Design principles for the temperature difference of the return water in a circulating water system

The design principle for the return water temperature difference in a circulating water system is to ensure effective heat transfer as well as energy savings and emission reduction, based on the system’s thermal load and flow requirements. The ideal temperature difference of the return water should meet the thermal efficiency requirements for equipment operation while also ensuring hydraulic stability within the system. The design basis usually includes: 1. The thermal load requirements of the system. 2. Head and flow characteristics of the water pump. 3. The dimensions of pipes and equipment, as well as their capacity to handle flow rates and temperatures. 4. Energy-saving effect and economic analysis. Possible reasons for an excessive temperature difference in the return water: 1. The system is overheating or the cooling load is insufficient. 2. There is poor local circulation within the system, which affects heat exchange efficiency. 3. Measure or control equipment failures. Possible reasons for too small a return water temperature difference: 1. The system’s heating or cooling is excessive compared to actual needs. 2. The flow rate of water is too high, resulting in insufficient heat transfer. 3. The equipment or piping layout is unreasonable, resulting in heat loss. Proper design and adjustment of the return water temperature difference are key to ensuring the efficient and stable operation of the system. .

Determine the water usage and temperature difference based on the maximum load

Generally, the temperature difference is ensured based on the volume and heat load

The temperature difference between the supply and return water in a circulating water system (typically referring to the chilled water system in central air conditioning or process cooling water systems) is a key design parameter, as it directly affects the system’s energy efficiency, initial investment, and operational stability. Its design principle is not a single value, but rather a systems engineering issue that requires comprehensive consideration. The following are its core design principles and considerations: 1. Core principle: Seek optimal technical and economic efficiency throughout the entire life cycle, while meeting end-user needs. In other words, while ensuring the cooling performance of the air conditioning system or the process, minimize the total cost of “initial investment + operating energy expenses”. II. Key considerations in temperature difference design 1. System type and terminal equipment: Conventional comfort central air conditioning systems (fan-coils, air handling units): The standard design temperature difference is 5°C (for example, supply water at 7°C, return water at 12°C). This is the most widely adopted standard, based on the balance point between the efficient operating range of chillers, the heat exchange capacity of terminal units, the power consumption of water pumps, and pipeline investment. Large temperature difference design: 6-8°C or even higher (for example: supply water at 5°C, return water at 13°C). Advantages: When transporting the same amount of cooling capacity, it significantly reduces the water flow rate, allowing for the use of smaller pipe diameters and pumps, which lowers the initial investment as well as the energy consumption associated with operating the pumps. Challenge: Chilled water units require a lower evaporation temperature (lower supply water temperature), which results in a decrease in the efficiency of the unit (COP) and an increase in power consumption. End devices: As the average temperature of the supply and return fluids decreases, the heat exchange capacity of these end devices declines; this may require an increase in the coil area (by increasing the fan volume or the number of fans), which in turn raises the investment costs associated with these end devices as well as the energy consumption of the fans. Units for large temperature differences that may require specialized design. Industrial process cooling or data center cooling: The temperature range is wider, potentially ranging from 3°C to over 15°C. The design depends entirely on the heat exchange characteristics of the process equipment. For example, some precision equipment requires a low flow rate and a small temperature difference to ensure temperature stability ； Some cooling tower circulating water systems use large temperature differences to save energy consumption of water pumps. 2. Chiller performance: The rated operating conditions of a chiller usually correspond to a standard temperature difference (such as 5°C or 6°C). Deviating from this operating condition, especially by increasing the temperature difference (reducing the supply water temperature), the compression ratio increases and the COP decreases. During design, it is necessary to evaluate whether the impact of temperature differences on the host’s energy consumption exceeds the energy savings provided by the water pump. 3. Pump energy consumption and pipeline investment: A large temperature difference leads to a low flow rate, which in turn reduces the flow velocity inside the pipes; this allows for smaller pipe diameters, thereby reducing the initial investment in pipes and valves. Low flow rate → The head and power required by the pump decrease significantly (pump power is proportional to the cube of the flow rate) → Operational energy consumption is greatly reduced. This is the main driving force behind the design for large temperature differences. 4. Terminal heat transfer performance The amount of heat transferred at the terminals (fan coil units, AHU cooling coils) depends on the log mean temperature difference. Increasing the temperature difference between the supply and return water usually means lowering the return water temperature or the average water temperature, which reduces the heat exchange capacity at the end points. To achieve the same cooling effect, it may be necessary to: increase the number of coil rows (which increases air resistance and fan energy consumption). Increase the coil size (increases cost). Reduce the supply air temperature (affects comfort). During design, it is necessary to verify whether the actual cooling capacity at the end of the coil under the selected temperature difference meets the room load. 5. System control and stability: In systems with large temperature differences, the water flow rate is low and the system’s water capacity is small; therefore, water temperature fluctuations may be more sensitive to changes in load, which places higher demands on the automatic control system. Small temperature difference system: It has a high water flow rate and large thermal inertia, resulting in more stable temperatures, but the control response may be slightly slower. 6. Codes and standards: Local or ** HVAC design codes must be adhered to (such as China’s “Code for Design of Heating, Ventilation and Air Conditioning in Civil Buildings” GB50736). These codes typically provide guidelines regarding the maximum design flow velocity and recommended temperature differences. III. Summary of the design decision-making process: Determine the boundary conditions: clarify the building type, cooling load characteristics, terminal types, and the range of options for selecting the main equipment. Predefined temperature difference options: Typically, 5°C is used as the standard option, while a larger temperature difference option is also provided (such as 7°C or 8°C). Conduct technical verification: On the chiller side: Calculate the annual energy consumption of the chiller under different supply water temperatures (partial load performance must be taken into account). Transmission and distribution side: Calculate the specifications of the water pumps and their annual energy consumption for different flow rates. Terminal side: Verify whether the capacity of the terminal coil is sufficient under the selected temperature difference, and whether a larger model is required. Piping system: Compare the investment costs of different pipe diameter options. Economic analysis (Life Cycle Cost Analysis LCCA): Calculates the difference in initial investment between the two options (host unit, end devices, pumps, pipes, insulation, etc.). Calculate the difference in annual operating energy costs for the two options (main machine electricity cost + pump electricity cost). Taking into account the time value of money, calculate the payback period or net present value. Make a decision: If the increased initial investment associated with the large temperature difference approach can be recovered through savings on electricity costs within an acceptable time frame (such as 3–5 years), and if it is technically feasible, then a large temperature difference design is recommended. For ordinary commercial buildings, if electricity prices are low or the system is small, a 5°C temperature difference remains a safe and economical choice. For large-scale district cooling systems, super-tall buildings, or areas with high electricity prices, the advantages of a large temperature difference design (6–8°C) are very evident, which is why it is widely used. IV. General Recommendations: Standard option – When no special requirements exist, a temperature difference of 5°C is recommended; this approach ensures safety, reliability, and good compatibility. Large-scale/energy-saving key projects: A detailed technical and economic comparison of the large temperature difference approach (7–8°C) is necessary, as it is often the key to achieving high energy efficiency. Avoid too small temperature differences: ones less than 4°C can result in excessive flow rates, leading to a sharp increase in the energy consumption of the water pump, which is not economical, unless there are special requirements in the process. In summary, the design of the circulating water temperature difference is an optimization problem, the core of which is to find the optimal balance among the efficiency losses of the chiller units, the energy savings from pump operation, the performance changes in the terminal equipment, and the initial investment cost. The design of modern high-efficiency systems increasingly tends to employ a moderately increased temperature difference, based on detailed simulation analysis.