ME 410 Mechanical Engineering Systems Laboratory Experiment 3 Mass and Energy Balances in Psychrometric Processes Asst.Prof.Dr. Tuba OKUTUCU, A 143
AIR CONDITIONING (A/C) Goal: Control temperature and humidity Types: 1. Comfort 2. Industrial
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)
AIR CONDITIONING (A/C) Human comfort conditions stated by ASHRAE*: Temperature Relative Humidity Winter 20 23 C 50±20% Summer 24 27 C 50±20% *American Society of Heating, Refrigerating and Air conditioning Engineers)
BASICS Atmospheric air: The air in the atmosphere that normally contains 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.
BASICS In 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
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 = P a + P v (kpa) subscripts a: dry air v: water vapor pressure water vapor would exert if it existed alone at the temperature and volume of the mixture
BASICS h v (T, low P) h g (T) At 0 C, enthalpy of water vapor = 2501.3 kj/kg Average c p of water vapor in 10 C < T < 50 C c p = 1.82 kj/(kg. C) h g (T) 2501.3 + 1.82 T (kj/kg) T in C.
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
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.
RELATIVE HUMIDITY Comfort level depends on the amount of moisture the air holds (m v ) relative to the maximum amount of moisture the air can hold at the same temperature (m g ). The ratio of the two is called the relative humidity.
Relative humidity: RELATIVE HUMIDITY Relative humidity in terms of the specific humidity:
ENTHALPY OF ATMOSPHERIC AIR Total enthalpy:
TEMPERATURE Dry bulb temperature: The ordinary temperature of atmospheric air. DEW POINT: T dp, the temperature at which condensation begins if the air is cooled at constant pressure. T dp is the saturation temperature of water corresponding to the vapor pressure: T dp = T sat@p v Knowing the dew point we can determine the vapor pressure and thus relative humidity.
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 channel o Saturated air at the exit o Φ = 100% o T 2 = Adiabatic saturation temperature
ADIABATIC SATURATION PROCESS Mass balance:
ADIABATIC SATURATION PROCESS Energy balance: per unit mass of dry air
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.
WET BULB TEMPERATURE A more practical way of achieving saturation conditions. Temperature measured this way is called the wet bulb temperature, T wb Twb T adiabatic saturation
PSYCHROMETRIC CHART h in kj/kg dry air For saturated air T db = T wb = T dp 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.
ASHRAE PSYCHROMETRIC CHART NO. 5
AIR CONDITIONING PROCESSES ω (g moisture / kg dry air) T db ( C)
EXPERIMENTAL SET UP 5 1 Discharge d Inlet Mixer 4 Rotating vane anemometer Evaporato r 3 Mixer Steam Injection Mixer 2 Fan Reheaters (3.6 kw) T.E.V. PreHeaters (2.88 kw) Feed Water Drier Condensate Boiler Compressor condenser unit Liquid Receiver Schematic drawing of the experimental set up 1.44 kw 2.5 kw 1.44 kw
SIMPLE HEATING AND COOLING Cooling Heating h 1, h 2 : enthalpies per unit mass of dry air at the inlet and the exit respectively
HEATING WITH HUMIDIFICATION In simple heating, relative humidity, Φ To eliminate this problem, heated air is humidified.
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.
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.
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
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.
ADIABATIC MIXING OF AIR STREAMS Mass and energy balances Eliminating
IDEAL VAPOR COMPRESSION REFRIGERATION CYCLE Each component is a steady flow device.
IDEAL VAPOR COMPRESSION REFRIGERATION CYCLE For each component in the cycle:
ACTUAL VAPOR COMPRESSION REFRIGERATION CYCLE
P h diagram for R 12
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