Numerical Studies of Refrigerating Liquid Overfeed Systems Working with Ammonia and R-134a

A numerical simulation of a liquid overfeed system working with pure refrigerants in steady state conditions has been performed. The global algorithm is based on the sequential resolution of the different elements of the system (compressor, evaporator, condenser, ... ). For each element, a simplified mathematical model based on global balances of mass, momentum and energy is used. For the evaporator and condenser, specially tailored lumped parameter models have been developed based on an advanced detailed simulation model. In this way, global heat transfer coefficient (U) and the flow configuration factor (F) have been obtained. The behaviour of two refrigerant working fluids, ammonia and Rl34a, has been compared for a specified system and for different working conditions. NOMENCLATURE w Q m rna maux p Tj 'A ~p ~Tim ~Tsh ~z A Cp D F f g h k m rna p power [W] heat flux [W] mass flux [kg/s] oil mass flux [kg/s] auxiliar heat exhanger secondary fluid mass flow [kg/s] condenser secondary fluid mass flow [kg/s] evaporator secondary fluid mass flow [kg/s] heat transfer coefficient [W/m K] density [kg/m] efficiency thermal conductivity [W/m K] pressure drop [Pa] logarithmic temperature difference [K] gas superheating [K] height difference [m] longitudinal area [m] specific heat capacity cte press.[J/kg K] diameter [ m] configuration factor friction factor acceleration due to gravity [ m/s] enthalpy [J/kg] coefficient of pressure drop in singularities mass [kg] quantity of oil after the oil separator [ppm] pressure [Pa] Fifteenth International Compressor Engineering Conference at Purdue University, West Lafayette, IN, USAJuly 25-28, 2000 S cross section area [m] T temperature [K] U global heat transfer coefficient [W/m K] v velocity [m/s] Xa oil mass fraction in low pressure receiver Xg vapour quality subscripts aux,i inlet secondary fluid auxiliar heat exchanger aux,o outlet secondary fluid auxiliar heat exchanger IllS insulation c condenser c,i inlet secondary fluid condenser c,o outlet secondary fluid condenser cp compressor dis discharge E electrical e evaporator e,i inlet secondary fluid evaporator e,o outlet secondary fluid evaporator em electrical-mechanical ext external fric friction i inner numbers (1, 2, 3, ... )-points (see Figure 1) 0 outer p pump r receiver sing singularities sue suction