COOLING AND PROTECTION
Heat is generated in the compression process and discharge temperature must be limited to avoid risk of oil or refrigerant decomposition. The temperatures encountered are dependent on the operating conditions and the refrigerant. Under many conditions cold suction gas combined with heat loss to atmosphere provides sufficient cooling for small machines. For some operating conditions a fan may be specified by the manufacturer. Refrigerants such as ammonia giving high discharge temperatures require the use of water-cooled cylinder heads. Oil coolers may be needed which may be water, air or refrigerant cooled. Discharge temperature will tend to be higher when operating at part load conditions.
Compressors can overheat if the mass flow rate becomes very low, for example, at conditions resulting from abnormally low suction pressure. This can be the result of loss of refrigerant charge. Low mass flow rate will also result in loss of oil to the system and the onus is on the system designer to ensure adequate oil return, otherwise lubrication will be impaired. Liquid refrigerant may enter the compressor under fault conditions and this can reduce the lubricant viscosity, and excessive amounts may cause severe damage.
These system issues can have the effect of shortening compressor life, and it is usual to provide compressors with fault protection. Temperature and oil protection are the most effective and usually take the form of ‘cut-outs’ which stop the compressor in the event of excessive temperature or insufficient oil pressure. Motor protection is normal for enclosed motor types and may take the form of temperature sensors embedded in the windings. For small compressors an internal line break protector which is sensitive to both temperature and electric current is sometimes used, Figure 4.14 . Each leg has a heater and a contact and all three open and close at the same time. It is positioned where the three motor windings meet and all three phases are taken out in the event of overheat and/or overload. Because it is built-in and internal there is no need to bring wires back out of the casing for external connections, and it cannot be accidentally by-passed.
Figure 4.14 L ine break protector located at the meeting point of the motor windings
Some applications are known to require the compressor to withstand liquid return. For example, it is not economically feasible to provide preventative system controls on small reversible air conditioners to ensure no liquid return on defrost cycles. Here the compressor manufacturer will ensure that the compressor design has been life tested to give many years of reliable operation under foreseeable conditions.
When first started, a refrigeration system may operate at a higher suction temperature and pressure than at normal operating conditions, and consequently a higher discharge pressure, taking considerably more power. Moreover, during the first seconds of operation, the motor is required to provide sufficient torque to accelerate the compressor. Assisted start devices are used for most commercial and industrial applications and may take the form of unloaded start bypass, or suction pressure regulation. On the electrical side a frequent requirement is to minimize the electrical surge occurring on start up. Start devices such as star delta or part-winding start motors are used. For open compressors the drive motors must be sized accordingly to provide this pulldown power and an allowance of 25% is usual. As a result, the drive motor will run for the greater part of its life at something under 80% rated output, and so at a lower efficiency, low running current and poor power factor. For semi-hermetic and hermetic compressors the starting characteristics are defined to ensure minimal over-sizing of the motor.
Posted in Refrigeration and Air Conditioning