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IEA SHC Task 38 Solar Air Conditioning and Refrigeration Subtask C1 Report, 31 October 2010
2.10.2 Flat Sheet Adiabatic Absorber
The falling film type absorbers presently used in lithium bromide absorption chillers are
characterized by a number of problems: low mass transfer, low heat transfer and large
volume.
The adiabatic spray absorber developed and patented by Ryan [28] as a solution, is not itself
free of drawbacks. According to its inventor [29], these include:
a) Low liquid flow rate, calling for many spraying units. As a result, the absorber
occupies a great deal of space.
b) Variable droplet diameter and a significant proportion with diameters under 150 µ.
c) Droplets with diameters under 150 µ and, consequently, a substantial portion of the
pumped mass flow is of no use for absorption purposes [29].
d) Considerable head loss. High power demand in the pump that drives the solution.
In an attempt to solve these problems, Warnakulasuriya and Worek [30] developed a spray
absorber with 400-micron diameter droplets. This device delivered higher mass transfer than
Ryan's spray and multiplied the performance of commercial falling film absorbers by about
four-fold.
Tackling the problem from a different angle, the Research Group modified the spray
absorber, first by building a parallel droplet spray absorber, patented in Spain, able to
generate droplets with no pressure loss and diameters of over 300 µ [31]. This absorber was
not commercially viable because the mass of the solution was difficult to control and the
mass transfer coefficient was similar to Ryan's. A jet absorber subsequently designed and
built, also featuring no pressure loss, improved mass transfer but solution mass remained
uncontrolled [32]. The next step was to develop a flat sheet absorber resting on a slanted
surface [33] able to operate at a higher flow rate and therefore greater mass transfer. While it
was also readily controlled, its need for material support constituted yet another difficulty. To
solve these problems, two new adiabatic absorbers were studied: one generating a coneshaped
sheet [34 and 38] and the other a flat fan-shaped sheet with an oval nozzle [39].
This flat sheet adiabatic absorber, which consists of a bank of sprayers with an oval crosssection
spaced at a minimum distance from one another, has been patented by Izquierdo et
al. [37]. This arrangement increases both the mass transfer coefficient and the area available
for mass transfer, significantly reducing volume. Moreover, since the flow rate is higher in
sheet than in droplet sprays, absorber volume can be reduced even further. The process is
readily controlled and pressure loss is small [39]. As an adiabatic unit, it transfers mass and
heat separately [29].
Its operation involves pumping the solution from the absorber to the single generator in the
single effect version or to the high and low temperature generators in the double-effect
prototype. Similarly, the solution is pumped from the absorber and recirculated in a heat
exchanger, where the absorption heat is transferred either to the outside air or to the water in
a cooling tower. This characteristic affords adiabatic absorbers greater heat transfer capacity
in a smaller exchange area than falling film absorbers [29].
After the device was tested, the results were published in the International Journal of
Refrigeration [36], Energy Conversion and Management [38] and Applied Energy [39]. The
paper “Evaluation of mass absorption in LiBr flat-fan sheets” [39] compared the mass
transfer of a flat-sheet spray generated by an oval spray nozzle to Ryan's and
Warnakulasuriya and Worek’s sprays. The conclusion drawn was that the mass transfer
coefficient delivered by the flat-sheet spray was five times greater than by the
Warnakulasuriya and Worek absorber. The improvement in mass transfer obtained with the
flat-sheet absorber constitutes a significant development in absorber technology. This new
generation of absorber is more efficient than Ryan's droplet spray and Warnakulasuriya and
Worek's absorbers and than any commercial absorber in use today. The patent granted in
2009 in the European Union (PCT/EP2009/057061 and European Patent EP09162208.4)
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