Refrigerant line sizing is an overlooked aspect of system application and installation. Incorrectly sized refrigerant lines can rob your system of capacity and reduce its efficiency. In a worst case scenario, incorrectly sized and applied refrigeration lines can cause multiple compressor failures. For most applications, the two lines involved are the liquid line and the suction line.
Pressure drop in the liquid line can create flash gas in the line before the refrigerant reaches the metering device. This causes a drop in system capacity because it reduces the amount of liquid entering the evaporator. Some pressure drop is inevitable. It is important to have enough liquid subcooling to offset the pressure drop through the liquid line.
Suction line pressure drop also hurts the system performance by increases the compression ratio and reducing the amount of refrigerant being circulated. The general rule is to try and keep suction line pressure drop below an equivalent saturation temperature drop of 2F. The actual amount of pressure this represents depends on the refrigerant and the evaporator saturation temperature. Suction lines have another very important design criteria: the refrigerant traveling through them must have sufficient velocity to return oil. While larger lines help reduce pressure drop, they also decrease refrigerant velocity. In general, you want your suction line to be as large as possible while still having enough velocity to return oil.
There are far too many variables to mention in a blog post, but I can point you toward some excellent materials available online. DuPont and Lennox both have excellent refrigerant piping handbooks available on the web in pdf form. The Dupont document is applicable to all forms of refrigeration while the Lennox material is primarily for air conditioning. Virginia Air has a great excel file which does a lot of the heavy lifting for you in calculating pressure drop and velocity. They also have several line sizing tables on another tab of the excel file. I guarantee that you will learn something about line sizing if you download and examine these three wonderful resources. I know I did.
DuPont - http://www.hvaceducationaustralia.com/Resources/PDF/DuPont%20Refrigerant%20Piping%20Handbook.pdf