web viewwhen air is fully saturated with water vapor, its vapor pressure is called the saturated...

PSYCHROMETRY

The capacity of air for moisture removal depends on its humidity and its temperature. The

study of a relationship between air and its associated water is called psychrometry.

Moist Air Properties

The drying medium used in drying cereal grains is moist air, which is a mixture of dry air and

water vapor. Dry air consists of a number of gases, mainly Oxygen and Nitrogen plus some

minor components such as Argon, Carbon dioxide, Neon, etc. Goff (1949), in determining the

thermodynamic properties of moist air, arbitrarily defined dry air as a gaseous mixture with a

molecular weight of 28.966 and a mole-fraction composition of 0.2095 Oxygen, 0.7809

Nitrogen, 0.0093 Argon and 0.0003 Carbon dioxide. Dry air may vary slightly from these

proportions at a given location; however, the Goff figures are sufficiently accurate for

engineering calculations.

In addition to the dry-air gases, moist air contains a varying amount of water vapor. Although

the weight fraction of water vapor in the air used for cereal grain drying is always less than

one-tenth, the presence of water vapor molecules has a profound effect on the drying process.

A number of terms are used to express the amount of water vapor in moist air. These and other

thermodynamic terms employed in describing moist air properties are defined in the following

section.

DEFINITION OF PSYCHROMETRIC TERMS

Three humidity terms are used in the grain-drying literature to characterize the amount of water

vapor held in the drying air:

Vapor pressure,

Relative humidity, and

Humidity ratio.

The temperatures of moist air may refer to the:

dry-bulb,

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dew-point or

Wet-bulb temperature.

Two additional moist-air properties frequently used in grain-drying calculations are:

Enthalpy and

Specific volume.

These nine moist-air thermodynamic properties are defined in the following paragraphs.

Vapor Pressure

The vapor pressure (Pv) is the partial pressure exerted by the water vapor molecules in moist

air. When air is fully saturated with water vapor, its vapor pressure is called the saturated vapor

pressure (Pvs).

Relative Humidity

The relative humidity () is the ratio of the mole fraction (or vapor pressure) of water vapor in

the air to the mole fraction (or vapor pressure) of the water vapor in saturated air at the same

temperature and atmospheric pressure. The relative humidity is expressed as a decimal or a

percentage. Relative humidity values between 0.0 and 100.0% are encountered in grain drying.

Humidity Ratio

The humidity ratio () is the weight of the water vapor contained in the moist air per unit

weight of dry air. Other terms used for humidity ratio are absolute humidity and specific

humidity.

Dry-bulb Temperature

The dry-bulb temperature (T) is the temperature of moist air indicated by an ordinary

thermometer. Whenever the term temperature is used in this book without a prefix, dry-bulb

temperature is implied.

Dew-point Temperature

The dew-point temperature (Tdp) is the temperature at which condensation occurs when the air

is cooled at constant humidity ratio and constant atmospheric pressure. Thus, the dew point

temperature can be considered as the saturation temperature corresponding to the humidity

ratio and vapor pressure of the moist air.

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Wet-bulb Temperature

A distinction should be made between the psychrometric and thermodynamic wet-bulb

temperatures. The psychrometric wet-bulb temperature (Twb) is the temperature of moist air

indicated by a thermometer whose bulb is covered with a wet wick. The airflow passing over

the wick should have a velocity of at least 5 m per sec.

The thermodynamic wet-bulb temperature (Twb*) is the temperature reached by moist air and

water if the air is adiabatically saturated by the evaporating water. The psychrometric and

thermodynamic wet-bulb temperatures of moist air are nearly equal.

Enthalpy

The enthalpy (h) of a dry air-water vapor mixture is the heat content of the moist air per unit

weight of dry air above a certain reference temperature. Since only differences in enthalpy are

of practical engineering interest, the choice of the reference temperature is inconsequential.

