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# Announcements - PowerPoint PPT Presentation

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
• Tonight ASHRAE Student Chapter Meeting
Objectives
• Finish with review thermodynamics
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)

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
• 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
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
• Best source is ASHRAE fundamentals (Chapter 6)
• Also in your text (back cover fold-out)
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
• 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