Seeking help: I’m not sure if there are any issues with the calculation for heating the steam coil in this water tank; I’d appreciate it if some experienced colleagues could take a look at it.
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Here’s how it works: there is a tank in which a low-temperature medium is added, while a high-temperature medium is expelled, thus achieving mass balance. A coil is used for heating to maintain the temperature inside the tank. For the heat exchange area, I used a shell-and-tube heat exchanger model as an approximation. Our company is engaged in PET cleaning, and the accuracy requirements for heat exchange calculations are quite low; therefore, it should be possible to use a heat exchanger to approximate this tank. The medium is a low-concentration sodium hydroxide solution; I used the properties of water for all calculations. The feed temperature is 70°C, with heating to 95°C; the flow rate is 100 m3/h. The coil is tentatively to be made of DN80 carbon steel pipes, and the design steam temperature is 170°C. The chemical tank previously used by the company had a flow rate of 25 m3/h, with a coiled tube of DN40; everything else was the same. I’ve been using it, and there are no problems. In this case, I calculated the flow rate to be 72 m/s. Using this table, I found that the flow rate for the chemical tanks used in the past was also over 70 m/s. I would appreciate it if experienced professionals and experts could help me analyze whether there is any mistake in my calculations.
Based on our calculation experience, a flow velocity of 70 m/s is no problem
Okay, thank you. One more question: the steam used requires a flow rate of 70 m/s. Are there any requirements regarding the steam supplied by the utility services? Such as the diameter and flow rate of the public steam pipes, etc
The main pipes and branch pipes should be selected at a speed of around 20-30 m/s; the speed can be reduced upon reaching the equipment inlet.
The steam flow rate isn’t a major issue, but using a plate heat exchanger for this purpose isn’t very appropriate. The heat transfer coefficient of plate heat exchangers is generally around 3000–5000 W/㎡·℃. For coil heating, a spiral-tube heat exchanger is a better choice, with a heat transfer coefficient of approximately 2000–3000 W/㎡*℃
Thank you. I looked up the overall heat transfer coefficient for steam condensation to water in a chemical engineering design manual and used a value of 1000. Is this value too low?
When selecting a heat exchanger, one must consider not only the heat transfer medium but also the path taken by the medium. For steam-water heat exchange as well, whether it is steam in the tube side and water in the shell side, or steam in the shell side and water in the tube side, the selected heat transfer coefficient is completely different. When you check the specifications and manuals, you need to pay attention to making the distinction.
This post was last edited by yyong110 on 2022-11-11 21:27. 1. The flow rate is a bit high; it is recommended to use two coils in parallel. Steam main inlet DN125, divided into two circuits with 80 coils each. For exports, it’s sufficient to summarize as DN80 with a reduction to DN50. 2. The overall heat transfer coefficient is on the high side; it cannot be referenced using that of shell-and-tube exchangers. For coil estimation, it is recommended to select a coiled tube heater according to the manual; for steam-water systems with natural convection, it is advisable to use the lower limit value, around 500. Forced convection; take the lower limit of 730. For your 100 m3 volume, depending on the size of your storage tank, it probably won’t achieve the effect of forced convection, but it will be better than natural convection. If you want to save money, do detailed calculations – it requires a lot of data and computation, which can be quite time-consuming. I remember that over 10 years ago, the company Heat Exchanger Master had a tool designed specifically for calculating coil configurations in tanks, but it required purchase and registration to use. No money, didn’t buy it: If there’s no other choice, create a detailed calculation table by yourself – it allows you to learn in depth while also saving money.
Thank you, senior! It’s exactly as the seniors in our company said. The steam is divided into two paths; both the inner and outer coils are DN80, with an outlet of DN50.
Thank you; the senior over there has already provided a reference value for the overall heat transfer coefficient: lol
Thank you; I’ll recalculate it by taking everyone’s experience into account