procesos psicrometricos

Psychrometric Processes

1. Sensible Heating or Cooling

Adding heat to or removing heat from air without changing its humidity ratio (e.g., winter heating or room air in animal buildings). The processes of sensible heating and cooling are represented on the psychrometric chart by straight horizontal lines parallel to the abscissa. Change occurs in: dry-bulb and wet-bulb temperatures, enthalpy, specific volume, and relative humidity. No change occurs in: humidity ratio, dewpoint temperature, and vapor pressure of the moist air. Governing Equation

� Qs = Ma*(h

2 - h

1)

� qs = m

a*(h

2 - h

1)

where:

� Qs = sensible heat added, Btu

� Ma = Mass of dry air, lb

= (volume of air)/(specific volume of moist air in ft3/lb da)

� h = Enthalpy of air, Btu/lb of dry air � q

s = Rate of sensible heat transfer, Btu/min

� ma = Mass airflow, lb/min

= (volume of moist airflow per min)/(specific volume of moist air in ft3/lb da)

2. Evaporative Cooling (Drying) Evaporative cooling is an adiabatic process, that is, there is no net heat loss or gain. Sensible heat in the air is converted to latent heat in the added vapor. The result of evaporative cooling is air that is cooler and more humid, and a surrounding that is dryer. For this reason, the process of evaporative cooling is sometimes referred to as drying. Evaporative cooling is represented on the psychrometric chart by constant wet-bulb temperature lines. Change occurs in: dry-bulb temperature, specific volume, relative humidity, humidity ratio, dewpoint temperature, and vapor pressure of the moist air. No change occurs in: wet-bulb temperature and enthalpy.

3. Heating and humidifying

Heating and humidifying is the process of simultaneously increasing both the dry-bulb temperature and humidity ratio of the air (e.g., animals in confinement add both sensible heat and moisture to the surrounding air). The process is represented by a dashed line or curve between the initial and final state on the chart. The total heat gained (Q or q) in going from the initial to the final condition can be broken into

Page 1 of 5Psychrometric Processes

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sensible and latent heat portions. To separate the total enthalpy into sensible and latent heat, consider a horizontal movement on the chart as sensible heat and a vertical movement as latent heat. The humidity ratio is constant for the horizontal movement (sensible) and the dry-bulb temperature is constant for the vertical movement (latent). If the condition of the air is changing from state 1 to state 2, consider that the intersection of a horizontal line through state 1 and a vertical line through state 2 occurs at state 0. Then the change in heat can be expressed as:

Note that sensible and latent heat losses from animals are determined by filling a room full of the creatures and monitoring the change in the temperature and humidity of the room with time.

4. Cooling and Dehumidifying

Cooling and dehumidifying is the process of lowering both the dry-bulb temperature and the humidity ratio of the moist air. The calculations are identical to those for heating and humidifying the only difference being that the initial state (state 1) is the warmer more humid state. As before, the total heat change (Q or q) in going from the initial to the final condition can be broken into a sensible and latent heat portion.

5. Adiabatic Mixing of Two Streams of Moist Air

When two streams of air with different properties and flow rates are mixed, the properties of the resulting mixture are obtained from energy and mass balances.

1. Conservation of air mass: ma1

+ ma2

= ma3

2. Conservation of water vapor: ma1

*W1 + m

a2*W

2 = m

a3*W

3

3. Conservation of energy: ma1

*h1 + m

a2*h

2 = m

a3*h

3

6. Example Problem 1: Given: Consider air at t

db = 100 F & t

wb = 80 F

Find: %RH, Humidity Ratio, tdpt

, enthalpy, specific volume

Solution: RH = 42% HR = 0.0175 lb/lb da tdpt

= 73 F

Enthalpy = 43.3 Btu/lb Specific volume= 14.5 cuft/lb

7. Example Problem 2: Determine the amount of sensible heat needed to increase the temperature of air from 50 F & 50% RH to 90 F.

