chapter 8 part twoedr

CHAPTER 8

(Elementary Principles of Chemical Processes, 3rd Ed., Felder & Rousseau, 2005)

Balances onNonreactive Processes

Psychometric Charts

Psychometric charts or humidity charts are charts that have several properties of a gas-vapor mixture.

The most common one is that for air-water system at 1 atm, shown in Fig. 8.4-1, where it can be used in the analysis of humidification, drying, and air-conditioning processes.

Below are some definitions for terms that are on these charts:1. Dry-bulb temperature, T-the abscissa of the chart. This is the air temp. as

measured by a thermometer or any other temp. measuring instrument.2. Absolute humidity, or moist content, ha {kg water vapor/kg dry air)-the

ordinate of the chart.3. Relative humidity, hr. The curve that forms the left boundary of the chart

is the 100% relative humidity or the saturation curve.4. Dew point, ddp the temp. at which humid air becomes saturated if it is

cooled at constant P.

Balances on Nonreactive Processes

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5. Humid volume (m3/kg dry air): which is the volume occupied by a kg of dry air (DA) plus water vapor that accompanies it.

for example, you wish to know the volume occupied by 150 kg of humid air at T = 30C and hr = 30%. From Figure 8.4-1, ha = 0.0080 kg H20(v)/kg DA and VH = 0.87 m3/kg DA. The volume may then be calculated as


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Humid volume

Wet bulb temperature


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Wet-bulb temp., Twb, which is the temperature measured for a stream of flowing air when the thermometer bulb is wrapped with a porous material like cotton that has been soaked in water to form a wick. Notice that evaporation of water from the wick into the flowing air is accompanied by a transfer of heat from the bulb which cause a drop in temp. The wet-bulb temperature of humid air depends on both the dry-bulb temp. and the moisture content of the air. If the air is saturated, no water evaporates from the wick and the wet-bulb and dry-bulb temp. are the same. The lower the humidity, the greater the difference between the two temperatures. The humid air conditions that correspond to a given wet-bulb temp. fall on a straight line on the psychometric chart, called constant wet-bulb temp. lines. These lines are shown in the figures as lines with negative slopes extending beyond the saturation curve that are less steep than the lines of constant humid volume. The value of Twb corresponding to a given line can be read at the intersection of the line with the saturation curve.

7. Specific enthalpy of saturation: the diagonal scale above the saturation curve on the psychometric chart shows the enthalpy of a unit mass (i.e., 1 kg) of dry air plus the water vapor it contains at saturation. The reference states are liquid water at 1 atm and 0 oC. To determine the enthalpy, follow the constant wet-bulb temp. line from the saturation curve at the desired temp. to the enthalpy scale.

8. Enthalpy deviation: the curves of enthalpy deviation (kJ/kg DA) on the psychometric chart are almost vertical with negative values and convex to the left. These curves are used to determine the enthalpy of humid air that is not saturated, according to the following steps: 1. For the conditions that you have, find on the chart the enthalpy deviation. 2. Follow the constant wet-bulb temp. line to the enthalpy scale above the saturation curve, read the value on the scale, and 3. Add this value to the enthalpy deviation value.


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Specific enthalpy of saturation

Example saturated air at 25C and 1 atm-which has an absolute humidity ha = 0.0202 kg H2O/kg DA

has a specific enthalpy of 76.5 kJ/kg DA. (Verify these values of both ha and H on Figure 8.4-1.)

Solution The enthalpy is the sum of the enthalpy changes for 1.00 kg dry air and 0.0202 kg water

going from their reference conditions to 25C. The computation shown below uses heat capacity data from Table B.2 for air and data from the steam tables (Table B.S) for water.


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Ref: Tao and Janis (2001)


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Ref: Tao and Janis (2001)


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Ref: Tao and Janis (20Balances on Nonreactive Processes

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Ref: Tao and Janis (20Balances on Nonreactive Processes

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Humidity refers to the amount of moisture (water vapor) in the surrounding air.

Relative Humidity is a measure of the amount of moisture in the air compared with the amount of moisture the air can hold.

Relative humidity is expressed as a percentage of how much moisture the air could possibly hold at the temperature it happens when you measure it.

The "wetter or damper" you feel,, the higher is the relative humidity. If you feel the air is dry around you, the relative humidity is low.

Relative Humidity


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We are very sensitive to humidity. Sweating keeps our body cool and maintain its current temperature. If the air is at 90% relative humidity, sweat will

not evaporate into the air. As a result, we feel much hotter than the actual temperature when the relative humidity is high. If the relative humidity is low, we can feel much

cooler than the actual temperature because our sweat evaporates easily, cooling the body

Humidity is measured by means of a hygrometer.

