2.2.2 Water Content of the Refrigerant Vapour
The first important difference is the vapor pressure of ammonia / water mixtures compared to that of water/lithium bromide. Here, ammonia is the refrigerant. The normal boiling point is the -33.350c. And therefore the pressure of such systems at the temperatures normally encountered in air-conditioning and refrigeration applications is relatively high. The pressure is comparable to vapor compression systems that utilize R22 as the refrigerant. The high vapor pressure leads to rather small pipe diameters, and relatively compact heat exchange as compare to Water/LiBr. A second important difference is that the absorbent (water) has a vapor pressure that is not negligible relative to that of ammonia. As a consequence, the vapor generated in the desorber contains a certain amount of water. (The mass fraction depends on the mass fraction of the liquid mixture in the desorber, the temperature and the desorber design. Any water contained in the desorber vapor is detrimental to the performance of the system). The water will pass with the vapor into the condenser and then into the evaporator where the water tends to accumulate if a poor boiler design is used. The vapor leaving the evaporator is rich in ammonia. Although it still contains some water, the ammonia mass fraction of the vapor is considerably higher than that of the vapor leaving the desorber. Thus water will remain in the evaporator. If no other measures are taken the evaporator temperature has to be increased considerably to evaporate the remaining water-rich solution. An accumulation of water will lead to decrease in evaporator pressure which in turn will affect the absorber conditions. As the water accumulates in the evaporator the pressure drops for a constant evaporator temp which is fixed by the application. The absorber has either to be cooled to a lower temp or the mass fraction of the solution has to change to lower ammonia content. Assuming that the desorber temps do not change, the condenser temp must drop which is usually prohibited by the cooling water temp available to the condenser. a consequence the high pressure level has to be raised and the desorber temp will increase as well. As the water continue to accumulate in the evaporator the operating conditions of the entire system drift and operation at design conditions is not possible. One way of preventing excessive water accumulation is to drain it periodically into the absorber. However this method represents loss of efficiency in 2 ways. First the water was evaporated in the desorber requiring desorber heat input, but it does not evaporate in the evaporator; thus it does not provide to the cooling capacity. Second it contains a considerable amount of ammonia that is retained as liquid, amplifying this effect.
2.2.2 น้ำเนื้อหาของไอน้ำยาครอบThe first important difference is the vapor pressure of ammonia / water mixtures compared to that of water/lithium bromide. Here, ammonia is the refrigerant. The normal boiling point is the -33.350c. And therefore the pressure of such systems at the temperatures normally encountered in air-conditioning and refrigeration applications is relatively high. The pressure is comparable to vapor compression systems that utilize R22 as the refrigerant. The high vapor pressure leads to rather small pipe diameters, and relatively compact heat exchange as compare to Water/LiBr. A second important difference is that the absorbent (water) has a vapor pressure that is not negligible relative to that of ammonia. As a consequence, the vapor generated in the desorber contains a certain amount of water. (The mass fraction depends on the mass fraction of the liquid mixture in the desorber, the temperature and the desorber design. Any water contained in the desorber vapor is detrimental to the performance of the system). The water will pass with the vapor into the condenser and then into the evaporator where the water tends to accumulate if a poor boiler design is used. The vapor leaving the evaporator is rich in ammonia. Although it still contains some water, the ammonia mass fraction of the vapor is considerably higher than that of the vapor leaving the desorber. Thus water will remain in the evaporator. If no other measures are taken the evaporator temperature has to be increased considerably to evaporate the remaining water-rich solution. An accumulation of water will lead to decrease in evaporator pressure which in turn will affect the absorber conditions. As the water accumulates in the evaporator the pressure drops for a constant evaporator temp which is fixed by the application. The absorber has either to be cooled to a lower temp or the mass fraction of the solution has to change to lower ammonia content. Assuming that the desorber temps do not change, the condenser temp must drop which is usually prohibited by the cooling water temp available to the condenser. a consequence the high pressure level has to be raised and the desorber temp will increase as well. As the water continue to accumulate in the evaporator the operating conditions of the entire system drift and operation at design conditions is not possible. One way of preventing excessive water accumulation is to drain it periodically into the absorber. However this method represents loss of efficiency in 2 ways. First the water was evaporated in the desorber requiring desorber heat input, but it does not evaporate in the evaporator; thus it does not provide to the cooling capacity. Second it contains a considerable amount of ammonia that is retained as liquid, amplifying this effect.
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