Objectives

Continue with Psychometrics• Psychometric chart

• Equations

Plot processes in AHU and buildings on psychrometric chart

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

Ref: Tao and Janis (2001)

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)

Examples

• What is enthalpy of air in the classroom right now?

• Condensation on windows when taking a shower

• How cold does it have to be outside for condensation to form on windows?

– Assumption is that windows are the same temperature as outside air

– 80 °F, RH = 80%

Alternate calculation for W

• PV = mRT

• What do we know about R ratio?

• P = Pw + Pa

w

a

a

w

TRVP

TRVP

a

w

R

R

P

P

m

mW

a

a

w

w

w

w

PP

PW

622.0

R = gas constantP = pressureV = volumeT = absolute temperatureW = 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 heath = enthalpyT = absolute temperaturet = temperature W = humidity ratio

Subscripts: w is water vapor, a is dry air, g is saturated water vapor

Adiabatic mixing

• Governing equation hmQhmoutin

External heat

AHU

Sensible heating

tcmQ p

Dehumidification by Cooling

Real Dehumidification Process

Mold in a duct

Transport of saturated air

tsurface < tdp Condensation

Humidification

hw Specific enthalpy of water added to system

hg Specific enthalpy of saturated water vapor

Evaporative Cooling (air washing)

Condition Line for a Space

Summary

• Describe psychrometric quantities

• Given any two psychrometric quantities, calculate any other quantity

• Use Tables A4 or psychrometric charts to look up psychrometric quantities

• Calculate psychrometric quantities at non-standard conditions

Reading assignment

• Chapters 7 & 8