Chapter 6 THE MECHANICAL ENERGY BALANCE

In some process, heat transfer (Q) and enthalpy changes are important energy component in the energy balance. While shaft work, PE and KE terms are usually taken as negligible energies, thus usually taken as zero value. Ex. Distillation, reactors, heat exchangers There are some process where shaft work and mechanical forms of energy ( KE, PE) are important factors in the energy balance. Ex. Compression of gases, pumping of liquids, flow of fluids in rough pipes For these processing an energy balance including the mechanical forms of energy is called Mechanical Energy Balance.

Mechanical energy balance is a type of energy balance that can tell us a great deal about simple flow systems Mechanical Energy balance is useful when : Only one input and one output system Steady flow Almost constant temperature No reaction No heat transfer No phase change Fluid is incompressible Shaft Work ( - Ws) is the work done by the process fluid on a moving part within the system (ex. pump motor ). Usually with negative sign

Mechanical Energy Balance for Open System, Steady State Ev = loss of mechanical energy (Q – ΔU) which are loss during the flow of the fluid, i.e. friction, zero for reversible process

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In a processing plant, milk flows from a storage tank maintained at 5ºC through a valve to pasteurizer via an insulated 10 cm diameter pipe at the rate of 1000 L/min. The upstream pressure is 290 KPa and downstream pressure is 140 KPa. Determine the lost work (Ev) in J/min and the temperature change which occurs in the milk as a result of this throttling process (Milk and water are sufficiently equivalent in properties for you to use those of water.)

If a turbine, driven by water flowing from a reservoir 80 m higher than the turbine delivers 200 kW, what is the flow rate of the water in kg/s? Assume that the friction losses in the system yield an overall efficiency (actual work/ideal reversible work) of 75%. The reservoir is open to the atmosphere, and the exit velocity of the water is 5 m/s at a pressure of 150 kPa from the turbine