Because of their better chemical resistance and fouling characteristics, plastic heat exchangers are of increasing interest for lower temperature applications. However, their lower thermal performance compared to that of metal heat exchangers has prevented their widespread use and acceptance. To overcome this constraint, polymeric hollow fiber heat exchangers (PHFHEs) are proposed as a new type of heat exchanger for lower temperature/pressure applications. In polypropylene-based PHFHEs, the overall heat-transfer coefficients achieved here, 647-1314 and 414-642 W m(-2) K-1 for the water-water and ethanol-water systems, respectively, are comparable with accepted design values for metal shell-and-tube heat exchangers; further, for 20% of our water-water runs, it was higher than any value reported for plastic heat exchangers. The extremely large surface area/volume ratio of PHFHEs makes them more efficient than metal heat exchangers. Devices less than 30 cm (1 ft) long yielded efficiencies of up to 97.5%, up to 3.7 number of transfer units (NTU) and a height of a transfer unit (HTU) as low as 5 cm; the latter is 20 times less than the lower limit for shell-and-tube exchangers and 10 times less than the typical values for plate heat exchangers. PHFHEs achieve conductance/volume ratios 3-10 times higher than shell-and-tube devices accompanied by low-pressure drops, as low as 1 kPa/NTU, compared to 30 kPa/NTU for metal heat exchangers. Considering the much lower cost, weight, and elimination of metal contamination, PHFHEs can substitute metal heat exchangers on both thermal performance and economical grounds.