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Thermodynamics Lecture Notes: Properties, Units, and Calculations - Prof. Jeffrey A. Siege, Study notes of Architecture

The University of Texas at AustinArchitecture
Prof. Jeffrey A. Siegel

Lecture notes on thermodynamics, covering topics such as thermodynamic properties, units, sensible and latent heat, work, energy, and power. It includes formulas, conversions, and examples. Students are encouraged to review material from chapter 2 for an upcoming homework assignment.

Typology: Study notes

Pre 2010

Uploaded on 08/30/2009

koofers-user-we8
koofers-user-we8 🇺🇸

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bg1
Admin
• Website was down (sorry), now back up
• Lecture notes and videos posted
• Make-up classes
• Monday, Wednesday, Friday mornings and
afternoons
• Do you have a conflict EVERY week?
Objectives
• Solve thermodynamic problems and use
properties in equations
• Calculate heat transfer by all three modes
including phase change
• Apply Bernoulli equation to flow in a duct and
use a pitot tube
• Differentiate heat exchangers
• Review heating and cooling loads (next week)
pf3
pf4
pf5
pf8
pf9
pfa

Partial preview of the text

Download Thermodynamics Lecture Notes: Properties, Units, and Calculations - Prof. Jeffrey A. Siege and more Study notes Architecture in PDF only on Docsity!

Admin

  • Website was down (sorry), now back up
  • Lecture notes and videos posted
  • Make-up classes
    • Monday, Wednesday, Friday mornings and afternoons - Do you have a conflict EVERY week?

Objectives

  • Solve thermodynamic problems and use properties in equations
  • Calculate heat transfer by all three modes including phase change
  • Apply Bernoulli equation to flow in a duct and use a pitot tube
  • Differentiate heat exchangers
  • Review heating and cooling loads (next week)

Units

  • Pound mass and pound force
    • lbm = lbf (on Earth, for all practical purposes)
  • Acceleration due to gravity
    • g = 9.807 m/s^2 = 32.17 ft/s^2
  • Pressure (section 2.5 for unit conversions)
  • Temperature (section 2.6 for unit conversions)

Thermodynamic Properties

  • ρ = density = mass / volume
  • v = specific volume = 1 / ρ
  • specific weight = weight per unit volume (refers to force, not to mass)
  • specific gravity = ratio of weight of volume of liquid to same volume of water at std. conditions (usually 60 °F or 20 °C and 1 atm) Both functions of t, P

Work, Energy, and Power

  • Work is energy transferred from system to surroundings when a force acts through a distance - ft·lbf or N·m (note units of energy)
  • Power is the time rate of work performance
    • Btu/hr or W
  • Unit conversions in Section 2.
  • 1 ton = 12,000 Btu/hr (HVAC specific)

Where does 1 ton come from?

  • 1 ton = 2000 lbm
  • Energy released when 2000 lbm of ice melts
  • = 2000 lbm × 144 BTU/lbm = 288 kBTU
  • Process is assumed to take 1 day (24 hours)
  • 1 ton of air conditioning = 12 kBTU/hr
  • Note that it is a unit of power (energy/time)
  • How big is one ton of ice? ( ρ = 57.4 lbm/ft^3 )

Thermodynamic Laws

  • First law?
  • Second law?
  • Implications for HVAC
    • Need a refrigeration machine (and external energy) to make energy flow from cold to hot

Internal Energy and Enthalpy

  • 1 st^ law says energy is neither created or destroyed
    • So, we must be able to store energy in a fluid
  • Internal energy ( u ) is all energy stored
    • Molecular vibration, rotation, etc.
    • Formal definition in statistical thermodynamics
  • Enthalpy
    • Composite energy (sensible + latent)
    • We always track this term in HVAC analysis
    • h = u + Pv h = enthalpy (J/kg, Btu/lbm) P = Pressure (Pa, psi) v = specific volume (m^3 /kg, ft^3 /lbm)

Ideal gas law

  • Pv = RT or PV = nRT
  • R is a constant for a given fluid
  • For perfect gasses
    • Δu = cvΔt
    • Δh = cpΔt
    • cp - cv= R M = molecular weight (g/mol, lbm/mol) P = pressure (Pa, psi) V = volume (m^3 , ft^3 ) v = specific volume (m^3 /kg, ft^3 /lbm) T = absolute temperature (K, °R) t = temperature (C, °F) u = internal energy (J/kg, Btu, lbm) h = enthalpy (J/kg, Btu/lbm) n = number of moles (mol)

Mixtures of Perfect Gasses

  • m = mx my
  • V = Vx Vy
  • T = Tx Ty
  • P = Px Py
  • Assume air is an ideal gas
    • -70 °C to 80 °C (-100 °F to 180 °F) Px V = mx Rx·T Py V = my Ry·T m = mass (g, lbm) P = pressure (Pa, psi) V = volume (m^3 , ft^3 ) R = material specific gas constant T = absolute temperature (K, °R)

Mass-Weighted Averages

  • Quality,^ x , is^ mg/(mf + mg)
    • Vapor mass fraction
  • ζ = v or h or s in expressions below
  • ζ^ =^ ζf + x^ ζfg
  • ζ^ = (1- x)^ ζf + x^ ζg
  • ζ^ =^ ζg - (1- x)^ ζfg s = entropy (J/K/kg, BTU/°R/lbm) m = mass (g, lbm) h = enthalpy (J/kg, Btu/lbm) v = specific volume (m^3 /kg) Subscripts f and g refer to saturated liquid and vapor states and fg is the difference between the two

Properties of water

  • Water, water vapor (steam), ice
  • Properties of water and steam (pg 675 – 685)
    • Alternative - ASHRAE Fundamentals ch. 6

Thermodynamic Properties of

Refrigerants

  • What is a refrigerant?
    • Usually interested in phase change
  • What is a definition of saturation?
  • Enthalpy of liquid is ~the same as saturated liquid at same temperature
  • ASHRAE Fundamentals ch. 20 has additional refrigerants

Homework Assignment 1

  • Review material from chapter 2
  • Mostly plug and chug
    • Depends on your memory of thermodynamics and heat transfer
  • You should be able to do any of problems in Chapter 2 (after videos)
  • Problems 2.2, 2.6, 2.12, 2.14, 2.20, 2.22, 2.16a
    • Due on Tuesday 2/3 (1.5 weeks)