Specific Volume

The specific volume (v) of moist air is defined as the volume per unit weight of dry

air. The specific density of the moist air is equal to the reciprocal of its specific

volume

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PSYCHROMETRIC CHART

10 20 • t 30

Dry bulb temperature n °C

Construction

The thermodynamic properties of the dry air-water vapor mixture are frequently

needed in analyzing grain-drying problems. To alleviate the frequent necessity of

making the time-consuming calculations, special charts containing the values of the

most common thermodynamic properties of moist air have been prepared. These are

called psychrometric charts.

There are a number of psychrometric charts in use. The charts differ with respect

to the barometric pressure, the temperature range, the number of thermodynamic

properties included, and the choice of coordinates.

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Use of the Psychrometric Chart

Psychrometric charts give the following thermodynamic properties of moist air at one

atmosphere:

(1) Dry-bulb temperature,

(2) Wet-bulb temperature,

(3) Dew point (or saturation) temperature,

(4) Humidity ratio,

(5) Relative humidity,

(6) Specific volume, and

(7) Enthalpy.

If two of these properties are known, the state point of the air can, in general, be

determined on the chart and the other properties found by reading the values of the

appropriate lines which pass through the point.

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Sensible Heating and Cooling

Several processes relative to grain conditioning can be represented conveniently on the

psychrometric chart. During sensible heating and cooling of the air at constant

humidity ratio, heat is added to or withdrawn from the drying air in a heat exchanger

as in an indirect heater (for grain drying) or in an evaporator (for grain chilling).

The processes of sensible heating and cooling are represented on the psychrometric

chart by straight horizontal lines parallel to the abscissa (Fig. 2.3), and result in

changes in the dry and wet-bulb temperatures, the enthalpy, the specific volume and

the relative humidity of the moist air. No change occurs in the humidity ratio, dew

point temperature and vapor pressure of the moist air.

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Heating with Humidifying

In most heated-air grain-drying systems, energy is added to the air by direct

combustion of gas in the air. During this process not only heat but also a small

amount of water vapor is added to the air. The result of this heating and humidifying

process is that the enthalpy, the humidity ratio, the vapor pressure, the dry-bulb, wet-

bulb and dew point temperatures, and the specific volume of the air are increased. The

change in the relative humidity is determined by the relative amounts of energy and

water vapor added to the air. In grain-drying installations, the relative humidity of the

drying air decreases during the combustion of a fuel in the heater (Fig. 2.4).

Cooling with Dehumidifying

In the process of grain chilling, air is often cooled to below the dew point temperature

by passing it over an evaporator. Since the air is saturated with water vapor at the dew

point temperature, water condenses out of the air as soon as its temperature drops below

Tdp. The humidity ratio of the air will then be decreased, as will the dew point, wet-bulb

and dry-bulb temperatures and the enthalpy and specific volume. The cooling and

dehumidifying process is illustrated in Fig. 2.5.

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Drying

The drying of a column of grain can be considered an adiabatic process. This implies

that the heat required for evaporation of the grain moisture is supplied solely by the

drying air, without transfer of heat by conduction or radiation from the surroundings.

As the air passes through the wet grain mass, a large part of the sensible heat of the air

is transformed into latent heat as a result of the increasing amount of water held in the

air as vapor. During the adiabatic drying process there is a decrease in the dry-bulb

temperature, together with an increase in the humidity ratio and relative humidity, the

vapor pressure and the dew point temperature. The enthalpy and the wet-bulb

temperature remain practically constant during the adiabatic drying process. The

process of grain drying is illustrated in Fig. 2.6.

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Mixing of Two Airstreams.

In a number of continuous-flow grain dryers two streams of air with different mass

flow rates, temperatures, and humidity ratios are mixed. The condition of the resulting

mixture can be determined directly on the ASHRAE psychrometric charts.

