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Changing Air Conditions by Heating, Cooling, Mixing, Humidifying or Dehumidifying the Air Overview of the basic changes of air conditions by heating, cooling, mixing, humidifying and dehumidifying air by adding steam or water. The different processes are shown in psychometric diagram and the Mollier chart. Sponsored Links The most common changes of air conditions are Heating of Air Mixing of Air Cooling and Dehumidifying of Air Humidifying by Adding Steam or Water Heating of Air The process of heating of air can be expressed in the Mollier diagram as:

Changing Air Conditions by Heating for Ambrose West

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Changing Air Conditions by Heating, Cooling, Mixing,Humidifying or Dehumidifying the Air

Overview of the basic changes of air conditions by heating, cooling,mixing, humidifying and dehumidifying air by adding steam or water. Thedifferent processes are shown in psychometric diagram and the Mollier chart.

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The most common changes of air conditions are

Heating of Air • Mixing of Air • Cooling and Dehumidifying of Air • Humidifying by Adding Steam or Water

Heating of Air

The process of heating of air can be expressed in the Mollier diagram as:

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Heating of air moves the air condition from A to B along a constant specifichumidity - x - line. The supplied heat - dH - can be read in the diagram asshown.

The heating process of air expressed can also be expressed in thepsychrometric chart as:

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The heat balance for the mix can be expressed as:

Q A h A + Q C h C = (Q A + Q C ) h B (1)

where

Q = volume of the air

h = heat of the air The moisture balance for the mix can be expressed as:

Q A x A + Q C x C = (Q A + Q C ) x B (1)

where

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x = water content in the air

When hot air is mixed with cold air the result may be fog . When the mixing

point is below the saturation line, parts of the moisture in the air condensatesas small droplets floating in the air. The fog process can be expressed in theMollier diagram as:

• More about Mixing Air

Cooling and Dehumidifying Air

With a cold surface in the air, the air may be cooled and even dehumidified.

If the temperature on the cooling surface is higher than the dew pointtemperature - t DP , the air cools along the constant specific humidity - x - line.

The cooling process of air can be expressed in the Mollier diagram as:

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It the temperature on the cold surface is lower than the dew point temperature -t DP , the air cools in the direction of a point C as shown below.

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Vapor in the air condensates on the surface, and the the amount of water condensated will be x A - x B.

• More about Cooling and Dehumidifying - Latent and SensibleHeat

Humidifying , Adding Steam or Water

If water is added to air without any heat supply, the air condition will changeadiabatic along a constant enthalpy line - h. The dry temperature of the air willdecrease as shown in the Mollier diagram below.

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If steam is added to the air, the air condition will change along a constant Air can be humidified by either dh/dx line as shown above.

• adding water, or • adding water vapor - steam

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ρ = density of air - vary with temperature, 1.293 kg/m 3 at 20 oC(kg/m 3)

x = specific humidity of air (kg/kg)

If the water added is at the same temperature as the air, the change in enthalpyis zero.

Example - Humidifying Air by adding Water

An airflow of 3000 m 3/h at 25 oC and 10% relative humidity (A) is humidified to60% relative humidity (C) by adding water through spray nozzles.

Using the Mollier diagram and following the constant enthalpy line 30 kJ/kgfrom A to 60% relative humidity, the state at C can be found at 14.7 oC.

The specific humidity at (A) is 0.002 kg/kg and at (C) 0.0062 kg/kg.

The amount of water added can be calculated as:

m w = ((3000 m 3/h) / (3600 s/h)) (1.293 kg/m 3) ( (0.0062 kg/kg) -(0.002 kg/kg) )

= 0.0045 kg/s = 16.3 kg/h

Humidifying Air by adding Steam

If steam is added to the air, the air condition will change along a constant dh/dx line for steam as shown above.

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When adding saturated steam at atmospheric pressure the constant line dh/dx= 2,502 kJ/kg (the evaporation heat of water at atmospheric pressure). Whenadding saturated steam at atmospheric pressure the temperature rise is very

small - in general less than 1o

C. For practical purposes the process of addingsaturated steam at atmospheric pressure approximates the horizontaltemperature line.

