Active Thermal Cooling Technologies - Low Energy Cooling for Sustainable Buildings - page 159

Table 5.4 Design conditions for a diffusion-absorption chiller

Generator Heating capacity 5.2 kW

Heating water in/out 130/120 ◦ C

Dephlegmator Recooling capacity 0.9 kW

Recooling water in/out 42/47 ◦ C

Condenser Recooling capacity 2.8 kW

35/37 ◦ C

Recooling water in/out

Evaporator Refrigerating capacity 2.5 kW

13/10 ◦ C

Cold brine in/out

Absorber Recooling capacity 4.0 kW

30/35 ◦ C

Recooling water in/out

COP 0.48

The used working pair for the solution circulation is an ammonia/water mixture. The
inert auxiliary gas used for the pressure compensation between the high and low
ammonia partial pressure level and for the gas circulation is helium.
The prototypes of the single effect solar-driven DACMs are designed for the ap-
plication area of air-conditioning as water chillers with an evaporator temperature of
6-12 ◦ C as well as for the operation of cooled ceilings with an evaporator temperature
of 15-18 ◦ C. Design temperatures and power levels are summarized in Table 5.4.

Gas Bubble Pump Performance

The directly driven bubble pump of DARs usually consists of a single lifting tube
where the heat input is restricted to a small heating zone by a heating cartridge or
the flame of a gas burner with a high heat flux density. The indirectly driven DACM
consists, on the other hand, of a bundle of tubes where the heating zone is spread and
lower heat flux densities are available. The developed bubble pumps of the DACM
pilot plants are basically vertical shell-and-tube heat exchangers where the solution
flows inside the tubes of small circular cross-section, forming slug flow at best, and
the heating medium flows through baffled tube bundles on the shell side.
The operation of the gas bubble pump is based on internal forced convection boiling,
commonly referred to as two-phase flow, and is characterized by rapid changes from
liquid to vapour in the flow direction. By external heating of the vertical pipes of the
bubble pump, the pipes are surrounded by a heat transfer medium such as a water-
glycol mixture or thermo-oil. The ammonia/water solution rests in the bundle of pipes
in such a way that when the tube walls are overheated ammonia is expelled from
the solution. Thus, bubbles are formed at the inner surface of the tube walls under
constant heat supply. In this case, the procedure is called flow boiling. The internal
flow boiling has to be distinguished by five two-phase flow regimes (bubbly flow, slug
flow, annular flow, transition flow and mist flow) as shown in Figure 5.48.