Adding heat to a substance does not always raise its  temperature. When a substance changes state, such as when  a liquid changes into a vapor, heat energy is absorbed. This  is called latent heat. When a vapor condenses into a liquid,  this heat energy is given off. The temperature of a substance  remains constant during its change of state. All energy  absorbed or given off, the latent heat, is used for the change  process. Once the change of state is complete, heat added to  a substance raises the temperature of the substance. After a  substance changes state into a vapor, the rise in temperature  of the vapor caused by the addition of still more heat is  called superheat.  into a liquid when heat is removed. The boiling point of any  substance varies directly with pressure. When pressure on  a liquid is increased, its boiling point increases, and when  pressure on a liquid is decreased, its boiling point also  decreases. For example, water boils at 212 °F at normal  atmospheric temperature (14.7 psi). When pressure on liquid  water is increased to 20 psi, it does not boil at 212 °F. More  energy is required to overcome the increase in pressure. It  boils at approximately 226.4 °F. The converse is also true.  Water can also boil at a much lower temperature simply by  reducing the pressure upon it. With only 10 psi of pressure  upon liquid water, it boils at 194 °F.  [Figure 16-73] Vapor pressure is the pressure of the vapor that exists  above a liquid that is in an enclosed container at any given  temperature. The vapor pressure developed by various  The temperature at which a substance changes from a liquid  into a vapor when heat is added is known as its boiling point.  This is the same temperature at which a vapor condenses  16-44 Figure 16-71.  The temperature control system of a Boeing 777 combines the use of zone and duct temperature sensors with trim air  modulating valves for each zone. Redundant digital left and right cabin temperature controllers process temperature input signals from  the sensors and temperature selectors on the cockpit panel and throughout the aircraft to modulate the valves. AUTO OVRD EQUP COOLING ON RECIRC UPPER AUTO OFF L PACK AUTO OFF R PACK ON FANS LOWER ON L TRIM AIR R ON FAULT FAULT MAN TEMP AUTO FLT DECK W C TEMP AIR COND RESET AIR CONDITIONING CABIN W C LEFT CABIN TEMPERATURE CONTROLLER RIGHT CABIN TEMPERATURE CONTROLLER AIMS ASCPC F/D F/D A B C D E LEFT AC PACK RIGHT AC PACK ARINC 629 Flight deck zone Passenger cabin zone Air conditioning panel Zone air temperature sensors Zone duct temperature sensors AFT upper  recirculation fan Trim air modulating valves (6) Forward upper recirculation fan Trim air pressure regulating and shutoff valve Ozone converter Pneumatic system Ozone converter Flow control and shutoff valves Flow control and shutoff valves Lower recirculation fans Mix manifold Trim air pressure regulating and shutoff valves