air conditioning ppt

1. CHAPTER-FIVE

2. Moisture content of the atmosphere.
The atmosphere always contains some moisture in the form of
water vapor;
The maximum amount depends on the temperature/amount of
vapor that will saturate the air increases with a rise in temperature/
example At 4.4° C , 454 kg of moist air contains a maximum of 2
kg of water vapor; at 37.8° C , the same amount of moist air
contains a maximum of 18 kg of water vapor.
When the atmosphere is saturated with water, the level of
discomfort is high, because the evaporation of perspiration with its
attendant cooling effect is impossible.
5.1 Humidity

3. The temperature-humidity index (THI) /the discomfort index/ expresses
in numerical values the relationship between comfort or discomfort and
temperature and humidity. It provides an apparent temperature, or how hot
the air feels. example, an air temperature of 38° C and a relative humidity
of 60 percent produces an apparent very hot temperature, or THI, of 54° C.
When the index is 20° C , most people are comfortable; an index of 25° C
means that half are uncomfortable.
Temperature Humidity Index
To determine the heat index or THI, first we have to find the temperature
and then find the dew point. Then read down the dew point column and
across the temperature row to where the two axes intersect to find the heat
index.
Value < 80 is considered comfortable. Value > 90 is considered extreme.
Value >100 is considered hazardous. Value > 110 is considered dangerous.

4. Heat Index Chart
Dew Point (°C)
Temp
(°C)
10 12.8 15.6 18.3 21 24 26.7 29.4
18.3 62.7 63.8 65.0 66.6 - - - -
21 67.8 68.7 69.8 71.1 72.6 - - -
24 73.1 73.9 74.8 75.9 79.2 80.7 - -
26.7 79.8 80.6 81.6 82.8 84.4 86.9 90.9 -
29.4 83.5 84.7 86.1 88.0 90.5 94.0 99.0 106.6
32.2 87.9 89.4 91.2 93.6 96.9 101.2 107.2 115.6
35 92.9 94.5 96.7 99.6 103.4 108.4 115.2 124.3
37.8 98.1 99.9 102.4 105.6 109.8 115.3 122.7 132.3
40.6 103.4 105.4 108.1 111.6 116.1 122.0 129.7 139.7
43.3 108.7 110.9 113.8 117.5 122.3 128.4 136.3 146.5

5. 5.2 Humidification
Is accomplished by either water or steam.
There are four methods of using water as a humidifying agent:
1. Wetted element- by passing air over wetted surfaces of water a
hygroscopic element is partially submerged in a water bath , used
for lowest capacity air conditioning
2. Evaporative pan - by force passing over pan filled water having
energy supply to evaporate, used for medium capacity AC.
3. Water spray method- by mixing pressurized water with
compressed air and is used for large capacity AC systems
4. Steam method- by passing live steam through a separator to remove
condensate, and then over a drying chamber, use sterilized water, applicable
for medium to large scale AC systems.

6. How do humidifiers work

7. The need to control the amount of water vapor in the air is felt in all
industrial, commercial, and institutional facilities.
Humidity control is important to human health and comfort.
Humidity control also improves the reliability of equipment,
production processes, and materials by controlling static electricity,
corrosion, and other factors.
In designing dehumidification systems, one of the most important
tasks is to quantify the water vapor loads that must be removed by the
system.
Types of dehumidification systems
Moisture can be removed from the air by
i. Refrigerant dehumidification method
(cooling air below the dewpoint temperature so condensation
occurs by air-to-air heat exchangers, which bring in dryer
outside air, or
ii. Chemical methods, and
5.3 Dehumidification

9. A. Refrigerant Dehumidifier
Is an air to air type heat exchanger
Is the most common dehumidifier used.
It is cheap,
easy to operate and effective in most domestic and commercial
applications.
The primary benefit of a refrigerant dehumidifier is that it performs
exceptionally well when used in warm, humid conditions.
is essentially a re-arranged portable air conditioning unit, where
the air is first passed across the evaporator coil to cool it, then
across the condenser coil to heat it.
The air becomes both warmer and drier.
The cold evaporator coil reduces the air temperature to a
point where condensation occurs. Condensation formed on
the cold coil then drips into a collection tray.
Water is either drained away or collected in a container in
the base of the unit.

