Can screw compressors and centrifugal chillers be used in parallel?

Feasibility of Parallel Operation Between a Screw Compressor and a Centrifugal Chiller

In large-scale central air-conditioning and industrial refrigeration systems, it is entirely feasible to operate screw-type chillers in parallel with centrifugal chillers. This configuration has been widely adopted in practical engineering applications, primarily to leverage the distinct operating characteristics of different compressor types and thereby optimize the overall system efficiency.

The main advantages of parallel operation

• Expand the load regulation rangeCentrifugal chillers typically achieve optimal efficiency at higher load rates, whereas screw chillers offer excellent modulation capabilities and energy efficiency under low-load conditions. Operating the two in parallel enables coverage of a broader load range.
• Enhance system reliability: By paralleling compressors that operate on different principles, effective equipment redundancy can be achieved. When one unit requires maintenance or experiences a failure, the other unit can continue to handle part of the load, ensuring continuous system operation.
• Optimize part-load performanceDuring transitional seasons or under low-load conditions, the system can prioritize operation of the screw compressor, thereby avoiding the potential efficiency degradation or surge risk that may occur with centrifugal compressors at low loads.

Key considerations when using devices in parallel

Although parallel operation offers numerous advantages, its design and operation must address the technical challenges arising from the differences between the two systems’ characteristics:

• Water System Resistance and Flow Rate Matching: The internal hydraulic resistance of evaporators and condensers in screw chillers and centrifugal chillers typically differs. When designing parallel piping systems, it is essential to accurately calculate pressure losses and, if necessary, install balancing valves to prevent flow short‑circuiting or uneven distribution, thereby ensuring that each unit achieves its designed water flow rate.
• Control Logic and Group Control System: The load‑following and unloading characteristics, as well as the protection logic, differ between the two types of chillers. A sophisticated centralized control system is essential to intelligently determine the start‑stop sequence and load distribution based on changes in terminal‑side demand, thereby preventing frequent cycling or load oscillations.
• Anti-Surge and Oil Return ManagementCentrifugal chillers are prone to surge under low-load conditions or when the cooling water temperature is too low, whereas screw chillers require careful attention to oil return at low loads. The group‑control strategy must take both cooling‑water and chilled‑water temperatures into account to ensure that both types of units operate within safe operating limits.

Engineering Design and Operational Recommendations

To achieve optimal performance when operating a screw compressor in parallel with a centrifugal chiller, the following recommendations should be followed in engineering practice:

First, optimize the capacity ratio: it is generally recommended to use centrifugal chillers as base-load units to handle the steady‑state load, while screw chillers serve as peaking units to accommodate load fluctuations. Second, select equipment that supports open communication protocols to enable seamless integration with a unified building automation system. Finally, enhance routine water‑quality management and condenser cleaning to maintain heat‑exchange efficiency—key factors for ensuring the long‑term energy‑efficient operation of the interconnected system.