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Presentation Transcript

Announcements

- Tonight ASHRAE Student Chapter Meeting

Objectives

- Finish with review thermodynamics
- Learn about Psychrometrics

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
- Can make your own
- 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

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