10. B- Chemical dehumidification
is carried out through the use of sorbent materials called desiccants,
which are solids or liquids that can extract moisture from the air and
hold it.
Desiccants naturally absorb moisture that's why we find little packets of
silica gel in new shoes or electronic goods. Because desiccant dehumidifiers
don't need to cool air before dehumidifying it, this technology is really ideal
for sub-zero conditions.
There are two classifications of sorbents:
Adsorbents which do not experience a phase change. Moisture is deposited
on the surface of the dry desiccant. Most adsorbents are solids.
Types of dry solid adsorbents :Silica gel, Titanium gel , Dry lithium
chloride, Zeolites, Synthetic zeolites (molecular sieves), Activated alumina,
Synthetic polymers
Absorbents—which change physically, chemically, or both during the
sorption process. Most absorbents are liquids or solids that become
liquid as they absorb moisture.

11. Liquid desiccant /absorption/ dehumidifier
Liquid desiccant dehumidification operates on the principle of
chemical absorption of water vapor from the air.
Some of the absorbents/liquid desiccant solutions used for
dehumidification are:
Lithium chloride (LiCl), Lithium bromide (LiBr), Calcium chloride
(CaCl2), Triethylene glycol (TEG) , Propylene glycol
Liquid absorption dehumidification is very similar to a chilled
water air-washer system.
When the air passes through the washer, its dew point approaches
the temperature of the water supplied.
Air that is more humid is dehumidified, and air that is less humid
is humidified.

12. Hygrometer is a common name given to instruments used to measure
atmospheric humidity.
Types of hygrometers- Mechanical, dry&wet thermometer and dew
point hygrometer
5.4.1-Mechanical Hygrometers
A simple form of hygrometer, used in houses and offices, utilizes the
change in length in an organic fiber (often a single human hair) brought
about by the absorption of moisture.
The fiber tends to lengthen in damp air, and the apparatus is so
arranged that the change in length of the fiber moves a pointer across a
dial, which is calibrated to give a reading in percent relative humidity.
This type of hygrometer gives only an approximate indication of
humidity and is not used for accurate, quantitative determinations.
5.4 Humidity Measurement Methods

13. Humidity stretches the hair (human / animal / synthetic) while
dryness shortens it.
The hygrometer has the job of stretching a hair between a fixed and
a movable point to measure humidity over time.
Mechanical/Hair Hygrometer

14. 5.4.2- Psychrometer /wet and dry bulb Hygrometer/
The two common psychrometer types are Aspiration and
whirling psychrometer
Is the instrument most commonly used in laboratories to measure
relative humidity.
whirling psychrometer
Aspiration psychrometer

15. ...Psychrometer /wet and dry bulb Hygrometer/
Two similar thermometers are mounted side by side; one,
the dry bulb, has its bulb exposed to the atmosphere,
and the bulb of the other, the wet bulb, is wrapped in a
suitable material, which is immersed in water and
serves as a wick.
The wet bulb is cooled by evaporation of the water, the
amount of evaporation and consequent cooling of the
thermometer depending on the humidity of the
atmosphere—the drier the atmosphere, the faster the
water evaporates.
A table accompanying the instrument gives the relative
humidity in terms of the readings of the wet-bulb and
dry-bulb thermometers.

17. • The difference in temperatures, and the absolute
temperature of the wet thermometer can be used to
calculate the relative humidity of the air.
• pw is the experimental partial vapour pressure of water
• ps is the saturation vapour pressure at temperature Tw
• A is the psychrometer constant (typically, values of A,
for Tw above 0°C is ≈6.66X10-4 ºC-1
• P is the total pressure
• TD is the dry bulb temperature
• TW is the wet bulb temperature

19. 5.4.3-The dew-point hygrometer measures relative
humidity by means of the dew point.
A small amount of ether is placed in a highly polished,
thin, metallic cup, and the evaporation of the ether,
accelerated by blowing air through it, lowers the
temperature of the cup.
When the dew point of the surrounding air is reached, a
film of moisture suddenly appears on the surface of
the cup.
The temperature is read by means of a thermometer, and
a table accompanying the instrument gives the
relative humidity in terms of the atmospheric and
dew-point temperatures.