The effect of impeller clearance and liquid level on the critical impeller speed (Njs) for various radial and axial flow impellers in 0.29 m ID agitated vessel has been studied. Five types of radial impellers: Rushton turbine (RT), Straight blade (SB), Curved blade (CB), Curved blade with disc (CBWD) and R130 impeller and four types of axial impellers: Rushton turbine 45o angle (RT 45), Pitched blade (PBT), A320 and HE3 impeller were used. Tap water and resin particle of 0.506 mm were used as liquid and solid phases, respectively. The impeller clearance to vessel diameter (T) was varied between 0.17 and 0.41. The liquid level (H) was also varied as H/T=0.5, H/T=0.75 and H/T=1. The R130 impeller and A320 impeller was found to be more efficient among radial and axial impellers respectively. A new expression for Zwietering constant 'S' was developed to predict critical impeller speed, considering impeller clearance and liquid level for all the impellers. The results obtained here show that the 'S' values increase with increase in clearance, and decrease with liquid level for all impellers and it also depends on the type of impeller.