SHUTTERS
Another method for controlling head pressure is with the use of shutters. Shutters can be opened or closed to control the airflow across the condenser.
Typical output voltage and temperature curve for a variable-speed fan cycle control. (Source: Delmar/Cengage Learning) They can be installed on the inlet or outlet side of the condenser fan. A pressure-operated piston is used to open the shutters and permit more air to flow. The piston is connected to the high side of the system in
much the same way as the pressure-operated fan cycle switch. If pressure decreases, the shutters close to restrict airflow. If the pressure rises, the piston pushes against a mechanical rod causing the shutters to open and permit more airflow.
An advantage of shutters is that they open and close slowly maintaining an even head pressure. If a condenser contains multiple fans, one fan is generally equipped with a shutter and the other fans will be cycled on or off with pressure-activated or temperature-activated switches.