Introduction to
Air Conditioning
System
Define air conditioning
Application of air conditioning
Learning Relationship between refrigeration and air
Objective conditioning
Working substance of air conditioning
Properties of moist air
Air conditioning is the process of altering
the properties of air (primarily
temperature, air movement, air quantity
and humidity) to the more favorable
conditions. More generally, air
conditioning can refer to any form of
technological cooling, heating, ventilation,
or disinfection that modifies the condition
of air.
APPLICATIONS
THE MOST COMMON USE
OF AIR CONDITIONING
HUMAN COMFORT
FOOD AND DRUG PROCESSES
LABORATORIES AND HOSPITALS
COMMUNICATION BUILDING AND SENSITIVE ELECTRONIC FACILITIES
Relationship of the refrigeration and air conditioning fields.
Air Conditioning Refrigeration
Industrial
Heating, Cooling and refrigeration,
humidifying, dehumidifying including food
and control of operations in air preservation,
air quality conditioning chemical and
process industries
Working substance in air conditioning
Moist air is a binary mixture of dry air and water
vapor.
Dry air is the noncondensing components of the
mixture, mainly nitrogen and oxygen.
Vapor is the condensable component of the
mixture, the water vapor or steam which may exist
in a saturated or superheated state.
• Psychrometry is the science dealing with the physical laws of air
– water
mixtures.
• When designing an air conditioning system, the temperature and
moisture content of the air to be conditioned, and the same
properties of the air needed to produce the desired air
conditioning effect, must be known.
Properties of Air
• Dry-bulb temperature
• Wet-bulb temperature
• Dew-point temperature
• Relative humidity
• Humidity ratio
Dry bulb temperature refers to the temperature of the
air as measured by a standard thermometer,
unaffected by moisture content or humidity. It's
essentially the air temperature that we typically refer
to and is what a regular thermometer displays.
Wet-bulb temperatures are read from a thermometer whose bulb is
covered by a wet wick. The difference between the wet-bulb
temperature and the dry-bulb temperature is caused by the cooling
effect produced by the evaporation of moisture from the wick. This
evaporation effect reduces the temperature of the bulb and, therefore,
the thermometer reading.
Measured in Dry Bulb
degrees
Fahrenheit (oF
or ℃)
Wet Bulb
Dew point Temperature
• When the dry-bulb, wet-bulb, and dew-point temperatures are the
same, the air is saturated. It can hold no more moisture. When air is at
a saturated condition, moisture entering the air displaces moisture
within the air. The displaced moisture leaves the air in the form of fine
droplets. When this condition occurs in nature, it is called fog.
Saturation Fog
DP, WB, DB same
Relative Humidity
• Relative humidity, is a comparison of the amount of moisture that a
given amount of air is holding, to the amount of moisture that the
same amount of air can hold, at the same dry-bulb temperature.
Amount of moisture that a given amount of air is
holding
Relative =
Humidity Amount of moisture that a given amount of air
can hold
100% RH -
50% RH Saturated
Humidity ratio describes the actual weight of water in an air – water vapor mixture. In other words, if
one pound of air were wrung completely dry, comparing the weight of the water to the weight of the
dry air would yield its humidity ratio.
Humidity ratio can be expressed as pounds of moisture per pound of dry air, or as grains of moisture
per pound of dry air. There are 7000 grains of water in a pound. To appreciate the magnitude of these
units of measurement, at sea level one pound of 70°F air occupies approximately 13.5 cubic feet, and
one grain of water in that air weighs about two-thousandths (0.002) of an ounce.
