【正文】 nsity occurs at or near noon. Solar heat gain through all glass surface area normal to the incident light is a substantial fraction of the cooling load. Horizontal and Reflected Radiation. The intensity is significantly less, but the glass area is large enough to merit 山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文 5 consideration. Surface Heating. The temperature of the surface is a function of the solar energy absorbed, the interior and ambient temperatures, and the automobile’s velocity. The temperature control should not produce objectionable sounds. During maximum heating or cooling operation, a slightly higher noise level is acceptable. Thereafter, it should be possible to maintain fort at a lower blower speed with an acceptable noise level. Compressorinduced vibrations, gas pulsations, and noise Surface Transportation must be kept to a minimum. Suction and discharge mufflers are often used to reduce noise. Beltinduced noises, engine torsional vibration, and pressor mounting all require particular attention. COMPONENTS Compressors Scroll and piston pressors are used in mobile applications. They have the following characteristics: Displacement. Fixed displacement pressors have displacements of to L/rev. Variable displacement pressors have a minimum displacement of about L/rev, about 10% of their maximum displacement. A typical variable capacity scroll pressor has a maximum displacement of L/rev and a minimum displacement of just 3% of the maximum. Physical Size. Fuel economy, lower hood lines, and more engine accessories all decrease pressor installation space. These features, along with the fact that smaller engines have less accessory power available, promote the use of smaller pressors. Speed Range. Because pressors are belt driven directly from the engine, they must withstand speeds of over 100 rev/s and remain smooth and quiet down to rev/s. In the absence of a variable drive ratio, there are instances when the maximum pressor speed may need to be higher in order to achieve sufficient pumping capacity at idle. Torque Requirements. Because torque pulsations cause or aggravate vibration problems, it is best to minimize them. Minimizing peak torque benefits the pressor drive and mount systems. Multicylinder reciprocating and rotary pressors aid in reducing vibration. An economical singlecylinder pressor reduces cost。 however, any design must reduce peak torques and belt loads, which would normally be at a maximum in a singlecylinder design. Compressor Drives. A magic clutch, energized by power from the car engine electrical system, drives the pressor. The clutch is always disengaged when air conditioning is not required. The clutch can also be used to control evaporator temperature (see the section on Controls). Variable Displacement Compressors. Wobble plate pressors are used for automobile air conditioning. The angle of the wobble plate changes in response to the evaporator and discharge pressure to achieve a constant suction pressure just above freezing, regardless of load. A bellows valve or electronic sensorcontrolled valve routes internal gas flow to control the wobble plate. Refrigerants and Lubricants 山東建筑大學(xué)畢業(yè)設(shè)計外文文獻(xiàn)及譯文 6 New Vehicles. The phaseout of R12 has led to the use of nonCFC alternatives in vehicles. The popular choice is R134a because its physical and thermodynamic properties closely match those of R12. Because its miscibility with mineral oil is poor, R134a requires the use of synthetic lubricants such as polyalkylene glycol (PAG) or polyol ester (POE). PAGs are currently preferred. These lubricants—especially PAGs—are more hygroscopic than mineral oil。 production and service procedures must emphasize the need for system dehydration and limited exposure of the lubricants to atmospheric moisture. Older Vehicles. Vehicles originally equipped with R12 do not require retrofit to another refrigerant as long as an affordable supply of new or reclaimed R12 exists. When circumstances require it, most air conditioners can be modified to use R134a and POE. When PAG oil is used in a retrofit, the remaining mineral oil should