heat transfer course solve with calculations

A heating contractor must heat 0.2kg/s of water from 15C to 35C using hot gases in cross flow over a thinwalled tube. Your assignment is to develop a series of design graphs that can be used to demonstrate acceptable combinations of tube dimensions ( D and L ) and of hot gas conditions ( T and V ) that satisfy this requirement. In your analysis, consider the following parameter ranges: D=20,30, or 40mm;L=3,4, or 6m; T=250,375, or 500C; and 20V40m/s