In refrigeration, what primarily causes pressure drops across high and low pressure sides?

In refrigeration systems, pressure drops across the high and low-pressure sides are mainly caused by friction within the components of the system. As refrigerant flows through various parts like pipes, valves, and filters, it encounters resistance due to the surfaces of these components. This resistance causes a loss of pressure, which manifests as a drop across the system.

Friction is an unavoidable phenomenon in any fluid dynamics scenario, and it plays a crucial role in the practical operation of refrigeration systems. The design and material of components significantly affect the level of friction. Higher flow rates, longer pipe runs, and various fittings can all contribute to increased friction, leading to more significant pressure drops.

The other factors listed, such as cooling capacity, thermal expansion, and evaporator malfunction, might influence system performance but they do not primarily account for the pressure drops across the refrigeration system. For instance, cooling capacity primarily relates to how much heat the system can remove, while thermal expansion refers to changes in refrigerant state rather than frictional effects. An evaporator malfunction could cause inefficiencies but is not the main cause of pressure drops. Therefore, the presence of friction in the components is the key reason for pressure drops in refrigeration systems.