Issues regarding the layout of the two-stage cooling process
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Prepare to cool a hot material using circulating water and chilled water. The hot material is at 60°C and cooled to 35°C, with a total flow rate of 200 m³/h. With the same cooler, there are two process layout options. 1. Divide the main pipeline into two pipelines and connect them in parallel. A pipeline is equipped with a circulating water cooler, with a flow rate of F1 ； A pipeline is equipped with a chilled water cooler, with a flow rate of F2. Circulating water supply temperature: 32°C, chilled water supply temperature: -5°C. 2. Connect the two coolers in series; the hot material is first cooled by circulating water and then by chilled water. Let’s discuss it: under the same conditions, taking all factors into account, which process layout is more reasonable? In Method 1, is there a certain relationship between F1 and F2 that makes this arrangement the most economical and reasonable? . If known conditions are lacking, one can make bold assumptions.
It is generally designed in a series configuration, with the second cooler serving as a backup. Under normal circumstances, the heat exchange area of the first stage should be sufficient; this approach takes into account both economic efficiency and the ability to achieve cooling even in special situations.
It still depends on the process requirements: series cooling provides better results, while parallel cooling results in a lower pressure drop. It is also possible to connect both circulating water and low-temperature water; circulating water is used during normal operation, while low-temperature water is used in summer or in case of an emergency. Ultimately, the decision must be based on the actual circumstances.
If there are no special process requirements, it is recommended to use a series connection； Materials at 60° are cooled to around 40°-45° using circulating water ； Then cool it down to around 35° using chilled water ； First-stage circulating water and second-stage chilled water enable better energy savings and better control; especially in winter, chilled water may not even be necessary ；
I really haven’t seen any with parallel connections split into two paths; they’re all in series. Series connection is what is known as secondary cooling. After connecting the two heat exchangers in parallel, their cooling temperatures are different – should you mix them then? More non-condensable gas is produced when only primary circulating water is used for cooling, resulting in material waste.
Select based on operating conditions. Generally, two stages are connected in series; after the material is cooled, its temperature drops further, which increases the demand for the secondary refrigerant at -5°C ； Two-stage condensation in parallel: most of the material is collected after being condensed by the primary circulating water; the temperature is relatively high, so the non-condensable gas goes to the secondary condenser, resulting in low demand for the refrigerant at -5°C ；
We have an old unit here that originally only had circulating water cooling. However, the temperature difference between the material and the circulating water is too small, requiring a large heat exchange area; as a result, the temperature cannot be controlled later on. Later, a cooler was added for cooling with low-temperature water; due to space and operational constraints of the setup, only a parallel configuration could be used. In normal design, I believe they should be connected in series, as this enables better energy savings and control, and also makes the cost of the heat exchanger more reasonable.
If there are no special process requirements, it is recommended to use a series connection； Materials at 60° are cooled to around 40°-45° using circulating water ； Then cool it down to around 35° using chilled water ； First-stage circulating water and second-stage chilled water enable better energy savings and better control; especially in winter, chilled water may not even be necessary ； Agree with the above views