Objectives Review thermodynamics - Solve thermodynamic problems and use properties in equations,...
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Transcript of Objectives Review thermodynamics - Solve thermodynamic problems and use properties in equations,...
Objectives
• Review thermodynamics - Solve thermodynamic problems and use properties
in equations, tables and diagrams
Systems: Heating
• Make heat (furnace, boiler, solar, etc.)
• Distribute heat within building (pipes, ducts, fans, pumps)
• Exchange heat with air (coils, strip heat, radiators, convectors, diffusers)
• Controls (thermostat, valves, dampers)
Systems: Cooling
• Absorb heat from building (evaporator or chilled water coil)
• Reject heat to outside (condenser)• Refrigeration cycle components (expansion valve,
compressor, concentrator, absorber, refrigerant)• Distribute cooling within building (pipes, ducts, fans,
pumps)• Exchange cooling with air (coils, radiant panels,
convectors, diffusers)• Controls (thermostat, valves, dampers, reheat)
Systems: Ventilation
• Fresh air intake (dampers, economizer, heat exchangers, primary treatment)
• Air exhaust (dampers, heat exchangers)• Distribute fresh air within building (ducts,
fans)• Air treatment (filters, etc.)• Controls (thermostat, CO2 and other
occupancy sensors, humidistats, valves, dampers)
Systems: Other
• Auxiliary systems (i.e. venting of combustion gasses)
• Condensate drainage/return
• Dehumidification (desiccant, cooling coil)
• Humidification (steam, ultrasonic humidifier)
• Energy management systems
Cooling coil•Heat transfer from air to refrigerant•Extended surface coil
Drain Pain•Removes moisture condensed from air stream
Condenser
Expansion valve
Controls
Compressor
Blower•Overcome pressure drop of system
Adds heat to air stream
Makes noise
Potential hazard
Performs differently at different conditions (air flow and pressure drop)
Duct system (piping for hydronic systems)•Distribute conditioned air•Remove air from space
Provides ventilation
Makes noise
Affects comfort
Affects indoor air quality
Diffusers•Distribute conditioned air within room
Provides ventilation
Makes noise
Affects comfort
Affects indoor air quality
Controls•Makes everything work
Temperature
Pressure (drop)
Air velocity
Volumetric flow
Relative humidity
Enthalpy
Electrical Current
Electrical cost
Fault detection
Heat Units
• Heat = energy transferred because of a temperature difference• Btu = energy required to raise 1 lbm of water 1 °F• kJ
• Specific heat (heat per unit mass)• Btu/(lbm∙°F), kJ/(kg∙°C)
• For gasses, two relevant quantities cv and cp
• Basic equation (2.10) Q = mcΔt Q = heat transfer (Btu, kJ)m = mass (kg, lbm)c = specific heatΔt = temperature difference
Sensible vs. latent heat
• Sensible heat Q = mcΔt
• Latent heat is associate with change of phase at constant temperature• Latent heat of vaporization, hfg
• Latent heat of fusion, hfi
• hfg for water (100 °C, 1 atm) = 1220 Btu/lbm
• hfi for ice (0 °C, 1 atm) = 144 Btu/lbm
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 Handouts
• 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)
Thermodynamic Laws• First law?
• Second law?
• Implications for HVAC• Need a refrigeration machine (and external energy) to
make energy flow from cold to hot
• Literature for Thermodynamics Review:• Fundamentals of Thermal-Fluid Sciences
-Yunus A. Cengel, Robert H. Turner, Yunus Cengel, Robert Turner
• or any thermodynamics book
Internal Energy and Enthalpy
• 1st law says energy is neither created or destroyed• So, we must be able to store energy
• Internal energy (u) is all energy stored • Molecular vibration, rotation, etc.
• Formal definition in statistical thermodynamics
• Enthalpy• Total energy
• We track this term in HVAC analysis
• h = u + Pv or H=U+PVh = enthalpy (J/kg, Btu/lbm)P = Pressure (Pa, psi)v = specific volume (m3/kg, ft3/lbm)
Second lawIn any cyclic process the entropy will either increase or
remain the same.
Entropy• Not directly measurable• Mathematical construct
or
• Note difference between s and S
• Entropy can be used as a condition for equilibrium
T
dQdS
S = entropy (J/K, BTU/°R)Q = heat (J, BTU)T = absolute temperature (K, °R)T
QdS
T-s diagrams
• dH = TdS + VdP (general property equation)• Area under T-s curve is change in specific energy
– under what condition?
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
Kkg
kJ314.8
R
lbf
lbm
ft1545
MMR
M = molecular weight (g/mol, lbm/mol)P = pressure (Pa, psi)V = volume (m3, ft3)v = specific volume (m3/kg, ft3/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∙TPy V = my Ry∙T
What is ideal gas law for mixture?
m = mass (g, lbm)P = pressure (Pa, psi)V = volume (m3, ft3)R = material specific gas constantT = absolute temperature (K, °R)
Mass-Weighted Averages
• Quality, x, is mg/(mf + mg)
• Vapor mass fraction
• φ= v or h or s in expressions below
• φ = φf + x φfg
• φ = (1- x) φf + x φg
s = entropy (J/K/kg, BTU/°R/lbm)m = mass (g, lbm)h = enthalpy (J/kg, Btu/lbm)v = specific volume (m3/kg)
Subscripts f and g refer to saturated liquid and vapor states and fg is the difference between the two