For the injection system with a constant injection pressure,
with moving the injection port location away from the start of
compression to the end of compression, the pocket pressure
will continue increasing and the actual injection mass flow
rate will decrease gradually. But the cooling capacity will keep
constant in the prior period due to over-quality injection
refrigerant and the main loop refrigerant temperature at the
outlet of IHX keeps constant as the saturation temperature at
the injection pressure. However, in the later period, the cooling
capacity will decrease due to the lack of injection refrigerant
for subcooling the main loop refrigerant. However, the
indicated work will decrease in this whole process. Therefore,
the maximum COP will appear when the injection port is
located between the location for optimal cooling capacity and
the highest possible position (at that position, the pocket
pressure at the start of injection equals the injection pressure).
In other words, the injection port location for optimal
COP should be higher than the injection port location for
optimal cooling capacity.