For a general cooler or heater, the calculations for heat transfer are based on the tube side and shell side. So 1) For a reboiler would the vaporizer be the "shell" since that's where the heat transfer occurs? 2) How do you account for the phase change? I used Q = m*cP*dT to calculate duty. Since steam is condensing in the tubes, I was told to use Q = m*Hv instead, where "Hv" is the latent heat of steam and "m" is the flow (kg/s) I have from operating data. I looked up Hv at the given temperature and it was around 1900 kJ/kg. So I calculated the duty to be 200 kW. I have cP and dT data for my process acid (which is being vaporized). So in order to check myself, I used Q = m*cP*dT for my process acid. I dont have operating data for flow on the process side, so I used the flow specified on the datasheet for the exchanger since they should reasonably be the same. The thing is, when I calculated the duty on the process side, I got 900 W. Thats no where near 200 kW. So I dont understand where I'm going wrong. If someone can explain whats wrong with my method above and how to make calculations for a reboiler, that would be wonderful.