ACHRNEWS

Size matters with chiller barrel installation

July 13, 2000
When installing chiller barrels, suction line sizing is very important. If suction line diameter is too small, you will see excessive pressure drops, which can cause a reduction in performance and inadequate compressor cooling. Too large of a diameter can reduce velocities enough that the oil will not remain entrained with the refrigerant and can cause oil to log in the evaporator and suction line.

Standard refrigeration suction lines are sized for long piping runs (100 ft). You may want to reduce line diameter one size for shorter runs.

Overall, keep the suction line as short and straight as possible and use as few elbows as possible, using long radius elbows where space permits. This will minimize pressure drop and enhance performance. Suction lines should always be insulated.

Oil traps: P-traps are made up with copper elbows and ells to help keep the oil mixed with the refrigerant as it travels to the compressor. Traps may not be necessary when the compressor is mounted below or level with the evaporator, unless you do not have a pumpdown control. In that case, you should have an inverted trap to prevent oil from draining into the compressor during the off cycle.

Simply pitching the suction line towards the compressor should be adequate when the compressor and evaporator are mounted at the same level.

A p-trap should be installed at the base of any suction line with a vertical rise. In long runs, a p-trap should be installed for every 20 ft of vertical rise and every 30 ft of horizontal run.

Suction accumulators: P-traps will not prevent large amounts of oil or liquid refrigerant from slugging the compressor. Systems that are prone to liquid damage, such as commercial and low-temperature systems, should use a suction accumulator. This will prevent floodback and slugging, and enhance oil return.

The accumulator should be installed at the same level as the compressor.

Suction line heat exchangers: These use suction gas to subcool the liquid refrigerant, boil off any liquid in the suction line, and add superheat. This increases system performance and adds a measure of protection for the compressor.

Smoldering-brazing guidelines

  • Only air conditioning and refrigeration tubing that is plugged and dehydrated should be used for piping the system.

  • Always use a torch tip that is large enough to bring the metal being soldered up to the appropriate temperature quickly, to prevent overheating the work piece.

  • Whenever possible, the component being soldered should be disassembled. This is especially important on ball valves. The seats in these valves are prone to damage from overheating and most are easily disassembled and reassembled.

  • Always clean both pieces being brazed with sand cloth or a wire brush.

  • Copper-to-steel joints: You must use solder with no less than 45% silver content and the corresponding flux.

  • Copper-to-copper joints: Solder with no less than 15% silver content is recommended. No flux is needed. Solders with a lower silver content can be used if necessary.

  • Nitrogen flood: The system should be flooded with nitrogen when soldering to prevent the formation of oxidation on the inside of the copper tubing being brazed. This is done by filling the system with nitrogen, setting the regulator at 2 to 4 lb, and letting it flow while you are brazing. There should be an opening large enough to prevent pressure from building up in the system.

    Wipe the joints clean with a wet rag when you are done brazing. Always pressure test with nitrogen when brazing is completed; 150 psig is recommended for testing.

    Note: Ensure that all of the system’s components and relief valves are rated for at least that pressure.

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