Calculation of heat transfer capacity for propane heat exchangers
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This post was last edited by 17156614 on 2018-6-20 at 13:25. Given conditions: shell-and-tube heat exchanger, propane flowing in the tube side, seawater flowing in the shell side. Propane is heated from 42°C (25 bar) to 0°C (1 bar) at a flow rate of 500 m3/h; the seawater inlet temperature is 15°C with a flow rate of 700 m3/h. The heat load is to be calculated. Using the enthalpy method, I found that the enthalpy of propane at 42°C (25 bar) is 102.86 kJ/kg, with a density of 593.8 kg/m3 ； At 0°C (1 bar), the heat enthalpy is 589.53 kJ/kg. Q = m(i1 – i2) = 500 * 593.8 / 3600 * (589.53 – 102.86) = 40,136.75 kW. Due to phase changes, if the specific heat method is used, Q = m(r + cp(t1 – t2)) = 500 * 593.8 / 3600 * (425.47 + 2.2339 * 42) = 43,191.36 kW. However, the value given in the specifications is around 7,600 kW, which is an order of magnitude different. I’m not sure what’s going on; I hope someone experienced can help me figure this out!
Your question is too technical. Can’t solve it
I have to figure this out myself; I don’t know how to calculate this phase-transformed liquid or how to select the parameters
The original poster, your calculation is correct; the specification sheet does not take into account the heat generated by the vaporization of propane.
Thank you! But the specifications also mention a seawater flow rate of 700 m3/h, as well as inlet and outlet temperatures of 5.4°C/15°C. I calculated that this corresponds to approximately 7600 kW. In that case, this amount of seawater is not sufficient to cool the propane, so I’m still not quite sure
The problem was found to be an excessively high pressure at the propane outlet; the pressure drop was too large. It should generally be around 50 kPa. With a propane outlet pressure of 24.5 bar, the values will then match
I did a rough calculation just now: for the seawater on the hot side, the flow rate is 700 m3/h, with inlet and outlet temperatures ranging from 15°C to 5.5°C; the inlet and outlet pressures are respectively 3 bar and 2.5 bar； For the cold-side propane, the flow rate is 500 m3/h; the inlet and outlet temperatures range from -42°C to 6.4°C, while the inlet and outlet pressures are 25 bar and 24.5 bar respectively. The total heat exchange load is approximately 8000 kW. It is worth noting, however, that under these operating conditions the gas phase fraction at the cold-side propane outlet is 0, meaning the flow is entirely in liquid form; thus this heat exchanger does not have any vaporization effect. To achieve complete vaporization of the seawater at 15°C, a temperature crossover would occur. At an operating pressure of 24.5 bar, the temperature at which propane is completely vaporized is 69°C, so the temperature of the hot-side fluid should be at least 90°C. If you require the propane to be completely vaporized, my suggestion is: if the propane flow rate is to remain constant, it should first be reduced in pressure to 4.5 bar, and then fully vaporized after heat exchange, at a temperature of 1.8℃ ； The inlet and outlet temperatures of the seawater remain at 15℃ and 5.5℃ respectively, but the flow rate of seawater needs to be changed to 2950 m3/h; the total heat load of the entire heat exchanger is 33500 kW, for reference only