A refrigeration circuit leaks at its joints, so build it with as few as possible: welded or flanged steel joints rather than threaded ones, supports that stop vibration cracking pipe, slopes that let liquid drain, and a relief path for any section where liquid can be trapped between closed valves. Warming trapped liquid refrigerant expands it against rigid walls and can burst the pipe with no heat source more dramatic than the sun.
Part of Refrigeration
Piping, joints, seals and assembly order for a refrigeration circuit, and the failure modes each choice carries.
Every Joint Is A Leak Path And Every Leak Of This Fluid Matters
A water pipe that weeps is a nuisance. A refrigerant circuit that weeps loses charge, lets moisture and air in on the low side, and, with ammonia, fills a room with irritant vapour. The corpus carrier names the building blocks: steel pipe of 12 to 25 mm for refrigerant lines, threaded pipe fittings, high-temperature gaskets, and valves. Using them well comes down to a few principles.
- Prefer welded joints; where you must separate, use flanges. Threads are the commonest leak path in any pressure circuit. The corpus covers flange loading in Bolted Flange and gasket seating in Gasket Seating and Flange Finish.
- Choose gaskets and sealants that the fluid will not dissolve. The corpus guides in Gasket Types and Gaskets and Seals concern steam and other fluids. None of them is a statement about ammonia, so soak-test any gasket or thread compound in the fluid’s solution before trusting it.
- Keep thread forms matched. Thread and Screw Standards explains why forms do not mix; a mismatched thread seals badly.
- Route the pipe for drainage and support. Slope lines so liquid runs to where you want it, and clamp them so a vibrating compressor does not fatigue a weld.
What You Need
- Steel pipe and fittings of one known grade, with welders tested on the actual procedure, as the vessel guidance in Vessel Construction sets out.
- Flanges and gaskets at the points that must come apart: compressor, vessels, instruments.
- A means of proof testing with water, then leak testing with soap; see Pressure Testing.
- Pipe supports, hangers and a way to isolate sections for repair.
- A relief device on each section that can be isolated, or none of the isolating valves allowed to close such a section.
Method: Assembly Order
Step 1: Fix the compressor and vessels first, on a rigid base that absorbs vibration.
Step 2: Run the pipe from the high side to the low side, with supports every few lengths and no pipe carrying the weight of a valve.
Step 3: Put a filter before the restriction and keep a drain at the lowest point.
Step 4: Fit valves only where you need to isolate for repair, and decide at that moment where each isolated section’s relief goes.
Step 5: Prove each pressure part and the finished circuit with water, then leak-test with soap; only then charge.
Step 6: Insulate cold lines last, after the circuit has been proved leak-free, with a vapour seal so moisture does not condense into the insulation and corrode the pipe under it (recalled, flagged).
Worked Example: Why A Trapped Liquid Section Is Dangerous
Qualitative. A liquid is nearly incompressible, as the corpus says for water in Pressure Testing. Refrigerant liquid also expands when warmed. If a length of pipe is filled with liquid and closed at both ends, a rise in temperature has nowhere to put the extra volume, and the pressure climbs steeply until something gives. The corpus makes a parallel point for hydraulic oil in Pressure Relief Valves, whose own description says trapped oil gains about 1 MPa per kelvin. That figure is for oil and is not carried over to a refrigerant; the lesson is the mechanism. The remedy is a relief path from every section that can be isolated with liquid in it, or a rule that such a section is never closed full.
Common Mistakes
- Tightening a leaking thread repeatedly. Strip it, clean it and remake it.
- Brass or copper “just for a gauge or valve” in an ammonia circuit. It leaks at that joint within the year.
- A pipe resting on the compressor. The vibration cracks a weld.
- Closing valves both sides of a liquid-filled run. Warmth then bursts it.
- Insulating before the leak test. A leak hidden under insulation is found by the smell, not the soap.
Safety
Never repair, cut or weld a pipe that has held refrigerant
Residual ammonia or oil can burn or release toxic gas when heated, and a joint that is still charged will vent at you when loosened. Shut off, vent to a safe place outdoors, purge, and test the atmosphere before hot work. Do not use a flame as the detector for flammable vapour; use a ventilated site and a wet indicator strip. See Pressure Vessels.
What’s Next
- Learn next: Insulation & Construction — keeping the cold in once the circuit works
- Combine with: Troubleshooting — finding the leak you did not prevent