CONTROL LOOP DIAGRAMS
Lubricating oil reduces friction between moving mechanical parts and also removes heat from
the components. As a result, the oil becomes hot. This heat is removed from the lube oil by a
cooler to prevent both breakdown of the oil and damage to the mechanical components it serves.
The lube oil cooler consists of a hollow shell with several tubes running through it. Cooling
water flows inside the shell of the cooler and around the outside of the tubes. Lube oil flows
inside the tubes. The water and lube oil never make physical contact.
As the water flows through the shell side of the cooler, it picks up heat from the lube oil through
the tubes. This cools the lube oil and warms the cooling water as it leaves the cooler.
The lube oil must be maintained within a specific operating band to ensure optimum equipment
performance. This is accomplished by controlling the flow rate of the cooling water with a
temperature control loop.
The temperature control loop consists of a temperature transmitter, a temperature controller, and
a temperature control valve. The diagonally crossed lines indicate that the control signals are air
The lube oil temperature is the controlled variable because it is maintained at a desired value (the
setpoint). Cooling water flow rate is the manipulated variable because it is adjusted by the
temperature control valve to maintain the lube oil temperature. The temperature transmitter
senses the temperature of the lube oil as it leaves the cooler and sends an air signal that is
proportional to the temperature controller. Next, the temperature controller compares the actual
temperature of the lube oil to the setpoint (the desired value). If a difference exists between the
actual and desired temperatures, the controller will vary the control air signal to the temperature
control valve. This causes it to move in the direction and by the amount needed to correct the
difference. For example, if the actual temperature is greater than the setpoint value, the
controller will vary the control air signal and cause the valve to move in the open direction.
This results in more cooling water flowing through the cooler and lowers the temperature of the
lube oil leaving the cooler.
(B) in Figure 9 represents the lube oil temperature control loop in block diagram form. The lube
oil cooler is the plant in this example, and its controlled output is the lube oil temperature. The
temperature transmitter is the feedback element. It senses the controlled output and lube oil
temperature and produces the feedback signal.
The feedback signal is sent to the summing point to be algebraically added to the reference input
(the setpoint). Notice the setpoint signal is positive, and the feedback signal is negative. This
means the resulting actuating signal is the difference between the setpoint and feedback signals.