Design a heat exchanger for a specific process in which a 50,000 kg/day of a mixture with a specific heat capacity of 3.5 kJ/kg.K needs to be heated from 20°C to 70°C using a hot stream of 100,000 kg/day of a fluid with a temperature of 150°C. The heat exchanger needs to operate with a heat transfer coefficient of at least 400 W/m2.K and the pressure drop across the heat exchanger should be no more than 0.5 bar. What type of heat exchanger would be most suitable and what are the dimensions required to achieve the desired performance?

Question

Design a heat exchanger for a specific process in which a 50,000 kg/day of a mixture with a specific heat capacity of 3.5 kJ/kg.K needs to be heated from 20°C to 70°C using a hot stream of 100,000 kg/day of a fluid with a temperature of 150°C. The heat exch

Answer 1

To design the heat exchanger, we first need to determine the required heat transfer rate  Q  and the log mean temperature difference  LMTD .1. Calculate the heat transfer rate  Q :Q = m*c_p*Twhere m is the mass flow rate of the mixture, c_p is the specific heat capacity, and T is the temperature difference.For the mixture:m = 50,000 kg/day = 50,000/24/3600 = 0.579 kg/sc_p = 3.5 kJ/kg.K = 3500 J/kg.KT = 70C - 20C = 50CQ = 0.579 kg/s * 3500 J/kg.K * 50 K = 101,325 W2. Calculate the log mean temperature difference  LMTD :LMTD =  T1 - T2  / ln T1/T2 where T1 is the temperature difference between the hot and cold fluids at one end of the heat exchanger, and T2 is the temperature difference at the other end.T1 = 150C - 20C = 130CT2 = 150C - 70C = 80CLMTD =  130 - 80  / ln 130/80  = 103.2C3. Determine the required heat transfer area  A :A = Q /  U * LMTD where U is the overall heat transfer coefficient.U = 400 W/m2.KA = 101,325 W /  400 W/m2.K * 103.2 K  = 2.46 m24. Select the type of heat exchanger and dimensions:A shell and tube heat exchanger would be suitable for this application due to its high heat transfer coefficient and ability to handle a wide range of flow rates and temperatures.To minimize the pressure drop, we can use a single-pass shell and tube heat exchanger with a larger shell diameter and a lower number of tubes. The dimensions of the heat exchanger will depend on the tube diameter, length, and layout.Assuming a tube diameter of 25 mm  0.025 m  and a tube length of 2 m, we can calculate the number of tubes required:Tube area per tube =  * D * L =  * 0.025 m * 2 m = 0.157 m2Number of tubes = A / Tube area per tube = 2.46 m2 / 0.157 m2 = 15.7  16 tubesTherefore, a shell and tube heat exchanger with 16 tubes, each with a diameter of 25 mm and a length of 2 m, would be suitable for this application. The shell diameter should be large enough to accommodate the tubes with sufficient space for the fluid to flow around them, typically around 200-250 mm. The pressure drop can be further minimized by optimizing the baffle spacing and orientation.