ME 410 - Mechanical Engineering Systems...

ME 410 - Exp. 3 - Mass and Energy Balances in Psychrometric Processes

ME 410 - Mechanical Engineering Systems Laboratory

Experiment 3 - Mass and Energy Balances in Psychrometric Processes

Assist.Prof.Dr. Özgür BAYER, A-123

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AIR-CONDITIONING (A/C)

Goal: Control temperature and humidity

Types:

1. Comfort

2. Industrial

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AIR CONDITIONING

Human comfort primarily depends on:

The (dry-bulb) temperature (22-27°C),

Relative humidity (40-60%), and

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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AIR-CONDITIONING (A/C)

Human comfort conditions stated by ASHRAE*:

*American Society of Heating, Refrigerating and Air-conditioning Engineers)

Temperature Relative Humidity

Winter 20-23°C 50±20%Summer 24-27°C 50±20%

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BASICS

Atmospheric air:

The air in the atmosphere that normallycontains some water vapor (moisture).

Dry air: Air that contains no water vapor.

amount of water vapor in the air is small, but

it plays a major role in human comfort.

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BASICSIn A/C applications,

-10°C < T < 50°C

In this range:

Air ≈ IDEAL GAS with constant specific heat.

Enthalpy and enthalpy change of dry air:

T : air temperature in °C

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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)

subscriptsa: dry air v: water vapor

pressure water vapor would exert if it existed alone at the temperature and volume of the mixture

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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.

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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

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RELATIVE HUMIDITY

Saturated air:

• As more vapor or moisture is added, thespecific humidity will keep increasing until theair can hold no more moisture.

• At this point, the air is said to be saturatedwith moisture, and it is called saturated air.

• Any moisture introduced into saturated air willcondense.

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RELATIVE HUMIDITY

• Comfort level depends on the amount ofmoisture the air holds (mv) relative to themaximum amount of moisture the air canhold at the same temperature (mg).

• The ratio of the two is called the relativehumidity.

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RELATIVE HUMIDITYRelative humidity:

Relative humidity in terms of the specific humidity:

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ENTHALPY OF ATMOSPHERIC AIR

• Total enthalpy:

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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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ADIABATIC SATURATION PROCESS

Long, insulated channel containing a pool of water

Steady stream of unsaturated air.

As air flows over water, some water will evaporate and mix with the air stream.

Moisture content of air ↑

Temperature of air ↓

(evaporative cooling)

Long enough channelo Saturated air at the exit

o Φ = 100%

o T2 = Adiabatic saturation temperature

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Mass balance:

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Energy balance:

per unit mass of dry air

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ADIABATIC SATURATION PROCESSThe specific humidity at the inlet,

and at the exit (Φ = 100%),

Absolute and relative humidities can be obtainedjust by measuring the temperature and pressureof the air stream at the inlet and exit of anadiabatic saturator.

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WET BULB TEMPERATURE

A more practical way of achieving saturation conditions.

Temperature measured this way is called the wet bulb temperature, Twb

Twb ≈ Tadiabatic saturation

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PSYCHROMETRIC CHART

State of the atmospheric air at a given total pressure can be determined by two independent intensive properties.

The rest can be calculated using above equations.

A more practical way is to use charts called psychrometric charts.

h in kJ/kg dry air For saturated air Tdb = Twb = Tdp

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ASHRAE PSYCHROMETRIC CHART NO. 5

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AIR CONDITIONING PROCESSES

Tdb (°C)

ω(g

mo

istu

re /

kg

dry

air

)

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EXPERIMENTAL SET-UP

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EXPERIMENTAL SET-UP (Contd.)

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Top row:• Fan• Humidifier

Bottom row:• Heaters• Evaporator


EXPERIMENTAL SET-UP (Contd.)

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SIMPLE HEATING AND COOLING

Heating

Cooling

h1, h2 : enthalpies per unit mass of dry air at the inlet and the exit respectively

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HEATING WITH HUMIDIFICATION

• In simple heating, relative humidity, Φ ↓

• To eliminate this problem, heated air is humidified.

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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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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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EVAPORATIVE COOLING

In desert (hot and dry) climates, the high cost of cooling can be avoided by using evaporative coolers.

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

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ADIABATIC MIXING OF AIR STREAMS

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.

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ADIABATIC MIXING OF AIR STREAMS

Mass and energy balances

Eliminating

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IDEAL VAPOR COMPRESSION REFRIGERATION CYCLE

Each component is a steady-flow device.

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IDEAL VAPOR COMPRESSION REFRIGERATION CYCLE

For each component in the cycle:

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ACTUAL VAPOR COMPRESSION REFRIGERATION CYCLE

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P - h diagram for R407C

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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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