what are suction risers. what are their use for

well if you are referring to refrigeration suction lines used as risers then they would be vertical pipes used to return the refrigerant and oil back to the compressor, some times when a compressor is located higher then the evaporator, returning oil is as important as returning the refrigerant thus the pipes are sized to maintain velocity while also trapping the oil at the base of the riser, this type of design uses two pipes in the rise(one smaller then the other to maintain velocity) Once the trap fills with oil, the higher velocity pulls it up the pipe to the compressor. Hope this is what you were referring to. Mike

Check with your equipment manufacture, many of them no longer approve suction line traps. Slope your suction line towards the condensing unit and depending on how high the vertical pipe is and refrigerant being used and the application, they may recommend decreasing the vertical line to a smaller size to keep the velocity higher. If you have two compressors operating with one common suction line make sure you size it correctly. Lack of oil is a compressor killer.....

the double riser on the suction line is a standard in the industry for more then fifty years , the smaller line is untrapped while the other line is the same size and trapped, by trapping the larger line the oil will fill the trap causing the refrigerant to go thru the smaller line thus increasing the velocity and then start pulling the oil from the trap , only one pipe is smaller and the other pipe that is trapped is the same size as the rest of the suction line.
The use of oil traps in certain installations are crucial to the longevity of the compressor, without the use of traps the oil would have a hard time getting back to the compressor, in larger commercial spilt systems using DX coils you will find a diagram showing the proper installation of oil traps in the installation instructions.
What are suction risers ? and what are they used for ? yes i think that was the question and i hope i have answered it .

Trane, Carrier and York no longer approve P-traps on suction lines. I just went thru this with a new Trane 70 ton split unit. I always used double risers with P-traps for years but, it's no longer approved and will void the warranty with Trane. Decreasing the riser size will increase the velocity to provide sufficient oil return without trapping oil. Most old timers like myself, added extra oil to allow for what the traps will hold, unfortunately now there's not much pride or interest in doing the job correctly.

That is not so in regards to oil traps ! they are still used and recomended by manufacturers, reducing the size of a suction line to increase velocity is a poor way to return oil , can cause a constant restriction in the low side of the system. Building maintaintenance personel who are not involved in the every day design and installation of commercial air conditioning systems are usually uninformed as to the proper methods of installation and design.