Question

 

Humid air at given : dry-bulb temperature, wet-bulb temperature and total pressure enters a drier at a given molar flow rate. Using psychometric chart to estimate :relative humidity, absolute humidity, dew point temperature and humid volume of air 

A- volumetric flow rate entering drier?
B-mass flow rate BDA entering drier?
C-Ha and Hm if more mass flow rate of water is added during passing through drier?
D-molar composition air leaving drier?
E-percentage humidity (Hp) air entering drier?

Answer 1

Dry-bulb temperature is the commonly measured temperature from a thermometer. It is called "dry-bulb" since the sensing tip of the thermometer is dry

Relative humidity is a measure of the amount of water air can hold at a certain temperature. It is "relative" with respect to the amount of water that air, at that same temperature, can hold at 100 percent humidity, or saturation. Air temperature (dry-bulb) is important because warmer air can hold more moisture than cold air. Air at 60 percent relative humidity contains 60 percent of the water it could possibly hold (at that temperature).

Humidity ratio of moist air is the weight of the water contained in the air per unit of dry air. This is often expressed as pounds of moisture per pound of dry air. Since the humidity ratio of moist air is not dependent on temperature, as is relative humidity, it is easier to use in calculations. Humidity ratio is found on the vertical, y-axis with lines of constant humidity ratio running horizontally across the chart.

Dewpoint temperature indicates the temperature at which water will begin to condense out of moist air. Given air at a certain dry-bulb temperature and relative humidity, if the temperature is allowed to decrease, the air is no longer able to hold as much moisture. When air is cooled, the relative humidity increases until saturation is reached and condensation occurs. Condensation occurs on surfaces which are at or below the dewpoint temperature. Dewpoint temperature is determined by moving from a state point horizontally to the left along lines of constant humidity ratio until the upper, curved, saturation temperature boundary is reached.

Wet-bulb temperature is determined when air is circulated past a wetted sensor tip. It represents the temperature at which water evaporates and brings the air to saturation.

Enthalpy is the heat energy content of moist air. It is expressed in Btu per pound of dry air and represents the heat energy due to temperature and moisture in the air. Enthalpy is useful in air heating and cooling applications. The enthalpy scale is located above the saturation, upper boundary of the chart. More accurate psychrometric charts use slightly different lines for wet-bulb temperature and enthalpy.

Specific volume indicates the space occupied by air. It is the increase of density and is expressed as a volume per unit weight (density is weight per unit volume). Warm air is less dense than cool air which causes warmed air to rise. This phenomena is known as thermal buoyancy.

Psychrometric Chart use

A psychrometric chart presents physical and thermal properties of moist air in a graphical form.The objective of this fact sheet is to explain characteristics of moist air and how they are used in a psychrometric chart.

Psychrometric charts are available in various pressure and temperature ranges.

Boundaries of the psychrometric chart are a dry-bulb temperature scale on the horizontal axis, a humidity ratio (moisture content) scale on the vertical axis, and an upper curved boundary which represents saturated air or 100 percent moisture holding capacity. The chart shows other important moist air properties as wet-bulb temperature; enthalpy; dewpoint or saturation temperature; relative humidity; and specific volume.

An understanding of the shape and use of the psychrometric chart will help in diagnosing air temperature and humidity problems. Note that cooler air (located along lower, left region of chart) will not hold as much moisture (as seen on the y-axis' humidity ratio) as warm air (located along right side of chart).
Wet-bulb temperature and enthalpy use the same chart line but values are read off seperate scales.

Step 1 Find air properties

A sling psychrometer gives a dry-bulb temperature of 78oF and a wet-bulb temperature of 65oF. Determine other moist air properties from this information. Two useful air properties for environmental analysis in agricultural buildings would be relative humidity and dewpoint temperature. Relative humidity is an indicator of how much moisture is in the air compared to desirable moisture conditions, and dewpoint temperature indicates when condensation problems would occur should the (dry-bulb) temperature drop.

Find the intersection of the two known properties, dry-bulb and wet-bulb temperatures, on the psychrometric chart. The dry-bulb temperature is located along the bottom horizontal axis. Find the line for 78oF, which runs vertically through the chart. Wet-bulb temperature is located along diagonal dotted lines leading to scale readings at the upper, curved boundary marked "saturation temperature". The intersection of the vertical 78oF dry-bulb line and the diagonal 65oF wet-bulb line has now established a "state point" for the measured air. Now read relative humidity as 50 percent (curving line running from left to right up through the chart) and dewpoint temperature as 58oF (follow horizontal line, moving left, toward the curved upper boundary of saturation temperatures).

What might we conclude from this information? The relative humidity of 50 percent is acceptable for most livestock and greenhouse applications. If we allowed the air temperature (dry-bulb) to decrease to 58oF (dewpoint) or below, the air would be 100 percent saturated with moisture and condensation would occur. The humidity ratio, as seen on the vertical, y-axis scale, is a reliable indicator of air moisture level since it reflects the pounds of moisture contained in a pound of dry air and does not fluctuate with dry-bulb temperature readings as does relative humidity. The humidity ratio for air in this example is about 0.0104 lb moisture/ lb dry air (move right horizontally from state point to humidity ratio scale).

Step 2

The air surrounding us is a mixture of dry air and moisture and it contains a certain amount of heat. We are used to dealing with air temperature, relative humidity and, oftentimes, the dewpoint as weather conditions are discussed. All these properties and more are contained in a psychrometric chart.You will find that the upper curved boundary of the chart has one temperature scale yet can represent three types of temperature: wet-bulb, dry-bulb, and dewpoint. This upper curved boundary also represents 100 percent relative humidity or saturated air.

Outdoor air at 40oF,80 percent relative humidity (point A is heated to 65oF (point B) for use in ventilation. Exhaust air (point C) at 75oF and 70% relative humidity contains three times the moisture of the fresh air

Step 3 Winter ventilation
Evaporative cooling uses heat contained in the air to evaporate water. Air temperature (dry-bulb) drops while water content (humidity) rises to the saturation point. Evaporation is often used in hot weather to cool ventilation air. The process moves upward along the line of constant enthalpy or constant web-bulb temperature, for example, from point D to point E in Figure 5. Notice that hot dry air (points D to E with a 24oF temperature drop) has more capacity for evaporative cooling than hot humid air (points F to G with only a 12oF temperature decrese).