The secondary pump system is designed as a variable water flow system, where load regulation is achieved by adjusting the amount of circulating water. Common methods for variable volume control include unit switching and speed adjustment.
3.1 Unit Switching
In traditional primary pump systems, differential pressure control is commonly used for load regulation. However, in secondary pump systems, flow rate control is more frequently applied, with load control being preferred in applications requiring higher precision. Differential pressure control relies on the parallel characteristics of the pumps, setting a range for return water pressure fluctuations. When the load changes, causing variations in pipeline flow, the supply and return pressures also fluctuate. If the pressure exceeds the set limit, additional pumps are activated; if it drops below the lower limit, some pumps are turned off.
Flow control, on the other hand, adjusts the operation of the pumps and chillers based on the direction and magnitude of the water flow within the bypass pipe. When user load decreases, the secondary flow reduces, leading to an excess flow that is redirected back through the bypass. If the flow exceeds 110% of a single pump’s capacity, a chiller and its corresponding pump are turned off. Conversely, when the load increases and flow is insufficient, cold water flows in reverse through the bypass. If the flow exceeds 20% of a single pump’s capacity, an additional pump and chiller are activated. Activating the chiller in advance helps avoid large temperature fluctuations in the secondary water supply.
Figure 4 illustrates the cooling capacity and flow relationship for fan coil units in typical air conditioning systems. Due to the non-linear thermal characteristics of terminal equipment, a reduction in flow does not necessarily mean a proportional reduction in user load. Therefore, load control is often required for more accurate regulation. Load control calculates the required cooling capacity based on the temperature difference and flow rate on the primary supply pipe. When the required cooling capacity corresponds to the output of one chiller, that pump and chiller are shut down. Compared to flow control, load control effectively addresses hydraulic and thermal imbalance issues.
3.2 Variable Speed Adjustment
The secondary pump must overcome resistance from the piping network, coils, balancing valves, and control valves. In a constant-speed variable flow system, as flow decreases, the pressure drop across the piping, coils, and balancing valves also decreases. However, the pump head does not reduce but instead increases, requiring the control valve (two-way valve) to compensate, as shown in Figure 5. This results in limited energy savings. At very low loads, the control valve may lose effectiveness due to excessive pressure drop, allowing excess cold water to pass through the coil.
Variable speed adjustment overcomes these issues by reducing both the pump head and flow when the load decreases, leading to significant energy savings. Considering inverter efficiency and motor cooling, variable speed systems should have a minimum speed limit—usually around 30% of the rated speed. For systems with large load variations, multi-pump parallel variable speed control is often used to achieve optimal energy efficiency.
Figure 6 shows the power consumption of different pump operation modes versus load. In a constant flow system, the pump operates at a fixed power. A single pump with constant speed only uses throttling via a two-way valve, resulting in minimal power change. In contrast, a multi-pump variable speed system maintains significant energy-saving potential even at low loads.
4. Pump Speed Regulation Control Curve
According to the similarity law, under similar conditions, pump power is proportional to the cube of its rotational speed. Ignoring static head, points on the system curve represent similar operating conditions that follow this law. In a variable speed water system, constant pressure control is commonly used, but the control curve and system curve do not align. As a result, the relationship between pump power and speed does not strictly follow the third power law.
Figure 7 illustrates the relationship between the pump’s variable speed control curve and the system curve. The pump head consists of two parts: a constant pressure differential controlled by a sensor, which remains unchanged regardless of flow, and a variable pressure related to the pipe network’s pressure drop, which is proportional to the square of the flow. By shifting the constant pressure control curve upward along the system curve, it becomes clear that a smaller constant pressure differential leads to better energy efficiency.
It should be noted that the control curve in Figure 7 assumes an average user load ratio. For example, if the system flow is reduced by 50%, each user’s demand is assumed to be 50%. In practice, user loads vary based on individual needs, and flow changes rarely occur uniformly across all users.
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