Moist Air Properties

PSYCHROMETRY

The capacity of air for moisture removal depends on its humidity and its temperature. The study ofa 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,
• 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.

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

PSYCHROMETRIC CHART

1020 • t 30

Drybulbtemperature 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.

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.

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-bulbtemperatures, 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.

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 itover 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.

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. Theenthalpy and the wet-bulb temperature remain practically constant during the adiabatic drying process. The process of grain drying is illustrated in Fig. 2.6.

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 mland m2, temperatures Tland T2and humidity ratios Wi and W2. The mixture will have a dry mass flow rate of m3, a temperature of T3and a humidity ratio of W3. The mass and energy balances for this process are:

ml + m2= m3

mlWl+ m2W2 = m3W3

mlhl+ m2h2 = m3h3

Eliminating m3 yields:

M1(h3-h1)= m2(h2-h3)

ml(W3-Wl)= m2(W2-W3)

and thus:

Re-arranging gives:

The condition of the mixture of the two air streams therefore lies on a straight line joining (h1, W1) and (h2, W2) on the h-Wpsychrometric chart. The point (h3, 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 Cin 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 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/hrof 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 forevaporation being supplied by the hot air passingover, a wet solid surface, the system behaves like theadiabatic saturation system. It is adiabatic becauseno 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 hotair. Looked at from the viewpoint of the Solid, this is a drying process; from the viewpoint of the air it is humidification.

HOME WORK

Write short notes on:

1. Equilibrium moisture content
2. Constant rate drying
3. Falling Rate drying

(Use sketches and graphs to illustrate your answer)

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 beddryer, 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 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 areconveyed rapidly in an air stream, the velocity andturbulence of the stream maintaining the particles insuspension. Heated air accomplishes the drying andoften some form of classifying device is included inthe equipment. In the classifier, the dried material isseparated, the dry material passes out as productand the moist remainder is re-circulated for furtherdrying.

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.

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.

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