Buildign Sertvices HVAc

1. •BUILDING SERVICES – IV
HVAC
3rd YEAR, SEMESTER 6

2. HVAC -Heating, ventilation, and air conditioning (HVAC) is the technology of indoor and
vehicular environmental comfort. Its goal is to provide thermal comfort and acceptable
indoor air quality.
HVAC
Air conditioning
•It is a system for controlling the humidity, ventilation, and temperature in a building
or vehicle, typically to maintain a cool atmosphere in warm conditions.
•Air conditioning is achieved by developing the principles of moving air in ducted
ventilation systems to include a number of physical and scientific processes which
enhance the air quality.
•For buildings with human occupancy, the design specification is likely to include an
internal air temperature of 21* to 26*C and relative humidity between 30 and 60%.

3. The following is a glossary of some of the terminology used in air
conditioning design:
•Specific humidity, mass of water vapour in a unit mass of moist air, usually
expressed as grams of vapour per kilogram of air.
•For example, the rate of evaporation of water from any surface is directly
proportional to the specific humidity difference between the surface and the
adjoining air.
•Relative humidity (RH) † Relative humidity definition, the amount of water vapor
in the air, expressed as a percentage of the maximum amount that the air could
hold at the given temperature; at a given pressure.
RH- actual moisture in air / max moisture air can hold x 100

4. •Latent heat - heat energy added or removed as a
substance changes state, whilst temperature remains
constant, e.g. water changing to steam at 100ƒC and
atmospheric pressure (W).
•.Solids can become liquids (ice to water) and liquids can
become gases (water to vapor) but changes such as these
require the addition or removal of heat. The heat that
causes these changes is called latent heat.
Sensible heat -
When an object is heated, its temperature rises as heat is
added. The increase in heat is called sensible heat.
Similarly, when heat is removed from an object and its
temperature falls, the heat removed is also called sensible
heat. Heat that causes a change in temperature in an
object is called sensible heat.
Ton( TR)
It is the unit of measure, to describe the cooling capacity of
a system. One ton of cooling is based on the amount of
heat needed to melt one ton (2000 lbs.) of ice in a 24 hour
period. One ton of cooling is equal to 12,000 Btu/hr.

5. •Psychrometrics
Psychrometrics (derived from the Greek: psuchron = cold) is study of air-water vapor
mixtures at different conditions.
To quote the 1989 ASHRAE Handbook of Fundamentals, "Psychometrics deals with the
thermodynamic properties of moist air and uses these properties to analyze conditions
and processes involving moist air.“
In air-conditioning system, we use psychometric properties for environment control.
A psychometric chart presents physical and thermal properties of moist air in a graphical
form.
Human comfort
The common notion is that as long as cooling or heating (in winters) takes place, the
environmental conditions are comfortable or the air conditioning is effective. Over a period of
time, experience as well as research has shown that there is much more to human comfort
than just temperature. There are four major factors that determine comfort
• Air temperature (dry bulb temperature or DBT)
• Humidity (relative humidity RH)
• Air movement (velocity fpm or m/s)
• Internal Quality of Air

7. A psychometric chart contains a
lot of information packed in an
odd-shaped graph.
The psychometric chart
highlights seven properties of air:
1. Dry Bulb Temperature (DBT)
2. Wet Bulb Temperature (WBT)
3. Saturation Temperature
4. Relative Humidity
5. Humidity Ratio
6. Volume
7. Enthalpy

8. Psychrometric Chart Boundaries
The horizontal X-axis denotes dry bulb
temperature (DBT) scale. Vertical lines
indicate constant dry bulb temperature.
DBT is the air temperature measured in
°C or °F and determined by an ordinary
glass thermometer.
Y-axis indicates humidity ratio or
absolute humidity, which is the weight
of the water, contained in the air per unit
of dry air. This is often expressed as
pounds of moisture per pound of dry air
or (g/Kg).

