There is a continuing quest for ever more compact heat exchangers. There is abundant patent literature on novel compact heat exchanger designs, and additive manufacturing has set free even more creativity. Where does all this effort lead?
shows a trend. The x-axis shows the hydraulic diameter of a flow port, for example, for refrigerant. The left y-axis shows the heat exchanger compactness in terms air-side surface area per total envelope volume, and the right y-axis shows the material utilization, defined here as volume of material (metal) to build the heat exchanger in terms of air-side surface area per material volume. The first measure is for physical size, the second for the amount of material used. For both axes, higher values describe a better heat exchanger.
The points plotted are estimates for heat exchanger compactness (red dots) and heat exchanger material utilization (blue dots) as a function of port hydraulic diameter. The values are based on the assumption of a round tube with a wall thickness that is only governed by the requirement that the tubes can safely contain fluids up to the same maximum pressure.
Following the x-axis from large values of 10 mm to smaller ones, a distinct increase in both compactness and material utilization can be observed. However, once the hydraulic diameter drops below 1 mm, especially below 0.5 mm, the graph takes off, and both compactness and material utilization increase drastically.
What does this tell us about the future of heat exchangers? For higher performance (more heat transfer capacity in smaller volumes) and lower cost (less material per heat transfer capacity), flow channels can be expected to become much narrower; however, dimensions below 0.5 mm raise a number of concerns. Clogging of channels by refrigerant or small debris particles, even compressor oil, becomes much more likely. Air-side fouling due to dust is a growing concern, and so is clogging due to water condensate from dehumidification. Thus, oil-free compressors, water-shedding surfaces, and filters will be part of the future of ever more compact heat exchangers.
Reinhard Radermacher, PhD
Fellow ASHRAE
Editor-in-Chief
Minta Martin Professor of Engineering
Director, Center for Environmental Energy Engineering
University of Maryland
College Park, MD, USA
Vikrant Aute, PhD
Member ASHRAE
Research Scientist and Director
Modeling and Optimization Consortium Center for
Environmental Energy Engineering
Department of Mechanical Engineering
University of Maryland
College Park, MD, USA