The water vapor added can be calculated using (1).

The enthalpy added can be found using the Mollier diagram.

Example - Humidifying Air by adding Steam

An airflow of 3000 m3

/h at 25o

C and 10% relative humidity (A) is humidified to60% relative humidity (C) by adding saturated steam at atmospheric pressure.

Using the Mollier diagram the process from (A) to (B) can be approximated byfollowing the constant temperature line 25 oC to 60% relative humidity andapproximately 25.5 oC (a temperature increase less than 1 oC).

The specific humidity at (A) is 0.002 kg/kg and at (B) 0.012 kg/kg.

The amount of water added can be calculated as:

m w = ((3000 m 3/h) / (3600 s/h)) (1.293 kg/m 3) ( (0.012 kg/kg) -(0.002 kg/kg) )

= 0.011 kg/s = 38.8 kg/h

The enthalpy change can be found using the Mollier diagram. The enthaply at(A) is 30 kJ/kg and at (B) 55 kJ/kg. The enthalpy difference is

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dh = 55 kJ/kg - 30 kJ/kg

= 25 kJ/kg

The total heat added by the steam can be calculated as:

q = ((3000 m 3/h) / (3600 s/h)) (1.293 kg/m 3) (55 kJ/kg - 30 kJ/kg)

= 26.9 (kJ/s, kW)

Density of Dry Air, Water Vapor and Moist Humid Air

Calculating the density of dry air, water vapor or the mixture of air andwater vapor - moist or humid air.

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Density of air will vary as the temperature and moisture content in the air varies. When the temperature increases, the higher molecular motion results inan expansion of volume and thus a decrease in density.

The density of a gas, either it is dry air, water vapor or a mixture of dry air andwater vapor - moist or humid air, can be calculated on basis of the Ideal GasLaw .

Density of Dry Air

The density of dry air can be expressed as:

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ρ a = 0.0035 p a / T (1)

where

ρ a = density dry air (kg/m 3)

p a = partial pressure of air (Pa, N/m 2)

T = absolute dry bulb temperature (K)

Density of Water Vapor

The density of water vapor can be expressed as:

ρ w = 0.0022 p w / T (2)

where

p w = partial pressure water vapor (Pa, N/m 2)

ρ w = density water vapor (kg/m 3)

T = absolute dry bulb temperature (K)

Density of Moist Air - an Air Vapor Mixture

The amount of water vapor in the air effects the density. Water vapor is arelatively light gas when compared to diatomic Oxygen and diatomic Nitrogen -the dominant components in air .

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ρ = ρ da (1 + x) / (1 + x R w / R a ) (5)

The gas constant ratio between water vapor and air is 455 / 286.9 = 1.608.

Inserting the ratio in (5):

ρ = ρ da (1 + x) / (1 + 1.608 x ) (6)

Note! As we can see from (6) increased moisture content reduces the densityof the moist air - Dry air is more dense than moist air.

Lesson: Condensation

Prerequisites:

• The Water Cycle• Evaporation

Objectives:

1. To demonstrate the concept of condensation

Materials:

• Glass Cups or Jars• Ice Cubes• Water • Paper Clips• 8-10 Petri Dishes or similar shallow plates• 4-5 tin lids• Salt

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Introduction:

Questions to stimulate thought:

• What are clouds?• How are they made?• Have you ever seen a cloud close to the ground?• What is that called? (fog)• How is fog made?• In the winter, when you walk outside, what happens to your breath when it hits the cold air?

Body:

Divide the class into partners. Distribute a glass cup or jar to each group. Ask: "Do the cups feel wet or dry?"

Have the students fill their cups with ice. Add cold water to everyone's cup. Ask: "Do the cups feel wet or dry? Do the cupsfeel hot or cold?"

Let the cups sit for about one half hour. Ask: "What do you notice about the outside of your cups? Are they wet or dry?Where did the water come from? (teacher may need to direct the students towards the idea that the water came from theair.) Students can leave the ice water glasses out overnight or for several hours. If the class has studied evaporation, thendiscuss with them what happened to the water on the outside of the glass. If necessary refer to the evaporation lesson.

