psychrometrics - nc state university3).pdf1 psychrometrics outline •what is psychrometrics?...

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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

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Psychrometrics

• 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?

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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 Air4

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% N2 and ~20% O2 (Ratio of N2:O2 = 4:1)– If 10 molecules of water are added to air, 8 molecules of N2 and 2

molecules of O2 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 RH5

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

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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’ (m3/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

Note 1: Each psychrometric chart is created at some constant pressure (most are for atmospheric pressure). So, psychrometric charts can not be used to analyze processes in which the pressure changes.Note 2: Human comfort zone is at ~70-80 °F & ~40-60% RH

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Measurement of Wet Bulb Temperature• Place a moist wick over the bulb of a mercury thermometer• Blow air at high speed over the wick• High energy water molecules from the wick evaporates since

vapor pressure of water vapor near the wick is higher than that of the bulk surrounding air

• Latent heat for evaporation (of high energy water molecules) is removed from the wick, causing a decrease in temperature

• As the temperature of the wick decreases, sensible heat from air flows to it

• Equilibrium is attained when latent heat lost from the wick equals sensible heat flowing into the wick

Note 1: If the relative humidity of the surrounding air is 100%, moisture will NOT evaporate from the wick and hence the reading of the wet bulb & dry bulb thermometers will the sameNote 2: Greater the difference between Tdb & Twb, lower the RH of the surrounding airNote 3: This evaporative cooling principle provides cooling of water in an earthen pot

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Dew Point Temperature (Tdp)

• It is the temperature at which moisture in a mixture of water vapor and air begins to condense (or form “dew”) when cooled

Q: Why does a soda can “sweat”?Q: When and why do we “see” our breath?

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Hygrometers to Measure RH (and Twb, Tdp)• Psychrometer– Two thermometers; one has moist wick (sling or mounted with fan)

• Mechanical– Metal-paper coil (paper strip attached to metal coil; coil changes

shape with moisture; dial rotates similar to that in a bimetallic thermometer; inexpensive & not very accurate)

– Hair tension (human hair attached to spring & dial; hair swells as RH inc.)

• Electronic– Change in electrical resistance of LiCl or semiconductor

• Chilled mirror– Optoelectronic mechanism; very accurate

• Capacitive, resistive, thermal conductivity, gravimetric• Other (Change in weight, volume or transparency of a material)

High RH: Sweating; promotes growth of mold during storage of foodsLow RH: Static electricity

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wet wick

wet bulb temp.

dry bulb temp.handle is rotated

Sling psychrometer

Hygrometers (contd.)

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Psychrometric Chart (Low Temp.)

Sensible heat factor

Moisture Content

(g/kg dry air)

Specific Volume (m3/kg dry air)Dry bulb Temperature (°C)

‐10 °C to +55 °CPage 819 of textbook

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Dry bulb Temperature (°C)

20 °C to 120 °C

Page 820 of textbook

Moisture Content

(g/kg dry air)

Psychrometric Chart (High Temp.)

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Constant Dry Bulb Temperature


Dry bulb Temperature (°C)14

Const. Wet Bulb Temp. & Const. Enthalpy


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Constant Moisture Content or Humidity Ratio


Moisture Content

(g/kg dry air)

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Constant Relative Humidity


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Constant Specific Volume


Specific Volume (m3/kg dry air) 18

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Lines of Constant Psychrometric Parameters



Specific Volume (m3/kg dry air)

Moisture Content

(g/kg dry air)

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Dew Point Temperature


Moisture Content

(g/kg dry air)

*A

<

Dew point temp. of air at “A” is determined by moving horizontally tothe left and intersecting the 100%RH line (saturation temp. line) &reading the temp. at that point.Note: Tdb = Twb at this point

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Cooling Air Below its Dew Point Temperature


Moisture Content

(g/kg dry air)

*A

<

When warm air contacts a cold surfacethat is below its dew point temp.,moisture from the air condenses ontothe surface of the cold surface

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Moisture Content (g/kg dry air)


Psychrometric Chart (Determining Properties)

Given any two properties on the chart, the condition of air-watervapor mixture can be identified on the chart and hence the remaining properties can be determined.

