Objectives

• Learn more about Psychrometrics • Describe equations for psychometric processes• Plot processes on psychrometric chart• Use protractor to calculate SHR

Alternate calculation for W

• PV = mRT (IGL)

• 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

Fog formation above riversSolution is in

Psychometric chart

Sensible heating

tcmQ p

Dehumidification by Cooling

Real Dehumidification Process

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

AHU

Single-Zone Systems

• Zone – any space with a thermostat

Air conditioning

Recirculation

Reheat

Evaporative Cooler

Air-to-air heat exchanger

Summary

• Show HVAC processes on a psychrometric chart

• Draw condition lines• Define sensible heat ratio

• Draw HVAC processes for real buildings• Single zone• Multiple zone

• Skipping Chapter 9