Now divide the class into four or five groups. Each group should get two petri dishes, two paper clips and one tin lid. Ask amember of each group to place the paper clips in one of the petri dishes, then set the tin lid on top of the paper clips.Have another group member give one shake of the salt shaker, or whatever it takes to get 6-10 salt crystals on the tin lid.Then the teacher will pour a thin layer of water into each petri dish so the tin lid doesn't get wet any water on it.

Now a member from each group should place an empty petri dish on top of each set up - careful to not bump any water onto the tin lid. (Could also use plastic wrap or some other clear cover.)

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Let the set up sit for about one half hour. Ask for predictions from students of what they think might happen. Have onegroup member be a recorder and record what they see happening. (Eventuallly water from the dish will collect of condense around the salt crystals leaving little drops of salt water on the tin lid.) Ask: "What happened to the salt? Why

are there drops on the lid instead of salt?"

Explain that the water evaporated from the dish and created air that had high humidity in the dish. Water from in the air collected around each salt crystal until the salt dissolved or melted. Now, all that is left are small drops of salty water onthe tin lid. This process is called condensation. This is the same thing that happens in clouds before it rains. (Studentsmay ask where the salt comes from or how does the salt get into the sky.) Explain that condensation can happen aroundother small particles like dust and that salt can get in the sky from the evaporation of ocean spray.

Each group can now take the lid off their set up and make predictions about what they think might happen to the drops.(After about half an hour, the water will evaporate and only salt will be left.) Ask the students to observe what they see on

their lid. (They can taste it to discover that it is salt that is left.

If the class has already studied evaporation, then review what happened to the water. If they haven't, then this may makea great introduction.

Conclusion:

Review with the whole group everything that has been studied in this lesson. Review with the class the process of evaporation -remind them of the puddle/plate experiment. Ask the class: "When water is evaporating where does it go? What happened to our coldglasses? Where did the water come from? What did we call the process where water comes from the air and collects into liquid?(condensation) What is the opposite of condensation, when liquid water goes into the air? (evaporation)"

Vocabulary Words:

• condensation• evaporation

Evaluation:

Defer evaluation until the end of the Water Cycle lesson.

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Followup Lessons:

• Precipitation

About the Authors

Post Comments about the Unit

Read Comments about the Unit

Return to Weather Unit Home Page

Prerequisites:

• Evaporation• Condensation• Precipitation

Objectives:

1. Prepare terrariums for initial observations2. Use terrariums to connect concepts of precipitation, condensation, and evaporation into a unified water cycle concept

Materials:

• Have each student supply their own plastic salad container from a fast food restaurant or some similar clear plastic container.(A ziplock baggie will work if there are not enough salad containers available.)

• soil• water • spray bottle

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• large sunny window or grow light• fast and reliably germinating seeds (marigolds, herbs, lima beans, etc.)

Introduction:Ask: "What are clouds? What are they made of? What is rain? What does the sky look like when it rains? Why does it rain? Wheredoes the rain go after it falls? What happens to puddles after it rains?" Get a discussion going about the different parts of the water cycle: evaporation, precipitation, and condensation. Use as many questions as possible to determine which concepts the students mayunderstand and where any misconceptions may be.Body:

1. Assemble the terrariums: Have each student build his/her own terrarium by putting about an inch of soil in the bottom of their plastic container, planting a seed according to the package instructions, and giving a thorough soaking of water. The initialwatering should be all that is necessary since the plastic container will create a closed environment which will not allow thewater to escape into the atmosphere. Label the terrariums and put them in a sunny window or under a grow light.

2. Observing the terrariums: Have the students make observations about their terrariums each day and record their observations intheir weather journals. Try to do the observations at different times each day. Have the students record what they see in writingand/or in pictures. Discuss as a class anything the students observe. Continue this throughout the lessons on the water cycle(evaporation, condensation, precipitation).

