Lubricating oil reduces friction between moving parts and also removes heat from the components. As a result, the oil becomes hot. This heat is removed from the lubricating oil by a cooler to prevent both the breakdown of the oil and damage to the mechanical components it serves.
The lubricating 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. Lubricating oil flows inside the tubes. The water and lubricating oil never make a physical contact.
As the water flows through the shell side of the cooler, it picks up heat from the lubricating oil through the tubes. This cools the lubricating oil and warms the cooling water as it leaves the cooler.
The lubricating 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, temperature controller, and a temperature control valve. The diagonally crossed lines indicate that the control signals are air (pneumatics).
The lubricating 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 lubricating oil temperature. The temperature transmitter senses the temperature of the lubricating oil as it leaves the cooler and sends an air signal that is proportional, to the temperature controller.
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The temperature controller compares the actual temperature of the lubricating oil to the setpoint (the desired value). If a difference exists between the actual and desired temperature, 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 lubricating oil leaving the cooler.
Related: Feedback Control System
From the lubricating oil temperature control block diagram above, the lube oil cooler is the plant, and its controlled output is the lube oil temperature. The temperature transmitter is the feedback element. It senses the controlled output and lubricating oil temperature and produces the feedback signal.
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The actuating signal passes through the two control elements: the temperature controller and the temperature control valve. The temperature control valve responds by adjusting the manipulated variable (the cooling water flow rate). The lubricating oil temperature changes in response to the different water flow rate, and the control loop is complete.
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