1. INTRODUCTION

1.1. Types of heat exchangers

REGENERATORS

In a regenerator, the heat transfer between two streams is transported by the passage of by the passage of alternating hot and cold fluids through a bed of solids, which has a considerable heat storage capacity. The fluid provides heat to the hot solid heat gradually, but before reaching equilibrium flows are exchanged and then the cooling fluid removes heat from the bed. In one type of regeneration, using identical twins as a desorbidor-absorber system. A second type uses a rotating bed in the shape of a thick rim, with cold fluid flowing axially through the sector (usually 180 degrees) of the bed, while the hot fluid flows in a reverse direction through the other sector. In rotary regenerators, the bed is often an array of bars, screens or corrugated sheets, for having a large surface area, but also a high fraction of voids and a pressure drop lower than a bed of particles.

The regenerative offer the advantage of a large surface area per unit volume and low cost compared to shell and tube exchangers. They are also easy to clean, and the frame can be easily replaced. The main problem with the rotating units is that some fluid leaks under the sheet deflectors that separate hot and cold sectors. In addition, almost no mixing of the currents due to fluid in any of the spaces is transported through the plates to another sector. For air preheated with hot combustion gases, the slight leakage of combustion gases into the air, and vice versa, is not a big problem, and rotary regenerators are widely used in power plants. They are also used in incinerators, furnaces and gas turbine engines. In general, are not suitable for regenerative liquid, because the heat capacity of liquid in the pores could be comparable to the solid matrix.

The effectiveness of a regenerator depends on the number of units of heat transfer and cycle time. For the same flow capacity and resistance negligible in the solid film coefficients are combined to obtain an effective global coefficient U:

The number of transfer units based on the total surface area of two beds or wheel rotation.



Where :



Below you can see the operation of a regenerator (click on the image to see it in motion):