Consider two air streams with dry mass flow rates of ml and m2, temperatures Tl

and T2 and humidity ratios Wi and W2. The mixture will have a dry mass flow rate of

m3, a temperature of T3 and a humidity ratio of W3. The mass and energy balances for

this process are:

ml + m2 = m3

m l W l + m2W2 = m3W3

m l h l + m2h2 = m3h3

Eliminating m3 yields:

M1(h3-h1) = m 2 (h 2 - h3)

m l (W 3 - W l ) = m 2 (W 2 - W3)

and thus:

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Re-arranging gives:

The condition of the mixture of the two air streams therefore lies on a straight line

joining (h 1 , W1) and (h2, W2) on the h-W psychrometric chart. The point ( h 3 , W3) can

be found algebraically or by applying the rule of the congruent right triangles directly

on the psychrometric chart. The mixing process is illustrated in Fig. 2.7.

EXAMPLE

If the wet-bulb temperature in a particular room is measured and found to be 20 C in air

whose dry-bulb temperature is 25 C (that is the wet-bulb depression is 5 °C) estimate the

relative humidity, the enthalpy and the specific volume of the air in the room.

On the humidity chart follow down the wet-bulb line for a temperature of 20°C until it of 20

meets the dry-bulb temperature line for 25°C. Examining the location of this point of

intersection with reference to the lines of constant Relative humidity, it lies between 60%

and 70%RH and about 4/10 of the way between them but nearer to the 60% line.

Therefore the RH is estimated to be 64%. Similar examination of the enthalpy lines gives

an estimated enthalpy of 57 kJ / kg and from the volume lines a specific volume of 0.862m3 /kg

Once the properties of the air have been determined other calculations can easily

be made.

EXAMPLE

If the air in the above Example is then heated to a dry-bulb temperature of 40°C, calculate

the heat needed for a flow of 1000 m3 /hr of the hot air to be supplied to a dryer, and the

relative humidity of the heated air.

On heating, the air condition moves, at constant absolute humidity as no water vapour is

added or subtracted, to the condition at the higher (dry bulb) temperature of 40°C. At this

condition, reading from the chart, the enthalpy is 73kJkg-1, specific volume is 0.906

m3 /kg and RH.27 %.

Mass of 1000m3 is 1000/0.906 - 1104kg, - (73 - 57) = 16kJ/kg.

So rate of heating required

- 1104 x 16 Kj/hr

- (1104 x 16)/3600= 5kW.

If the air is used for drying, with the heat for evaporation being supplied by the hot air

passing over, a wet solid surface, the system behaves like the adiabatic saturation system. It is

adiabatic because no heat is obtained from any source external to the air and the wet solid, and

the latent heat of evaporation must be obtained by cooling the hot air. Looked at from the

viewpoint of the Solid, this is a drying process; from the viewpoint of the air it is

humidification.

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HOME WORK

Write short notes on:

a. Equilibrium moisture content

b. Constant rate drying

c. Falling Rate drying

(Use sketches and graphs to illustrate your answer)

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DRYING EQUIPMENT

In an industry so diversified and extensive as the food industry, it would be expected that a

great number of different types of dryer would be in use. This is the case and the total range of

equipment is much too wide to be described in any introductory course such as this. The

principles of drying may be applied to any type of dryer, but it should help the understanding

of these principles if a few common types of dryers are described.

The major problem in calculations of real dryers is that conditions change as the drying air and

the drying solids move along the dryer in a continuous dryer, or change with time in the batch

dryer. Such implications take them beyond the scope of the present course, but the principles

of mass and heat balances learned in FEB 423 are the basis and the analysis is not difficult

once the fundamental principles of drying are understood.

Tray Dryers

In tray dryers, the food is spread out, generally quite thinly, on trays in which the drying takes

place. Heating may be by an air current sweeping across the trays, by conduction from heated

trays or heated[shelves on which the trays lie, or by radiation from heated surf aces. Most tray

dryers are heated by air which also removes the vapours.

Tunnel Dryers

These may be regarded as developments of the tray dryer, in which the trays on trolleys move

where the heat is applied and the vapours removed . In most cases, air is used in tunnel drying

and the material can move through the dryer either parallel or countercurrent to the air flow.