Actual weight of water in an air – water
vapor mixture
Pounds of moisture per pound of dry air
𝒑𝒕 = 𝒑𝒂 + 𝒑𝒔
𝒑𝒕 =total pressure of mixture
𝒑𝒂 =partial pressure exerted by the dry air
𝒑𝒔 =partial pressure exerted by the vapor (Psat at dew point temperature)
𝑝𝑠 𝑉
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑣𝑎𝑝𝑜𝑟 𝑚𝑣 ൗ𝑅 𝑇
𝑠
𝜔= = =
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑑𝑟𝑦 𝑎𝑖𝑟 𝑚𝑎 𝑝𝑎 𝑉ൗ
𝑅𝑎 𝑇
𝑝𝑠 𝑅𝑎 𝑝𝑠 𝑅𝑎 287𝑝𝑠
𝜔 = = =
𝑝𝑎 𝑅𝑠 (𝑝𝑡 − 𝑝𝑠 )𝑅𝑠 461.5(𝑝𝑡 − 𝑝𝑠 )
𝑝𝑠
𝜔 = 0.622
𝑝𝑡 − 𝑝𝑠
𝐽
𝑅𝑎 = 287 , 𝑔𝑎𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑜𝑓 𝑑𝑟𝑦 𝑎𝑖𝑟
𝑘𝑔 − 𝐾
𝐽
𝑅𝑠 = 461.5 , 𝑔𝑎𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑣𝑎𝑝𝑜𝑟
𝑘𝑔 − 𝐾
Relative humidity is the ratio of the partial pressure of water
vapor in the air to the saturation pressure corresponding to the
temperature of the air.
𝒑𝒔
∅=
𝒑𝒅
𝒑𝒅 = 𝒔𝒂𝒕𝒖𝒓𝒂𝒕𝒊𝒐𝒏 𝒑𝒓𝒆𝒔𝒔𝒖𝒓𝒆 𝒂𝒕 𝒂 𝒈𝒊𝒗𝒆𝒏 𝒕𝒆𝒎𝒑𝒆𝒓𝒂𝒕𝒖𝒓𝒆 of the mixture
Enthalpy of a mixture of dry air and water vapor is the sum of the enthalpy of
the dry air and the enthalpy of the water vapor. Enthalpy values are always
based on some reference condition and the zero value of the dry air is chosen
as air at zero degree Celsius and the zero value of the water vapor is saturated
liquid at zero degree Celsius.
𝒉 = 𝑪𝒑 𝑻 + 𝝎𝒉𝒈 kJ/kg dry air
𝑘𝐽
𝐶𝑝 = 1.0062 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑒𝑎𝑡 𝑜𝑓 𝑑𝑟𝑦 𝑎𝑖𝑟 𝑎𝑡 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒
𝑘𝑔 − 𝐾
ℎ𝑔 =enthalpy of saturated steam at a given temperature t
Specific Volume: V
Specific volume is the number of cubic meters of mixture per kilogram
of dry air.
𝑹𝒂 𝑻 𝑹𝒂 𝑻 𝒎𝟑ൗ
𝒗= = 𝒌𝒈 𝒐𝒇 𝒅𝒓𝒚 𝒂𝒊𝒓
𝒑𝒂 𝒑 𝒕 − 𝒑𝒔
1) Compute the humidity ratio of air at 65% relative humidity and 34℃
when the barometric pressure is 101.3 kPa. Calculate also the
enthalpy.
2) What is the specific volume of an air vapor mixture at 30℃ and a
relative humidity of 40% at 101.3 kPa pressure.
3) A mixture of dry air and water vapor is at a temperature of 21℃
under a pressure of 101.3 kPa. The dew point temperature is
15℃. Find (a) partial pressure of water vapor (b) relative humidity
(c) humidity ratio (d) enthalpy (e) specific volume
Compute the humidity ratio of air at 65% relative humidity and
34℃ when the barometric pressure is 101.3 kPa. Calculate also
the enthalpy.
What is the specific volume of an air vapor mixture at 30℃ and
a relative humidity of 40% at 101.3 kPa pressure.
A mixture of dry air and water vapor is at a temperature of
21 ℃ under a pressure of 101.3 kPa. The dew point
temperature is 15℃. Find (a) partial pressure of water vapor (b)
relative humidity (c) humidity ratio (d) enthalpy (e) specific
volume
