What could cause a significant drop in compressor discharge temperature along with a high suction line pressure in a reciprocating refrigeration system?

A significant drop in compressor discharge temperature along with a high suction line pressure in a reciprocating refrigeration system can indeed be attributed to an expansion valve that is open too wide. When the expansion valve is excessively opened, it allows an abnormally large amount of refrigerant to flow into the evaporator. This excessive flow can lead to a rapid increase in the cooling effect within the evaporator, which results in less heat absorption from the refrigerant. As a consequence, the refrigerant reaches the compressor at lower temperatures, which explains the drop in discharge temperature.

Moreover, the high suction line pressure suggests that the evaporator is effectively utilizing the refrigerant, but since it's flowing in too rapidly due to the malfunctioning expansion valve, it prevents the refrigerant from absorbing sufficient heat. This situation leads to inefficient compressor operation and changes in the thermodynamic characteristics of the system, reinforcing why an open expansion valve can cause this specific combination of symptoms.

High suction line pressure might confuse the diagnosis, but it aligns with the excessive amount of refrigerant moving through the system without being adequately vaporized or heated. The other options do not provide the exact coupling of symptoms as seen here; particularly, issues such as a leak or compressor malfunction would tend to have different or more complex