Vapor Cycle Air Conditioning Cabin Temperature Control System Typical System Operation Theory of Refrigeration 16-43 Figure 16-70.  Typical temperature selectors on a transport category aircraft temperature control panel in the cockpit (left) and a business  jet (right). On large aircraft, temperature selectors may be located on control panels located in a particular cabin air distribution zone. NORMAL MAN COCKPIT STANDBY NORMAL MAN CABIN STANDBY COLD HOT TEMP COLD HOT TEMP TEMPERATURE CONTROL 0 20 40 −40 −20 CABIN TEMP 0° C Cabin temperature pickup units and duct temperature sensors  used in the temperature control system are thermistors. Their  resistance changes as temperature changes. The temperature  selector is a rheostat that varies its resistance as the knob is  turned. In the temperature controller, resistances are compared  in a bridge circuit. The bridge output feeds a temperature  regulating function. An electric signal output is prepared and  sent to the valve that mixes hot and cold air. On large aircraft  with separate temperature zones, trim air modulating valves  for each zone are used. The valves modulate to provide the  correct mix required to match the selected temperature. Cabin,  flight deck, and duct temperature sensors are strategically  located to provide useful information to control cabin  temperature.  [Figure 16-71]  Heat is an expression of energy, typically measured by  temperature. The higher the temperature of a substance, the  more energy it contains. Heat always flows from hot to cold.  These terms express the relative amount of energy present in  two substances. They do not measure the absolute amount  of heat present. Without a difference in energy levels, there  is no transfer of energy (heat). Energy can be neither created nor destroyed; however, it can  be transformed and moved. This is what occurs during vapor  cycle air conditioning. Heat energy is moved from the cabin  air into a liquid refrigerant. Due to the additional energy, the  liquid changes into a vapor. The vapor is compressed and  becomes very hot. It is removed from the cabin where the very  hot vapor refrigerant transfers its heat energy to the outside  air. In doing so, the refrigerant cools and condenses back  into a liquid. The refrigerant returns to the cabin to repeat  the cycle of energy transfer.  [Figure 16-72] The absence of a bleed air source on reciprocating engine  aircraft makes the use of an air cycle system impractical  for conditioning cabin air. Vapor cycle air conditioning is  used on most nonturbine aircraft that are equipped with air  conditioning. However, it is not a source of pressurizing air  as the air cycle system conditioned air is on turbine powered  aircraft. The vapor cycle system only cools the cabin. If an  aircraft equipped with a vapor cycle air conditioning system  is pressurized, it uses one of the sources discussed in the  pressurization section above. Vapor cycle air conditioning  is a closed system used solely for the transfer of heat from  inside the cabin to outside of the cabin. It can operate on the  ground and in flight.  Most cabin temperature control systems operate in a similar  manner. Temperature is monitored in the cabin, cockpit,  conditioned air ducts, and distribution air ducts. These  values are input into a temperature controller, or temperature  control regulator, normally located in the electronics bay. A  temperature selector in the cockpit can be adjusted to input  the desired temperature.  [Figure 16-70]  The temperature  controller compares the actual temperature signals received  from the various sensors with the desired temperature input.  Circuit logic for the selected mode processes these input  signals. An output signal is sent to a valve in the air cycle air  conditioning system. This valve has different names depending  on the aircraft manufacturer and design of the environmental  control systems (i.e., mixing valve, temperature control valve,  trim air valve). It mixes warm bleed air that bypassed the  air cycle cooling process with the cold air produced by it.  By modulating the valve in response to the signal from the  temperature controller, air of the selected temperature is sent  to the cabin through the air distribution system. 