There are different types of hygrometers. The most common hygrometers are Wet- and Dry-

Bulb Psychrometer and Hair Hygrometer

Relative Humidity


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(1) Wet- and Dry Bulb Psychrometer It consists of two identical mercury

thermometers, one of which has a wet cotton or linen wick

around its bulb. Evaporating water from the wick absorbs heat

from the thermometer bulb, causing the thermometer reading to drop.

The difference between dry-bulb and wet-bulb temperatures is compared on psychrometric charts.


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How relative humidity (RH) is measured?

Wet - and Dry Bulb Psychrometer


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If the difference between wet and dry bulb is 6 F and the temperature is 72 F (dry bulb), then the RH is 54%.


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Humidity stretches the hair (human / animal / synthetic) while dryness shortens it.

The hygrometer has the job of stretching a hair between a fixed and a movable point to measure humidity over time.


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Hair Hygrometer


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The Psychrometric Chart Use the psychrometric chart to estimate (1) the absolute humidity, wet-

bulb temperature, humid volume, dew point, and specific enthalpy of humid air at 41C and 10% relative humidity, and (2) the amount of water in 150 m3 of air at these conditions.


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Material and Energy Balances on an Air Conditioner

Air at 80F and 80% relative humidity is cooled to 51F at a constant pressure of 1 atm. Use the psychrometric chart to calculate the fraction of the water that condenses and the rate at which heat must be removed to deliver 1000 ft3/min of humid air at the final condition.

Basis: 1 Ibm Dry Air? Since the psychrometric chart plots the mass ratio kg H2O/kg dry air rather than the mass

fraction of water, it is usually convenient to assume a quantity of dry air in a feed or product stream as a basis of calculation if the chart is to be used in the solution.


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Adiabatic Cooling In adiabatic cooling, a warm gas is brought into contact with a cold liquid, causing

the gas to cool and some of the liquid to evaporate. Heat is transferred from the gas to the liquid but no heat is transferred between the gas-liquid system and its surroundings (hence "adiabatic cooling).

Some common processes of this type are described below. Spray cooling, spray humidification. Liquid water is sprayed into a relatively

dry warm air stream. Some of the water evaporates and the temperature of the air and of the unev aporated liquid both decrease. If the object is to cool the water or the air, the operation is called spray cooling; if the point is to increase the moisture content of the air, the operation is spray humidification.

Spray dehumidification. Warm humid air is dehumidified by spraying cold water into it. Provided that the liquid temperature is low enough, the air is cooled below its dew point, causing some of the water vapor in it to condense.

Drying. Spray drying. (Dried milk)


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Adiabatic Cooling It can be shown, air undergoing adiabatic cooling through contact with

liquid water moves along a constant wet-bulb temperature line on the psychrometric chart from its initial condition to the 100% relative humidity curve

Further cooling of the air below its saturation temperature leads to condensation and hence dehumidification.


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Adiabatic Humidification A stream of air at 30C and 10% relative humidity is humidified in an adiabatic spray tower

operating at P = 1 atm. The emerging air is to have a relative humidity of 40%1. Determine the absolute humidity and the adiabatic saturation temperature of the entering

air.2. Use the psychrometric chart to calculate (i) the rate at which water must be added to

humidify 1000 kg/h of the entering air, and (ii) the temperature of the exiting air.


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MIXING AND SOLUTION When two different liquids are mixed or when a gas or solid is dissolved

in a liquid, bonds are broken between neighboring molecules-and possibly between atoms- of the feed materials, and new bonds are formed between neighboring molecules or ions in the product solution.

If less energy is required to break the bonds in the feed materials than is released when the solution bonds form, a net release of energy results. This energy goes into raising the solution temperature

An ideal mixture is one for which the heat of mixing or solution is negligible


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Heats of Solution and Mixing The heat of solution is defined as the change in enthalpy for a process

in which 1 mole of a solute (gas or solid) is dissolved in r moles of a liquid solvent at a constant temperature T.

As r becomes large, heat of solution approaches a limiting value known as the heat of solution at infinite dilution

The heat of mixing has the same meaning as the heat of solution when the process involves mixing two fluids rather than dissolving a gas or solid in a liquid.


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Production of Hydrochloric Acid Hydrochloric acid is produced by absorbing gaseous HCI (hydrogen chloride) in

water. Calculate the heat that must be transferred to or from an absorption unit if HCI(g) at 100C and H20(l) at 25C are fed to produce 1000 kg/h of 20.0 wt% HCI(aq) at 40C.


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chapter 8 part twoedr

Documents

enthalpy of humid air

dry air da

humid air conditions

airwater system

water vaporkg dry air

constant wetbulb

humid volume m3kg dry

drybulb temperature