How can system pressure be adjusted manually in refrigeration systems?

Adjusting system pressure manually in refrigeration systems primarily involves manipulating the flow of refrigerant, which is most effectively achieved by adjusting expansion devices. These devices, such as thermostatic expansion valves or electronic expansion valves, regulate the refrigerant flow into the evaporator based on the system's current conditions. When you adjust the setting or opening of these devices, you modify the pressure difference across them, allowing you to control how much refrigerant enters the evaporator, thereby adjusting the pressure within the system.

Adding more refrigerant may affect system pressure, but it is not a direct method of control and can lead to overcharging if not done carefully. Changing the temperature setting influences operation but does not directly adjust system pressure. Opening service valves can affect the system's pressure; however, this method is more about access and does not provide a precise means of managing pressure levels under normal operating conditions.

Thus, the best practice for manually adjusting system pressure is through the adjustment of expansion devices, making this choice the most accurate response.