The efficiency and reliability of a split air-conditioning system hinges largely on the piping that connects the refrigerant-condensing and air-handling sides of the system. Operation difficulties are inevitable if this interconnecting piping is improperly designed or installed, regardless of how carefully the equipment was selected and applied. Interconnecting piping designs that successfully avoid these difficulties share several common traits: A simple, direct layout that reduces the amount of system refrigerant, a refrigerant tube size that consistently returns oil to the compressors and a refrigerant tube size that doesn’t cause excessive pressure drops which reduce compressor efficiency and capacity.
The success of any HVAC application begins with choosing the right system and properly sizing the equipment. If a split air-conditioning system best suits the project, then observe these guidelines: Base your selection on a minimum coil suction temperature of 40°F for constant-volume systems and 43°F for variable-air-volume (VAV) systems. These guidelines give good performance over the system’s entire operating range.
Select the system using design conditions and check for proper operation at the expected operating extremes, and avoid using hot gas bypass in comfort cooling applications. It squanders energy, and is seldom necessary for properly designed systems. You want to limit overall line length, including the vertical suction and liquid risers. Enough sub-cooling may be lost as refrigerant travels up the liquid riser to cause flashing. Review any questionable applications with the manufacturer.
Use Type L copper tubing and for copper-to-copper joints, use BCuP-6 (silfos) without flux. For copper-to-steel or copper-to-brass joints, use BAg-28 (Silver Brazing Alloy) with a non-acid flux. A pressure drop of more than 3 lbs in the suction line adversely affects unit capacity and efficiency. With that in mind, consider these suction-line recommendations:
1. Route the suction line from the evaporator to the compressor by the shortest path.
2. Use different pipe sizes for horizontal and vertical lines to make it easier to match line pressure drop and refrigerant velocity to suction-line requirements.
3. To assure proper oil entrainment and avoid sound transmission, size the suction line so that the refrigerant velocity equals or exceeds the required minimum velocity and remains below 4,000 fpm.
Figure 1 illustrates one method of trapping the evaporator coil. The suction lines from the evaporator headers should join at a point lower than the lowest suction header outlet. This arrangement drains the coils and prevents oil and refrigerant from “back feeding” from one coil to the other(s). The common connection should then rise above the height of the coil before proceeding to the compressor to prevent the refrigerant and oil in the evaporator from “free-draining” toward the compressor.
Provide a 1-inch pitch toward the evaporator for every 10 feet of run to prevent any refrigerant that condenses in the suction line from flowing to the compressor when the unit is off. Specify suction-line insulation if moisture condensation and dripping pose a problem, or to prevent refrigerant from condensing inside the lines if long runs will be exposed to low temperatures. Don’t install suction lines underground. The likelihood of corrosion, vibration, condensation of water outside—and refrigerant inside—the line, combined with inaccessibility and difficulty in
maintaining cleanliness, make this practice unwise. If underground installation is unavoidable, make provisions to insulate, waterproof and encase the lines in a hard sleeve (PVC)
Sufficient sub cooling must be maintained at the expansion valve. To provide proper operation throughout the range of operating conditions, the liquid-line pressure drop should not exceed the unit’s minimum sub cooling value less 5°F. To achieve this objective, keep these liquid-line considerations in mind:
Select the smallest, practical line size for the application. Limiting the refrigerant charge improves compressor reliability.
When designing the liquid line for a typical air-conditioning application (i.e. one with an operating range of 40°F to 115°F), remember that every 10 feet of vertical rise will reduce sub-cooling by 2.8°F, while every 10 feet of vertical drop will add 1.1°F of sub-cooling.
Specify one expansion valve per distributor. An expansion valve serving more than one distributor will distribute the refrigerant unevenly and possibly contribute to a hunting of the expansion valve.
Provide a 1-inch pitch toward the evaporator for every 10 feet of run. Since this pitch equals that of the suction line, the two may be run together. If the system is designed with a liquid line rise, a column of liquid refrigerant remains atop the volume of refrigerant gas in the condenser whenever the unit stops. The liquid refrigerant will eventually drain down the line and may fill the condenser … perhaps even overflow to the compressor. Sloping the liquid line toward the evaporator creates a gas trap at the line’s highest point, preventing liquid refrigerant from passing.
Solenoid valves are required. They prevent liquid refrigerant from filling the evaporator when the compressor stops and slugging when the compressor restarts. Adding solenoid valves also prevents siphoning which could allow an elevated column of liquid to overcome the gas trap and flow back into the condenser and compressor.
And finally, if the liquid line must be routed through an area warmer than outdoor air temperature, insulate the line to prevent the refrigerant from flashing.
copyright(c)2009 Roger J. Desrosiers
About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.