Suction riser

[hb]

What is the purpose of a double suction riser on a split system?

[The Penguin]

increased velocity = oil return back to the compressor

[Dowadudda]

If piped properly.

You will see double suction risers on systems that often don't run at full compressor capacity. Capacity reduction like unloading. One thing to remember, most built up HVAC or Refrigeration systems are often designed for peak load. But most systems only run at peak load at around 20% OF THE TIME, ALTHOUGH IT DEPENDS ON THE DESIGN.

During the 80% of time when it is not required to run full out, we reduce suction capacities, we reduce or elevate head pressures. We do a lot of things to compensate for this system we got that for the most part is too big 80% of the time.

In a system that has the ability for this to take place such as the case is for compressor unloading, the compressor begins to run at less than full capacity. That means we have less velocity. One thing always rings true. During operation a compressor will pump refrigerant but also oil. It's always discharging a little oil. Since there is only so much oil in a system, if it leaves the pump, we got it get it back some how. At reduced suction gas velocity due to a compressor at part load, the velocity of the gas will not on it's own bring that oil back. So what we do is we trap exiting the evaporator. The oil will collect and pool in the trap. Once there is enough there, and the suction needs to get out of that coil, the suction gas will naturally begin to pick up oil,
push oil along with the suction gas. The double riser, that can get real interesting.

We need to do the trapping obviously, but that second reduced pipe riser, it's allowing gas to get back but it is also causing resistance, (increased velocity the result) so that oil in that trap will eventuall be overcome by that suction pressure behind it pushing it back to the compressor.

If there were no capacity reduction, the piping and the system was designed to operate at a certain amount of suction velocity that there is no need for treatment of oil return. There is such adequate velocity, that suction is just simply pushing, dragging that oil back. The oil, inside that pipe has no choice with the amount of velocity, but to be pushed and dragged by the suction gas.

Kapish??

[condenseddave]

Either that or the guy had a whole bunch of small pipe and fittings, and no big pipe, and was too cheap to go buy big pipe.

[hvacfella]

Dowa gave a very good explanation on DSR's.


Ill just add one comment though :

Dowa :'If there were no capacity reduction, the piping and the system was designed to operate at a certain amount of suction velocity that there is no need for treatment of oil return. There is such adequate velocity, that suction is just simply pushing, dragging that oil back. The oil, inside that pipe has no choice with the amount of velocity, but to be pushed and dragged by the suction gas.
'

ME: Even with no capacity reduction, on long suction vertical piping...its still important to use p traps every so many feet in order to be sure the oil is entrained back to the compressor .

[Dowadudda]

Good catch. I was talking like a dinky system where these sorts of things don't mean bo diddly. But good point.

I'd love to add this. I only ever deal with built up systems, be it larger splits, or packaged stuff in the 100 ton VAV range. I see alot of incorrect piping practice.

[hvacfella]

Although im nearly all residential, i see alot of bad piping practices too; alot of undersized piping, kinks, bad flaring, bad soldering, no driers, not pitched, no p traps, and unsupported piping. Ive spent time on the 'big stuff' when i did pneumatic temperature controls -- my favorite was with malfunctioning static pressure sensor 2/3 rds of the way downstream of a 100 h.p supply fan ...than had the vortex damper jammed wide open creating a very dangerous suction on the entrance door to the airhandler. Good thing nobody turned the handle to the door.

[R12rules]

D4, you did a great job of articulating what I lack words to describe.
Thank you.

Had I knew what you were gonna say before you wrote that fine piece ... I would have done so myself. And in the process of this endeavor ... I may have set a new site record for length of post.

[fatboy]

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

[Dowadudda]

Fatboy,

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

[fatboy]

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

[hvacfella]

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

[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?

[Green Mountain]

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.

[Dowadudda]

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.

[icemeister]

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,

[lwarren]

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.

[Green Mountain]

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

[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.

[Green Mountain]

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.