The relation is known as Newton’s low of cooling and can be used to express the overall effect of convection. In the equation 4 h is heat transfer coefficient and  is the temperature difference.is the wall temperature and  is temperature of  the surrounding. For many engineering applications equation 4 is very useful. Equation 4 is consistent with equation 1 in which  is shown to be proportional to surface area and temperature gradient. When the heat transfer between the fluid and the surface composed of a series heat path, we may define thermal resistance (R) due to convection

(5)

 is an empirically derived value from the heat transfer experiments. It is to be noted that the heat transfer coefficient is an empirically derived quantity and its numeral value depends in addition to physico-chemical-thermal variables on the design of experiments.