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.
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 .
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.