Surging:

Let A as the point of operation at full load. While the refrigeration load decreases, the point of operation move to the left until point B of maximum head is attained. If the load continues to reduce to the left of B, say to C, the pressure ratio make by the compressor becomes less than the ratio needed between the condenser & evaporator pressure. Like as.,

p₄/p₁ < p_k/p₀

Therefore some of gas flows back from the condenser to the evaporator, so increasing the evaporator pressure & decreasing p_k/ p₀ .The pt of operation suddenly move to A. Since the refrigeration load is still less, the cycle shall repeat itself. This fact of reversal of flow in centrifugal compressors is known as surging. It takes place when the load reduces to below 35 % of the rated capacity & causes severe stress conditions in the compressor as a consequence of hunting.