PSYCHROMETRYPSYCHROMETRY(A Review)(A Review)

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Dr. Khairul Habib

PsychrometryPsychrometryWhat is psychrometry?

is the study of the properties of mixtures of air and water vapor. Example: how wet is the air ?Why study psychrometry?

To be able to analyse various processes involving moist air.Example: if the air is warm and wet, what will be the air-conditioning design required

PsychrometryPsychrometry

Gibbs-Dalton Law

For a mixture of perfect gases, the mixture pressure is equal to the sum of partial pressures of the constituents

Therefore for moist air:

P = PN2 + PO2 + PCO2 + PAr + Pv

PsychrometryPsychrometry

Various constituents (N2, O2, CO2, Ar) of dry air may be considered to be one gas.

Therefore, the moist air pressure can be simplified as: Pa = partial pressure of dry air Pv = partial pressure of water vapor

P = Pt =Pa + Pv

Partial pressure for vapor, Pv :

Also known as vapor pressure. is the saturated partial pressure of water vapor at the dry bulb temperature.

Can be obtained directly from the saturated water table in Thermodynamics text books.

Example: At 30°C, the saturated water vapor pressure is ?

Pv = 4.246 kPa

Humidity Ratio :

The ratio of mass of the vapor (mv) to the mass of dry air in the mixture (ma)

Humidity RatioHumidity Ratio

a

v

m

m

Humidity ratio can also be expressed as:

(kg water vapor / kg dry air)

Humidity RatioHumidity Ratio

a

v

aa

vv

aa

vv

a

v

P

P

RP

RP

TRVP

TRVP

m

m622.0

/

/

)/(

)/(

v

v

PP

P

622.0Not easy to physically measure Pv

Relative Humidity, Relative Humidity,

Relative Humidity (RH) is the mole fraction of the water vapor in a mixture to the mole fraction of the water vapor in the saturated mixture at the same temperature and pressure.

In simpler words: The amount of moisture that air holds (mv) relative to the maximum amount of moisture the air can hold (ms) at the same temperature

Relative Humidity (RH) can be written as :

Relative Humidity, Relative Humidity,

s

v

m

m

s

v

vs

vv

P

P

TRVP

TRVP

)/(

)/(

sP

P

)622.0(

Dew Point Temperature, Tdp

The temperature of saturated moist air at the same pressure and humidity ratio of a given air mixture.

This is the temperature at which condensation begins when the air is cooled at constant pressure

Any example?

PsychrometryPsychrometry

Enthalpy, h

•Is a thermodynamic property.•The enthalpy of moist air is the sum of the enthalpy of the dry air and the enthalpy of the water vapor. •The enthalpy of moist air is given by

Wet bulb temperature

The wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor. Wet bulb temperature is the lowest temperature that can be reached by the evaporation of water only. It is the temperature one feels when one's skin is wet and is exposed to moving air.

Specific volume

The specific volume is defined as the number of cubic meters of moist air per kilogram of dry air.

PSYCHROMETRYPSYCHROMETRY

Adiabatic SaturationAdiabatic Saturation

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ADIABATIC SATURATIONADIABATIC SATURATION

There is no way to directly measure , or h

Can determine RH () if know the dew point temperature, which helps determining the Pv value, by:

s

v

P

P

EXAMPLE: EXAMPLE: and T and Tdpdp

Based on Cengel, Example 13-2:

In cold weather, condensation occurs on inner surface of windows due to lower air temperatures near the window surface. A house contains air at 20°C and 75% RH.

At what window temperature will moisture in the air start condensing on the inner surfaces of the window?

Tdp = T sat@Pv,where Pv = P g@20°C = 0.75 x 2.339 kPa = 1.754 kPa

so, Tdp = T sat@1.754kPa = 15.3°C

But this method is not practical!

ADIABATIC SATURATIONADIABATIC SATURATION

Air leaving the outlet (2) has RH () of 100%.

T2 : adiabatic saturation temperature

Better known as wet bulb temperature, Twb

ADIABATIC SATURATIONADIABATIC SATURATION

Using energy balance and mass balance relations, results in:

22

22

622.0

v

v

PP

P

21

22121

)(

fv

fgp

hh

hTTC

See Cengel’s example 13-3

ADIABATIC SATURATIONADIABATIC SATURATION

How to measure wet bulb temperature?How to measure wet bulb temperature?

Simplest tool is the sling psychrometer

The device has 2 thermometers. One with a cotton wick saturated with water.

Operation principle of sling psychrometerOperation principle of sling psychrometer

When unsaturated air passes (by the swing motion) over the wet wick, water in the wick evaporates.

The water evaporation causes temperature of the wick water to drop. Thus the wet bulb temperature (Twb) is recorded by the wet wick thermometer.

Operation principle of sling psychrometerOperation principle of sling psychrometer

With the use of two thermometers, the following temperatures can be recorded:

a. wet bulb temperature (Twb)b. dry bulb temperature (Tdb)

Ref: http://www.aslltd.co.uk/humidity.htm

Electronic Electronic PsychrometerPsychrometer

PSYCHROMETRIC CHARTPSYCHROMETRIC CHART

To ease the psychrometric calculations, a psychrometric chart was invented.

Example

By obtaining the values for Twb and Tdb using sling psychrometer, can just refer to the psychrometric chart for various information.

Otherwise, need to use formula, which is quite tedious.

PSYCHROMETRIC CHARTPSYCHROMETRIC CHART

A psychrometric chart consists of the following information:

1. Humidity ratio, 2. Relative humidity, 3. Enthalpy, h4. Wet bulb temperature, Twb

5. Dry bulb temperature, Tdb

6. Dew point temperature, Tdp

7. Specific volume,

Problem 1

Assume that the outside air temperature is 32°C with a relative humidity φ = 60%. Use the psychrometric chart to determine the specific humidity ω, the enthalpy h, the wet-bulb temperature Twb, the dew-point temperature Tdp, and the specific volume of the dry air v. Indicate all the values determined on the chart.

PSYCHROMETRIC CHARTPSYCHROMETRIC CHART

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