MEC653

HVAC SYSTEMS

CHAPTER 3

PSHYCHROMETRY

PSHYCHROMETRY OF AIR

Constituent By volume By mass

Nitrogen 78.03% 75.47%

Oxygen 20.99% 23.19%

Argon 0.94% 1.29%

Carbon dioxide 0.03% 0.05%

Hydrogen 0.01% -

Psychrometry is the science dealing with the physical laws of air water mixtures.

The molecular mass of dry air is taken as 28.97 M kg/kmol and the ideal gas

constant is 0.2871 kJ/kgK.

Daltons law of partial pressure states that the total pressure of the gases is the

sum of their partial pressures

va PPP

DRY & ATMOSPHERIC AIR

From the definition of relative humidity the mass and enthalpy of mixture can be

expressed as:

11 a

a

v

ava mm

mmmmm

va

a

vvaa

a

va

a

m hhm

hmhm

m

HH

m

Hh

Equation

TCh paa Thv 82.13.2501

RELATIVE HUMIDITY

RH is a measure of the amount of moisture the air holds relative to the maximum

amount the air can hold at the same temperature.

The relative humidity can be expressed as the ratio of the vapor pressure to the

saturation pressure of water at that temperature.

g

v

g

v

P

P

m

m

SPECIFIC HUMIDITY

v

v

a

v

PP

P.

m

m

6220

Specific humidity (also called absolute humidity or humidity ratio) is the mass of

water vapor present in 1 kg of dry air i.e. the ratio of mass of water vapor to the

mass of dry air

Using the definition of the specific humidity, the relative humidity may be

expressed as

gg

g PP

P

P

P

622.0and

622.0

AIR TEMPERATURES

DBT is the ordinary temperature of atmospheric

air.

WBT is temperature measured by using a

thermometer whose bulb is covered with a

cotton wick saturated with water.

DPT is the temperature at which vapor

condenses when cooled at constant pressure.

PSHYCHROMERIC CHART

AIR MIXTURE

SENSIBLE AND LATENT HEAT

Sensible heat heat that causes a change in temperature.

Latent heat - heat that causes a change of state with no change in temperature.

AIR QUANTITY

CAV

VAV

TONS OF REFRIGERATION