Heat exchangers: Basic facts

Simple mixing of hot and cold fluids

When hot and cold miscible fluids are mixed, heat transfer occurs from the hot fluid to the cold fluid. During the process, the temperature difference between hot and cold fluids drives the heat transfer until both fluids reach the same temperature. However, while the final temperature reaches a constant temperature, the rate of heat transfer of both fluids occurs at different rates. The rate of heat transfer is dependent in addition to the temperature difference between the fluids, the specific heat capacity of the fluids, and the mass or volume of the fluids being mixed.

Heat transfer in a heat exchanger

The same process as above becomes different when hot and cold fluid are separated by a metal plate.

Approach temperature

In a heat exchanger, the metal plate acts as a common boundary for both the hot and cold fluids. The heat transfer coefficient (h) represents the rate at which heat is transferred through the boundary, and it is indeed different for hot and cold fluids. This difference in heat transfer coefficients contributes to the existence of an approach temperature in the heat exchanger.

The approach temperature is a result of the temperature difference between the hot and cold fluids at the interface with the metal plate. This difference in temperature is partially due to the different heat transfer coefficients of the two fluids, which can result in varying rates of heat transfer through the boundary. As a result, an approach temperature is observed across the metal wall, reflecting the temperature difference between the hot and cold fluids at the point of heat exchange.

Pinch temperature and temperature cross

In the context of heat exchangers and process engineering, the terms "temperature cross" and "pinch temperature" are related but not synonymous."Temperature cross" refers to a situation in a heat exchanger where the temperature profiles of the hot and cold fluids intersect at some point within the exchanger. This can lead to decreased efficiency in the heat exchanger due to reduced temperature driving force for heat transfer.

"Pinch temperature" is a concept used in process heat integration. It refers to the minimum temperature difference between the hot and cold streams in a heat exchanger network. The pinch temperature is an important consideration in designing heat exchanger networks to optimize energy efficiency.

To summarize, in the case of temperature cross, the point of intersection between the temperature profiles of the hot and cold fluids results in a very low or zero temperature difference (dt) at that point, which can reduce the effectiveness of heat transfer in contrast, pinch temperature refers to the minimum temperature difference (dt) between the hot and cold streams in a heat exchanger network, and it is used to optimize energy efficiency in the design of heat exchanger networks. A temperature cross occurs when the outlet temperature of the hot fluid is lower than the outlet temperature of the cold fluid.

While the pinch point represents a minimum temperature difference, it is not necessarily reduced to zero, and the objective is to minimize this temperature difference to optimize the heat exchanger network.

The upside of temperature cross

In the case of a temperature cross, the Logarithmic Mean Temperature Difference (LMTD) can be higher compared to a non-temperature cross situation. This is due to the way the LMTD formula is calculated, as the temperature difference can be capitalized on when calculating the LMTD.

LMTD = dt A - dt B / ln dtA/dt B

where dt A is the temperature difference between the two streams at end A, and dt B is the temperature difference between the two streams at end of B. A and B are two ends of a heat exchanger

When there is a temperature cross, at some point dt A and dt B add to each other, dt B becomes more than dt A giving a temperature difference dt A + dt B.

The LMTD becomes: LMTD = (dT A + dT B) / ln(dT A / dT B)

This increases the heat transfer rate

Temperature cross: Counter current vs co-current heat exchangers

Counter-current heat exchangers are more vulnerable to temperature cross than co-current heat exchangers. This is because in a counter-current heat exchanger, the difference in temperature between the hot and cold fluids [dt A - dt B] is more consistent across the heat exchanger, with h being h = Q/dt [Q is same] and the film coefficients (h) on the hot and cold sides are often closer in value than co-current heat exchangers making temperature cross more likely to occur.