Analysis of the Advantages and Disadvantages of Fixed‑Frequency Screw Chillers and Variable‑Frequency Screw Chiller Units

Technical Features of Fixed‑Frequency Screw Chiller Units

Fixed‑speed screw chillers employ compressors driven by motors operating at a constant speed and utilize a slide‑valve mechanism for capacity modulation. This design has established a broad and well‑rooted presence in the HVAC industry.

Key Advantages:

• Mature and reliable technology:The control system is relatively simple and has been thoroughly validated by the market over the long term, ensuring high operational stability.
• Lower initial investment cost:There is no need to install costly variable-frequency drives or complex VFD control systems, giving the equipment a significant cost advantage.
• Relatively easy to maintain:Due to the simplicity of the electrical control system, routine maintenance and troubleshooting are relatively straightforward, and maintenance costs remain manageable.

Limitations:

• High inrush current:Direct-on‑line starting or reduced‑voltage starting can impose significant inrush currents on the power grid, necessitating adequate grid capacity.
• Part-load efficiency degradation:At low-load operation, poppet‑valve unloading incurs significant throttling losses, resulting in a marked decline in part‑load efficiency.

Technical Features of Variable‑Frequency Screw Chiller Units

The variable‑frequency screw chiller uses a frequency inverter to adjust the supply frequency of the compressor motor, enabling stepless speed control and ensuring precise matching between cooling capacity and actual load.

Key Advantages:

• Excellent part-load energy efficiency:Under off‑design conditions, the motor speed decreases as the load drops, significantly reducing throttling losses and resulting in outstanding performance in terms of the integrated part‑load value.
• Smooth startup:By employing a soft-start method, the inrush current is minimized, effectively reducing the impact on the power grid and extending the service life of electrical equipment.
• Precise temperature control:The stepless modulation capability ensures extremely minimal fluctuations in the outlet water temperature, delivering a higher‑quality chilled water supply.

Limitations:

• Higher initial investment:The high cost of variable-frequency drives and associated control components has driven up the overall procurement price of the equipment.
• High maintenance requirements:The complexity of variable-frequency drives and control systems increases the technical challenges and potential costs associated with subsequent maintenance.
• Full-load energy efficiency decreases slightly:Under full-load operation, the variable-frequency drive itself incurs a certain amount of electrical energy loss, and its energy efficiency may be slightly lower than that of a fixed-frequency unit with the same specifications.

Engineering Selection and Application Scenario Recommendations

In real-world engineering projects, the decision between a constant‑speed and a variable‑speed screw chiller should be based on a comprehensive assessment of load characteristics, the project budget, and operating costs.

Application scenarios for fixed-frequency units:It is suitable for buildings or industrial projects with relatively stable cooling loads, short annual operating hours, limited initial investment budgets, and ample grid capacity.

Application scenarios for variable-frequency units:It is suitable for large commercial complexes or data centers characterized by highly variable cooling loads, year-round operation, stringent temperature‑control accuracy requirements, and a strong emphasis on long‑term energy‑efficiency benefits.