ME 410 ‐ Mechanical Engineering Systems Laboratory
Experiment 3 ‐ Mass and Energy Balances in Psychrometric Processes Asst.Prof.Dr. Tuba OKUTUCU, A‐143
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
AIR‐CONDITIONING (A/C)
Goal: Control temperature and humidity
Types:
1. Comfort
2. Industrial
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
AIR CONDITIONING
Human comfort primarily depends on:
¾ The (dry‐bulb) temperature (22‐27°C),
¾ Relative humidity (40‐60%), and
¾ Air motion (enhances heat transfer by convection and evaporation)
speed ≈ 15 m/min
e.g. Environment at 10°C with 48 km/h wind feels as cold as ‐7°C and 3 km/h wind as a result of body‐chilling effect (the wind chill factor)
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
AIR‐CONDITIONING (A/C)
Human comfort conditions stated by ASHRAE*:
Winter
Summer
Temperature
Relative Humidity
20‐23°C
24‐27°C
50±20%
50±20%
*American Society of Heating, Refrigerating and Air‐conditioning Engineers)
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
BASICS
Atmospheric air: The air in the atmosphere that normally
contains some water vapor (moisture).
Dry air: Air that contains no water vapor.
¾ amount of water vapor in the air is small, but
¾ it plays a major role in human comfort. 12:31
ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
BASICS
In A/C applications, ¾ ‐10°C < T < 50°C
In this range: ¾ Air ≈ IDEAL GAS with constant specific heat.
Enthalpy and enthalpy change of dry air:
T : air temperature in °C
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
BASICS
Water vapor in the air => ideal gas (Pv = RT, h = h(T))
(behaves as if it existed alone)
Atmospheric air: an ideal gas mixture
Its pressure = sum of the partial pressures of air and water vapor.
P = Pa + Pv
(kPa)
pressure water vapor would exert if it existed alone at the temperature and volume of the mixture
subscripts
a: dry air
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v: water vapor
ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
BASICS
hv(T, low P) ≈ hg(T)
• At 0°C, enthalpy of water vapor = 2501.3 kJ/kg
• Average cp of water vapor in ‐10°C < T < 50°C
cp = 1.82 kJ/(kg.°C)
hg(T) ≈ 2501.3 + 1.82 T (kJ/kg) T in °C. 12:31
ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
SPECIFIC HUMIDITY
Absolute (specific) humidity (humidity ratio):
The mass of water vapor present in a unit mass of dry air, ω.
Using the ideal gas relations
or 12:31
ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
RELATIVE HUMIDITY
Saturated air:
• 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.
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
RELATIVE HUMIDITY
• Comfort level depends on the amount of
moisture the air holds (mv) relative to the
maximum amount of moisture the air can
hold at the same temperature (mg).
• The ratio of the two is called the relative
humidity.
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
RELATIVE HUMIDITY
Relative humidity:
Relative humidity in terms of the specific humidity:
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
ENTHALPY OF ATMOSPHERIC AIR
• Total enthalpy:
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
TEMPERATURE
• Dry bulb temperature: The ordinary temperature of atmospheric air.
• DEW POINT: Tdp, 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:
Tdp = Tsat@Pv
Knowing the dew point we can determine the vapor pressure and thus relative humidity.
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
ADIABATIC SATURATION PROCESS
¾ Long, insulated channel containing a pool of water
¾ Steady stream of unsaturated air.
¾ As air flows over water, some water will evaporate and mix with the air stream.
¾ Moisture content of air ↑
¾ Temperature of air ↓
(evaporative cooling)
¾ Long enough channel
o Saturated air at the exit
o Φ = 100%
o T2 = Adiabatic saturation temperature
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
ADIABATIC SATURATION PROCESS
Mass balance:
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
ADIABATIC SATURATION PROCESS
Energy balance:
per unit mass of dry air
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
ADIABATIC SATURATION PROCESS
The specific humidity at the inlet,
and at the exit (Φ = 100%),
Absolute and relative humidities can be obtained just by measuring the temperature and pressure of the air stream at the inlet and exit of an adiabatic saturator.
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
WET BULB TEMPERATURE
¾A more practical way of achieving saturation conditions.
¾Temperature measured this way is called the wet bulb temperature, Twb
¾Twb ≈ Tadiabatic saturation
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
PSYCHROMETRIC CHART
¾State of the atmospheric air at a given total pressure can be determined by two independent intensive properties.
¾The rest can be calculated using above equations.
¾A more practical way is to use charts called psychrometric charts.
¾h in kJ/kg dry air
¾ For saturated air Tdb = Twb = Tdp
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
ASHRAE PSYCHROMETRIC CHART NO. 5
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
ω (g moisture / kg dry air) AIR CONDITIONING PROCESSES
Tdb (°C)
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
EXPERIMENTAL SET‐UP
5
1
d
Discharge
Rotating vane anemometer
Mixer
4
Fan
Reheaters
Evaporato
r
Mixer
3
Mixer
Steam Injection
2
Feed Water
PreHeaters
T.E.V.
(3.6 kW)
Inlet
(2.88 kW)
Drier
Boiler
Condensate
Compressor condenser unit
Liquid Receiver
Schematic drawing of the experimental set‐up
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1.44 kW
ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
2.5 kW
1.44 kW
SIMPLE HEATING AND COOLING
Cooling
Heating
h1, h2 : enthalpies per unit mass of dry air at the inlet and the exit respectively
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
HEATING WITH HUMIDIFICATION
• In simple heating, relative humidity, Φ ↓
• To eliminate this problem, heated air is humidified.
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
HEATING WITH HUMIDIFICATION
• If steam is introduced in the humidification section => additional heating.
• If humidification is accomplished by spraying water into the airstream => cooling of the heated airstream.
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
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.
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
EVAPORATIVE COOLING
¾ In desert (hot and dry) climates, the high cost of cooling can be avoided by using evaporative coolers.
¾ As water evaporates, the latent heat of vaporization is absorbed from the water body and the surrounding air. ¾ As a result, both the water and the air are cooled during the process
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
ADIABATIC MIXING OF AIR STREAMS
Many air conditioning applications require the mixing of two airstreams. This is particularly true for ¾ large buildings, ¾ most production and process plants, and ¾ 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.
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
ADIABATIC MIXING OF AIR STREAMS
Mass and energy balances
Eliminating
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
IDEAL VAPOR COMPRESSION REFRIGERATION CYCLE
Each component is a steady‐flow device.
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
IDEAL VAPOR COMPRESSION REFRIGERATION CYCLE
For each component in the cycle:
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
ACTUAL VAPOR COMPRESSION REFRIGERATION CYCLE
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
P ‐ h diagram for R‐12
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes
REFERENCES
1. Thermodynamics, An Engineering Approach, Third Edition, Yunus A. Çengel, Michael A. Boles, McGraw Hill, 1998.
2. ASHRAE 1989 Fundamentals Handbook (SI)
3. ASHRAE Psychrometric Chart No. 5
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ME 410 ‐ Exp. 3 ‐ Mass and Energy Balances in Psychrometric Processes