# objectives learn basics about ahus review thermodynamics - solve thermodynamic problems and use...

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- Slide 1
- Objectives Learn basics about AHUs Review thermodynamics - Solve thermodynamic problems and use properties in equations, tables and diagrams
- Slide 2
- 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)
- Slide 3
- 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)
- Slide 4
- 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, CO 2 and other occupancy sensors, humidistats, valves, dampers)
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- Systems: Other Auxiliary systems (i.e. venting of combustion gasses) Condensate drainage/return Dehumidification (desiccant, cooling coil) Humidification (steam, ultrasonic humidifier) Energy management systems
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- Cooling coil Heat transfer from air to refrigerant Extended surface coil Drain Pain Removes moisture condensed from air stream Condenser Expansion valve Controls Compressor
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- Heating coil Heat transfer from fluid to air Heat pump Furnace Boiler Electric resistance Controls
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- 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)
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- Duct system (piping for hydronic systems) Distribute conditioned air Remove air from space Provides ventilation Makes noise Affects comfort Affects indoor air quality
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- Diffusers Distribute conditioned air within room Provides ventilation Makes noise Affects comfort Affects indoor air quality
- Slide 11
- Dampers Change airflow amounts Controls outside air fraction Affects building security
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- Filter Removes pollutants Protects equipment Imposes substantial pressure drop Requires Maintenance
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- Controls Makes everything work Temperature Pressure (drop) Air velocity Volumetric flow Relative humidity Enthalpy Electrical Current Electrical cost Fault detection
- Slide 14
- Review Basic units Thermodynamics processes in HVAC systems
- Slide 15
- 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)
- Slide 16
- Thermodynamic Properties = density = mass / volume v = specific volume = 1 / 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
- Slide 17
- 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/(lbmF), kJ/(kgC) For gasses, two relevant quantities c v and c p Basic equation (2.10) Q = mct Q = heat transfer (Btu, kJ) m = mass (kg, lbm) c = specific heat t = temperature difference
- Slide 18
- Sensible vs. latent heat Sensible heat Q = mct Latent heat is associate with change of phase at constant temperature Latent heat of vaporization, h fg Latent heat of fusion, h fi h fg for water (100 C, 1 atm) = 1220 Btu/lbm h fi for ice (0 C, 1 atm) = 144 Btu/lbm
- Slide 19
- Work, Energy, and Power Work is energy transferred from system to surroundings when a force acts through a distance ftlbf or Nm (note units of energy) Power is the time rate of work performance Btu/hr or W Unit conversions in Section 2.7 1 ton = 12,000 Btu/hr (HVAC specific)
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- 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)
- Slide 21
- Thermodynamic Laws First law? Second law? Implications for HVAC Need a refrigeration machine (and external energy) to make energy flow from cold to hot
- Slide 22
- Internal Energy and Enthalpy 1 st 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 h = enthalpy (J/kg, Btu/lbm) P = Pressure (Pa, psi) v = specific volume (m 3 /kg, ft 3 /lbm)
- Slide 23
- Second law In any cyclic process the entropy will either increase or remain the same. Entropy Not directly measurable Mathematical construct Note difference between s and S Entropy can be used as a condition for equilibrium S = entropy (J/K, BTU/R) Q = heat (J, BTU) T = absolute temperature (K, R)
- Slide 24
- Thermodynamic Identity Use total differential to H = U + PV dH=dU+PdV+VdP, using dH=TdS +VdP TdS=dU+PdV Or: dU = TdS - PdV
- Slide 25
- T-s diagrams dH = TdS + VdP (general property equation) Area under T-s curve is change in specific energy under what condition?
- Slide 26
- T-s diagram
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- h-s diagram
- Slide 28
- p-h diagram
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- Ideal gas law Pv = RT or PV = nRT R is a constant for a given fluid For perfect gasses u = c v t h = c p t c p - c v = 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)
- Slide 30
- Mixtures of Perfect Gasses m = m x m y V = V x V y T = T x T y P = P x P y Assume air is an ideal gas -70 C to 80 C (-100 F to 180 F) P x V = m x R x T P y V = m y R y T What is ideal gas law for mixture? m = mass (g, lbm) P = pressure (Pa, psi) V = volume (m 3, ft 3 ) R = material specific gas constant T = absolute temperature (K, R)
- Slide 31
- Enthalpy of perfect gas mixture Assume adiabatic mixing and no work done What is mixture enthalpy? What is mixture specific heat (c p )?
- Slide 32
- Mass-Weighted Averages Quality, x, is m g /(m f + m g ) Vapor mass fraction = v or h or s in expressions below = f + x fg = (1- x) f + x g 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
- Slide 33
- Properties of water Water, water vapor (steam), ice Properties of water and steam (pg 675 685) Alternative - ASHRAE Fundamentals ch. 6
- Slide 34
- Psychrometrics What is relative humidity (RH)? What is humidity ratio (w)? What is dewpoint temperature (t d )? What is the wet bulb temperature (t*)? How do you use a psychrometric chart? How do you calculate RH? Why is w used in calculations? How do you calculate the mixed conditions for two volumes or streams of air?
- Slide 35
- Thermodynamic Properties of Refrigerants What is a refrigerant? Usually interested in phase change What is a definition of saturation? ASHRAE Fundamentals ch. 20 has additional refrigerants
- Slide 36
- Homework Assignment 1 Review material from chapter 2 Mostly thermodynamics and heat transfer Depends on your memory of thermodynamics and heat transfer You should be able to do any of problems in Chapter 2 Problems 2.3, 2.6, 2.10, 2.12, 2.14, 2.20, 2.22 Due on Thursday 2/3 (~2 weeks)