REFRIGERATION
& AIR CONDITIONING
Air conditioning—deals with artificial tampering of the
conditions of air that may involve cooling as well as heating coupled with
ventilation, filtration and air circulation.
Refrigeration—means the artificial withdrawal of heat,
producing in a substance or within a space a temperature lower than that which
would exist under the natural influence of the surroundings.
Application of refrigeration—(a) Ice making, (b)
Preservation and transportation of perishables, (c) Special industrial
processes, (d) Air conditioning.
Application of air conditioning—(a) For human comfort
(b) Industrial air conditioning
PSYCHROMETRIC TERMS
Psychometric—is that branch of
engineering science, which deals with the study of moist air i.e. dry air mixed
with water vapor or humidity.
Dry air—the pure dry air is a mixture of a number of gases such as
nitrogen, oxygen, carbon dioxide, hydrogen, argon etc.
Moist air—it is a mixture of dry air and water vapor.
The amount of water vapor, present in the air, depends upon the absolute
pressure and temperature of the mixture.
Saturated air—it is a mixture of dry air and water vapor,
when the air has diffused the maximum amount of water vapor into it. However, when the saturated air is cooled, the water
vapor in the air starts condensing, and the same may be visible in the form of
moist, fog or condensation on cold surfaces.
Degree of saturation—it is the ratio of actual mass of water vapor
in a unit mass of dry air to the mass of water vapor in the same mass of dry
air when it is saturated at the same temp.
Humidity—it is the mass of water vapor present in 1 kg
of dry air, and is generally expressed in terms of g/kg of dry air. It is also called specific humidity or humidity ratio.
Absolute humidity—it is the mass of water vapor present in 1 m3
of dry air, and is generally expressed in terms of g/m3 of dry air.
Relative humidity—it is the ratio of actual mass of water vapor
in a given volume of moist air to the mass of water vapor in the same volume of
saturated air at the same temperature and pressure.
Specific humidity—it can be defined as the ration of mass of
water vapor to the mass of dry air in a given volume of the air-vapor mixture.
Dry bulb temperature—it is the temperature of air recorded by a
thermometer, when it is not affected by the moisture present in the air.
Wet bulb temperature—it is the temperature of air recorded by a
thermometer, when its bulb is surrounded by a wet cloth exposed to the air.
Dew point temperature—it is the temperature of air recorded
by a thermometer, when the moisture present in it begins to condense.
Dew point depression—it is the difference between the dry bulb
temperature and dew point temperature of air.
Sensible heating—the heating of air, without any change in its
specific humidity, is known as sensible heating.
Sensible cooling—the cooling of air, without any change is its
specific humidity, is known as sensible cooling.
Humidification—the addition of moisture to the air, without
change in its dry bulb temperature, is known as humidification.
Dehumidification—the removal of moisture from the air, without
change in its dry bulb temperature, is known as dehumidification.
Cooling with dehumidification—this process involves
lowering of both the air temperature and the specific humidity. This process is
commonly used in summer air conditioning in which air passes over a cooling
coil. When moist air is cooled below its
dew point, vapor is condensed from the air resulting in simultaneous cooling
and dehumidification. The final relative humidity is generally higher than that
at the start.
Heating and humidification—in this process, which
is reverse of the cooling and dehumidification process, air is simultaneously
heated and humidified. While both the
final specific humidity and temperature rise, the relative humidity may be
lower or higher than the initial value.
Sensible heat factor—the ratio of the sensible heat to the total
heat is known as sensible heat factor. (Briefly written as SHF or sensible heat
ratio written as SHR)
By-pass factor—a little consideration will show that when air
passes over a coil, some of it (say x kg) just by-passes unaffected while the
remaining (1-x) kg comes in direct contact with the coil.
