Tube-to-tube heat transfer degradation effect on finned-tube heat exchangers

Tube-to-tube heat transfer is suspected to affect the overall capacity of m ost finned-tube heat exchangers running subcooled (single-phase) simple sub stances, or zeotropic mixtures, as coolants. A simplified numerical model i s presented and used for simulating the heat transfer process of air flowin g in between two parallel fins of a finned-tube heat exchanger with known t ube temperatures. The objective of the numerical simulation is to detect th e existence of tube-to-tube heat transfer and the corresponding detrimental impact it might have on the overall capacity of the heat exchanger. The nu merical simulation considers a 4 x 3 finned-tube, single-phase, air-water h eat exchanger with data provided by the National Institute of Standards and Technology (NIST). Results indicate the existence of strong tube-to-tube h eat transfer along the fins (particularly among neighboring tubes presentin g very different temperatures, as expected). The tube-to-tube heat transfer phenomenon is shown to account for approximately 20% of the heat exchanger capacity. Thin adiabatic layers placed along the fin surface are shown, al so through numerical simulations, to be very effective in interrupting the tube-to-tube heat transfer phenomenon. A simplified analytical procedure to estimate the tube-to-tube heat transfer effect of any finned-tube heat exc hanger, suggested as a design tool, is satisfactorily tested against the nu merical results.