MEC653 HVAC SYSTEMS CHAPTER 3 PSHYCHROMETRY
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