3. Proceed through the following lessons in this order : evaporation, condensation, precipitation.

Conclusion:

Possible questions to ask the students: "We only watered the soil in our terrariums once; how did the water get on the lid? Take your

lid off the terrarium and feel the soil. Why is the soil still wet? Do you think that any water has evaporated from the soil? Why? If water evaporated, where did the evaporated water go? Did it ever rain in your terraium? How do you know? Where did the rain comefrom? Is there anything in your terrarium that reminds you of a cloud or cloud drops?"

Teacher may want to make a connection between the water cycle in the terrarium and in the real world with a discussionusing the following: "If the terrarium is a model of the real world, what do you see outside that reminds you of the plant inour terrarium? reminds you of the soil in our terrarium? reminds you of the small water droplets on the lid? The soil in our

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terrarium stays moist, the ground outside never dries out completely. Why? What keeps it moist? Water collects on the lidof the terrarium, water also collects in the sky as clouds, where does the water in the clouds come from?

Vocabulary Words:noneEvaluation:

1. In the students make a picture model in their weather journal which represents their terrariums. Have the students include allthe parts of the terrarium like, container, soil, water, lid, plants, etc. Also, have them draw and label the processes (evaporation,condensation, precipitation) they see happening and their locations in the terrarium.

2. Have the students complete the water cycle page in their weather journal by labelling the evaporation, condensation and precipitation processes.

Followup Lessons:

• Willy the Water Molecule• Debate: Hey, You Stole My Rain!

About the Authors

Post Comments about the Unit

Read Comments about the Unit

Return to Weather Unit Home Page

Lesson: Precipitation

Prerequisites:

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• The Water Cycle• Condensation• Evaporation

Objectives:

1. To demonstrate the concept of precipitation

Materials:

• Hot pot• glass bowl with ice• pie tin• Water colors• shiny paper • "The Story of Rain" • Red, yellow, blue, green, white construction paper

Introduction:

1. Questions to stimulate thought:o What is rain?o How is rain made?o How are puddles formed?o What are clouds made of?

2. Read aloud : "The Story of Rain"

Body:

Start some water boiling in a hot pot for use in section 2.

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1. Students can sit at their desks while they do this section of the lesson. Pass out two sheets of shiny paper, a cup of water and aset of water colors to each student. First have the students get their brushes soaking wet and dip their brushes into the yellow

paint. Have the students make many large yellow drops all over one sheet of the shiny paper. Rinse the brushes and make

many large blue drops between the yellow drops being careful not to mix the drops. With a second sheet of shiny paper covering the desk, have the students lift their paper so that it is perpendicular to the desk and the drops start to run down the paper. The drops should slide down the paper and mix with each other dripping off the bottom of the paper as large greendrops. Ask: "What happened to the blue and yellow drops when you lifted your paper? What happened to the paper flat onyour desk? Is there a new color on your papers? What is the color? How did the new color get there? Were the drops which felloff the bottom of your paper the same size as the blue and yellow drops?"

2. The class should gather around a common work place where they can all view the hot pot and bowl of ice. Explain to the classthat you are going to hold the pot of ice water over the boiling water. Ask: "What do you think will happen - to the bowl of ice? to the steam? to the bottom of the bowl?" Once the water is boiling, hold the bowl of ice over the steam. Place a pie tin sothat the water which drips from the bottom of the bowl will collect in the tin.

The class should observe and share what they observe happening. Some questions to help: "What do you seehappening on the bottom of the bowl? What do you see happening in the pie tin? How does the water get on thebowl? Are the water drops on the side of the bowl the same size? Why? Which drops are falling from the bowl?Why? Which drops look like rain? Which drops look like a cloud? How are the big drops formed?"

Explain that the small misty drops which have condensed onto the side of the bowl of ice represent a cloud. Thewinds in a cloud blow the small drops around so that they collide with one another. During these collisions, somedrops will combine with others making bigger and bigger drops. When the drops become so large that the windscannot keep them in the sky, the drops fall as rain or precipitation . This is similar to the large drops falling from the

bottom of the bowl.Conclusion:

Play "The Rain Game" Vocabulary Words:

• condensation

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