KEJ3163 Chapter 3 - 2024 PDF

12/23/2024 At the end of this lecture, students should be able to: ►Explain key concepts related to energy and the first law of...

12/23/2024 At the end of this lecture, students should be able to: ►Explain key concepts related to energy and the first law of thermodynamics... including internal, kinetic, and potential energy, work and power, heat transfer and heat transfer modes, heat transfer rate, power cycle, refrigeration cycle, and heat pump cycle. ►Analyze closed systems including applying energy balances, appropriately modeling the case at hand, and correctly observing sign conventions for work and heat transfer. ►Conduct energy analyses of systems undergoing thermodynamic KEJ3163 CHEMICAL PROCESS PRINCIPLES cycles, evaluating as appropriate thermal efficiencies of power cycles and coefficients of performance of refrigeration and heat pump cycles. 1 2 1 2 WORK 1ST LAW OF ISOBARIC AND THERMODYNAMIC THERMODYNAMICS ISOCHORIC CYCLES PROCESSES 3 4 3 4  12/23/2024 Work is a force F acting through a displacement x, in the direction of the force. THE THERMODYNAMIC DEFINITION OF WORK Since F = PA, work can be expressed Thermodynamic Definition of Work: Work is done by a system on the surroundings if the in terms of pressure sole effect on everything external to the system could have been raising a weight. and area: This equation is based on a mechanical Compression or definition of work. However, we need a broader expansion of gas (or definition of work that will allow us to liquid) understand other forms of work. 5 The dashed line represents our system boundary (the imaginary surface that separates the system of study from the surroundings). The region of space inside the dashed line is our 6 closed system and everything else is considered the surroundings. 5 6 WHICH DIRECTION IS WORK TRANSFERRED? Consider System A: the paddle wheel transfers work across the system boundary into We have drawn the boundaries of two systems, A and B. the gas. This is work done by the surroundings (paddle wheel, motor and battery) on the system (gas). Work is crossing the boundary of each system. There is no inherent sign convention for work because it depends on your perspective. Can you tell in which direction work is moving? Into or out of each system ? However, once you choose a sign convention for a problem it is important to be consistent. System A System A Surroundings Our sign convention: 7 8 System B WA < 0: work done on the system 7 8  12/23/2024 Consider System B: the generator transfers work across the system boundary. This is This diagram illustrates our definition of work involving the electrical work done by the system on the surroundings. raising of a weight. Work has units of Joules (J) in SI units. Again, there is no inherent sign convention for work because it depends on your perspective. 1 J = 1 N*m However, once you choose a sign convention New idea: what is the rate at for a problem it is important to be consistent. which work is being done ? the dot over the W indicates a rate. In SI, the rate at which work is done has units of Joules per second or Watts (W): 1 W = 1 J/s Surroundings Our sign A Watt is a unit of Power. convention: English System: WB > 0: work done on the surroundings 9 10 System B Power [=] Btu/s or ft-lbf/s or hP (horsepower) 9 10 WORK IS NOT A PROPERTY: IT IS INEXACT This is an inexact Work is not a property of the system differential. So far, we or the surroundings. Work is a path have only dealt with exact variable differentials. and path variables have inexact differentials. State 2 To understand this better State 2 consider this process path from State 1 to 2 Exact Differentials: Volume, temperature, pressure, Let this process be the expansion internal energy, and enthalpy are of a gas contained in a piston-and- State 1 state functions (they depend only on the State 1 cylinder device. state and not the details of the process). 11 12 The differential of every property is exact. 11 12  12/23/2024 BOUNDARY WORK AND ITS PROCESS PATH POSITIVE AND NEGATIVE BOUNDARY WORK Here we see our process from We are going to investigate the Here is a P-V Diagram showing relationship between work and our the initial and final states of our State 1 (high pressure, small process path. system. volume) to State 2 (low pressure, large volume). Remember that work is the product of the force driving the expansion and the displacement in the direction of Think about our sign convention for the force. work. P > 0, always. If the piston never accelerates, then: When dV > 0, the gas expands and W > 0. When dV < 0, the gas is compressed and W < 0. Initial State: P1 V1 Final State: P2 V2 Where: P1>P2 V1