HVAC SYSTEMS - HFT Stuttgart

2.2.2 COOLING SYSTEM
2.2.2.1 Static Absorption Chiller Model
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CHAPTER 02
The performance of absorption cooling machines strongly depends on the
external fluid temperatures and mass flow rates. Apart from the generator
temperature the COP of absorption chillers at given cold water temperature is
mainly influenced by the cooling water temperatures for the absorption and
condensation process. For a description of the behaviour of absorption cooling
machines a number of models have been developed during the last two
decades. The most commonly used model is the so called characteristic
equation which has been developed by (Ziegler, 1998) and is based on the
internal mass and energy balances in all components. Detailed derivations of
the characteristic equation with its coefficients were described in (Schweigler, et
al., 1999). (Albers, et al., 2003) conclude that the application of one single
equation is not sufficient for absorption chillers with thermally driven bubble
pumps for internal solution transport.
The characteristic equation is derived from the heat transfer equations and the
internal enthalpy balance. Internal temperatures are written with a capital T and
standard t is used for all external temperatures. For simplification in the
standard characteristic equation, all UA values and the internal enthalpy
differences are calculated only for the design conditions and are assumed to be
constants for the calculation of the cooling power for other external temperature
conditions. Fig. 2-6 shows a system scheme of a single stage absorption chiller
with all main components like the evaporator (E), the absorber (A), the
generator (G) and the condenser (C) and the interconnection and flow
directions between the components.