Document 6rY1EoMGgNppO7a1e21YoO31

674 CHAPTER 41 1. Temperature controller located in the compartment. 2. Temperature viator located in the compartment. 3. Dud air temperature anticipator located in the duct connect ing the water separator ana the compartment. 4. Compartment air temperature pickup located in the com partment. 5. Hot air modulating valve located in the duct <nn>itin the hot air supply duct with the cold tor duct downstream of the turbine discharge. Hie temperature controller converts signals from the tem perature anticipator, temperature pickup, and temperature selector into movement of the actuator of the hot air modu lating valve. Positioning of the valve permits hot air to by pass the heat exchangers and the cooling turbines. This hot air b mixed with the cold air from the turbine discharge in the proper proportions to satisfy the compartment temperature for which the selector is set. An all-pneumatic compartment temperature control system consists of the same number of components as the electronic temperature control system. Each component performs the same function as its corresponding part in the electronic sys tem. Air-Row Control The type of air-flow control system shown in fig. 8 for the basic air-cycle refrigeration system is a maximum air-flow limiting system in which the flow is limited by a simple Ven turi. flow is limited only when the throat of the Venturi b choked, Le., when sonic velocity exists in the nozzle throat. If the pressure of the high-pressure air source varies over a wide range, it b found that without an air-flow control system, more air b sometimes forced into the compartment than desired, while at other times less air is delivered to the compartment than b required for proper ventilation. In order to eliminate these undesirable conditions, a high air-flow con trol system and a low air-flow control system are snmatimea incorporated in the air-conditioning system. They are inde pendent of each other. A typical air-flow control system com monly utilized b the so-called constant AP type control. The AP referred to b the differential between Venturi upsteeam pressure and Venturi throat pressure. Two independent air-flow control systems are shown in fig. 9 for the bootstrap air-cycle air-conditioning system. One b a high airflow control system, which limits the air flow to the compartment to a maximum value, and the other b a tow air-flow control system which limits air flow to the compart ment to a mininmm value. Both systems consist of the follow ing items: L Air-flow sensor (Venturi), located in the high-pressure air duct upstream from the primary heat exchanger the high air-flow control valve. A singleVenturi is used. 2. High air-flow control head, mounted on the Venturi. 3. Low air-flow control bead, mounted on the Venturi. 4. High air-flow control valve, located in the high-pressure air duct upstream of the primary heat exchanger. 5. Low air-flow control valve, located in the duct which by passes the secondary heat exchanger, cooling turbine, and water separator. When the high air-flow control system b controlling system air flow, a constant Venturi AP b maintAmad by the auto matic positioning of the high air-flow control valve which throttles the supply air pressure. When the high air-flow con trol valve b throttling, the tow air-flow control valve b always closed. When the low air-flow control system b controlling system 1965 Guide And Data Book air flow, a constant Venturi AP b.maintained by the automatic positioning of the low air-flow control Valve. The Venturi AP maintained by the low air-flow control b a value less than the Venturi AP maintained by the high air-flow con trol system when that system b controlling. Modulation of the low air-flow control valve permits cool air from the discharge of the primary heat exchanger to bypass the secondary h^t exchanger, the cooling turbine, and the water separator, and then mix with the cold discharge air from the water separator. When the tow air-flow control valve b rtnyfoluting the high air-flow control valve b always wide open. Actuation of tire high and tow air-flow control valves b usually accomplished by pneumatic actuators because highpressure air b conveniently available.1 Water Separator and Turbine Anti-Ice Control The pneumatic water separator anti-ice control systems shown in figs. 8 and 9 consist of the following items: 1. Pneumatic thermostat and control head, located in the duct upstream from the water separator. 