A dirty evaporator and subsequent low evaporator pressure will cause which condition?

A dirty evaporator can significantly affect the performance of a refrigeration system, primarily by impeding the heat exchange process. When the evaporator coil is dirty, it reduces the surface area available for heat absorption from the refrigerant, resulting in insufficient evaporation of the refrigerant. This insufficient evaporation leads to lower pressure in the evaporator, known as low evaporator pressure.

When the evaporator pressure is low, it requires the compressor to work harder to draw the refrigerant vapor and compress it to the discharge pressure, which is indicative of a higher compression ratio. The compression ratio is defined as the ratio of the discharge pressure to the suction pressure. Therefore, with a low evaporator pressure, the suction pressure is also low, and this situation results in a higher compression ratio as the compressor struggles to achieve a higher discharge pressure from the low suction pressure.

In summary, a dirty evaporator results in low evaporator pressure, which translates to high compression ratios in order to maintain the required cooling capacity of the system.