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Objectives

Objectives. Define basic HVAC and AHU components Learn about Psychrometrics . 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

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Objectives

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  1. Objectives • Define basic HVAC and AHU components • Learn about Psychrometrics

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

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

  4. 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)

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

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

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

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

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

  10. p-h diagram

  11. 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)

  12. 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)

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

  14. Humidity Ratio, W • W = mw/ma • Degree of saturation, µ = W/Ws • Humidity ratio is hard to measure, but very useful in calculations • What are units? • Is W a function of temperature? What about Ws? Ws = humidity ratio at saturation ma = mass of dry air mw = mass of water vapor

  15. Relative Humidity • Φ = xw/xw,s = Pw/Pws • Function of T Easy to measure and useful in some contexts, but often need to know temperature as well x = mole fraction P = pressure μ = degree of saturation W = humidity ratio

  16. Dew-point temperature, td • Temperature at which condensation will form • Under appropriate surface conditions • Vapor is saturated • Φ = ? • Ws(P, td) = W

  17. Wet-bulb temperature, VBT (t*) • Temperature of wet surface or • Temperature at which water, by evaporating into the air, will bring air to saturation adiabatically • * superscript is designation that variable is evaluated at the wet-bulb temperature • Note, distinct from that measured by a sling psychrometer • Section 9.5

  18. Tables for Moist Air (P = 1 atm) • Tables A.4 in your text • Ability to get Ws for calculations • Subscripts: • a = dry air, s = saturated air v = va+µvas h = ha+µhas s = sa+µsas

  19. Psychrometric Chart • Need two quantities for a state point • Can get all other quantities from a state point • Can do all calculations without a chart • Often require iteration • Many “digital” psychrometric charts available • Can make your own • Best source is ASHRAE fundamentals (Chapter 6) • Also in your text (back cover fold-out)

  20. Alternate calculation for W • PV = mRT (IGL) • What do we know about R ratio? • P = Pw + Pa R = gas constant P = pressure V = volume T = absolute temperature W = humidity ratio Subscripts: w is water vapor, a is dry air

  21. Calculation of psychometric quantities • For an ideal gas, • hda = ∫cpadT, hw = ∫cpwdT • So, hda = cp,dat which assumes a reference state of 0 °F or 0 °C – Tables A4 • Note different reference • hw = cpwt + hg0 • h = cp,dat + W(cpwt + hg0) Or you can use: • h = cpt + W∙hg0, cp = cp,da + Wcpw cp = specific heat h = enthalpy T = absolute temperature t = temperature W = humidity ratio Subscripts: w is water vapor, a is dry air, g is saturated water vapor

  22. Adiabatic mixing • Governing equation External heat

  23. Sensible heating

  24. Dehumidification by Cooling

  25. Real Dehumidification Process

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