humidification section 3
TRANSCRIPT
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S I M U L T A N E O U S H E A T &
M A S S T R A N S F E R
•Humidity data for air water system
•Temperature humidity chart
•Enthalpy humidity chart
•Methods of increasing humidity
•Methods of decreasing humidity
•Determination of humidity
HUMIDITY DATA FOR AIR WATER SYSTEM
To facilitate calculations, various properties are
plotted on a chart for air water system.
These are called HUMIDITY CHARTS or
PSYCHROMATERIC CHARTS.
Two types of these charts are commonly used
Temperature humidity chart
Enthalpy humidity chart
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TEMPERATURE HUMIDITY CHART
Following quantities are plotted against
temperature
Humidity
Specific volume of dry gas
Saturated volume
Latent heat of vaporisation
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TEMPERATURE HUMIDITY CHART
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EXAMPLE 13.4
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ENTHALPY HUMIDITY CHART
For calculation, standard reference is required.
Enthalpy at that state is taken as zero
Commonly melting point is taken as reference
temperature and liquid state as a reference state.
Enthalpy of unsaturated gas (H) = Ha + Hw * H
Ha = Ca (θ – θo)
Hw = Cw (θ – θo ) * λ
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ENTHALPY HUMIDITY CHART
H = enthalpy of humid gas per unit mass of dry gas
Ha = enthalpy of dry gas per unit mass
Hw = enthalpy of vapours per unit mass
Ca = specific heat of gas at constant pressure
Cw = specific heat of vapours at constant pressure
θ = temperature of humid gas
θo = reference temperature
λ = latent heat of vaporization of liquid at θo
H = humidity of gas
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ENTHALPY HUMIDITY CHART
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MIXING OF TWO STREAMS OF HUMID GAS
Stream 1 and stream 2 are mixed
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Gas 1 Gas 2 Resultant
Gas
Mass m1 m2 m
Humidity H1 H2 H
Enthalpy H1 H2 H
Temperature θ1 θ2 θ
MIXING OF TWO STREAMS OF HUMID GAS
Taking a balance on dry gas
m1 + m2 = m -------------i
m1 H1 + m2 H2 = m H -------------ii
m1H1 + m2H2 = mH -------------- iii
m1 H1 + m2 H2 = (m1+m2) H
m1 (H – H1) = m2 (H2 -H)
H – H1 m2
H2 –H = m1 13
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MIXING OF TWO STREAMS OF HUMID GAS
The conditions of resultant gas is therefore
represented by a point on the straight line joining
(H1,H1) and (H2 ,H2)
The gas obtained by mixing of two unsaturated
gases may be either unsaturated, saturated or
supersaturated. The possibility of producing
supersaturated gas arises because the 100
percent humidity line on the enthalpy humidity
diagram is concave toward the humidity axis 14
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MIXING OF TWO STREAMS OF HUMID GAS
For enthalpy
m1H1 + m2H2 = mH
m1H1 + m2H2 = (m1 + m2)H
M2 (H2 – H1) = m1(H-H1) ------ iv
Dividing iii & iv
H – H1 H – H2
H – H1 = H – H2
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ADDITION OF LIQUID OR VAPOUR TO A GAS
Liquid mass = m3
enthalpy = H3
Humidity balance
m1(H – H1 ) = m3
H – H1 = m3/m1
Enthalpy balance
m1(H – H1) = m3H3
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H – H1
H – H1 H3
MIXING OF TWO STREAMS OF HUMID GAS
The composition and properties of mixed stream are
therefore represented by a point on the straight line of
slope H3, relative to the humidity axis, which passes
through the point (H1,H1).
Addition of live steam has only a small effect on the
temperature of the gas.
Addition of water spray, even if the water is considerably
above the temperature of the gas, results in a lowering of
the temperature after the water has evaporated. 17
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METHODS OF INCREASING HUMIDITY
1. Addition of live steam
2. Water spray
- Complete vaporization of water sprayed
3. Mixing of gas with a gas of higher humidity
- Mixing of humid gases
4. Partial vaporization of water - gas brought
into contact with liquid vapours in such way
only a part of liquid is evaporated 18
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METHODS OF DECREASING HUMIDITY
Condensation
Gas is brought in contact with cold surface, either
liquid or solid, if temperature is lower than dew
point condensation takes place.
Compression
When air is compressed, water/moisture present is
condensed.
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DETERMINATION OF HUMIDITY
Chemical method
Determination of wet bulb temperature
Determination of dew point
Measurement of change in length of a hair or fibre
Measurement of conductivity of fiber
Electrolytic hygrometry
Piezo-electric hygrometry
Capacitance meters
Observation of colour change. 20
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DETERMINATION OF HUMIDITY BY WBT
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DETERMINATION OF DEW POINT
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HAIR HYGROMETER
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HAIR HYGROMETER
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Measurement of conductivity of a fibre
If a fibre is coated with an electrolyte, such as
lithium chloride, its electrical resistance will be
governed by its moisture content. Which in turn
depends upon the humidity of atmosphere in which
it is situated.
Electrolytic hygrometry
Quantity of electricity required to electrolyse the
water absorbed from the atmosphere on to a thin
film of desiccant is measured.
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Piezo-electric hygrometry
This method employ a quartz crystal with a hygroscopic
coating in which moisture is alternatively absorbed
from a wet gas and desorbed in a dry gas stream; the
dynamics is a function of gas humidity.
Capacitance meters
Change in capacitance is measured which is a function
of deposition of moisture from the atmosphere
Observation of colour change.
Change of colour is observed in active ingredients such
as cobaltous chloride. 28
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Example No. 13.4,13.5,13.6
& related numericals
Thank you Questions