Compressed Air Systems for Starting and Control
(a) Starting Air Pressure:
The typical pressure used for starting air in compressed air systems can range from 25 bar (363 psi) to 40 bar (580 psi). This can vary depending on the size and type of engine being started.
(b) Reason for High Starting Air Pressure:
High starting air pressure is necessary for overcoming the following challenges during engine startup:
- Overcoming Engine Compression: The engine pistons compress the air within the cylinders as they rotate during startup. The starting air pressure needs to be high enough to overcome this compression and force the pistons down to initiate the combustion cycle.
- Friction Losses: Friction exists between various moving parts within the engine during startup. High starting air pressure ensures sufficient force to overcome these frictional losses and crank the engine effectively.
- Low Engine Speed: Initially, the engine speed is low during startup. This reduces the air intake efficiency, requiring a higher starting air pressure to deliver the necessary air volume for combustion.
(c) Control Air Pressure:
The pressure used for control air in compressed air systems is typically much lower than starting air pressure, ranging from 5 bar (73 psi) to 10 bar (145 psi). The specific pressure depends on the control equipment being operated.
(d) Reason for Different Pressures:
The pressure requirements for starting air and control air are different due to their distinct functions:
- Starting Air: Needs high pressure to overcome the high resistance encountered during engine startup, as explained in part (b).
- Control Air: Operates various control equipment and actuators within the system. These components typically require lower pressure to function effectively and efficiently. Using excessively high pressure for control air can damage delicate components or lead to over-actuation.
(e) Higher Control Air Pressure Than Minimum Requirement:
The control air pressure is typically chosen to be higher than the minimum pressure required to operate the control equipment for a few reasons:
- Safety Margin: A higher pressure provides a buffer against pressure drops within the system due to factors like long pipelines, bends, or leaks. This ensures reliable operation of the control equipment even with some pressure loss.
- Accounting for Future Expansion: Maintaining a higher pressure allows for future expansion of the control system with additional equipment that might require slightly higher pressure for operation.
- Standardization: Standardized control air pressure levels are often used within facilities or industries to simplify system design and component selection.
Even though a higher pressure ensures reliable operation, it’s important to avoid excessively high pressure that can waste energy and potentially damage control equipment. Choosing the optimal control air pressure involves considering the specific needs of the equipment, potential pressure losses, and overall system efficiency.