From a heat transfer perspective, why do plate heat exchangers have a higher heat transfer coefficient than tube heat exchangers? !

The heat transfer coefficient is high at the same flow rate, and the pressure drop is low at the same heat transfer coefficient. It mainly causes turbulence.

Tubular heat exchangers also generate turbulence. Is the plate Reynolds number high? !

Let’s take a look at the Reynolds number. Re = d * density * u / viscosity; the density and viscosity are determined. Only d and u can change Re; with the same speed, where does a high Re come from? !

Turbulence still forms at low Reynolds numbers.

It’s true that low Reynolds numbers can lead to turbulence. But where does the higher Reynolds number of plate heat exchangers come from compared to tube heat exchangers? !

Features of plate heat exchangers (comparison with shell-and-tube heat exchangers): a. High heat transfer coefficient. Due to the inverted arrangement of the different corrugated plates, complex flow channels are formed, allowing the fluid to flow in a three-dimensional rotational manner within these channels. Turbulence can occur at relatively low Reynolds numbers (typically Re=50~200), resulting in a high heat transfer coefficient; it is generally considered to be 3~5 times higher than that of shell-and-tube heat exchangers. b. The logarithmic mean temperature difference is large, while the temperature difference at the ends is small. In shell-and-tube heat exchangers, the two fluids flow separately in the tube side and the shell side, resulting in a cross-flow pattern; hence, the logarithmic mean temperature difference correction factor is low. In plate heat exchangers, the fluids usually flow in parallel or counterflow, and the correction factor is also around 0.95. Additionally, in plate heat exchangers, the cold and hot fluids flow parallel to the heat exchange surface without any bypass flow, which results in a small temperature difference at the ends – it can be less than 1°C for heat exchange with water, whereas in shell-and-tube heat exchangers it is generally around 5°C. c. Smaller footprint. Plate heat exchangers have a compact structure, with a heat exchange area per unit volume that is 2 to 5 times that of shell-and-tube heat exchangers. Moreover, there is no need to reserve space for removing the tube bundles for maintenance, as in shell-and-tube heat exchangers. Therefore, to achieve the same heat transfer capacity, plate heat exchangers require only about 1/5 to 1/10 of the space needed by shell-and-tube heat exchangers. d. It is easy to change the heat exchange area or the configuration of the process; by adding or removing a few plates, it is possible to increase or decrease the heat exchange area ； By changing the arrangement of the plates or replacing a few of them, it is possible to achieve the desired combination of processes and adapt to new heat exchange conditions, whereas it is almost impossible to increase the heat transfer area of shell-and-tube heat exchangers. e. Light weight: The thickness of the plates in plate heat exchangers is only 0.4–0.8 mm, while the thickness of the heat exchange tubes in shell-and-tube heat exchangers is 2.0–2.5 mm. The shell of a shell-and-tube heat exchanger is much heavier than the frame of a plate heat exchanger; generally, a plate heat exchanger weighs only about 1/5 as much as a shell-and-tube heat exchanger. f. Low cost: Using the same materials and with the same heat exchange area, the cost of plate heat exchangers is about 40%~60% lower than that of shell-and-tube heat exchangers. g. Easy to manufacture: The heat transfer plates of plate heat exchangers are produced by stamping, allowing for a high degree of standardization and mass production, whereas shell-and-tube heat exchangers are generally manufactured manually. h. Easy to clean: In frame-type plate heat exchangers, the plate bundles can be separated by loosening the clamping bolts, and the plates can be removed for mechanical cleaning, which is very convenient for heat exchange systems that require frequent cleaning. i. Low heat loss: In plate heat exchangers, only the shell plates of the heat transfer plates are exposed to the atmosphere; therefore, heat loss is negligible, and no insulation measures are required. Shell-and-tube heat exchangers have high heat losses and require insulation layers. j. The capacity is smaller, at 10%~20% of that of a shell-and-tube heat exchanger. k. High pressure loss per unit length: Due to the small gaps between the heat transfer surfaces and the irregularities on these surfaces, the pressure loss is higher compared to that of traditional smooth tubes. l. It is resistant to scaling: Due to the strong turbulence inside, scaling does not occur easily; its scaling coefficient is only 1/3 to 1/10 that of shell-and-tube heat exchangers. m. The operating pressure should not be too high, nor should the temperature of the fluid be too high, as this may lead to leaks. Plate heat exchangers use gaskets for sealing, and the operating pressure generally should not exceed 2.5 MPa; the fluid temperature should also be below 250°C, otherwise leaks may occur. n. Prone to clogging: Due to the narrow channels between the plates, which are usually only 2–5 mm wide, the channels can easily become blocked when the heat exchange medium contains large particles or fibrous substances.

Please do not copy content from the Internet without thinking; it’s pointless. Let’s take a look below. 1. The more corrugated plate channels there are, the greater the pressure drop. This is basic common sense in life. 2. Who says tubular systems are all counterflow? ! That is a flow method used only when counterflow is not permitted. 3. The other ones are meaningless. 4. I’m going on a date; I’ll discuss it with you when I get back tonight.

The tubular type has baffle plates – isn’t that countercurrent flow?