2. Antilles valve, located in the duct connecting the bot-air supply duct with the cold-air duct between the turbine dis charge and the water separator inlet. The anti-ice control system limits the dry-bulb temperature Of the air entering the water separator to a minimum nominal value of 35 F. The thermostat senses the temperature of thi air entering the water separator and maintains that air at a temperature of 35 F by positioning tire anti-ice valve so that the proper amount of hot air b mixnH with cold turbine dis charge air. . Another type of water separator anti-ice control system^ which b frequently employed, is the so-called AP-Type. This system consists of two pressure sensing* lines, a control and an anti-ice valve. The pressure differential .across'the water separator is sensed and the control differential pressure b set at a value .higher than the maximum pressure drop existing across the water separator under normal operating conditions. Thus, if icing conditions are not prevalent, the anti-ice valve will be closed. If ice begins to collect in the water separator, the water separator pressure drop begins to increase until the control differential pressure b reached. The anti-ice valve b then modulated to maintain the control differential pressure constant. The water separator inlet-air temperature will be maintained at approximately 32 F (dry bulb) or at the dew point of the air, if the dewpoint b less than 32 F. Under certain operating conditions, ice may form on the cooling turbine wheel. If this occurs to an extent sufficient to cause wheel unbalance and damaging vibration, it may be necessary to add some sort of turbine anti-ice control. This may be accomplished by using a back pressure device at .the turbine discharge, variable area turbine nozzles, or by moving the water separator anti-ice valve thermostat to the turbine discharge. SOURCES OF HK3H-PRESSURE AIR The following are common sources of high-pressure air used in air-cycle refrigeration systems for aircraft: 1. Jet engine and prop-jet engine compressors. 2. Auxiliary air compressors driven by main engine, either through an air turbine or by means of a shaft. 3- Auxiliary gas turbine compressor. A commonly-used source of high-pressure air for air-cycle air conditioning. system in jet-propelled airplanes is the \ Air<yde Equipment compressor of the turbojet engine. However, the bleed air from some jet engines b found at times to* be v^ntuminated with oil or toxic products formed by the decomposition of oil at big*1 temperature. If, for this reason, air from a turbojet atgine compressor cannot be used, then one or more auxiliary ajr compressor are installed in the airplane. The auxiliary air compresor can be equipped with an air turbine drive and can utilize engine bleed air as a power source, or it can be shaftdriven from the main engine accessory gearbox. Bleeding of high-pressure air from prop-jet engines for air conditioning is another common practice, but it b not so widely practiced as b bleeding of air from turbojet engines. Many prop-jet engines are limited to a relatively small amount of air that can be bled. In many instances this is I_, thnn the amount of air required for proper ventilation of the aircraft cabin. Under these circumstances, the usual pro cedure b to equip the airplane with auxiliary air compressors which are mechanically driven from an engine gearbox. In some instances a gas turbine compressor b used for sup plying high-pressure air for air conditioning of airplanes on the ground. Some' airplanes carry an auxiliary gas turbine on board and use it during flight for cabin pressurization and air conditioning. estimating performance of fixed-size AIR-CYCLE REFRIGERATION UNITS The method of determining the quantity and'temperature of refrigerated air delivered by a fixed-size air-cycle refrigera tion system is illustrated by Example* 1 and- S. Example ! deals with a basic system of fixed sue, represented by the component test performance curves shown in figs. 10 to 14. The entire system b shown in fig. 15. Example t deals with a bootstrap system of fixed size, represented by the component test performance curves shown in figs. 16.to 22. The entire system is shown in Fig. 23. 675 ' The symbols used in Examples 1 and t are defined here for convenient reference: A -- geometric area of turbine nozzle, square inches. A. = effective area of turbine nozzle, square inches. Cf specific heat at constant pressure. For air, c, = 0.24 Btu per (Cb) (F deg). T> s tip diameter of compressor, impeller, ten wheel, or turbine wheel, inches. E * effectiveness of heat exchange (basic system), dimension- less. Ei > effectiveness of primary heat exchanger (bootstrap sys tem), dimensionless. Et * effectiveness of secondary heat exchanger (bootstrap System), dimermyinlega Fcr ~ compressor flow factor, dimensionless. Frr m turbine flow factor, dimensMinlraa. Ftp " turbine power factor, dimeasionlea. N Ft " turbine velocity factor ** ' Ky/dli where K * Jy/ Jgc, /f1--2-- --X--60J\*, dimensionless. < g = 32.2 feet per (second) (second). J mechanical equivalent of heat, 778 foot pounds per Btu. M -- Mach number, dimensionless. N rotational speed of cooling turbine shaft, revolutions per minute. P -- total pressure of air, inches Hg absolute. Pt ~ average total pressure of air between two points in the system, inches Hg absolute. * for Alr-to-Alr Heat Exchanger to a Sctfe Alr-Cyd* Miigerotton that. Re- 10.... Test Performance .Curve---Effectiveness v* Bleed Air Row Rate* * For Alr-t>Mr Heat Exchanger to a Berk Air-Cyde ttoMgeratton Unit. fig, 11 .... Test Performance Curve--Pressure Drop vs Air Row Rate* ` `