What will happen if an 8-kg screw air compressor head is used on a 5-kg system?

In industrial production, the selection of pressure for screw air compressors directly affects equipment efficiency and service life. Occasionally, companies find themselves using high-pressure compressor heads in low-pressure applications—for example, installing a head designed for an 8‑bar discharge pressure in a 5‑bar system. Such practices can trigger a series of physical and mechanical changes during operation, necessitating careful consideration.

Changes in Displacement and Volumetric Efficiency

The air displacement of a screw compressor head is closely related to its operating pressure. When an 8‑bar compressor head operates at a lower back pressure of 5 bar, the pressure ratio inside the head decreases significantly.

• Leak reduction: A reduced pressure ratio implies a decrease in gas leakage across the rotor clearance, thereby improving volumetric efficiency.
• Displacement increased: At a constant rotational speed, the actual displacement of the compressor head will exceed its rated displacement under an 8‑kg operating condition.

Risks of Motor Load and Power Matching

An increase in displacement does not necessarily translate into improved overall system efficiency; on the contrary, it may introduce the risk of motor overload. The shaft power of an air compressor is closely related to the product of its displacement and pressure ratio.

• Shaft power increasesAlthough the pressure ratio decreases, the substantial increase in displacement may cause the total shaft power to exceed the rated power of the original motor.
• Overload protectionIf the motor’s power rating is not upgraded accordingly, prolonged operation under overload conditions will trigger thermal relay protection, resulting in frequent shutdowns and potentially even motor burnout.

Internal Operating Conditions and Life‑Cycle Degradation of the Aircraft Nose

During the design phase, the rotor profile, bearing arrangement, lubrication system, and cooling oil circuit of the compressor head were all optimized based on the rated pressure. Altering the operating pressure will disrupt the original design equilibrium.

• Change in force application: Changes in the axial and radial forces acting on the rotor may lead to uneven bearing loads, accelerating bearing wear.
• Lubrication and Temperature AbnormalitiesAn increase in displacement can lead to changes in fuel‑injection demand; if the cooling system is not adjusted accordingly, it may cause abnormal exhaust‑gas temperatures at the cylinder head, compromising lubricant performance and ultimately reducing the service life of the cylinder head.

Scientific Recommendations for Pressure Selection

To avoid the aforementioned issues, companies should adhere to the following principles when selecting pressure equipment:

• For exclusive use on the special aircraft: Select a compressor head with the appropriate pressure rating based on the actual gas‑pressure requirements; for a 5‑bar operating condition, choose a head designed for 5 bar or 6 bar.
• System MatchingEnsure that the nose, motor, cooling system, and control system operate in concert under design operating conditions to achieve optimal energy efficiency.
• Variable-frequency controlIf air demand fluctuates significantly, it is recommended to use a variable-frequency screw air compressor, which adjusts its speed to accommodate changes in air volume rather than compromising by altering the compressor head’s design pressure.