Air conditioning: Heat exchanger

Printer-friendly version

Heat exchangers are important building blocks of technical systems, which take care of the appropriate environmental conditions in apartments as well as in industrial manufacturing. A very frequently used type of heat exchangers is the tube-bundle heat exchanger. It features numerous tubes, which are oriented either in a vertical or horizontal direction, and the tubes can be aligned in a staggered or in-line fashion. In case dry air shall be conditioned by a heat exchanger, established relations for its design and dimensioning can be employed. In case moist air is the fluid to be treated, the possibly large mass fraction of water can lead to the occurrence of more complex thermodynamic processes within the heat exchanger than just pure heat conduction.

If moist air, having a vapor content approaching the saturation condition, is cooled down, condensation will take place. The cooling tubes of the heat exchanger will be covered by a water film, which decreases the local heat transfer coefficients and hence impair the efficiency of the heat exchanger significantly. In case the heat exchanger is equipped with horizontal tubes, the condensed water of the tubes located higher will trickle down onto lower tubes, increase the water-film thickness there and deteriorate the heat transfer locally.

Such effects can not be account for employing the known framework of engineering guidelines for the design of heat exchangers, as available e.g. for dry air.

However, a three-dimensional numerical simulation gives valuable information also for complex multiphase flows with phase changes.

In cooperation with an international renowned corporation of the refrigeration and air-conditioning engineering branch, a tube-bundle heat exchanger has been analyzed. The heat exchanger exhibited vertical tubes aligned in an in-line pattern. Moist air having properties close to saturation conditions was cooled down in the device. The numerical investigation captured all relevant thermodynamic effects, as there are

- Inhomogeneous, swirled flow approaching second and subsequent cooling tubes (see Figure 1),

- Condensation at the tubes, depending on the local mass fraction of the vapor (see Figure 2),

- Establishment of water films at cooling tubes (see Figure 3), and

- Flow of the condensed water from forward to rear stagnation point on the tube due to the oncoming flow, and to the bottom of the tube due to gravity.

These detailed analysis enabled a substantially improved heat exchanger design, especially in case condensation takes place within the device.