What are the applicable scenarios, advantages, and disadvantages of the primary-pump and secondary-pump schemes in an air-conditioning water system?
I am an HVAC expert. This issue involves the core selection strategy for air-conditioning water system design and is directly related to system energy consumption, investment cost, and operational stability. In large buildings or district cooling and heating projects, the choice between primary-pump and secondary-pump schemes is often the key point in the trade-off between energy savings and initial investment. The following is a detailed analysis based on the relevant clauses of GB50736-2012 Code for Design of Heating Ventilation and Air Conditioning of Civil Buildings and engineering practice.
First, the definitions of the two schemes are clarified:
- **Primary-pump system**: The cooling source side (e.g., chillers) and the load side (e.g., air-conditioning terminals) share the same pumps, and the pump head must overcome the resistance of both the cooling source equipment and the piping network.
- **Secondary-pump system**: The cooling source side is equipped with primary pumps (constant frequency), and the load side is equipped with secondary pumps (variable frequency), achieving hydraulic decoupling through bypass pipes and differential pressure control.
**Applicable scenario analysis:**
1. **Primary-pump system**: Suitable for occasions where the system scale is relatively small, the resistance loss is not large (usually total resistance ≤30 mH₂O), and the load characteristics of each loop are similar. For example, a standalone office building, shopping mall, or small hotel. According to Clause 8.5.4 of GB50736-2012, when the pressure losses of each loop differ little, a primary-pump system should be adopted. This type of system has simple design and low initial investment, but pump energy consumption has a low correlation with load changes, and the problem of "large flow and small temperature difference" is likely to occur at partial load.
2. **Secondary-pump system**: Suitable for projects with a large system scale, long service radius (such as super high-rise buildings, large commercial complexes, and district cooling systems), large resistance differences among loops, or complex load characteristics. For example, in a certain ultra-luxury five-star hotel, the air-conditioning water system needs to supply both the guest room area (low-resistance terminals) and the banquet hall (high-resistance terminals). The secondary-pump system can precisely match the demand of each loop through variable-frequency regulation of the secondary pumps. Clause 8.5.4 of GB50736 also clearly states that when the pressure losses of each loop in the system differ greatly, a secondary-pump system should be adopted.
**Comparison of advantages and disadvantages:**
- **Primary-pump system**:
- Advantages: Simple system, fewer equipment items, direct control logic, and initial investment can be reduced by about 10%-15%; small plant room footprint and low maintenance workload.
- Disadvantages: The pump head must be determined according to the most unfavorable loop, and at partial load the pump energy consumption cannot decrease synchronously with the load; chillers and pumps require interlocked start-stop, resulting in poor flexibility; improper bypass pipe design can easily lead to the risk of chiller evaporator freezing.
- **Secondary-pump system**:
- Advantages: Primary pumps operate at constant frequency, ensuring stable chiller flow; variable-frequency secondary pumps can greatly reduce partial-load energy consumption (usually saving 20%-30%); each loop is independently regulated, suitable for differentiated cooling supply in multiple areas; switching the number of operating chillers does not affect terminal pressure.
- Disadvantages: Increased initial investment (an additional set of pumps, frequency converters, and control valves); complex control strategy, requiring differential pressure sensors and bypass pipes; large plant room space occupation; if the bypass pipe design is unreasonable, the phenomenon of "low-efficiency mixing" may occur.
**Actual engineering case:**
A large commercial complex (building area 120,000 ㎡) was originally designed with a primary-pump system. During actual operation, it was found that in the transition season (load rate 40%), pump energy consumption accounted for as much as 18% of the total system energy consumption, and terminal cooling capacity was uneven. After retrofitting to a secondary-pump system (constant-frequency primary pumps and variable-frequency secondary pumps) and adding differential pressure bypass control, measured pump energy consumption decreased by 32%, while also solving the problem of insufficient cooling supply in remote areas. However, it should be noted that the retrofit took 2 months, the initial investment increased by about 450,000 yuan, and the payback period was about 2.5 years.
**Summary:** The primary-pump system is suitable for projects with a small scale and balanced loop resistance, with low investment but relatively high energy consumption; the secondary-pump system is suitable for large, multi-zone projects, with significant energy savings but complex control. It is recommended to carry out hydraulic calculations and full life-cycle cost analysis in the early stage of the project, and to select the optimal scheme in combination with Clause 8.5.4 of GB50736 and the actual load characteristics of the project.
The above are reference opinions. The actual situation of the project still needs to be comprehensively evaluated and implemented in combination with specific operating conditions, site conditions, and other factors.