Exception: Constant enthalpy and constant wet bulb temperature lines are the same. Thus, given enthalpy & wet bulb temperature, we can not identify the point that depicts the properties of the air-watervapormixture on the chart and hence the remaining properties can not be determined.

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Mixing Two Streams of Air


Moisture Content

(g/kg dry air)

*

*A

B

Specific Volume (m3/kg dry air)


A: ma kg/sB: mb kg/sC: Conditions of mixtureLA to C: [mb/(ma + mb)]*LA to BLB to C: [ma/(ma + mb)]*LA to B *

C

Example: LA to B = 10 cmma = 6 kg/s, mb = 2 kg/sThen, LA to C = [2/(2+6)]*10 = 2.5 cm

LB to C = [6/(2+6)]*10 = 7.5 cm

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. .

..

. . .

. . .

Spray Dryer

A

B

C

Moist Product(Atomized)

Ambient air

Hot Dry Air

Warm Moist Air + Dry Product

Heater

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Blower

Dry Product

Air + Tiny Particles

of Product

CycloneSeparator

“Atomization” involves breaking up a liquid product into tiny droplets by forcing the product & compressed air into an atomizer (disc with multiple slots at periphery that spins at a high rpm) at the TOP. This increases the surface area of the product, thereby increasing the rate of heat transfer, and thus the rate of evaporation. In this lab, we are using a nozzle at the CENTER instead of a true atomizer at the TOP.

Atomizer

Wet Product

Spray dryer calculations involve: 1. Energy balance equation for air between

points ‘A’ & ‘B’(heater adds energy to air at point ‘A’)

2. Water balance equation for air between points ‘B’ & ‘C’(product adds moisture to air at point ‘B’)

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Heating of Air (Constant Moisture Content or Humidity Ratio)


Moisture Content

(g/kg dry air)

*A B>


Q: Why do we feel dry in a heatedroom?

)H(mQ)H(m:BalanceEnergy BaAa . .

'VV

m:Note a .

.

*

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Drying of Product (Constant Enthalpy & Wet Bulb Temp.)


Moisture Content

(g/kg dry air)

B

C

**


Adiabatic Process (Q = 0)

If Q = 0 & work done = 0, then, H = Constant

)W(m)moisture(%m)W(m:BalanceWater CapBa . . .

Part of sensible heat of air is converted to latent heat of water vapor; thus, temp. drops; m.c. inc.

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Heating Ambient Air & Drying a Product


Moisture Content

(g/kg dry air)

B

C*

A


* > *

A: Ambient airB: Heated airC: Exit air (after heating product)

Energy Balance

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Further Applications of Psychrometrics:Heating, Cooling, Humidification, and Dehumidification

*

Humidification(Latent Heat Addition)

Dehumidification(Latent Heat Removal)

Heating & Humidification

Cooling & dehumidification Heating & dehumidification

Cooling & Humidification

Sensible HeatingSensible Cooling

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Summary• Psychrometric chart (6 quantities + dew point temp., Tdp)

– Dry bulb temperature, Tdb (°C)– Wet bulb temperature, Twb (°C)– Moisture content or specific humidity, W (kg water / kg dry air)– Relative humidity, (%)– Specific volume, V’ (m3/kg dry air)– Enthalpy, H (kJ/kg dry air)

• Mixing of two streams of air– Straight line split in the inverse ratios of mass flow rates

• Heating of air– Const. m.c. line (horizontal line); energy balance

• Drying of a product– Const. enthalpy (inclined line – const. Twb); water balance

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psychrometrics - nc state university3).pdf1 psychrometrics outline •what is psychrometrics?...

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