface area to achieve a partial vacuum, but would bleed through. That, I agree is definately a bad valve. On the other hand if the suction valve was in perfect shape, it would pull and hold. Right?
But what if a small peice of the discharge valve is broken. The compressor would still hold the vacuum, but the dicharge valve is broken, therefore, his statement that the pupm down and hold vacuum test confirms that BOTH valves are good is wrong.

Can someone please help me to understand this if I am wrong, or help me to explain it to him if I'm right.

The compressor we are discussing is a standard recip, hermetic, residential 2 - 5 ton.

thanks for the help,

Terry from Fort Lauderdale

[Grizzly]

Personally I can't see how you would pump down into a vacuum. With a Discharge valve gone the non return effect. Ie Compressed gasses being expelled would flow straight back into the cylinder.
1) preventing the piston from drawing (suction) gasses from the crankcase.
2) Head Temperature would increase dramatically as compressed gases are recompressed and recompressed.
Basically your Boss is correct.
There is a earlier post somewhere that discusses this at length, but can't remeber where?
Grizzlyhttp://www.refrigeration-engineer.com/forums/images/icons/icon14.gif

[frank]

Terry

I had a problem with a Carlysle recip a few months ago

Some of the info here may enlighten you http://www.refrigeration-engineer.com/forums/showthread.php?p=50153#post50153

[absolute-zero]

I know this as to how it is applied in recips in the transport units.

If the compressor is unable to pull vacuum via compressor pump down, this would indicate that either suction reeds are not holding maybe cracked or nott seating properly, also may be too much piston to cylinder wall clearance.

If only one suction reed valve is bad the compressor can still use the remainder cyliders to pump down, the indication here would be slow pumpdown.

If you are able to pull 15 hg vacuum but soon as you stop the compressor and the suction gas rises in the lowside to equalized pressures whether it is slow or fast, this would indicate more of a discharge valve plate not holding.

This takes a little expeirence of watching pressures and timing the pull down cycles. compared to a normal opertaing system.

I have had many compressors with good capacity, pumpdown well and hold the vaccum, with cracked suction reed valves.

Here is where it gets a little more interesting, does this compressor utilize Hotgas unloaders or suction unloaders intagrated in to the compressor. if so you may have one of these unloader valves hanging up causing poor pump down issues.

There are variable factors when considering compressor pump down tests.

I do have an animation of a recip compressor with piston suction reeds and discharge valveplates and hot it works in slow motion.

Perhaps i will figure out a way to upload it here.

A-Z

[nike123]

I do have an animation of a recip compressor with piston suction reeds and discharge valveplates and hot it works in slow motion.

Perhaps i will figure out a way to upload it here.

A-Z

Just open account here:
http://www.mediafire.com/
and then upload file there, and after that, post link to that file here in forum. That is my way for similar purposes.

[US Iceman]

If there is capacity control on the compressor it has to be fully loaded, then close the suction service valve and allow the compressor to pull a vacuum. If it takes too long (more than a few minutes) then you either have weak valves or leaking piston rings.

Shut the compressor off. If the suction pressure does not rise , then the discharge valves are OK. If the suction pressure rises, the discharge valves are leaking.

It could also be an internal relief valve too (if one exists).

[absolute-zero]

If there is capacity control on the compressor it has to be fully loaded, then close the suction service valve and allow the compressor to pull a vacuum. If it takes too long (more than a few minutes) then you either have weak valves or leaking piston rings.

Shut the compressor off. If the suction pressure does not rise , then the discharge valves are OK. If the suction pressure rises, the discharge valves are leaking.

It could also be an internal relief valve too (if one exists).

Sorry but i feel i must demonstarte my ignorance again, But did you say piston rings? I have never heard of piston rings in a recip compressor.

Please educate me here,,, in what refrigerant compressors have Piston rings?:confused:

[Grizzly]

Basically any recipricating (up & down) compressor has piston rings!
They provide a gas tight seal between the moving piston and cylinder wall.
Allowing the gas tight space between the cylinder head and piston crown. To "Compress" the refrigerant gases.
Therefore if the rings pass by the high side pressure will drop to the lower pressure (suction) in the crankcase.
I will see what info I can dig up for you as some knowledge in this area is fundermental!
Grizzly

[absolute-zero]

Basically any recipricating (up & down) compressor has piston rings!
They provide a gas tight seal between the moving piston and cylinder wall.
Allowing the gas tight space between the cylinder head and piston crown. To "Compress" the refrigerant gases.
Therefore if the rings pass by the high side pressure will drop to the lower pressure (suction) in the crankcase.
I will see what info I can dig up for you as some knowledge in this area is fundermental!
Grizzly

We do not have or use piston rings in our compressors in TK or Carrier units, just very tight or close piston to cylinder wall clearnce 1-3 thousands TIGHT.

I have never seen compression/oil rings in anything other than a recip internal combustion engine typically found in mowers, automobiles, trucks, and even the diesel engines that run to turn the compressors in transport refrigeration units.

that is very cool, i would like to see this information on this.

Is there compressors out there with camshafts, along with a automotive type 3 angle intake or exhaust valve type designs?

Thanks Griz.

[monkey spanners]

I think it might depend on the size of the compressor, none of the small copeland/prestcold compressors (3/4 to 4hp) ive stripped have had rings, just a few grooves machined around the piston. Also the pistons apeared to be made of steel or iron as opposed to aluminium so presumably they would expand at the same rate as the block and maintain the piston to bore clearance.

I was once doing a pump test on a 3hp 2 cylinder prestcold which pulled down to a vacuum ok and then all hell let loose. Turns out it had snapped a rod and once the pressure in the crankcase dropped below that above the piston and snapped conrod, the piston was pushed down onto the crank, made one hell of a noise. I just run for cover:D and after it hadn't blown up i went back and turned it off:o

Jon

[absolute-zero]

:eek: Its funny what noises and or destruction you can hear a compressor do when you take away the refrigerant to it.

I have seen this many time in the field where a compressor sounds ok as No abnormal sounds, but as soon as you pull the compressor int o a vacuum the noise starts and then gets steadily louder.

I was charging a carrier O5G compressor via liquid phase R-404a through the compressor suction service valve to finish charge top off.

I did everything by the book, metered the charge in, and then out of know where, I heard the engine ramp down to lowere rpms, almost to a stall, Then I heard a horrific sudden noise from the compressor then the enginge piocked back up to full rpm, and my gauge pressures were almost erqualized still running.:eek:

sometimes when the time is up for these components there time is up.

I always do simulation of high ambient temp conditions to simulate a normal unit load conditions, I would rather have the component fail while it is in my care then to see it go back to the customer, get loaded with expensive product to only fail then.:rolleyes:


A-Z

[absolute-zero]

Just open account here:
http://www.mediafire.com/
and then upload file there, and after that, post link to that file here in forum. That is my way for similar purposes.


I found that piston demo for tk recip compressors, I will email link to whom PMs me.

A-Z