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Water ring heat pump system variable water volume operation and its related problems
In the real estate sector, water ring heat pump air conditioning systems have gained widespread adoption due to their high efficiency compared to air-cooled systems, ease of installation, and ability to allow independent billing for each unit. These systems offer economic benefits, but they also face challenges in operation, particularly regarding energy consumption during low-load periods or transitional seasons when only a few users are present. The question arises: how can we reduce the power consumption of the pumps under such conditions?
Currently, many real estate projects use sub-district, independently controlled water ring heat pump units. Each district or household manages its own cooling system, adjusting temperature and operating time as needed. While this allows for flexible control and accurate billing, it also leads to inefficiencies in the centralized pumping and cooling tower operations, which are shared among all users. This raises concerns about optimizing pump performance during times of low demand.
To address this, variable-speed pumping is often considered. However, traditional central air conditioning systems have limitations in terms of the minimum flow rate that chiller evaporators can handle—typically 50% to 70% of rated flow. In contrast, water ring heat pump systems can operate at partial loads as low as 10% or even less. So, how can the water system maintain economic operation under these extreme conditions? What design measures should be taken?
Several strategies can be implemented:
1. Install two-way valves on the cooling water inlet of each unit, which open and close in sync with the unit’s operation.
2. Link the pump operation on the unit side with the two-way valve; the pump starts whenever any valve is open.
3. Use frequency control based on supply and return pressure to adjust pump speed dynamically.
4. Design the pump size according to the load characteristics of the system to avoid over-sizing.
5. Control the cooling tower (or boiler) side pump based on return water temperature, activating it when the temperature exceeds 32°C or drops below 16°C.
The key challenges in implementation are determining the appropriate pump configuration and developing an effective control strategy. Let's explore these in more detail.
**Pump Configuration on the Unit Side**
When using variable-speed pumps, two critical factors must be considered:
- Avoiding resonance between the pump’s vibration frequency and the natural frequency of the vibration isolation system.
- Preventing the pump from entering the surge zone due to changes in pipeline characteristics caused by closed two-way valves.
For example, consider a project where the total cooling water flow is 600 t/h, and the system may operate at as low as 5% of the design load. Normally, three pumps of 200 t/h each would be used. At 5% load, the pump speed would drop to around 218 rpm, reducing the vibration frequency to approximately 3.6 Hz. If the shock absorber has a natural frequency of 6 Hz, this could lead to resonance, causing excessive vibration and potential damage. To prevent this, the pump speed should not fall below 900 rpm, corresponding to a 20% load. Therefore, a smaller pump (e.g., 120 t/h) is added to handle lower loads efficiently.
By adding a small pump, the system can switch between large and small pumps depending on the load, ensuring that the pump always operates within its safe and efficient range. For even lower loads, another smaller pump (e.g., 75 t/h) can be introduced, further reducing power consumption.
**Variable Volume Control System**
A DDC (Direct Digital Control) system is commonly used to manage variable volume pumping. This approach offers several advantages, including reliability, energy savings, cost-effectiveness, ease of use, and scalability. It also supports soft starting, reducing the impact on the power grid during startup.
Each floor or area has a local controller that monitors the status of the two-way valves. When a unit is activated, the valve opens, sending a signal to the central controller, which then adjusts the pump accordingly. When all units are off, the system automatically turns off the pumps.
During operation, the pump speed is adjusted based on the pressure difference between the supply and return lines. As more units are activated, the pressure drop increases, prompting the pump to increase its speed. If the pressure continues to drop, additional pumps are engaged in sequence until the system reaches full capacity.
On the cooling tower (or boiler) side, the pump operates in a fixed-volume mode, controlled by the return water temperature. It activates when the temperature exceeds 32°C or drops below 16°C.
**Economic Analysis of Variable Volume Operation**
To assess the energy savings, let’s look at an example. A real estate project with an annual air-conditioning runtime of 6000 hours (250 days) uses three main pumps (200 t/h, 22 kW each), one medium pump (120 t/h, 11 kW), and one small pump (75 t/h, 7.5 kW). The energy consumption at different load levels shows significant savings when using variable-speed pumping.
Without the smaller pumps, the system would consume more energy at low loads. With the addition of the small pumps, the overall energy consumption decreases by nearly 50%, demonstrating the economic benefit of a well-designed variable volume system.
In summary, while low-load operation may not require frequent use of small pumps, including them in the design can significantly improve efficiency over time. The choice of configuration depends on the specific operational profile of the building, but implementing variable-speed pumping and smart control systems is essential for maximizing energy savings and system performance.