Refrigeration and reciprocating engine cycles
(a) A refrigeration system operating with Refrigerant 134a consists of a compressor, a condenser, a throttle valve and an evaporator. The refrigerant enters the compressor dry saturated at a temperature of -150C and is compressed to pressure of 13.174bar, with an isentropic efficiency of 85%. The refrigerant leaves the condenser sub-cooled to 300C.
(i) Draw the T – s diagram for the system, marking appropriate state points.
(ii) Evaluate the coefficient of performance of the system acting as a refrigerator.
(iii) The refrigerator input is required to be 1kW. Calculate the mass flow rate of refrigerant needed for this.
(b) The system is now re-configured into a two-stage compressor with flash chamber. The flash chamber and mixer operate at a saturation temperature of 00C. The same condenser and evaporator temperatures and pressures apply as in (a). Both compressors are 85% efficient.
(i) Sketch the revised T -s diagram, marking appropriate state points.
(ii) Obtain the fraction of the mass of refrigerant that flashes and re-evaluate the coefficient of performance.
Q2. A reciprocating engine is designed to operate on a dual cycle. At the start of compression the air temperature and pressure are 150C and 1.1bar respectively and the volumetric compression ratio is 12. Combustion adds 2000kJ of heat to each kg of the air, equally divided between the constant pressure and constant volume processes. The compression and the second part of expansion are isentropic. For the analyses in (a), use estimated cv and/or cp values evaluated at the mid point of the temperature range of the process under consideration. Iterate no more than 2 times.
(i) Draw the p – v diagram for this cycle, marking in the necessary state points.
(ii) Evaluate the pressure ratio for the constant volume heat input process, the peak pressure, the cut-off ratio, the net work output, the cycle efficiency and the mean effective pressure.
(iii) If combustion results in the release of 43,000 kJ of heat per kg of fuel, determine the air to fuel ratio by mass.
