Download ME 410 - EXPERIMENT NO . 3 Dr. Cemil Yamalı Heat Transfer Lab

ME 410 - EXPERIMENT NO . 3
Dr. Cemil Yamalı
Heat Transfer Lab.
MASS AND ENERGY BALANCES IN PSYCHROMETRIC PROCESSES
Air Conditioning
Control Temperature and Humidity
Industrial
Comfort
ASHRAE Comfort conditions
Winter:
20 - 23 oC
50% +/- 20 RH
Summer: 24 - 27 oC
BASICS
The air in the atmosphere that normally contains some water vapor is called
atmospheric air. Air that contains no water vapor is called dry air. Although the amount of
water vapor in the air is small, it plays a major role in human comfort.
The temperature of air in air conditioning applications ranges from about -10 to about
50°C. In this range, the dry air can be treated as an ideal gas with a constant specific heat.
The enthalpy and enthalpy change of dry air
T is the air temperature in °C
Treat the water vapor in the air as an ideal gas. Water vapor in the air behaves as if it
existed alone and obeys the ideal-gas relation Pv = RT
hv(T, low P) = hg(T)
The amount of water vapor in the air:
humidity (also called humidity ratio)
As more vapor or moisture is added, the specific humidity will keep increasing until the
air can hold no more moisture. At this point, the air is said to be saturated with moisture, and
it is called saturated air. Any moisture introduced into saturated air will condense.
Comfort level depends on the amount of moisture the air holds relative to the maximum
amount of moisture the air can hold at the same temperature
The enthalpy of air is expressed in terms of the enthalpies of the dry air and the water
vapor
Dry-bulb temperature: The ordinary temperature of atmospheric air
Dew-point temperature Tdp is defined as the temperature at which condensation
begins if the air is cooled at constant pressure
Tdp is the saturation temperature of water corresponding to the vapor pressure:
Relative humidity and specific humidity are not easy to measure directly we must relate
them to easily measurable quantities such as temperature and pressure.
Another way of determining the absolute or relative humidity is related to an adiabatic
saturation process
Wet bulb temperature:
Psychrometric charts:
Air- water vapor mixture properties presented in
the form of a chart at a given total pressure
The dry-bulb temperatures are shown on the
horizontal axis, and the specific humidity is shown on
the vertical axis.
Lines of constant enthalpy (in kJ/kg dry air) lie
very nearly parallel to the lines of constant wet-bulb
temperature.
For saturated air, the dry-bulb, wet-bulb, and dewpoint temperatures are identical
The psychrometric chart also serves as a valuable
aid in visualizing the air conditioning processes
HUMAN COMFORT AND AIR CONDITIONING
The comfort of the human body depends primarily on three factors: the (dry-bulb)
temperature, relative humidity, and air motion.
The temperature of the environment is the most important index of comfort.
Most people feel comfortable when the environment temperature is between 22 and
27°C
The relative humidity also has a considerable effect on comfort since it affects the
amount of heat a body can dissipate through evaporation
Most people prefer a relative humidity of 40 to 60 percent.
Air motion also plays an important role in human comfort. It removes the warm, moist
air that builds up around the body and replaces it with fresh air. Therefore, air motion
improves heat rejection by both convection and evaporation
Most people feel comfortable at an airspeed of about 15 m/min. Very high-speed air
motion causes discomfort. For example, an environment at 10°C with 48 km/h winds feels as
cold as an environment at -7°C with 3 km/h winds as a result of the body-chilling effect of the
air motion (the wind-chill factor).
AIR CONDITIONING PROCESSES
Simple Heating and Cooling
Heating with Humidification
Problems associated with the low
relative humidity resulting from simple
heating can be eliminated by humidifying the
heated air
If
steam
is
introduced
in
the
humidification section, this well result in
humidification with additional heating.
If humidi fication is accomplished by
spraying water into the airstream instead,
part of the latent heat of vaporization will
come from the air, which will result in the
cooling of the heated airstream
Cooling with Dehumidification
It may be necessary to remove some moisture from the air, i.e., to dehumidify it. This
requires cooling the air below its dew-point temperature.