The bypass factor may
be defined as follows:
Where, Tla is the DBT of air leaving the coil
Tea is the DBT of air entering the coil
Tcoil
is the DBT of the coil
surface temperature
REFRIGERATION HARDWARE
Compressors—the main functions of a compressor are:
- Removing the refrigerant vapor from the evaporator
- Increasing the pressure of the refrigerant vapor through the process of compression, and simultaneously increasing the temperature of the vapor
Condenser—is the component in the refrigeration cycle
where heat is removed and rejected. Condenser are thus heat exchangers designed
to get rid of the heat absorbed by the refrigerant in the evaporator and the
heat of compressor added by the compressor. The
condenser receives the hot, high pressure gas from the compressor and cools it
to remove first the superheat and then the latent heat, so that the refrigerant
will condense back to a liquid.
Evaporators—are heat exchangers in which a refrigerant is
evaporated at low temperature and pressure for the purpose of removing heat from the refrigerated space or
material.
Defrosting—when an evaporator operates at a temperature
below 00C, the exchanger surface progressively becomes covered with
a layer of frost, produced by moisture condensation on the cold surfaces, the
moisture being deposited by the circulating air on the heat exchanger surface.
Frost is undesirable from operational standpoint for two reasons:
- Thick layers of frost act as insulation, greatly reducing the heat transfer capability of the evaporator and this condition becomes critical when the frost bridges the gap between adjacent fins, thus also greatly reducing the surface area available for HT
- In forced convection coils the frost reduces the air flow.
Expansion devices—the main purpose of the expansion device is
twofold:
- It must reduce the pressure of the liquid refrigerant
- It must regulate the flow of refrigerant to the evaporator
Receiver—this device basically acts as a storage space and surge tank for the
liquid refrigerant in a refrigerating system. During operation it provides
storage space for excess liquid refrigerant to accommodate large load changes.
During the shutdown period, the whole charge is pumped into the receiver and
isolated by using the valves, thus preventing
the loss of refrigerant.
Filter-drier—the device is installed in the liquid line to remove foreign particles as well as
water. It also prevents foreign particles from clogging the orifice of the
expansion valve. The common desiccants used in the drier are silica gel and
activated alumina.
Oil separator—the purpose of the oil separator is to separate the lubricating oil from the
refrigerant and return the same to compressor. The detrimental effects of
oil in the evaporator are:
- It thickness and gets coated on the inside walls of the evaporator (reduce the HT)
- Often the wax separates out from the oil at low temperature
- Boiling temperature of the refrigerant gets changed due to oil, resulting in more work for the compressor.
Accumulators—the device is installed in the suction line to ensure that no liquid refrigerant from
the evaporator reaches the compressor. It is essential to be used in a
system where there is inherent vulnerability to liquid flood back.
A refrigerant is any substance, which acts as a cooling agent by
absorbing heat from another body or substance.
Common refrigerants are—(a) Ammonia, (b) Carbon dioxide, (c)
Water, (d) Sulphur dioxide, (e) Methyl chloride, (f) Halocarbon refrigerants
are—(1) Refrigerant-11(CCl3F), (2) Refrigerant-12, (3)
Refrigerant-22, (4) Refrigerant-134a.
COP—it is the ratio of rate of refrigeration to the rate of
heat supply to the generator.
TOR or TR—a ton of refrigeration is defined as the
amount of refrigeration effect produced by the uniform melting of one ton
(1000kg) of ice form and at 00C in 24 hours. Since the latent heat
of ice is 335kJ/kg, therefore one ton of refrigeration,
1TR =
1000*335 kJ in 24 hours; = (1000*335)/ (24*60) = 232.6 kJ/min; = 3.5 kW
Comfort chart—could then be constructed by drawing lines
through the points at which the majority of people equally clothed and equally
active reported the same feeling of comfort. These lines are called the effective-temperature (ET). The summer
comfort zone for most people and spans the range of effective-temperatures from
around 19 to 240C. The winter comfort zone for most individuals and
spans the range of effective temperature from 17 to 220C.
Alignment Circle—is the point on Psychometric
Chart, where Dry Bulb Temperature is 26°C and relative humidity
is 50%.
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