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Announcements. Tonight ASHRAE Student Chapter Meeting. Objectives. Finish with review thermodynamics Learn about Psychrometrics . p-h diagram. Ideal gas law. Pv = RT or PV = nRT R is a constant for a given fluid For perfect gasses Δ u = c v Δ t Δ h = c p Δ t c p - c v = R.

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Presentation Transcript
announcements
Announcements
  • Tonight ASHRAE Student Chapter Meeting
objectives
Objectives
  • Finish with review thermodynamics
  • Learn about Psychrometrics
ideal gas law
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
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)

mass weighted averages
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
Properties of water
  • Water, water vapor (steam), ice
  • Properties of water and steam (pg 675 – 685)
    • Alternative - ASHRAE Fundamentals ch. 6
humidity ratio w
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
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 t d
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
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 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
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
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

calculation of psychometric quantities
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

adiabatic mixing
Adiabatic mixing
  • Governing equation

External heat

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