THERMODYNAMICS I

ME 226 / EN 226

Lecture 8
Psychrometry

Dr. J. Kihedu
1
 Overview
• Air is a mixture of nitrogen, oxygen, and small amounts of some
other gases.
• Air in the atmosphere normally contains some water vapor (or
moisture) and is referred to as atmospheric air.
• Controlling the moisture level in the atmosphere is important aspect
for maintaining a desirable environment in buildings.
➢ Although the amount of water vapor in the air is small, it plays a
major role in human comfort.
➢ Therefore, it is an important consideration in air-conditioning
applications.
• Psychrometry is a field of engineering concerned with the physical
and thermodynamic properties of gas-vapor mixtures.
• Psychometrics is the relationship of the physical and thermal
properties of an air-vapor mixture.
➢ Air vapor mixture consists of air with both moisture and heat.
➢ Amount of moisture air can hold depends on the amount of heat.
 Properties of Air
Vapor pressure of water vapor
• It is the pressure water vapor would exert if it existed alone at the
temperature and volume of atmospheric air.
• Then the atmospheric air can be treated as an ideal-gas mixture
whose pressure is the sum of the partial pressure of dry air Pa
and that of water vapor Pv

Dew-point Temperature Tdp

• Temperature at which condensation begins when the air is cooled
at constant pressure.
• Saturation temperature of water corresponding to vapor pressure

➢ When the temperature of a cold drink is below the dew-point

temperature of the surrounding air, it “sweats.” Formation of
dew indicates that the temperature of the drink is below the
dew-point temperature of the surrounding air.
• It can be read on the saturation curve horizontally to the left of the
point where the dry bulb and wet bulb meet.
 Properties of Air (Cont.)
Absolute humidity, specific humidity or humidity ratio (ω)
• Mass of water vapor present in a unit mass of dry air (kg water
vapor / kg dry air). It is located on the “Y” axis of the chart.
ms va psV RsT ps Rs p
= = = = = 0.622 s [ Note : pv = mRT ]
ma vs paV RaT pa Ra pa
ps Rs=Rvapor=0.4615 kJ/kg.K
 = 0.622 [ Note : p = pa + ps ]
p − ps Ra=Rair=0.287 kJ/kg.K

Relative humidity (Φ)

• Ratio between amount of moisture the air holds (mv) to the
maximum amount of moisture the air can hold at the same
temperature (mg)
• Ratio of actual pressure of water vapor in the air to the pressure if
the air were saturated at the same temperature.
• Ranges from 0 for dry air to 1 for saturated air.
 Properties of Air (Cont.)
Saturation Curve
• It includes the wet bulb and dew point temperatures.
• It completes the curved chart outline.
Dry Bulb Temperature
• Ordinary temperature of atmospheric air measured by using an
ordinary thermometer, independent of moisture.
• It is located on the “X” axis of a psychometric chart.
Wet Bulb Temperature
• The temperature measured by using
thermometer whose bulb is covered with
cotton wick saturated with water.
• The temperature at which an equilibrium exists
between an air-vapor mixture and water.
➢ It is dependent on moisture in the air.
Values are on the saturation curve, and
lines inclined downward to the “X” axis.
 Properties of Air (Cont.)
Specific Enthalpy
• Total enthalpy of atmospheric air is the sum of the enthalpies of dry
air and the water vapor.

• In air-conditioning, the amount of dry air in the air–water-vapor

mixture remains constant, but the amount of water vapor changes.
• Therefore, enthalpy of atmospheric air is expressed per unit mass of
dry air instead of per unit mass of the air–water vapor mixture.

Values in the chart are read by using a ruler placed diagonally.

Specific Volume
• The volume occupied by 1kg of dry air (m3/kg of dry air).
• The number of cubic meters of moist air per kilogram of dry air.
Values in the chart are read below the dry bulb readings.
 Psychrometric Chart
• The state of the atmospheric air at a
specified pressure can be specified
by two independent properties.
• The rest of the properties can be
calculated from relations or easily
readable chart.

