Calculation of heat exchange area for heat exchangers

The calculation of the heat exchange area of a heat exchanger needs to be based on actual conditions, and it mainly involves the following steps: 1. Determine the thermal conduction resistance coefficient (U-value). The thermal conduction resistance coefficient (U-value) refers to the thermal conduction resistance that must be overcome per unit time for a certain amount of heat to flow into the heat exchanger per unit area. The U-value is related to the process parameters of the heat exchanger, including the material of the heat exchanger, pipe diameter, wall thickness, and properties of the fluid. Under normal circumstances, the U value can be found in relevant manuals or the parameters provided by the manufacturer. 2. Calculate heat transfer: Using the principle of heat conservation, determine the heat transfer between the hot water inlet and outlet. The thermal conduction resistance coefficient can be calculated using the following formula: Q = U * A * ΔT, where Q represents the heat transfer amount of the heat exchanger, measured in watts ； A is the heat exchange area of the heat exchanger, with the unit in m² ； ΔT is the temperature difference between the hot water inlet and outlet, measured in °C. 3. Determine the inlet and outlet temperatures of the cooling water. According to the problem description, the inlet temperature of the cooling water is 30°C, while the outlet temperature is unknown. Due to the law of conservation of energy, the above formula can be used for calculations. 4. Calculate the heat exchange area: Using the heat transfer amount and cooling water temperature obtained from the above three steps, the required heat exchange area can be calculated. The formula for calculating the heat exchange area is: A=Q/(U*ΔTm), where ΔTm represents the average logarithmic temperature difference, which can be calculated based on specific conditions. In summary, in accordance with the requirements of the question, the calculation process for the heat exchange area is as follows: 1. Determine the thermal conduction resistance coefficient (U value). Since the material specified in the question is 304 stainless steel, the U value for 304 stainless steel can be found in manuals or relevant literature. Assume this value to be 500 W/m·°C. 2. Calculate heat transfer: Based on the conditions given in the problem, the inlet temperature of the hot water is 75°C and the outlet temperature is 50°C. The formula for calculating the heat transfer amount is: Q = U * A * ΔT, where ΔT = 75 – 50 = 25°C. Let the heat exchange area of the heat exchanger be A; by substituting the value of U, the following formula is obtained: Q=500*A*25. 3. Determine the outlet temperature of the cooling water. Since the outlet temperature of the cooling water is unknown, it is denoted as Tc°C. According to the conditions given in the question, the inlet temperature of the cooling water is 30°C. 4. Calculating the heat exchange area: Based on the principle of heat conservation, the following formula can be obtained: Q = m1*Cp1*(T1-T2) = m2*Cp2*(T2-Tc), where m1 is the flow rate of hot water, measured in t/Hr ； Cp1 is the specific heat capacity of hot water, with the unit of J/Kg·℃ ； T1 is the hot water inlet temperature ； T2 is the hot water outlet temperature ； m2 is the cooling water flow rate, in units of t/Hr ； Cp2 is the specific heat capacity of the cooling water, with the unit of J/Kg·℃. Simplification yields the following formula: A = Q/(U*ΔTm), where ΔTm is the average logarithmic temperature difference, which can be calculated using the following formula: ΔTm = ((T1-Tc) – (T2-Tc)) / ln((T1-Tc)/(T2-Tc)). Following these steps, the heat transfer rate of the heat exchanger, the temperature of the cooling water, and other parameters can be substituted into the aforementioned formula to determine the required heat exchange area. .

Thank you, the explanation is very clear

The heat transfer coefficient needs to be adjusted based on experience

This applies only when neither the hot nor the cold stream undergoes phase change. But I wonder: since the medium will vaporize at any temperature and pressure, and its heat of vaporization varies with temperature and pressure, should the heat of vaporization under the specific operating conditions always be taken into account?