The surge drum on a flooded evaporator performs various important functions. They are:

(1) To separate the liquid out of the gas returning from the evaporator; allow the gas to pass to the compressor through the suction line; to return the liquid to the evaporator for re-circulation.

(2) To absorb such surges in the evaporator as may occur due to variation of load during operation. The coil circuits will carry an amountof liquid in them in some what inverse proportion to the load. With light duty less gas isformed in each coil circuit and there is room for more liquid. When the load becomes heavier and duty is greater, more gas is formed and less liquid is carried in the coil circuits. Even room coils have changes in load, principally due to the addition of warm products to the room.

(3) The third function of a surge drum is to absorb the violent liquid surging that occurs when the suction line of a flooded evaporator is quickly opened. The immediate drop in pressure, on the liquid lying in the evaporator, following the instant the suction line is opened creates a tremendous amount of vapor. Practically the entire charge of liquid left in the evaporator at shut-down is suddenly thrown up into the surge drum.