Etiket Arşivleri: Dry-Bulb Temperature

Psychrometrics

Psychrometrics

Outline

• What is psychrometrics?
• Psychrometrics in daily life and food industry
• Psychrometric chart
– Dry bulb temperature, wet bulb temperature, absolute humidity, relative humidity, specific volume, enthalpy
– Dew point temperature
• Mixing two streams of air
• Heating of air and using it to dry a product
• Psychrometrics is the study of properties of mixtures of air and water vapor
• Water vapor
– Superheated steam (unsaturated steam) at low pressure
– Superheated steam tables are on page 817 of textbook
– Properties of dry air are on page 818 of textbook
– Psychrometric charts are on page 819 & 820 of textbook
• What are these properties of interest and why do we need to know these properties?

Psychrometrics in Daily Life

• Sea breeze and land breeze
– When and why do we get them?
• How do thunderstorms, hurricanes, and tornadoes form?
• What are dew, fog, mist, and frost and when do they form?
• When and why does the windshield of a car fog up?
– How do you de-fog it? Is it better to blow hot air or cold air? Why?
• Why do you feel dry in a heated room?
– Is the moisture content of hot air lower than that of cold air?
• How does a fan provide relief from sweating?
• How does an air conditioner provide relief from sweating?
• When does a soda can “sweat”?
• When and why do we “see” our breath?
• Do sailboats perform better at high or low relative humidity?

Key factors: Temperature, Pressure, and Moisture Content of Air

Do Sailboats Perform Better at low or High RH?
• Does dry air or moist air provide more thrust against the sail?
• Which is denser – humid air or dry air?
– Avogadro’s law: At the same temperature and pressure, the no. of molecules in a given volume is the same, no matter what the gas is
– Air has ~80% N

2 and ~20% O
2 (Ratio of N
2:O
2 = 4:1)
– If 10 molecules of water are added to air, 8 molecules of N

2 and 2
molecules of O
2 are displaced

– Atomic weight gained by air = 10 x 18 = 180
– Atomic weight lost by air = 8 x 28 + 2 x 32 = 288
– Thus, there is a net decrease in weight and hence density
– So, humid air is less dense than dry air

Thus, there is more thrust for the sail with dry air and hence sailboats perform better at low RH

Psychrometrics in the Food Industry

• Heating or cooling of air
– To heat or cool a product
• Mixing different streams of air
• Drying a food product using hot and dry air
– Drier the air, better the drying efficiency

Psychrometric Chart: 6 Quantities

• Dry bulb temperature, Tdb (°C)
– Temperature recorded by a regular thermometer
• Wet bulb temperature, Twb (°C)
– Temp. of a thermometer with air blowing over a moist wick on its bulb
• Moisture content or specific humidity, W (kg water / kg dry air)
– Amount of moisture in air (also called, absolute humidity)
• Relative humidity (RH),

 (%)

– Ratio of amount of moisture in air to max. amt. of moisture air can hold
• Specific volume, V’ (m

3/kg dry air)

– Volume of moist air per unit mass of dry air (specific volume = 1/density)
• Enthalpy, H (kJ/kg dry air)
– Energy content of air

Source: https://projects.ncsu.edu/project/foodengineer/231/notes/psychrometrics/Psychro-ppt.pdf

Mollier Diagram

MOLLIER DIAGRAM

Dew Point – Tdp

The Dew Point is the temperature at which water vapor starts to condense out of the air, the temperature at which air becomes completely saturated. Above this temperature the moisture will stay in the air.

If the dew-point temperature is close to the air temperature, the relative humidity is high, and if the dew point is well below the air temperature, the relative humidity is low.

The Dew Point temperature can be measured by filling a metal can with water and ice cubes. Stir by a thermometer and watch the outside of the can. When the vapor in the air starts to condensate on the outside of the can, the temperature on the thermometer is pretty close to the dew point of the actual air.

The dew point temperature can be read by following a vertical line from the state-point to the saturation line. Dew point is represented along the 100% relative humidity line in the Mollier diagram.

Dry-Bulb Temperature – Tdb

Dry bulb temperature is usually referred to as air temperature, is the air property that is most common used. When people refer to the temperature of the air, they are normally referring to its dry bulb temperature. Dry-bulb temperature – Tdb, can be measured by using a normal thermometer. The dry-bulb temperature is an indicator of heat content and is shown along the left axis of the Mollier diagram. The horizontal lines extending from this axis are constant-temperature lines.

Wet-Bulb Temperature – Twb

Wet bulb temperature is associated with the moisture content of the air. Wet bulb temperature can be measured with a thermometer that has the bulb covered with a water-moistened bandage with air flowing over the thermometer. Wet bulb temperatures are always lower than dry bulb temperatures but they will be identical with 100% relative humidity in the air (the air is at the saturation line). On the Mollier diagram, the wet-bulb lines slope a little upward to the left (dotted lines).

Heating of Air

Cooling and Dehumidfying Air

Mixing of Air of different Conditions

Humidifying ,
Adding Steam or Water (liquid)

Psychrometric Chart

The psychrometric chart is a variant of the Mollier diagram used in some parts of the world.

The process transforming a Mollier diagram to a psychrometric chart is shown below. First it has to be reflected in a vertical mirror, then rotated 90 degrees.

Evaporation from Water Surfaces

The amount of evaporated water can be expressed as:

Problem 6 (page 22)

An indoor pool evaporates a certain amount of water, which is removed by a dehumidifier to maintain +25ºC, f=70% RH in the room (state 1 in figure). The dehumidifier, shown in figure, is a refrigeration cycle in which moist air flowing over the evaporator cools such that liquid water drops out, and the air continues flowing over the condenser. The air after the evaporator (state 2) has a temperature of +14ºC. For an air flow of 0,10 kg/s dry air the unit has a coefficient of performance COPR =3,0.

Total pressure in the room is constant 101325 Pa.

Calculate

  1. a) the amount of water that evaporates from the pool ( steady state)

  2. b) the compressor work input

  3. c) the absolute humidity and enthalpy (kJ/kg of dry air) for the air as it returns

to the room (state 3 in figure)

Problem dryer

Problem dryer with heat pump