Qs = M

a*(h

0 - h

1) q

s = m

a*(h

o- h

1)

QL

= Ma*(h

2 - h

0) q

L = m

a*(h

2 - h

0)



Solution: Enthalpy (50 F, 50% RH) = 16 Btu/lb (HR = 0.0038 lb/lb da) Enthalpy (90 F, same HR) = 26 Btu/lb Heat added = 26 - 16 = 10 Btu/lb

8. Example Problem 3: How much moisture is added to 20 lb of air going from 50 F, 50% RH to 80 F, 60% RH? Solution: HR (50 F, 50% RH) = 0.0038 lbm/lb da HR (80 F, 60% RH) = 0.0132 lbm/lb da Water added = 20 lb * (0.0132 - 0.0038) lb/lb = 0.188 lb-m

9. Psychrometric Exercises:

1. Use the equations to determine the following air-water properties. Given: t

db = 27 C; RH = 50%; and the atmospheric pressure is 101.3 kPa psia (standard

barometric pressure at sea level) Find:

� Water vapor Saturation Pressure, Pws

� Partial Pressure of the Water Vapor, Pw

� Humidity Ratio, W � Specific Volume, v � Enthalpy, h

2. Use the equations to determine the following air-water properties. Given: t

db = 27 C; RH = 50%; and the atmospheric pressure is 84.3 kPa (barometric

pressure at 5000 ft) Find:

� Water vapor Saturation Pressure, Pws

� Partial Pressure of the Water Vapor, Pw


3. Use a psychrometric chart to determine the following air-water properties: Given: t

db = 27 C; RH = 50%; and the atmospheric pressure is 101.3 kPa (standard




� Dewpoint temperature, tdpt

� Wet-bulb temperature, twb


Graph

4. Use a psychrometric chart to determine the following air-water properties: Given: t

db = 20 C; t

wb = 13 C and the atmospheric pressure is 101.3 kPa (standard


� Dewpoint temperature, tdpt

� Relative humidity, RH � Humidity Ratio, W � Specific Volume, v � Enthalpy, h

Graph

5. How much heat must be added to 100 ft3 of moist air with a dry bulb temperature of 50oF

and a relative humidity of 60% to raise the temperature of the air 30oF? What will be the relative humidity of the air once this heat is added? Graph

6. A "perfectly" insulated room 8' x 10' x 20' contains air at 90oF and a relative humidity of 20%. How much water (in gallons) must be added to the room to adiabatically saturate the air in the room? What will be the temperature of the saturated air? (Assume that the heat required to change the temperature of the added water is negligible.) How much water

would be required to saturate the air in the room if the initial conditions were tdb

= 50oF and

RH = 20%? Graph

7. How much heat must be removed to cool 1000 cfm of air at 90oF dry-bulb and 85oF

dewpoint to 70oF dry-bulb and 100% RH? How much moisture is removed from the air? What is the ratio of latent to sensible heat removed in this process? Graph

8. If 1000 cfm of air at 95oF dry-bulb and 88oF wet-bulb is mixed with 100 cfm of air at 55oF

dry-bulb and 50oF wet-bulb, what are the dry-bulb temperature and relative humidity of the resulting mixture? Graph



10. Quiz:

1. Given: air at 30 C and 40% RH Find: The other important air properties (W,N,t

dpt,t

ub,h)

Solution:

� Humidity ratio (W) = 10.5 g/kg [10.2 - 10.8]

� Specific volume (v) = 0.873 m3/kg [0.870 - 0.877]

� Dewpoint temperature (tdpt

) = 15 C [14.8 - 15.3]

� Wet-bulb temperature (twb

) = 20 C [19.6 - 20.4]

� Enthalpy (h) = 57.5 kJ/kg [57.2 - 57.8]

2. Given: airflow rate (final state) 0.888 m3/s; initial conditions: 10 C, 50% RH; final conditions: t

db = 30 C, t

wb= 27 C

Find: Final RH, sensible heat required, moisture added



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