Field-assembled refrigeration systems require the installing technician to select the appropriately sized refrigerant lines. Choosing the correct line size is very important to the overall success of the installation. Incorrectly sized refrigerant lines can lead to system problems, excessive energy consumption, and higher installation and operation costs.
Equipment manufacturers generally provide charts for technicians to reference when choosing refrigerant lines for an installation. The correct line sizes provide an acceptable pressure drop across the line, while, at the same time, providing a minimum velocity to ensure good oil movement throughout the line. Excessive pressure or poor oil movement through a refrigerant line is detrimental to the overall operation of the system.
Choosing the correct size for the liquid line tends to be less problematic than the suction and discharge lines, but is still important to the overall success of the installation. This is because the refrigerant in its liquid form tends to mix well with the compressor’s oil in circulation, which allows for good oil movement through this section of the system. The major concern with sizing this line is ensuring a solid column of liquid is fed to the metering device. Excessive pressure drop or warming of the refrigerant creates the possibility of the refrigerant returning to a saturated condition, meaning a mixture of liquid and vapor refrigerant will be fed to the metering device. This will cause a severe drop in the system’s capacity.
Proper sizing of the liquid line and subcooling of the refrigerant in the condenser will compensate for most liquid line pressure losses. A major cause of pressure drop across a liquid line is the weight of the refrigerant as it is forced up a vertical section of line. Generally speaking, every 2 feet of lift will result in a pressure drop of approximately 1 psig. For example, if a condenser is located 30 feet below an evaporator, then a pressure drop of approximately 15 psig must be accounted for in the system design of the liquid line.
Although the velocity of the refrigerant traveling through the liquid line is generally not a concern, if it’s excessive it can cause issues, such as possible damage to the piping from pressure surges or liquid hammering caused by the rapid closing of liquid line solenoid valves. The velocity of the liquid line should not exceed 300 feet per minute (fpm).
Another installation concern associated with the liquid line, which is not directly related to its sizing, is the increase of the refrigerant’s temperature as it travels to the metering device. If the liquid line will travel through a high-temperature area, it may be necessary to insulate the line to prevent excessive warming of the refrigerant.
Again, it is best to always follow the manufacturer’s installation guidelines when selecting and installing the liquid line as well as all of the other refrigerant lines. Those guidelines will help ensure a successful installation for you and your customer.
Publication date: 2/9/2015