Outwardly equalized TXVs compensate for the large pressure drops. As a refrigerant flows dirough evaporator, he experienced a drop in pressure due to friction forces in the refrigerant coil pipes. This differential pressure decreases the saturation temperature of the refrigerant in accordance with its correlation between pressure and temperature. Thus, the pressure and temperature of the die refrigerant is always lower evaporator than it was at the entrance. When the pressure drop across the evaporator is relatively small, the associated drop the saturation temperature is also small and is usually neglected. However, when the pressure drop of a significant size, the saturation temperature of the refrigerant from the evaporator to be significantly lower than at the inlet of the evaporator. This condition
impairing the operation of the expansion valve, as this requires more suction superheat to create equilibrium condition between its internal forces. The volume of overheating required to achieve equilibrium is tuning the vernal plus the difference in temperature of saturation caused by die pressure drop.
With more surface of the evaporator will be needed to absorb the sensible heat overheating, increased requirements, the amount of the evaporator surface that can be used for more effective cooling is reduced. In other words, TXV will start to starve evaporator to provide the necessary increase of overheating, thereby reducing system performance.
To illustrate this answer, R-134a systems evaporator and internally equalized TXV described earlier. If the system experiences a 5 psi (34.5 kPa) pressure drop through the evaporator, the saturation pressure at the outlet will be reduced to approximately 28.1 psia (196 kPa).IN the saturation temperature corresponding 28.1 psi (196 kPa) is approximately 12 F. based on these characteristics, approximately 18F (10C) suction superheat is now necessary to ensure 30 F suction pair desired sensing bulb. To satisfy this more overheating requirement, the evaporation of the liquid must be completed in point B'. This response increases the surface area of the coil, dedicated absorbing sensible heat required to raise the temperature of vapor. Therefore, part of evaporators surface becomes ineffective for absorption of refrigerants latent heat, which reduces productivity and efficiency of the cooling system...
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