This work experimentally investigates the effect of the geometrical parameters of a vertical helically coiled tube in tube (HCTIT) evaporator on the coefficient of performance (COP) of a vapour compression refrigeration system (VCRS) used as a water chiller. Five HCTIT evaporators through which R134a flows in the internal tube, are constructed and tested at different flow rates of the heating water in their annulus. They are geometrically divided into two groups; the first group is fabricated such that the internal tube curvature ratios (δ_(ev,i)) is varied from 0.0238 to 0.0334 while the other group represents another three-different coil torsions (λ) from 0.0522 to 0.0948. The results show that the COP of the VCRS is enhanced with decreasing δ_(ev,i), increasing λ, and increasing the condenser to evaporator pressures ratio. Finally, an experimental correlation is developed to predict the COP of the VCRS.