Reply to Thread

While there may not be any dark secrets behind Trane's re-inventing refrigerant piping practices, it's still a little scary to me if you consider a 100-120 ton split system with four scrolls in parallel on a direct expansion VAV system running minimum load in cold weather......and no oil traps.

Here's a link to their engineering newsletter article that discusses the "new" piping rules:
[url http://www.trane.com/commercial/libr...l274/en274.pdf[/url]

You can get away with a lot of piping shortcuts if you're willing to size suction lines at 6#/100ft pressure drop.
Trane's standard pressure drop is half that at 3#/100. Supermarket piping practice is nearly half that again at 1.5 -2#/100 ft typically.

...and if you note the section on suction line piping pitch, they want the suction lines pitched opposite the direction of flow, or back toward the evaporator so the oil won't drain to the compressor during an off cycle. That's just another thing that's contrary to what I've ever seen.

Thankyou. So it's not like it's any more special a pump or anything. They just keep the same size pipe, they lose capacity, but keep oil velocity up. I got ya.

I don't get all that mumbo jumbo. Tell me in plain english then.

can someone answer my question or not?

Originally posted by icemeister
The second thing that comes to mind is that this guy may actually have purposely called for reduced single risers on all jobs to absolutely insure a constant positive oil return, even when the the single-riser pressure drops might adversely affect system capacity at design conditions. [/B]

The more I think about this, the more it makes sense that Trane has decided that since their published performance figures and efficiencies aren't likely to be field measured, they can go with the higher pressure drop losses and insure positive oil return all the time.....thus reducing in-warranty compressor failures and the like. Maybe Trane is simply trying to make field built-up systems idiot-proof.

Even with LMTDs explanation of the ability of the scroll to take slugs, I really wonder about that. That's what Copeland said about the Discus oh so many years ago.

The multiple scrolls in parallel is an intersesting thought as well. Don't these scrolls have a smaller oil sump capacity and therefore be more susceptible to oil being trapped out in the system during long periods of low-low operation?

I dunno, just another thought.

You aint gettin what I'm laying down brother. I don't doubt the Trane idea, atleast not at this point. I want the knolwedge of why Trane think's thats good???? I am thinking to myself they know something good..

Dow,

I understand why your asking

My problem is after I pipe up the job, Trane has the contractor assisted startup.

The Trane s/u tech arrives at the site and starts questioning the piping if he sees risers.

Trane has detailed manuals on scroll units, as a contractor I will follow these manuals to cover my a??.

It's been 8-10 yrs since I have piped up a new job that had a Trane semi- they had different rules than scrolls

fatboy

Lets not just talk good here. I am asking a question.

When I first read scrolls from trane are speced out not to have it, my thoughts were along the lines that somehow they pump better or something. The point is. I really want to know, if some one knows, the reasoning behind this. Given two systems of exact same piping requirements. A system where good piping practices call for a double suction riser. The recip compressor needs it but the scroll don't. Why?

Originally posted by lwarren

Originally posted by benncool
Does Trane have a scroll compressor that unloads? Or do they have systems with multi scroll compressors on one circuit?

I have not seen one of their scrolls that unloads just multi compressors per circuit.

Well then they might have something there. Hmmm, I'll have to look into this.

Originally posted by benncool
Does Trane have a scroll compressor that unloads? Or do they have systems with multi scroll compressors on one circuit?

I have not seen one of their scrolls that unloads just multi compressors per circuit.

Does Trane have a scroll compressor that unloads? Or do they have systems with multi scroll compressors on one circuit?

Trane does not recommend traps on single or double suction risers. There statement if the suction lines are sized and installed correctly they will have adequate velocity to achieve proper oil return.

That being said, who knows. We have installed units this way per Trane that have been running several years with no problem.

I always do a layout of the piping system with line lengths, 90's, and lift heights and have the mfg wether it be Trane, Carrier or whoever design the piping. It's free and If you have problems they can't come back and say you didn't install it right.

Originally posted by Dowadudda
I just want to know the basic idea as to why? Whats special about the scroll.


I mean is it just some hotshot at trane or is there some actual science behind the idea?

This one has puzzled me as well. I've have been fortunate enough to have worked with some pretty sharp old-timers in the refrigeration system design biz and proper piping practice is just like you say, great when it's done right. But poor piping installations can cause true nightmares.

I have two thoughts on this "new" Trane riser design criteria. One is that as Dow said, there's a hotshot engineer sitting up there in his Ivory Tower who figures all that's gone before is all a lot of bunk because he knows better. I would guess he'd one of those who only design for full load conditions and the best performance at rated capacity is all he knows about.

The second thing that comes to mind is that this guy may actually have purposely called for reduced single risers on all jobs to absolutely insure a constant positive oil return, even when the the single-riser pressure drops might adversely affect system capacity at design conditions.

As I'm beginning to hear more talk of how scrolls really don't tolerate liquid or oil slugs as well as they were once proclaimed to do so well at, I'm leaning toward the my second theory on piping practice. You see, with double risers, the trapped riser trap fills with oil during prolonged periods of low loads. As the load drops further the system shuts off for a period and then comes back on at full bore......bringing back a hefty slug of oil to the compressor, directly into the scroll.....no motor housing and no crankcase like in a recip.

Just a thought,

Much of equipment that I started out on was alot of this variety.

And I always wonder, if every job went in as it is supposed to go in, piped. Many service calls would be hugely eliminated. Flow. Oil and Gas. You do it by the book and you have given your customer a fine and reliable system.

Excellent explanation Many D's. Back when I was a kid there where lots of great built up systems that used dual risers and the was my specialty.

When sizing a dual riser the horizonial line of course should handle the full load. The smaller riser should be sized for the load when the compressor is fully unloaded.

Example: If you have a 20 ton compressor that can be unloaded down to 25% (5 Ton) Then the horizonal line would be 21/8 the risers would be 11/8 and 13/8. 11/8 would be good for 5 ton and 1 3/8 would be good for 15 ton. Air conditioning.

I just want to know the basic idea as to why? Whats special about the scroll.


I mean is it just some hotshot at trane or is there some actual science behind the idea?

Thats interesting . Seems it would reduce the systems capacity by using a smaller suction riser .

Dow,

Trane has a few articles /documents on this out

Checkout their Comm website under Engineered Newsletters/Archived for

Split Systems and Interconecting Refrig Lines to read up

hey- they make the rules we just follow ( if we chose)

fatboy

Fatboy,

Thats really interesting. Can you tell us why they do not prefer the double suction riser.

all great inputs on double suction risers

just a reminder, Trane does not recommend risers/traps on their scroll cond units that unload. They recommend to downsize the riser to keep the velocity up

fatboy