What Determines If a System Is Open Or Closed?

Worksheet: ENERGY BALANCES II Leader: Shawn V. Dr. Lamm Course: CH E 210 Supplemental Instruction Instructor: Dr. Lamm Iowa State University Date: 3-27-2016 Warm Up

What determines if a system is open or closed?

What are two conditions for which heat transfer across a boundary can be neglected?

Practice Problems

For the following systems, define the system, state whether the system is open or closed, and write a simplified energy balance.

Water passes through the sluice gate of a dam and falls on a turbine rotor, which turns the shaft connected to a generator. The fluid velocity on both sides of the dam is negligible, and the water undergoes insignificant pressure and temperature changes between the inlet and outlet.

A perfectly insulated piston-fitted cylinder contains chemicals that react with one another at 100 degrees Celsius. The temperature of the surroundings is 20 degrees Celsius. A clamp on the piston keeps it from expanding.

A liquid stream flows through a heat exchanger in which it is heated from 25 degrees Celsius to 80 degrees Celsius. The inlet and outlet pipes have the same diameter, and there is no change in elevation between the inlet and the outlet.

1060 Hixson-Lied Student Success Center v 515-294-6624 v [email protected] v http://www.si.iastate.edu The specific enthalpy of liquid n-hexane at 1 atm varies linearly with temperature and equals 25.8 kJ/kg at 30 degrees Celsius and 129.8 kJ/kg at 50 degrees Celsius.

Determine the equation that relates the specific enthalpy to temperature and calculate the reference temperature on which the given enthalpies are based. Then derive an equation for the internal energy as a function of temperature at 1 atm.

Calculate the heat transfer rate required to cool liquid n-hexane flowing at a rate of 20 kg/min from 60 degrees Celsius to 25 degrees Celsius at a constant pressure of 1 atm. Estimate the change in specific internal energy (kJ/kg) as the n-hexane is cooled at the given conditions.

Liquid water at 30.0 degrees Celsius and liquid water at 90.0 degrees Celsius are combined in a ratio (1 kg cold water : 2 kg hot water)

Use a simple calculation to estimate the final water temperature without using an energy balance.

Now assume a basis of calculation and write a closed-system energy balance to calculate for the process, assuming the mixing is adiabatic. Determine the final temperature of the mixed water.

Critical Thinking

Consider the weather on a sunny 75 degree F day in June. As you walk to your car, you notice that you feel comfortable (even a little warm) in the sun. However, as you drive you stick your hand out the window and feel that it is uncomfortably cold. Explain this phenomenon.