Heat transfer enhancement of ammonia pool boiling with an integral-fin tube

• Pool boiling of ammonia on plain and integral-fin tubes is experimentally studied.
• Integral-fin tube enhances heat transfer up to 30% compared with plain tube.
• Onset of nucleate boiling hysteresis exists and it is time-dependent.

Ammonia has been and is used as refrigerant in many industrial refrigeration systems. Flooded shell-and-tube evaporators are normally employed in such systems. However, it is crucial to reduce the size and refrigerant charge of these evaporators, which can be achieved by improving their heat transfer performance using tubes with enhanced surfaces.Tests were performed to analyse the enhancement achieved with a commercial integral-fin (32 f.p.i., 1260 f.p.m.) titanium tube, if compared to a plain tube of the same nominal external diameter, under pool boiling and using ammonia. The test conditions were those than can be found in water chillers and heat transfer coefficients were determined both in increasing and decreasing heat flux order to analyse nucleation hysteresis.The average enhancement factor achieved with this tube was of 1.2 (maximum 1.3). Hysteresis on nucleation occurred and should not be neglected, particularly with the integral-fin tube. An experimental correlation is proposed for both the plain and the enhanced tubes and it predicts the experimental results within 5.5%.