Document xzRBn8OB9kVgLzKL9o96Db82G

1086 CHAPTER 48 1955 Guide tem consisted either of a steam boiler and radiator or a single stage or double stage heat exchanger. On both types, the cabin temperature was adjusted by positioning the face and bypass dampers. While these were sometimes moved by an automatic modulating control, in the majority of cases they were positioned by one of the ship's crew, with results which, while not satisfactory, were passable. As these planes cruised at less than 200 mph and normally operated at low altitudes, changes in outside air temperatures were generally gradual enough so that manual readjustment of controls could maintain reasonably comfortable cabin conditions. Nearly all of these heating systems, were marginal in respect to heat available, and the main problem was lack of heat, rather than inadequate control. Non-Pressurized Cabins With the advent of the combustion type heater, and use of larger and faster planes, use of manual controls became impracticable. The com bustion type heaters reach full rating in less than a minute after being turned on, and as they are rated at 100,000 Btu. per hr and up, and since several .heaters are generally used, it would take full time of one crew member to keep cabin temperature regulated. As ships of this type are not pressurized, the heating system is still comparatively Simple. In one type, two 100,000 Btu heaters are placed in parallel positions and the ram air from an external scoop is passed through the heaters and dis charged through a series of distributing outlets located in the cabin ceiling. The cabin air is discharged through grilles located in the bottom walls of the cabin. An auxiliary nose heater is used by the crew to obtain additional heat for the cockpit or for windshield defrosting. The cabin is maintained at the desired temperature by means of an auto matic control which operates both heaters simultaneously. This control consists of two duct thermostats, one being mounted in the air inlet duct between the air scoop and the heaters so that it is affected by outside ambi ent temperatures, and the other being mounted in the heater outlet duct so that it is affected by the discharge air temperatures. A thermostat in the cabin is so located that a continuous stream of cabin air passes through it. This type of control has been found to respond to a 1 deg temperature change in less than a second. Its theory of operation follows. As the outside temperature starts to drop, the outside air duct thermo stat decreases in resistance, unbalancing an electronic bridge. This un balance is amplified by vacuum tubes and causes a power tube to close a relay, turning on the combustion heaters. The resulting increase in tem perature is sensed by the warm air duct thermostat which increases in resistance, thus re-balancing the bridge and causing the relay to open. If there were no loss by radiation or convection from the aircraft cabin, these two duct thermostats would be sufficient for adequate control. However, the cabin thermostat is given approximately 30 times more influence than the duct thermostats and so acts as the master controller, and the duct thermostats prevent over-heating or under-heating and keep the discharge air from alternating between extreme cold and extreme heat. In a slightly more elaborate system, two combustion heaters supply a plenum chamber which is maintained at a constant temperature. Air from the plenum chamber is then mixed with outside air to maintain desired cabin temperature. All of the warm air is discharged into the cabin through the walls. The discharge grilles are located on the floor under seats, and a modulating type controller varies proportions of heated and Transportation Air Conditioning 1087 outside air necessary to maintain desired cabin temperature. The same type of control system as previously described is used, except that an ampli fier operates a two-phase motor capable of positioning control dampers instead of operating a relay which would merely open, and close the: fuel valve. An auxiliary duct, running from the plenum chamber, is used by the pilot as a source of windshield defrosting air. Pressurized Cabins With the advent of the new high speed pressurized transport planes, and the addition of cabin cooling in addition to heating, the control problem becomes more complex. On all of these airplanes, the heat of compression from cabin supercharger must be controlled, the air cycle or expansion turbines must be turned on and also, the heat exchanger or combustion heaters, which are used in the system when cooling is required for additional heat, must be automatically controlled. Assuming that one of these airplanes is operating in an extremely cold climate, the sequence of operation would be as follows: The automatic controller for the supercharged-air inter-cooler would be in full ciosed position, so that none of the heat of compression would be removed, and the air would by-pass the expansion turbine and its compressor and the secondary after cooler. An additional automatic controller would be operating the combustion heater and supplying the additional heat necessary to maintain the desired cabin temperature. If a heat exchanger were used as a supplemental source of heat, a modulating control operating a damper on this exchanger would run towards full heat position. As the airplane enters a warm climate and heat requirements drop, the combustion heater would cease operation or the heat exchanger would go to full cold position, and the modulating control on the supercharger compressor would move towards the cold position. When the outside ambient temperature rises so high that cooling is desired, the cabin supercharger intercooler would be opened wide. 'If further cool ing were Required, the air would to into an air cycle turbine, which is modulated to deliver the required amount of cold air to maintain a comfortable cabin temperature. Pressure in the cabin is maintained by providing a controlled, constant rate of air flow into the cabin sufficient to maintain ventilation, and adjust ing the cabin-pressure relief valve setting, by means of a cabin pressure se lector, to maintain the desired cabin pressure. Limits on maximum inside to outside pressure may be of the order of 4 or 5 psi, and safety controls should be provided to prevent exceeding this limit. There is a maximum rate at which the cabin can change to a newly selected value, this rate being in some cases also adjustable. The requirements for controls of this nature are extremely rigid. It is commonplace for ships of this type to experience changes in outside ambient temperatures of as much as 100 deg in a space of 5 min. For this reason, speed of sensing a change and rapidity of response in the control system is essential if satisfactory control is to be accomplished. The older type thermostats cannot be used in airplanes, due to mass of the thermo stat and to vibration experienced on all airplanes. All modem controls use some type of bridge system with temperature sensitive resistors as -ensing elements. In some types of controls, the bridge system feeds a sensitive balanced relay, which in turn runs a modulating motor or controls an on-off power relay. A recent sensitive and quickly responding type uses an electronic amplifier, which in turn controls a two-phase motor, or, r ,rough relays, controls a d-c motor, or merely closes and opens a power e nv for on-off applications. tn addition to extreme speed and accuracy which are required of all aircraft temperature controls, they must be able to operate under great ex-