Consider three lakes in series with a pulse (instantaneous) input of a conservative contaminant into lake 1 that results in an initial concentration of 10 mg/l. lakes 2 and 3 initially have no contaminant initially. the inflow to lake 1 contains no contaminant. the hydraulic residence time of each lake is 3 months. write the appropriate mass balance equations and derive an equation (by solving the differential equations) for the concentration in each lake as a function of time. you can solve the mass balance equations sequentially. remember, the inflow concentration to the second lake is the outflow concentration from the first, and the inflow concentration to the third lake is the outflow concentration from the second.

Aheat pump with refrigerant-134a as the working fluid is used to keep a space at 25°c by absorbing heat from geothermal water that enters the evaporator at 500c at a rate of 0.065 kg/s and leaves at 40°c. the refrigerant enters the evaporator at 20°c with a quality of 23 percent and leaves at the inlet pressure as saturated vapor. the refrigerant loses 300 w of heat to the surroundings as it flows through the compressor and the refrigerant leaves the compressor at 1.4 mpa at the same entropy as the inlet. determine: (a) the degrees of subcooling of the refrigerant in the condenser, b)-the mass flow rate of the refrigerant . (c) the heating load and the cop of the pump, and d)-the theoretical minimum power input to the compressor for the same heating load.