A good example of heat-exchange effectiveness is shown by comparing the surface area and surface-area-to-volume ratio of the two heat exchangers pictured below. The large copper tube-in-tube exchanger was originally installed on a solar thermal system heating an indoor swimming pool. It has a total heat-exchange surface area of 65 square inches. The five 1/2-inch-diameter inner tubes total a volume of 36 fluid ounces, for a ratio of 1.8 to 1. The plate-type stainless-steel heat exchanger shown in front of the copper exchanger has a total exchange surface area of 720 square inches and total volume of only 15 fluid ounces, a 48:1 ratio—offering both a higher surface area and a higher surface-to-volume ratio.
The proof was in exchanging the exchangers. The tube-in-tube heat exchanger was unable to heat the pool successfully, while the plate exchanger allowed the system to perform as designed and heat the pool without any other modifications.