9. Relative humidity (RH) : The term
relative humidity is simply a ratio
between the actual moisture content
of the air compared with the
moisture content of the air required
for saturation at the same
temperature, i.e. at 100% relative
humidity (also known as saturation
point).
Wet-bulb temperature (WBT) is
determined when air is circulated
past a wetted sensor tip. It
represents the temperature at which
water evaporates and brings the air
to saturation. Inherent in this
definition is an assumption that no
heat is lost or gained by the air

10. Specific volume
indicates the space occupied by air. It is the
reciprocal of density and is expressed as a
volume per unit weight in ft3/ lb or m3/kg
(density is weight per unit volume). Warm air is
less dense than cool air, which causes warmed
air to rise.
Enthalpy (E)
is the heat energy content of moist air. It is
expressed in Btu ( British thermal unit) per
pound of dry air (or kJ/Kg) and represents the
heat energy due to temperature and moisture
in the air. Lines of constant enthalpy run
diagonally downward from left to right across
the chart. Lines of constant enthalpy and
constant wet-bulb are the same on this chart
but values are read off separate scales.

11. Dehumidification-
•The process in which the moisture or water vapor or the humidity is removed from the air keeping its dry
bulb (DB) temperature constant is called as the dehumidification process.
•This process is represented by a straight vertical line on the psychrometric chart starting from the initial
value of relative humidity, extending downwards and ending at the final value of the relative humidity.
Cooling and Dehumidification Process-
•The process in which the air is cooled sensibly and at the same time the moisture is removed from it is
called as cooling and dehumidification process.
•Cooling and dehumidification process is obtained when the air at the given dry bulb and dew point (DP)
temperature is cooled below the dew point temperature.
•The cooling and dehumidification process is most widely used air conditioning application.
•It is used in all types of window, split, packaged and central air conditioning systems for producing the
comfort conditions inside the space to be cooled.
•In the window and split air conditioners the evaporator coil or cooling coil is maintained at temperature
lower than the dew point temperature of the room air or the atmospheric air by the cool refrigerant
passing through it.
•When the room air passes over this coil its DB temperature reduces and at the same time moisture is
also removed since the air is cooled below its DP temperature.

13. Dewpoint temperature-
The dew point temperature is the temperature at which the air can no longer "hold"
all of the water vapor which is mixed with it, and some of the water vapor
must condense into liquid water.The dew point is always lower than (or equal to)
the air temperature

14. PRINCIPLE OF AIR CONDITIONING

15. The Compressor
The compressor is the heart of the system. Just like your
heart pumps blood through your body at a specific flow
rate and pressure, the compressor pumps the refrigerant
through the air conditioning system at a designed flow rate
and pressure.
When the refrigerant enters the compressor it is in a vapor
state the compressor compresses the vapor as it is being
pumped through it. When a vapor is compressed both the
pressure and temperature of that vapor increases. The
vapor leaving the compressor is very hot.
The Condenser
The high temperature refrigerant passes into a condenser
coil. As the vapor refrigerant travels through the coil, air
from a fan passes over the coil to cool the vapor
refrigerant. As the vapor cools it condenses and becomes
a liquid, this is referred to as a “change of state”. This
“change of state” from vapor to liquid is essential.

16. The Metering Device
The metering device controls the flow of the liquid
refrigerant to the next component which is the
evaporator. This is a dividing point between the high
pressure and low pressure sides of the system. As this
high pressure liquid is passing through the metering
device and into the evaporator the pressure drops.
The Evaporator
After leaving the metering device the refrigerant
immediately enters a coil called the evaporator. This
coil or evaporator has a fan blowing across it. As the
refrigerant enters the coil at a lower pressure it begins
to bubble and boil and “change state” back to a vapor.
During this process of changing state, energy in the
form of heat is being removed from the air passing over
the coil and is being absorbed by the refrigerant. The
heat that was in the air is transferred into the
refrigerant. Since heat was removed from the air
blowing over the evaporator coil, the air leaving the
evaporator coil is cold.