Objectives Review thermodynamics - Solve thermodynamic problems and use properties in equations, tables and diagrams Used prometheus

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

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

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

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

Dampers Change airflow amounts Controls outside air fraction Affects building security

Filter Removes pollutants Protects equipment Imposes substantial pressure drop Requires Maintenance

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

Review Basic units Thermodynamics processes in HVAC systems

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

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

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)

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

T-s diagram

h-s diagram

p-h diagram

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)

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 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)

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

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