Experimental heat transfer coefficients of pool boiling and spray evaporation of ammonia on a horizontal plain tube

• Boiling processes of ammonia on a titanium plain tube are experimentally studied.
• Spray evaporation outperforms pool boiling if a proper liquid distribution exists.
• Dry patches lead to a spray evaporation heat transfer coefficient deterioration.

We have determined experimentally both spray evaporation and pool boiling heat transfer coefficients, using ammonia and titanium plain tubes, to compare both processes and to contribute to the existing database. In the paper we have detailed the experimental equipment used and the methodology followed. We have also exposed the data analysis process followed to obtain spray evaporation or pool boiling heat transfer coefficients from the experimental data measured and we present and discuss the results obtained.Spray evaporation heat transfer coefficients depend on heat flux and on the refrigerant film flow rate. At the high heat flux range, spray evaporation heat transfer coefficients decrease with decreasing film flow rates, suggesting the existence of dry patches, which we confirmed visually. Spray evaporation is particularly beneficial at the low heat flux range studied. Under certain conditions of high heat flux, pool boiling outperforms spray evaporation due to the heat transfer coefficient deterioration caused by dry patches.