Thermodynamic Effects of Water in Ammonia on Evaporator Perf
Thermodynamic Effects of Water in Ammonia on Evaporator Performance (2010)-There is increasing interest in operating ammonia evaporators at reduced liquid overfeed rates (less than n=2), down to and including dry expansion (DX). Benefits include: reduced or eliminated liquid pumping power, dry or near dry suction lines, reduced liquid and suction line sizes, greatly reduced charge inventory of ammonia evaporators (as much as 30X reduction for dry expansion compared to pumped ammonia), reduced first cost, etc. A small amount of water (typically 2•6% by weight) is found in most industrial ammonia systems. This water is always concentrated in the low pressure side of the system and has the negative thermodynamic effect of increasing the bubble point of the ammonia liquid along the length of the evaporator circuits. This shift in the bubble point is normally neglected by evaporator manufacturers and designers since the shift is small (only 1•2°F) for pumped ammonia systems. However, as the liquid overfeed rate is reduced below n=2, and particularly in the case of dry expansion, this shift in the bubble point toward the end of the evaporator circuits may easily exceed the initial temperature difference (Entering Air Temperature • Evaporating Temperature), severely reducing evaporator thermal performance. Additionally, the shift in bubble point can “confuse” dry expansion control valves, further reducing performance. These thermodynamic effects of water in ammonia have not been fully recognized or well understood in the past and are the subject of this paper. The author also reviews and recommends methods for managing and removal of water from these systems.
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