The heat transfer efficiency issue of liquefied gas coolers
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Could everyone please help analyze this: The control of the catalytic stabilizer tower is a critical aspect; there is no thermal bypass. After the pressure control valve, there are air coolers and water coolers, all of which are located above the reflux tank. The temperature at the top of the tower is 66°C; after air cooling, the temperature drops to 47°C, and after water cooling, it drops to 46°C. The pressure in the stabilizer tower is currently controlled at around 1.1 MPa, while the pressure in the reflux tank is slightly over 0.9 MPa. The air-cooled temperature is okay, but why can the water-cooled version only cool by one degree? Last year, new heat exchanger cores were installed; they are arranged in a 2+2 series-parallel configuration. The water flow rate in one set of tubes is 0.4 m/s, while it is 1.1 m/s in the other set. Even if only one set of heat exchangers is in operation, it shouldn’t be possible to achieve a cooling effect of just one degree. The capacity is 240 t/h of liquefied gas; each heat exchanger has a length of 1 meter and a heat exchange area of 328 square meters. Please ask the experts to help analyze it.
I used to have similar doubts too; the best approach is to calculate the actual performance based on the heat exchange efficiency of the heat exchanger. It’s not necessary to conduct very precise calculations – a rough estimate will suffice to determine whether it makes sense or not!
I calculated based on the assumption of no phase change with only a liquid phase; in that case, the design load is more than sufficient. But how should I calculate it if there is some phase change and condensation?
Many things can’t necessarily be solved through calculation; the designers have already done thorough research on those matters that require calculation. My advice is that for things done on-site, one needs to focus on the principles of use there, both in terms of implementation and other aspects. For example, placing the liquid-phase gas in coolers, and using the outlet valves of the coolers to control the distribution of the fluid to each cooler. It is most likely a problem with usage; for reference only.
Although the water cooling reduces the temperature by only 1 degree, it seems that the main heat load is consumed in the latent heat of phase change during the liquefied gas’s condensation. I don’t quite understand what you mean by “2+2 series + parallel arrangement”; a simple diagram would help clarify it. There is a significant difference in the cooling water flow rates between the two groups; this is likely due to an asymmetric layout of the cooling water pipes, which is unreasonable.
Thank you, 2+2 means four heat exchangers: two in series, and these two groups are connected in parallel. If the initial design did not take condensation heat into account, wasn’t that an unreasonable design? ?
Thank you for your participation. First, we need to check whether the design is reasonable; only if it isn’t reasonable is there a possibility for technical modifications. Did the original design take phase change heat into account? I’m not very good at calculating phase change heat yet – should we calculate condensation first and then cooling? Is heat absorption used first for condensation? Making adjustments on-site doesn’t seem to change much, after all, the difference from the design is too great.
The heat exchanger has a considerable area, and the heat of phase change must have been taken into account during its design. You can determine it by checking the saturation temperature of liquefied gas at 0.9 MPa.