Question

which is best refrigerant liquid ammonia or liquid nitrogen

Answer 1

To use nitrogen for regular refrigeration would require very high pressures and thick wall expensive piping. x-rayed welds, etc. Leaks at say 2000 psi are dangerous.

Anhydrous ammonia is a clear liquid that boils at a temperature of -28°F. In refrigeration systems, the liquid is stored in closed containers under pressure.

When the pressure is released, the liquid evaporates rapidly, generally forming an invisible vapor or gas. This is why ammonia is used in refrigeration systems. Hence, ammonia is the best refrigerant liquid.