Environmental Engineering Reference
and evaporator inlet during the machine start-up, the distribution of condensate into
the evaporator tubes and the surface wetting must have been too uneven to generate
sufficient driving pressure between the ammonia-rich gas and the helium-rich gas after
the absorber. Therefore the cooling power dropped significantly after about 1 hour of
operation. The bubble pump performance was satisfactory, but the chosen shell-and-
tube heat exchangers had a maximum of only 50% heat recovery efficiency (SHX).
Furthermore, the large reservoir between the absorber and generator led to very slow
heating up of the rich solution entering the generator and COPs were correspondingly
The second compacted prototype showed stable and continuous temperature and
pressure levels. The COPs reached were between 0.2 and 0.45 and the continuous
cooling performance was between 1.0 and 1.6 kW. A maximum cooling capacity of
2.0 kW could be reached if the evaporator temperature was set to a value of 25 ◦ C.
Again, condensate distribution into the evaporator tubes and surface wetting seemed to
be the crucial problems of the second prototype, whereas the bubble pump performance
was good. Coaxial solution heat exchangers gave much better performance results than
the initially chosen plate heat exchangers.
The third prototype was set up in October 2005; this new prototype now has mar-
ketable dimensions. The design cooling power of 2.5 kW was reached at generator
temperatures of 120-130 ◦ C at COPs of nearly 0.4 (see Table 5.7).
An expanded, steady-state model of the DACM based on the characteristic equation
of sorption chillers is described in the next chapter. It gives a good agreement between
experimental and simulated data. Efficient evaporation with high surface wetting fac-
tors is essential for high performance.
Further development is required regarding the evaporator cooling capacity and
COPs at lower heating inlet temperatures as well as weight and production cost
reduction. With the second and third pilot plants, promising steps regarding these
requirements have been made.
Table 5.7 Summary of prototype development
Shell-and tube SHX
1.5 × 1.5
0.8 × 0.8
0.8 × 0.8
0.6 × 0.6
of COP and
COP and high weight
Generator temp. / ◦ C