Heat exchanger effectiveness: Role of transfer units [NTU]

Heat transfer in a heat exchanger occurs through the process of conduction, convection, and/or radiation. Heat is transferred from one fluid to another through a separating surface, such as a metal wall, in a heat exchanger. The resistance to heat transfer is represented by the overall heat transfer coefficient U.

Role of dT

The driving force for heat transfer in a heat exchanger is the temperature difference (dT) between the two fluids. The temperature difference, or dT, is what allows heat to flow from a higher-temperature fluid to a lower-temperature fluid, and it is a critical factor in determining the rate of heat transfer.

The heat transfer rate, Q, can be expressed as Q = U A dT, where U is the overall heat transfer coefficient, A is the surface area for heat transfer, and dT is the temperature difference between the two fluids.

The effectiveness of heat transfer depends on how much heat (Q) can be transferred through the heat exchanger with a minimum approach temperature, which is the smallest temperature difference between the two fluids. The design of the heat exchanger must consider the pinch temperature, which is the temperature at which the hottest stream entering the heat exchanger matches the coldest stream exiting the heat exchanger. Operating near the pinch temperature ensures efficient heat transfer.

However, there can be extreme situations such as temperature cross, where the temperature difference between the two fluids changes direction along the length of the heat exchanger. In such cases, careful consideration must be given to the design and operation to ensure efficient heat transfer.

Temperature cross

In a given heat exchanger design, if too much heat (Q) is pushed through without a sufficient surface area (A), the temperature difference (dT) diminishes to a point where it becomes zero, reducing the heat transfer rate. This highlights the importance of proper design considerations to ensure optimal heat transfer.

Role of transfer units [NTU]:

?The effectiveness of a heat exchanger is closely related to the number of transfer units (NTU). The NTU is a dimensionless measure of the size of the heat exchanger and the rate of heat transfer. It is related to the heat exchanger's size, flow configuration, and overall heat transfer coefficient.

The effectiveness (ε) of a heat exchanger can be calculated using the NTU, along with the heat capacity rate of the cold fluid (Cp cold ) and the heat capacity rate of the hot fluid (Cp hot). The formula for the effectiveness of a heat exchanger, in terms of NTU, is:

ε = 1 - exp(-NTU*(1+C_r))

Here, C_r is the capacity ratio, which is defined as the ratio of the heat capacity rate of the cold fluid to the heat capacity rate of the hot fluid. The effectiveness of a heat exchanger provides a measure of how well the heat exchanger is performing in transferring heat from the hot fluid to the cold fluid.

Target effectiveness of a heat exchanger

The target effectiveness (ε) for a heat exchanger depends on the specific requirements of the heat transfer application and the desired level of thermal efficiency. In general, a higher effectiveness indicates better heat transfer performance and is often a desirable objective in heat exchanger design.

The choice of target effectiveness is influenced by various factors, such as the temperature approach (minimum temperature difference) between the hot and cold fluids, the desired heat transfer rate, and the overall energy efficiency goals. In many cases, the target effectiveness is determined based on the thermodynamic and operational requirements of the heat transfer process.

For parallel-flow and counter-flow heat exchangers, the maximum possible effectiveness is limited by the temperature approach, which is the minimum temperature difference between the hot and cold fluids at the end of the heat exchanger. The target effectiveness is commonly set to achieve the highest possible thermal performance while considering practical constraints and economic considerations.

The relationship between effectiveness and NTU is crucial in the design and analysis of heat exchangers. By calculating or determining the NTU, engineers, and designers can assess the performance and efficiency of the heat exchanger, allowing them to optimize its design to achieve the desired heat transfer characteristics.

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