Unit - 2
First Law of Thermodynamics
Q1) Write Procedure for Energy Balance Calculations?
A1)
1. Make material balance calculations and find flow rates (or masses) of all streams.
2. Write the generalized energy balance equation and cancel all the terms that are either zeros or can be neglected.
3. Choose reference states for each species involved. By reference state we mean T, P and the phase of the species. A proper choice of the reference states enables easy calculation of enthalpies and hence, energy balances.
4. Construct an inlet-outlet enthalpy table with mass or molar flow rates for open systems and initial-final amounts of species and internal energies for closed systems.
5. Estimate the specific enthalpies or internal energies and insert the values in the Table constructed in step 4. You need to construct process paths to determine specific enthalpies and internal energies.
6. Solve the simplified energy balance equation in step 2 for the unknown.
Q2) Explain Energy Balances on Closed Systems?
A2)
A system is closed if mass does not cross the system boundary during the period of time covered by energy balance. Energy balance for a closed system written between two instants of time is
∆U + ∆Ek + ∆Ep = Q − W
∆U is change internal energy, ∆Ek is change in kinetic energy and ∆Ep is change in potential energy, Q is heat transferred to the system and W is work done by the system. We take heat lost to surroundings as –ve and heat transferred to the system as +ve.
If the system is adiabatic, there is neither gain by the system not heat loss and Q is zero. If there are no moving parts, then W is zero.
Q3 What is Nozzles?
A3
Nozzles are steady flow engineering devices that are used to increase a fluids velocity at the expense of pressure. For subsonic flows a nozzle will have a wider cross-sectional area at its entrance. The cross-sectional area will slope to a smaller cross-sectional area at its exit. On the other hand when the flow is supersonic, the entrance cross-sectional will be smaller than the exit cross-sectional area. The image below shows a basic subsonic nozzle.
Fig 1
Q3) What is Diffusers?
A4)
A diffuser is the exact opposite of a nozzle. Unlike a nozzle, it is used to decrease a fluids velocity. For subsonic flows the entrance of a diffuser is larger than its exit. In contrast, for supersonic flows the entrance is smaller than its exit. The image below shows a basic subsonic diffuser.
Fig 2
For both nozzles and diffusers, the rate of heat entering or exiting the boundary layer can be considered zero. In addition, the work and potential energy can also be considered zero. Taking these factors into consideration the energy balance equation for a nozzle and diffuser will be the following.
Q4) What is Turbines?
A5)
Turbines are steady flow engineering devices that rely on shaft work to drive an electric generator. They are found in steam, gas, and hydrostatic power plants. Heat transfer is negligible. In addition, potential energy is negligible unless otherwise stated. Finally, even though kinetic energy is present within the system, the change in enthalpy is normally much greater. As a result, kinetic energy is also negligible. During operation super-heated steam will be pushed through the turbine. As this is occurring it is possible that the steam could become a saturated gas when it leaves. This must be taken into consideration when you are calculating the exit enthalpy. Finally, liquids can damage the blades of a turbine. As a result, it is best to keep a fluid in a gas state as it passes through the turbine.
The equation below represents the basic energy balance for a turbine.
Fig 3
Q6) Explain Compressors?
A6)
A compressor, unlike a turbine, is a steady flow engineering device that uses shaft work to create power. Hence, shaft work is required to run a compressor. In addition, heat transfer, as well as potential and kinetic energy are negligible. The equation below represents the basic energy balance for a compressor.
Fig 4
Other devices that are similar to compressors are fans and pumps. There are, however, some application differences between these devices. For instance, a fan does not raise the pressure of a fluid to a high pressure. Instead it will mobilize a gas. Pumps, on the other hand, are used to move or pressurize liquids. Pumps can only be used for liquids, while compressors can only be used on gases. If a pump tried to pump a gas it could be damaged. The same goes for a compressor if it were used to compress a liquid it would most likely ruin the compressor.
Q5) Explain Throttling Valves?
A7)
A throttling valve is a steady flow engineering device that is used to restrict flow. A considerable pressure drop will result from the restricted flow. In turn, a large temperature drop will result. Hence, throttling valves are widely used in refrigeration devices. There are many devices that can be considered throttling valves. Some examples are porous plugs, capillary tubes, and adjustable valves. Fluid flowing through a throttling valve undergoes an adiabatic process. In addition, no work is done and potential energy is negligible. Kinetic energy is also negligible even though there is a large change in velocity. Instead change in internal energy is much more pronounced in comparison to kinetic energy. Finally, enthalpy values at the entrance and exit of the throttling valve are approximately the same. Taking all of this into consideration the following energy balance equation for a throttling valve will result.
Q6) Explain Types of Throttling Valves?
A8)
Throttling valves are very simple steady flow engineering devices. As mentioned above throttling valves can be a porous plug, capillary tubes put in between two large tubes, or simply an adjustable valve like a ball valve. The whole point is to restrict the flow to drastically drop the pressure which will cause a large temperature drop within the fluid. Refer to the image below to see different types of throttling valves.
Fig 5
For a fun fact, besides refrigerators, another common device that uses a throttling valve is the carburettor in a small engine. The throttling valve in a small engine is not there to adjust the temperature of the gas. Instead it is used to control the amount of air that will mix with the gas to control combustion. If there isn’t enough air, the engine will run rich causing poor fuel economy. On the other hand, if there is too much air, the engine will run lean. This will cause the engine to produce more heat. As a result, less power will be produced.
The type of valve used on carburettors is typically a butterfly valve. The throttling valve is normally controlled by the choke. Closing the choke purposely causes the engine to run rich. This is done to make it easier to start the engine. The choke is than opened to allow optimal running conditions once the engine has warmed up. The stoichiometric air-fuel ratio which 14.7 lb of for ever 1 lb of fuel is the ideal air to fuel ratio for lowest emission, however, the best ratio for power is 12.5 lb of air for 1 lb of fuel.
Q7) What is Mixing Chambers?
A9)
Mixing chamber are steady flow engineering devices that mix two streams of fluid to cause a temperature change. The temperature alteration is highly dependent on the mass flow of the two streams in comparison to each other. When solving for these types of problems the conservation of mass must be taken into consideration. For this device the mass of fluid entering must equal the mass of fluid exiting. In addition, how well insulated the device is must be considered. A well-insulated device will have negligible heat transfer to the outside environment. Finally, both potential and kinetic energy are negligible. Taking all of this into consideration the following energy balance is used for a well-insulated mixing chamber.
Fig 6
Q10) what is Heat Exchangers?
A10)
Heat exchangers are similar to mixing chambers except they do not allow the two streams of fluid to mix. As a result, the mass flow entering each pipe will remain constant. In addition, for well insulated heat exchangers heat transfer can be considered negligible. In addition, kinetic and potential energy as well as work is negligible. The following energy balance equation is used for a well insulating heat exchanger.
This equation is meant for a heat exchanger scene in the image below. If the heat exchanger you are looking at is different you may have to modify equation 8.
Fig 7
