250' Condensing unit piping

[nuhvac]

I had a rep from Carrier tell me today that if he used his condensing unit with Semi's that he could pipe 250' of refridgerant no problem. He doesnt know that I have a Carrier book as well, and all he is using is a chart that has tonange vs. length of line on a logrithmic scale. This scale goes up to 500'! I think we are going to have serious problems getting oil back to the compressors. Does anyone have any experience with refridgerant piping this long? How did it work out?

Thanks,

CB

 

[TwinScrew]

Which unit are you referring to?

From what you've already said, it appears to be at least a 10-Ton C/U (since that is the smallest C/U Carrier still offers with Semi-Hermetic's).

Regarding Piping Sizing: Are you referencing Chapter 3 of the System Design Manual?

I have done long lines applications before, but not with units that large. I have used 38HDR's with no issues yet... making sure to include a LLSV, Crankcase Heater, Hard Start Kit, and hard shut-off TXV at the AHU/FCU.

I would give the attached PDF a look, and would also check out Chapter 3 (in case that is not what you're referring to).

Hopefully someone else chimes in with experience on the larger units. I typically stress that other avenues where the length can be reduced be pursued instead (if possible).

 

[nuhvac]

Units are 40 ton units. Thank you for the link. That helps some. If nothing else it gives me some ammo since the rep gave me a condensing temp of 105F when the ambient temp is 105F, I think we all know that isnt going to happen.

CB
EngA

 

[nuhvac]

Thanks TwinScrew, this was ALOT of help. I did have two more questions though. How old is this letter? Where does this 4lbs of refridgerant change per ton of the unit come from? If you know those answers that would be great if not I can work without them.

CB
EngA

 

[TwinScrew]

That letter was from November, 1998.

You'll notice within the unit's Product Data that the 'typical refrig. charge' is about 62 lbs with 'standard' line lengths for that machine (38AH044). (See it here:

http://www.docs.hvacpartners.com/idc/groups/public/documents/techlit/38ah-11pd.pdf

Check out page 123- for line sizing. You will notice there is no data for over 200+ linear ft.

Are you able to reduce the line length in this application? I would seriously look at all other options if you haven't already...

Re: the 4lbs/ton guideline... I'm not entirely sure where that is derived from... I will see what I can find out, sorry.

Are you in AB/BC?

 

[Waramanga]

oil return is not neccessarily a problem on such long line lengths. the unit most likely has oil recovery function like vrf units have. the new fujitsu units have continuos operation during oil recovery, just dont ask me how that works.

 

[engair]

Well in general oil return on long line lengths IS a problem, thats why companies dont like to do them. I am very interested in this oil recovery function though. The only way I can imagine that working is for the unit to turn off.

 

[hoorain]

I think another thing to worry about apart from oil return is the pressure drop that would certainly occur with the increased lengths accompanied by dropped performance and most probably having to adjust the charge aswell thus increasing load on the compresor.

 

[engair]

Adjusting the charge in the line is standard for split systems. Dropped performance only occurs in the sense of line loss in temperature. The real problem with it is the migration of oil in the refridg because it will go to the coldest point, and you can slug a compressor.

 

[hoorain]

thanks for correcting me as i understand from my book migration means the refrigerant migrating to compressor e.g.during the off cycle can you please explain why it would do so if the line lengths are increased thanks

 

[engair]

not the refridg, its the oil in the refridg. There is mineral (and POE depending on the refridg) in the refridg. The oil will always try to migrate towards the coldest point. The longer the line length the harder it is for the compressor to pull the oil back to the compressor. If it is excessivly long sometimes the compressor is unable to pull the oil back until enough of it has accumulated. This makes the compressor run with out lubrication for a period of time, and causes a large slug of oil to hit the compressor all at once.

 

[hoorain]

I see thanks so thats when they use oil accumilators to prevent that happening