Objectives Define basic HVAC and AHU components Learn

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 • 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

p-h diagram

Ideal gas law • Pv = RT or PV = n. RT • R is a constant for a given fluid • For perfect gasses • Δu = cvΔt • Δh = cpΔt • 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)

Mixtures of Perfect Gasses • • • m = m x my Px V = mx R ∙T V = V x Vy Py V = my R ∙T T = T x Ty What is ideal gas law for mixture? P = P x Py Assume air is an ideal gas x y • -70 °C to 80 °C (-100 °F to 180 °F) m = mass (g, lbm) P = pressure (Pa, psi) V = volume (m 3, ft 3) R = material specific gas constant T = absolute temperature (K, °R)

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

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

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

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

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

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

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)

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

Calculation of psychometric quantities • For an ideal gas, • hda = ∫cpad. T, hw = ∫cpwd. T • So, hda = cp, dat which assumes a reference state of 0 °F or 0 °C – Tables A 4 • Note different reference • hw = cpwt + hg 0 • h = cp, dat + W(cpwt + hg 0) Or you can use: • h = cpt + W∙hg 0, 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

Adiabatic mixing • Governing equation External heat

Sensible heating

Dehumidification by Cooling

Real Dehumidification Process