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HVAC Systems: Thermodynamics Review & Problem-Solving Guide

Learn about heating, cooling, ventilation systems, and more in HVAC. Solve thermodynamic problems using equations and diagrams. Understand components like coils, compressors, and controls. Enhance your HVAC knowledge efficiently.

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HVAC Systems: Thermodynamics Review & Problem-Solving Guide

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  1. Objectives • Review thermodynamics - Solve thermodynamic problems and use properties in equations, tables and diagrams

  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)

  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)

  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, CO2 and other occupancy sensors, humidistats, valves, dampers)

  5. Systems: Other • Auxiliary systems (i.e. venting of combustion gasses) • Condensate drainage/return • Dehumidification (desiccant, cooling coil) • Humidification (steam, ultrasonic humidifier) • Energy management systems

  6. Drain Pain • Removes moisture condensed from air stream Cooling coil • Heat transfer from air to refrigerant • Extended surface coil Condenser Expansion valve Controls Compressor

  7. Heating coil • Heat transfer from fluid to air Heat pump Furnace Boiler Electric resistance Controls

  8. 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)

  9. Duct system (piping for hydronic systems) • Distribute conditioned air • Remove air from space Provides ventilation Makes noise Affects comfort Affects indoor air quality

  10. Diffusers • Distribute conditioned air within room Provides ventilation Makes noise Affects comfort Affects indoor air quality

  11. Dampers • Change airflow amounts Controls outside air fraction Affects building security

  12. Filter • Removes pollutants • Protects equipment Imposes substantial pressure drop Requires Maintenance

  13. Controls • Makes everything work Temperature Pressure (drop) Air velocity Volumetric flow Relative humidity Enthalpy Electrical Current Electrical cost Fault detection

  14. Review • Basic units • Thermodynamics processes in HVAC systems

  15. 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 heat Δt = temperature difference

  16. 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

  17. 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)

  18. 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)

  19. 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

  20. 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+PV h = enthalpy (J/kg, Btu/lbm) P = Pressure (Pa, psi) v = specific volume (m3/kg, ft3/lbm)

  21. Second law In 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 S = entropy (J/K, BTU/°R) Q = heat (J, BTU) T = absolute temperature (K, °R)

  22. T-s diagrams • dH = TdS + VdP (general property equation) • Area under T-s curve is change in specific energy – under what condition?

  23. T-s diagram

  24. h-s diagram

  25. p-h diagram

  26. 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 (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)

  27. 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) PxV = mx Rx∙T PyV = 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 constant T = absolute temperature (K, °R)

  28. 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

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

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