A heat exchanger for a heating, ventilating and air conditioning system comprises a plurality of heat exchange tubes extending between a pair of spaced header tanks and arranged in groups of tubes with varying number of tubes in each group to cause a refrigerant to flow in multiple passes in the interior of the tubes across another fluid flowing on the exterior of the tubes. The heat exchange tubes comprise a plurality of flow passages having at least one corner formed by a pair of straight or arcuate sides with an included angle of less than or equal to ninety degrees, more preferably less than or equal to thirty degrees, to promote intense pool boiling within the flow passages. In addition to at least one corner region, the flow passage has a passage-specific optimal hydraulic diameter determined by the relationship between the optimal hydraulic diameter of the passage and the optimal hydraulic diameter of a baseline circular passage.