Confusions about heat exchanger replacement
Thread Content
I would like to ask the seniors in the group for advice regarding a heat exchanger issue. There is a gas-to-gas heat exchanger (for transformed gas/propane) at the site, and it is planned to convert this device into a liquid-to-liquid cooler (using methanol/circulating water). The design pressure, temperature, and heat exchange area all seem appropriate; however, I’m concerned that the desired results may not be achieved, as the original design was for high-pressure gas with high flow rates, while at present it involves low-pressure methanol with lower flow rates
Your concerns are valid. Although the design pressure, temperature, and area appear to be appropriate on the surface, switching from gas/gas heat exchange to liquid/liquid heat exchange results in significant differences in operating conditions, with notable changes in the heat transfer coefficient and flow velocity: 1. Under the high-pressure gas conditions of the original design, the flow velocity is high, convective heat transfer is effective, and the heat transfer coefficient is high ； 2. After switching to liquid/liquid mode, especially when the flow rate on the low-pressure methanol side decreases, the heat transfer coefficient drops significantly, and the actual effective heat transfer amount may be far lower than the design capacity of the original gas-to-gas heat exchanger ； 3. Low flow rates can also lead to flow stagnation, dead zones, or dirt deposition within the channels, further reducing heat transfer efficiency. Therefore, based on experience, it is not sufficient to consider only the design pressure, temperature, and area; it is essential to verify: – the actual heat transfer coefficients of the fluids on both sides after the modification ； - Reynolds number and flow velocity under the new flow conditions, to avoid entering the laminar flow region ； - Check whether the heat load calculation meets the process requirements. It is recommended that the design firm conduct detailed thermodynamic calculations again; otherwise, it is likely that the desired results will not be achieved. .
Damn... not only is the flow rate different, but there are also significant differences in the properties of the gas and liquid. The thermal conductivity of liquids is more than ten times that of gases, and the difference in density is even greater. Take water-cooled and air-cooled units as examples – can their heat transfer modules be interchanged? This idea is really wild
It is necessary to run the process calculations (heat exchange calculations) again; once they are appropriate, it can be used. Based on conventional experience, the heat transfer coefficient of liquids is higher than that of gases; therefore, it is highly likely that the desired results will be achieved (subject to the calculation results).
It is necessary to follow the original equipment structure and use the current process data for calculations
There are no specific figures, but the option can be considered.
Let’s do the modeling calculation. It’s not some kind of confidential device anyway; there are already mature models available for it
This issue relates to heat exchanger verification. Several factors need to be considered to determine whether a heat exchanger can meet the requirements of a new process: 1. Whether the design temperature and design pressure are appropriate ； 2. Can the material of the heat exchanger meet the requirements? ； 3. Does the type of heat exchanger meet the requirements? ； 4. Verification of the heat exchanger structure: determine whether the heat exchange area is sufficient; if not, assess whether the heat transfer coefficient can be improved through simple structural modifications in order to reduce the heat exchange area required by the new process ； 5. Assess risks.