However, if the fluids are such that they do not mix, a direct contact heat exchanger may be used, for example a water chiller. Heat exchangers are also typically classified by flow arrangement and type of construction. It is common to classify these according to how many shell and how many tube passes the fluid must traverse. A special class of heat exchanger is the compact heat exchanger , which is characterized by very large values of surface area available for heat transfer per unit volume. The typical case here is that at least one of the fluids is a gas, which has a low convection coefficient, and thus the large surface area is designed to compensate for this.
Lungs make a significant contribution to removing waste heat from the body.
A qualitative description of heat exchangers can be made by the use of simple thumbnail schematics. Conversely, if the cool fluid is boiling, its temperature is constant, while the hot fluid is being cooled. For a parallel flow arrangement, the cool fluid warms and the hot fluid cools, however, the outlet temperature of the cool fluid can't be greater than that of the hot fluid, since the temperature gradient eventually approaches zero. Therefore, the effectiveness of these units is limited and they are typically not used for heat recovery.