What is the purpose of a double suction riser on a split system?
increased velocity = oil return back to the compressor
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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.
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.
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 .
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.
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.
D4, you did a great job of articulating what I lack words to describe.
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.
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
Thats really interesting. Can you tell us why they do not prefer the double suction riser.
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)
Thats interesting . Seems it would reduce the systems capacity by using a smaller suction riser .
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?