A Specific Humidity
E Dew Point

C Dry Bulb Temperature

 Example 1
The dry bulb reading is 20 °C and the wet bulb is 11 °C. Using the
chart determine the following;
a. Relative Humidity?
✓30 %
b. Dew Point?
✓2 °C
c. Absolute Humidity?
✓0.0045 kg /kg dry air
d. Specific Volume?
✓0.836 m3 / kg dry air
e. Specific Enthalpy?
✓31 kJ / kg dry air
 Example 2
The dry bulb reading is 40 °C and the RH is 40 %. Using the chart
determine the following;
a. Wet Bulb Temperature?
✓28.5 °C
b. Dew Point?
✓24.95 °C
c. Absolute Humidity?
✓0.0195 kg / kg dry air
d. Specific Volume?
✓0.915 m3 / kg dry air
e. Specific Enthalpy?
✓91 kJ/ kg dry air

NOTE: For saturated air, the dry-

bulb, wet-bulb and dew-point
temperatures are identical.
 Human Comfort and Air-Conditioning
Human beings have an inherent
weakness; they want to feel comfortable.
• Living in environment that is neither hot
nor cold, neither humid nor dry.
✓ Temperature between 22 and 27°C.
✓ Relative humidity of 40 to 60 %.
✓ Air speed of about 15 m/min.
• Air conditioners are used to heat, cool,
humidify, dehumidify, clean and
deodorize the air.
The human body as heat engine with
food as energy input, generates waste
heat that must be rejected;
• 87 W when sleeping, 115 W when resting or doing office work,
• 230 W when bowling, 440 W when doing heavy physical work.
✓These values are for adult male, for adult female take 15 % less.
Air-Conditioning Processes
 Air-Conditioning Processes (Cont.)
Simple Heating and Cooling
• Cooling occurs from right to left, heating occurs from left to right.
➢ Heat must be added (see heating coils below)
➢ Change in sensible heat (Q = ma(h2 – h1) of air-vapor mixture.
➢ Amount of moisture in air remains constant since no moisture is
added to or removed from the air but relative humidity changes.
Resulting relative humidity of heated/cooled air may be below/higher
than comfortable levels, simple heating/cooling is not common.
h2

Enthalpy
h1

RH %
Heat addition

T1 T2
 Air-Conditioning Processes (Cont.)
Cooling with Dehumidification
• Dehumidification removes moisture from the air which decrease
the absolute humidity.
➢ This requires cooling the air below its dew point temperature.
➢ Hot and moist air enters the cooling section where its
temperature decreases and its relative humidity increases at
constant specific humidity. Further cooling of air results in the
condensation of part of the moisture in the air.

2 1

Dehumidification
 Air-Conditioning Processes (Cont.)
Heating with Humidification
• Problems associated with the low relative humidity resulting from
simple heating can be eliminated by humidifying the heated air.
➢ Humidification adds moisture to air and hence absolute
humidity.
➢ Accomplished by passing the air first through a heating
section and then through a humidifying section.

1 2

Humidification
 Example (Cooling with Dehumidification)
Air-conditioner at Block O takes air at 1 atm, 30°C, and 80 percent
relative humidity at a rate of 10 m3/min, and the air leaves as saturated
air at 14°C. Part of the moisture in the air that condenses during the
process is also removed at 14°C. Determine the rates of heat and
moisture removal from the air.
Solution
• Air with 30°C dry bulb, 80% relative humidity
➢Absolute Humidity ω1 = 0.0216 kg H2O/kg dry air
➢Specific Enthalpy h1 = 85.4 kJ/kg dry air
➢Specific Volume v1 = 0.889 m3/kg dry air
• Saturated air at 14°C
➢Absolute Humidity ω2 = 0.0100 kg H2O/kg dry air
➢Specific Enthalpy h2 = 39.3 kJ/kg dry air
➢Specific Volume v2 = 0.823 m3/kg dry air
• Saturated liquid water at 14°C
➢hf14°C = 58.8 kJ/kg (steam table!)
 Example (Cooling with Dehumidification) Cont.

Hence, 0.131 kg/min moisture removal and 511 kJ/min moisture removal.