Production Technology [Weekly Topic]: Q&A on Heat Exchanger Design (2011-01-24~30)

This is due to process requirements: the cold medium needs to be heated, while the hot medium needs to be cooled. It is also a mechanical requirement, as it helps to enhance convection and reduce resistance. Parallel connection is used only when specific process requirements dictate it.

Reply 1# shmily327 Counterflow operation: High driving force, small heat transfer area required； Reduce the amount of heat carrier used.

In a heat exchanger, as heat is transferred, the temperatures of the cold and hot fluids also change simultaneously. For example, in a switched heat exchanger, the air temperature changes from 30°C to -172°C℃ ； The temperature of the contaminated nitrogen increased from -175°C to 28°C. Therefore, the temperature difference between the cold and hot fluids also changes continuously at different sections along the heat transfer surface. In heat transfer calculations, an average temperature difference is generally used to represent the temperature difference between the cold and hot fluids throughout the heat exchanger. The magnitude of the average temperature difference is related to the relative flow direction of the cold and hot fluids. When two fluids flow parallel to each other in the same direction, it is called \"co-current flow\"“ ； Parallel counterflow is called “counterflow”” ； Vertical cross-flow is called “cross-flow”. Under the same conditions of properties, flow rates, inlet and outlet temperatures, and heat exchange area for the cold and hot fluids, a counterflow arrangement results in the largest average temperature difference between the cold and hot fluids, followed by cross-flow, with co-flow having the smallest difference. Other things being equal, the greater the average temperature difference, the greater the heat transfer amount, and the better the heat exchange efficiency. To transfer the same amount of heat, the required heat transfer area can be reduced. Furthermore, at the same cross-sectional position within the heat exchanger, the temperature of the hot fluid is always higher than that of the cold fluid; if a co-current arrangement is used, the final temperature at the outlet of the hot fluid will still be higher than that at the outlet of the cold fluid ； With a counterflow arrangement, the outlet temperature of the hot fluid can be much lower than that of the cold fluid. This is the case with the main heat exchanger. Therefore, when designing heat exchangers, the flow directions of the cold and hot fluids are almost always arranged in a counterflow configuration.

During reverse flow, the temperature difference between the inlet and outlet is large, which improves heat transfer efficiency and yields good heat transfer results.

The cold medium needs to be heated, while the hot medium needs to be cooled; at the same time, there are mechanical requirements to enhance convection and reduce resistance.

1) In a heat exchanger, as heat is transferred, the temperatures of the cold and hot fluids also change simultaneously. For example, in a switched heat exchanger, the air temperature changes from 30°C to -172°C℃ ； The temperature of the contaminated nitrogen increased from -175°C to 28°C. Therefore, the temperature difference between the cold and hot fluids also changes continuously at different sections along the heat transfer surface. In heat transfer calculations, an average temperature difference is generally used to represent the temperature difference between the cold and hot fluids throughout the heat exchanger. 2) The magnitude of the average temperature difference is related to the relative flow direction of the cold and hot fluids. The parallel and concurrent flow of two fluids is called \"co-current flow\"” ； Parallel counterflow is called “counterflow”” ； Vertical cross-flow is called “cross-flow”. Under the same conditions of properties, flow rates, inlet and outlet temperatures, and heat exchange area for the cold and hot fluids, a counterflow arrangement results in the largest average temperature difference between the cold and hot fluids, followed by cross-flow, with co-flow having the smallest difference. Other things being equal, the greater the average temperature difference, the greater the heat transfer amount, and the better the heat exchange efficiency. To transfer the same amount of heat, the required heat transfer area can be reduced. 3) At the same cross-sectional position within the heat exchanger, the temperature of the hot fluid is always higher than that of the cold fluid; in the case of a co-current arrangement, the final temperature at the outlet of the hot fluid remains higher than the final temperature at the outlet of the cold fluid ； With a counterflow arrangement, the outlet temperature of the hot fluid can be much lower than that of the cold fluid. This is the case with the main heat exchanger. Therefore, when designing heat exchangers, the flow directions of the cold and hot fluids are almost always arranged in a counterflow configuration.

Under the conditions of identical properties, flow rates, inlet and outlet temperatures, and heat exchange area for the cold and hot fluids, a counterflow arrangement results in the largest average temperature difference between them. When all other conditions remain the same, the greater the average temperature difference, the greater the heat transfer rate and the better the heat exchange efficiency. To transfer the same amount of heat, the required heat transfer area can be reduced. Furthermore, with a counterflow arrangement, the outlet temperature of the hot fluid can be much lower than that of the cold fluid.

High driving force, small heat transfer area required, reducing the amount of heat carrier needed

Counterflow increases the driving force for heat transfer, reduces the heat exchange area, and lowers operating costs and equipment investment!

It provides high driving force, requires a small heat exchange area, and can reduce the amount of heat transfer medium needed.