A process flow diagram (PFD) is a diagram commonly used in chemical and process engineering to indicate the general flow of plant processes and equipment. As shown in the condensation water heating PFD diagram, the PFD displays the relationship between major equipment of a plant facility and does not show minor details such as piping details and designations. The condensation water heater diagram shows when natural or propane gas burns in gas water heaters, a lot of moisture in products of combustion are released. As the process flow diagram suggests, when the water vapor is cooled below the dew point, water heater condensation occurs. The dew point is the temperature at which water vapor turns into a liquid state, called condensate.