The secondary pump system is a variable flow system designed to regulate user load by adjusting the amount of circulating water. Common methods for variable volume control include varying the number of pumps and adjusting their speed.
3.1 Pump Number Adjustment
In traditional primary pump systems, differential pressure control is commonly used for load regulation. However, in secondary pump systems, flow control is more frequently applied, with load control being preferred in applications requiring higher precision.
Differential pressure control works by utilizing the parallel characteristics of pump curves. A return water pressure fluctuation range is set, and when load changes cause variations in pipeline flow, the supply and return pressures also fluctuate. If the pressure exceeds the set upper limit or drops below the lower limit, the number of pumps is adjusted accordingly.
Flow control, on the other hand, monitors the direction and magnitude of water flow within the bridge pipe to manage pump operation and the corresponding activation or deactivation of chillers. When user load decreases, secondary flow reduces, causing excess flow to be redirected from the cold water side back into the system. If the flow exceeds 110% of a single pump’s capacity, one chiller and its corresponding pump are turned off. Conversely, when load increases and flow is insufficient, cold water flows in reverse through the bridge, and if the flow exceeds 20% of a single pump’s capacity, an additional pump and chiller are activated. Activating chillers in advance helps prevent large fluctuations in secondary water temperature.
Figure 4 illustrates the cooling capacity and flow relationship for fan coil units in typical air conditioning systems [7]. Due to the non-linear thermal characteristics of terminal equipment [8]–[10], a reduction in flow to that of one pump does not necessarily mean the user load has dropped to the equivalent of one chiller’s capacity. Therefore, load-based control is often required for higher-precision applications. Load control calculates the required cooling capacity based on temperature difference and flow rate on the primary supply pipe. When the required cooling capacity corresponds to that of one chiller, the corresponding pump and chiller are turned off. Compared to flow control, load control effectively addresses mismatches between hydraulic and thermal conditions [1].
3.2 Variable Speed Control
The head of the secondary pump is used to overcome resistance from the pipeline, coils, balancing valves, and control valves. In a constant-speed variable-flow system, as flow decreases, the pressure drop across the pipeline, coils, and balancing valves also decreases. However, the pump head does not decrease but instead increases, leading to excessive pressure that must be managed by the control valve, as shown in Figure 5. This results in limited energy savings in such systems. At very low loads, the control valve may lose effectiveness due to excessive pressure drop, allowing too much cold water to pass through the coil.
Variable speed control overcomes these issues by adjusting the pump speed to reduce both head and flow when load decreases, resulting in significant energy savings. Considering inverter efficiency and motor cooling, variable speed systems should have a minimum speed limit (typically 30% of rated speed). For systems with wide load variations, multi-pump parallel variable speed control is often used to optimize energy efficiency. Figure 6 shows the pump power versus load curve under different operating modes. In a constant flow system, the pump operates at a fixed condition with consistent power. In a single pump constant speed system, power changes only slightly due to throttling via a two-way valve. In contrast, a multi-pump variable speed system retains significant energy-saving potential even at low loads.
4. Pump Speed Regulation Curve
According to the similarity law, pump power is proportional to the cube of its rotational speed under similar conditions. Ignoring static head, points on the system curve represent similar operating conditions that satisfy this law. In a variable speed water system, constant pressure control is commonly used, but the control curve and system curve do not align perfectly. As a result, the relationship between pump power and speed does not strictly follow the third-power law.
Figure 7 shows the relationship between the pump variable speed control curve and the system curve. The pump head consists of two parts: a constant pressure portion controlled by a differential pressure sensor, which remains unchanged regardless of flow, and a variable pressure portion related to the pipeline pressure drop, which is proportional to the square of the flow. By shifting the constant pressure line upward along the system curve, the control curve is established. From the figure, it is evident that a smaller constant pressure difference leads to better energy savings.
It should be noted that the control curve in Figure 7 assumes an average load ratio across all users. For example, if the system flow is reduced by 50%, each user is assumed to have a 50% load. In reality, user load varies based on individual needs, and the flow changes among users are rarely uniform.
GSH For Face Cream,glutathione materials,l glutathione powder,L-Glutathione reduced,Healthy L Glutathione
Changshu Enzyme Biotechnology Co., Ltd. , https://www.nmnglutathione.com