Temperature control:

The temperature control loop consists of a temperature transmitter, a temperature controller and a temperature control valve.  The diagonally crossed lines denotes in which the control signals are air (pneumatic).

A lube oil temperature is the controlled variable since it is maintained at a desired value (the setpoint). A Cooling water flow rate is the manipulated variable since it is adjusted through the temperature control valve to manage the lube oil temperature. A temperature transmitter senses the temperature of the lube oil as it leaves the cooler and sends an air signal which is proportional to the temperature controller.  Further, the temperature controller compares the real temperature of the lube oil to the setpoint (the desired value). The controller will vary the control air signal to the temperature control valve if a difference exists among the actual and desired temperatures. This causes it to move in the direction and through the amount needed to correct the difference. For  instance,  if  the  actual  temperature  is  greater  than  the  setpoint  value then the controller will vary the control air signal and cause the valve to move in the open direction.

This conclusion in more cooling water flowing by the cooler and lowers the temperature of the lube oil leaving the cooler.

(B) In Figure represents the lube oil temperature control loop in block diagram form. The lube oil cooler is the plant in this instance, and its controlled outcome is the lube oil temperature.  A temperature transmitter is the feedback element.   It senses the controlled outcome and lubes oil temperature and generates the feedback signal.

The feedback signal is sent to the summing point to be algebraically added to the reference input (the setpoint). Remember the setpoint signal is positive, and the feedback signal is negative.  That means the resulting actuating signal is the difference among the setpoint and feedback signals.

The actuating signal passes by the two control components: the temperature controller and the temperature control valve. A temperature control valve responds through adjusting the manipulated variable (the cooling water flow rate). A lube oil temperature changes in response to the different water flow rate, and the control loop is finished.