Roller or Drum Dryers

In these the food is spread over the surface of a heated drum. The drum rotates, with the food

being applied to the drum at one part of the cycle. The food remains on the drum surface for

the greater part of the rotation, during which time the drying takes place, and is then scraped

off. Drum drying may be regarded as conduction drying:

Fluidized Bed Dryers

In a fluidized bed dryer, the food material is maintained suspended against gravity in an

upward-flowing air stream. There may also be a horizontal air flow to convey the food

through the dryer. Heat is transferred from the air to the food material, mostly by convection.

Spray Dryers

In a spray dryer, liquid or fine-solid material in a slurry is sprayed in the form of a fine

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dispersion into a current of heated air. Drying occurs very rapidly, so that this process is very

useful for materials which are damaged by exposure to heat for any appreciable length of time.

The dryer body is large so that the particles can settle, as they dry, without touching the walls

on which they might otherwise stick.

Pneumatic Dryers

In a pneumatic dryer, the solid food particles are conveyed rapidly in an air stream, the

velocity and turbulence of the stream maintaining the particles in suspension. Heated air

accomplishes the drying and often some form of classifying device is included in the

equipment. In the classifier, the dried material is separated, the dry material passes out as

product and the moist remainder is re-circulated for further drying.

Rotary Dryers

The foodstuff is contained in a horizontal inclined cylinder through which it travels, being

heated either by air flow through the cylinder, or by conduction of heat from the cylinder

walls. In some cases, the cylinder rotates and in others the cylinder is stationary and a paddle

or screw rotates within the cylinder conveying the material through.

Trough Dryers

The materials to be dried are contained in a trough-shaped conveyor belt, made from mesh,

and air is blown through the bed of material. The movement of the conveyor continually turns

over the material, exposing fresh surfaces to the hot air.

Bin Dryers

In bin dryers, the foodstuff is contained in a bin with a perforated bottom through which warm

air is blown vertically upwards, passing through the material and so drying it.

Belt Dryers

The food is spread as a thin layer on a horizontal mesh or solid belt and air passes through or

over the material. In most cases the belt is moving, though in some designs the belt is

stationary and the material is transported by scrapers.

Vacuum Dryers

Batch vacuum dryers are substantially the same as tray dryers, except that they operate under a

vacuum, and heat transfer is by conduction or by radiation. The trays are enclosed in a large

cabinet which is evacuated. The water vapour produced is generally condensed, so that the

vacuum pumps have only to deal with non-condensable gases. Another type consists of an

evacuated chamber containing a roller dryer.

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Freeze Dryers

The material is held on shelves or belts in a chamber which is under high vacuum. In most

cases, the food is frozen before being loaded into the dryer. Heat is transferred to the food by

conduction or radiation and the vapour is removed by vacuum pump and then condensed. The

pieces of food must be shaped so as to present the largest possible flat surface to the expanded

metal and the plates to obtain good heat transfer. A refrigerated condenser may be used to

condense the water vapour.

Various types of dryers are illustrated in Fig. 7.8.

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MOISTURE LOSS IN FREEZERS AND CHILLERS

When a moist surface is cooled by an air flow, and if the air is unsaturated, water will

evaporate from the surface to the air. This contributes to the heat transfer, but a more

important effect is to decrease the weight of the foodstuff by the amount of water removed.

The loss in weight has serious economic consequences, since food is most often sold by

weight, and also in many foodstuffs the moisture loss may result in a less attractive surface

appearance. To give some idea of the quantities involved, meat on cooling from animal body

temperature to air temperature loses about 2 % of its weight, on freezing it may lose a further

1 % and thereafter if held in a freezer store it loses weight at a rate of aboutJX25 % per month.

After a time, this steady rate of loss in store falls off, but over the course of a year the total

store loss may easily be of the order of 2-2.5 %.

