Heat exchanger phase change: Signs and Symptoms

A heat exchanger is a device used for the transfer of heat between two fluids at different temperatures. In many heat exchangers, phase separation can occur under certain conditions.

A single phase refers to a fluid system where all the components are in the same state of matter, such as a liquid or a gas. When the conditions in a heat exchanger cause the single phase to convert into two phases, such as a liquid-gas system, it is known as a two-phase flow.

The thermodynamic trigger for phase change in a heat exchanger is typically a change in pressure, temperature, or both. When the conditions inside the heat exchanger cause the fluid to reach its saturation point, phase separation occurs. This can lead to the formation of bubbles or droplets within the fluid stream.

In a two-phase system, the physical and thermodynamic properties of the fluid change significantly. For example, the heat transfer coefficient is typically higher in a two-phase system compared to a single-phase system due to generation of latent heat, turbulence etc. In addition, the density and viscosity of the two phases are different, which can affect the flow behavior in the heat exchanger.

Thermodynamics

A phase change occurs when a substance in its current phase, at a given pressure and temperature, reaches a point of maximum energy.

For example, consider saturated water at 100°C and 1 bar pressure, with its maximum enthalpy of 419 kJ/kg. At this point, the substance has the maximum entropy and the minimum free energy (G is at a minimum, dG = TdS).

Any addition of heat at this critical point begins breaking intermolecular bonds, increases entropy, and causes the free energy to become negative, making the bond break into vapor spontaneously. This results in a single-phase transition to two phases, such as in the case of water boiling.

The molecules overcome intermolecular forces, transition from liquid to gas, and require an input of energy to break bonds and increase entropy. This

process leads to a decrease in the free energy of the system, making the transition to the vapor phase spontaneous.

In summary, a phase change occurs when a substance reaches a critical point of maximum energy in its current phase, and any additional energy input leads to bond breaking and a transition to a new phase with lower free energy.

Phase change occurs on the hot side of HX

In a heat exchanger, the phase change of the cold stream on the cold side is unlikely to occur under normal operating conditions because the heat supply comes from the hot side. When the heat exchanger is operating under designed conditions, the temperature difference (ΔT) between the hot and cold streams allows for efficient heat transfer between the two fluids. However, in extreme cases where the heat transfer rate exceeds the design limits of the heat exchanger, the temperature difference between the hot and cold streams can approach zero. When the hot and cold streams reach thermal equilibrium, heat transfer between them stops, which can prevent phase change of the cold stream on the cold side of the heat exchanger. This abnormal situation can lead to a disruption in the heat transfer process and a failure to achieve the desired phase change in the cold stream.

Symptoms of phase change in HX

The phase change is more likely to occur on the hot side, which adds enthalpy and turbulence, increasing the heat transfer coefficient. This increase in heat transfer rate is likely to reduce the temperature difference (dt), and the increased velocity due to phase change can cause a higher pressure drop.