Help calculate the amount of hot water needed
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Could you help calculate the amount of hot water required? It would be great if you could also provide a detailed breakdown of the calculation process. Thank you very much! The medium in question is high-purity water, with a flow rate of 7500 kg/h. The water needs to be heated from 4°C to 70°C. The heat exchange area is 60 square meters, and the heat source is hot water at 90 degrees Celsius under a pressure of 6 kg. Please calculate the amount of hot water needed per hour. The tube side of the heat exchanger is made of stainless steel, while the shell side is made of carbon steel. Thank you
This post was last edited by xjhaooo on 2017-11-9 at 17:12. There are many uncertainties, so estimates can only be made through assumptions. Assumptions: 1. The heat exchanger can operate in counterflow mode, with a heat transfer coefficient of 1300 W/m2·K. 2. The ultra-pure water to be heated is at a pressure of 0.1 MPa; for a pressure of 0.6 MPa and a temperature of 90 degrees Celsius, the required volume of hot water is 5864 kg/Hr, with an outlet temperature of 5.7 degrees Celsius. If the heat transfer coefficient of the actual heat exchanger is higher than 1300, less water is required, but not by a significant amount. Here is a brief description of the general process: 1. The total heat transfer amount is calculated: for water at 0.1 MPa, heating from 4 degrees to 70 degrees results in an enthalpy increase of 276.2 KJ/kg. Multiplying this by 7500 kg/Hr gives a heat transfer power of 575 KW. 2. Using the formula: Heat transfer area = Heat transfer power / (Heat transfer coefficient * Logarithmic mean temperature difference), with a heat transfer area of 60 m2, heat transfer power of 575 KW, and a heat transfer coefficient of 1300 W/m2·K, the logarithmic mean temperature difference is calculated to be 7.37 K. 3. Using the formula for the logarithmic mean temperature difference in a counterflow configuration, as specified in GB151: given the inlet and outlet temperatures on the cold side as 4–70 degrees Celsius, and the inlet temperature on the hot side as 90 degrees Celsius, along with a logarithmic mean temperature difference of 7.37 K, Excel can be used to determine that the outlet temperature on the hot side is 5.66 degrees Celsius. 4. Once the inlet and outlet temperatures on the hot side are known, and considering the pressure of the hot water at 0.6 MPa, the enthalpy drop of the hot water can be determined from enthalpy-entropy tables to be 353 KJ/kg. With a heat transfer power of 575 KW, the flow rate is 575 KW / 353 KJ/kg = 1.63 Kg/s, which equals 5864 kg/Hr. Without taking into account heat losses or other factors, the estimated water flow rate is 5864 kg/Hr. Prepare hot water at 6000 Kg/Hr. However, the temperature rise of one device is 66K, and the temperature difference at the low-temperature end is very small; it’s not clear whether this can be achieved in practice. I haven’t done any calculations in a long time; if there are any issues, please point them out so we can discuss them.
The heat exchanger calculation result is 6.2 tons per hour
The heat exchanger calculation result is 6.2 tons per hour
This is to calculate the temperature difference ΔT; once ΔT is determined, the water temperature can be calculated, and the water volume can be figured out based on the inlet and outlet water temperatures
Yes, my idea is to calculate △T, that is, the logarithmic mean temperature difference. To calculate △T, it is necessary to assume the heat transfer coefficient U, and then use the known heat transfer power and heat transfer area to determine △T.
Plate heat exchangers can be used; they offer high heat exchange efficiency and require less space.