Drying

To minimize these weight losses, the humidity of the air in freezers, chillers and stores and the

rate of chilling and freezing, should be high. The design of the evaporator equipment can help

if a relatively large coil area has been provided for the freezing or cooling duty. The large area

means that the cooling demand can be accomplished with a small air-temperature drier. This

may be seen from the standard equation

q= UAT

For fixed q (determined by the cooling demand) and for fixed U (determined by the design of

the freezer) a large A will mean a small T, and vice versa. Since the air leaving the coils will

be nearly saturated with water vapour as it leaves, the larger the T the colder the air at this

point, and the dryer it becomes. The dryer it becomes (the lower the RH) the larger drying

capacity for absorbing water from the meat. So a low T decreases the drying effect. The

water then condenses from the air, freezes to ice on the coils must be removed from time to

time, by defrosting. Similarly for fixed U and A, a large q means a large T and therefore

better insulation leading to a lower q will decrease weight losses.

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SUMMARY

1. In drying:

(a) The latent heat of vaporization must be supplied,

(b) The moisture must be transported out from the food.

2. Rates of drying depend on:

(a) vapour pressure of water at the drying temperature,

(b) vapour pressure of water in the external environment,

(c) The equilibrium vapour pressure of water in the food,

(d) The moisture content of the food.

3. For most foods, drying proceeds initially at a constant rate given by:

dw/dθ = k'gA(Ys - Ya) = hcA(ta - ts)/ = q/

for air drying. After a time the rate of drying decreases as the moisture content of the food

reaches low values.

4. Air is saturated with water vapour when the partial pressure of water vapour in the air

equals the saturation pressure of water vapour at the same temperature.

5. Humidity of air is the ratio of the weight of water vapour to the weight of the dry air in the

same volume.

6. Relative humidity is the ratio of the actual to the saturation partial pressure of the water

vapour at the air temperature.

7. Water vapour/air humidity relationships are shown on the psychrometric chart.

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PROBLEMS

1. Cabbage containing 89% of moisture is to be dried in air at 65°C down to moisture content

on a dry basis of 5%. Calculate the heat energy required per tonne of/raw cabbage and per

tonne of dried cabbage, for the drying. Ignore the sensible heat.

2. The efficiency of a spray dryer is given by the ratio of the heat energy in the hot air supplied

to the dryer and actually used for drying, divided by the heat energy supplied to heat the air

from its original ambient temperature. Calculate the efficiency of a spray dryer with an inlet

air temperature of 150°C, an outlet temperature of 95°C, operating under an ambient air

temperature of 15°C Suggest how the efficiency of this dryer might be raised.

3. Calculate the humidity of air at a temperature of 65°C and in which the RH is 42 % and

check from a psychrometric chart.

4. Water at 36°C is to be cooled in an evaporative cooler by air which is at a temperature of

18°C and in which the-RH is measured to be 43 %. Calculate the minimum temperature to

which the air could be cooled, and if the air is cooled to 5°C above this temperature, what is

the actual cooling effected. Check your results on a psychrometric chart.

5. In a chiller store for fruit, which is to be maintained at 5°C, it is important to maintain a

daily record of the relative humidity. A wet- and dry-bulb thermometer is available so prepare

a chart giving the relative humidity for the store in terms of the wet-bulb depression.

6. A steady stream of 1300 m3 h] of room air at 16°C and 65 % RH is to be heated to 150°C

to be used for drying. Calculate the heat input required to accomplish this. If the air leaves the

dryer at 92°C and at 98 % RH calculate the quantity of water removed per hour by the dryer.

7. In a particular situation, the heat-transfer coefficient from a food material to air has been

measured and found to be 25 Jm-2 s-1 °C-1. If this material is to be dried in air at 90°C and 15 %

RH, estimate the maximum rate of water removal.

8. Considering apples to be spheres of diameter 0.07 m and of density 960 kg m -3, estimate

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the rate of drying of apples, per cent per week, if they are in air at 12°C and 65 % RH flowing

over the apples at 0.5ms-1.

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web viewwhen air is fully saturated with water vapor, its vapor pressure is called the saturated...

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