DX AMMONIA PIPING HANDBOOK
Ammonia refrigeration systems have traditionally employed evaporators supplied with liquid by either gravity flooding (with surge drums), or pumped overfeed (either with mechanical pumps or discharge gas-driven vessels). Both of these designs typically use bottom feed coil circuiting which feeds liquid ammonia at the lowest point in the coil circuit and causes the ammonia to flow upward and “percolate” through the coil in ascending passes to the outlet at the top of the circuit. These coil designs also typically use large diameter tubing which means relatively large coil internal volume. This combination of refrigerant feed, circuiting, and tube diameter, results in the greatest evaporator charge inventory possible.
End users of ammonia refrigeration systems are increasingly interested in reducing the charge of ammonia in evaporators (and in the overall system) in the interest of minimizing the risk to workers and products associated with ammonia leaks. One very effective way to significantly reduce evaporator ammonia charge is to design and operate the evaporator using dry expansion (DX) circuiting and controls. Using DX ammonia can reduce the evaporator charge by as much as 30 to 50 times compared to bottom feed flooded or pumped designs. The magnitude of this reduction in ammonia charge may also mitigate regulatory requirements (PSM, RMP), and potentially reduce insurance risk and premiums.