Evaporative Cooling
In desert (hot and dry) climates, one can avoid the high cost of cooling by using
evaporative coolers
Evaporative cooling is based on a simple principle: As water evaporates, the latent heat
of vaporization is absorbed from the water body and the surrounding air. As a result, both the
water and the air are cooled during the process
Twb = constant
h = constant
Adiabatic Mixing of Airstreams
Many air conditioning applications require the mixing of two airstreams. This is
particularly true for large buildings, most production and process plants, and hospitals, which
require that the conditioned air be mixed with a certain fraction of fresh outside air before it is
routed into the living space
REFRIGERATION
THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE
The impracticalities associated with the reversed Carnot cycle can be eliminated by
vaporizing the refrigerant completely before it is compressed and by replacing the turbine
with a throttling device.
Isentropic compression in a compressor
P = constant heat rejection in a condenser
Throttling in an expansion device
P = constant heat absorption in an evaporator
ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLES
Two common sources of irreversibilities are fluid friction (causes pressure drops) and
heat transfer to or from the surroundings
Instead, the system is designed so that the refrigerant is slightly superheated at the
compressor inlet
This slight overdesign ensures that the refrigerant is completely vaporized when it
enters the compressor
The line connecting the evaporator to
the compressor is usually very long, thus the
pressure drop caused by fluid friction and
heat transfer from the surroundings to the
refrigerant can be very significant. the line
connecting the evaporator to the compressor
is usually very long, thus the pressure drop
caused by fluid friction and heat transfer
from the surroundings to the refrigerant can
be very significant.
The refrigerant is subcooled somewhat
before it enters the throttling valve
REFRIGERANTS
Cholorofluorocarbons, ammonia, hydrocarbons(propane, ethane, ehtylene, etc), carbon
dioxide, air (in the air conditioning of aircraft
R-11, R-12, R-22, and R-502 account for a large percent of the market
The industrial and heavy commercial sectors were satisfied with ammonia, and still are,
although ammonia is toxic.
The advantages of ammonia over other refrigerants are its low cost, higher COPs (and
thus lower energy cost), more favorable thermodynamic and transport properties and thus
higher heat transfer coefficients (requires smaller and lower-cost heat exchangers), greater
detectability in the event of a leak, and no effect on the ozone layer
Refrigerants used in the light commercial and household sectors such as sulfur dioxide,
ethyl chloride, and methyl chloride were highly toxic
The versatility and low cost of CFCs
R-11 is used primarily in large-capacity water chillers serving air conditioning systems
in buildings.
R-12 is used in domestic refrigerators and freezers.
R-22 is used in window air conditioners, heat pumps, air conditioners of commercial
buildings, and large industrial refrigeration systems, and offers strong competition to
ammonia.
R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial
refrigeration systems such as those in supermarkets because it allows low temperatures at
evaporators while operating at single-stage compression.
The ozone crisis: CFCs allow more ultraviolet radiation into the earth's atmosphere
while preventing the infrared radiation from escaping the earth and thus contributing to the
greenhouse effect which causes global warming.
Fully halogenated CFCs (such as R-11, R-12, and R-115) do the most damage to the
ozone layer . The nonfully halogenated refrigerants such as R-22 have about 5 percent of the
ozone-depleting capability of R-12.
Chlorine-free R-134a is expected to replace R-12.
The lowest pressure in a refrigeration cycle occurs in the evaporator, and this pressure
should be above atmospheric pressure to prevent any air leakage into the refrigeration system
The saturation pressure of the refrigerant at this temperature should be well below its
critical pressure if the heat rejection rocess is to be approximately isothermal. If no single
refrigerant can meet the temperature requirements, then two or more refrigeration cycles with
different refrigerants can be used in series
Cascade Refrigeration Systems
In actual cascade refrigeration systems, the two cycles would overlap somewhat since a
temperature difference between the two fluids is needed for any heat transfer to take place.
Compressor work decreases and the amount of heat absorbed from the refrigerated
space increases as a result of cascading. Therefore, cascading improves the COP of a
refrigeration system
Multistage Compression Refrigeration Systems
When the fiuid used throughout the cascade refrigeration system is the same, the heat
exchanger between the stages can be replaced by a mixing chamber (called a flash chamber),
since it has better heat transfer characteristics. Such systems are called multistage
compression refrigeration systems
The compression process in this system resembles a two-stage compression with
intercooling, and the compressor work decreases
Multipurpose Refrigeration Systems with a Single Compressor
Some applications require refrigeration at more than one temperature
A practical and economical approach would be to route all the exit streams from the
evaporators to a single compressor and let it handle the compression process for the entire
system.