An important part of the refrigeration design process is determining what size refrigerant lines returning to a compressor or an accumulator/recirculator vessel should be. These return lines are often simply called suction lines and are sized based on the tonnage of the refrigerated load. Recommended pipe size will change based on tonnage as well as what refrigerant your system operates with and for that reason, we will not dive into specific dimensions but rather a general overview.
Suction lines, like any other piping system, are subject to pressure drop. The faster the fluid or gas is forced to move through the piping, the larger the pressure drop will be. When systems lose pressure between the refrigerated load and the compressors the compressors will have to operate at a lower pressure to continue delivering the proper pressure at the evaporators. This causes a decrease in refrigeration capacity. Both issues lead to either additional upfront cost or additional operational cost. For this reason it is important to make sure the lines are adequately sized so that way we don’t cause undue pressure loss in our refrigerant lines.
Assuming we have sized our refrigerant lines appropriately for our selected refrigerant using either IIAR (International Institute of Ammonia Refrigeration) or manufacturers guidelines we still have considerations to make. Oil return is a vital part of all mechanical refrigeration systems and we must take certain precautions to ensure oil returns to the compressor properly. One such precaution is sloping our horizontal refrigerant lines back towards the compressor or accumulator/recirculator vessel. Sloping the lines is good practice for the return of liquid refrigerant also. Avoiding dips and drops in piping that may cause accidental trapping of oil and liquid refrigerant between the evaporator and the compressors is critical for proper oil return. Liquid pooling in low spots can also cause hydraulic shock or liquid slugging, both of which are significant safety concerns. Pitch vapor lines back towards accumulator vessels also, even if you don’t anticipate liquid presence as they may see some liquid spillover from time to time.
Vertical refrigerant lines where refrigerant is rising up the pipe (or risers) have an additional design consideration that may contradict some of our original suggestions. Vertical refrigerant lines should be sized to maintain a sufficiently high velocity to return oil or liquid up the entire length of the riser. This of course does increase pressure drop, but it is a necessary evil to ensure proper equipment operation. In dry suction applications without gas defrost, it is often best practice to include a small P-Trap at the bottom of suction risers to ensure liquid and oil are pulled up the line routinely and in smaller more manageable quantities. In recirculated applications, where liquid refrigerant is intentionally flowing through the suction line, proper sizing of the piping riser is particularly crucial. In installations where the refrigerant load through a riser deviates, such as a refrigerant main, it may be wise to consider installing a double riser. A double riser, as the name implies, uses two riser pipes instead of one. This configuration allows a reasonably low pressure drop during high load, but still promotes pulling liquid up the riser during lower load periods.
Suction piping plays a vital role in the longevity and operational efficiency of mechanical refrigeration systems. Not every system is the same, but every system deserves some consideration to ensure it is installed properly. Whether you are cooling birds, beef, beverages, or anything in-between, Carlson and Stewart would be happy to discuss your unique installation and help you design the optimal piping system. We offer not only design but full installation and service capabilities on new and existing systems as well.