An ammonia vapour-compression refrigerator works between −6.7 °C and 26.7 °C. The vapour is dry saturated at the end of compression and there is no under-cooling of the liquid, which is throttled to the lower temperature. Estimate the theoretical COP. Properties of ammonia: at −6.7 °C, h_{f} = −29.3, h_{fg} = 1293.8, h_{g} = 1264.5 kJ/kg, s_{f} = −0.113, s_{g} = 4.752 kJ/kg·K; at 26.7 °C, h_{f} = 125.6, h_{fg} = 1172.4, h_{g} = 1297.9 kJ/kg, s_{f} = 0.427, s_{g} = 4.334 kJ/kg·K. (Answer as a number.)