Adiabatic cooling line

In Figure 2 the lines, volume, m /kg dry air, indicate humid volume, which includes the volume of 1.0 kg of dry gas plus the volume of vapor it carries. Enthalpy at saturation data are accurate only at the saturation temperature and humidity however, for air—water vapor mixtures, the diagonal wet bulb temperature lines are approximately the same as constant-enthalpy adiabatic cooling lines. The latter are based on the relationship ... 

Cg = humid heat for humidity H in units of kj / (kg-K) and = latent heat of vaporization at / in kj /kg. The slope of the constant-enthalpy adiabatic cooling line is —C j which is the relationship between temperature and humidity of gas passing through a totally adiabatic direct-heat dryer. The humid heat of a gas—vapor mixture per unit weight of dry gas includes the specific heat of the vapor... 

For each of the. (bur sets of shelves, the condition of the air is changed to 60 per cent humidity along an adiabatic cooling line. 

Since complete humidification is not always attained, an allowance must be made when designing air humidification cycles. For example, if only 95 per cent saturation is attained the adiabatic cooling line should be followed only to the point corresponding to that degree of saturation, and therefore the gas must be heated to a slightly higher temperature before adiabatic cooling is commenced. 

Calculations involving to systems where the Lewis relation is not applicable are very much more complicated because the adiabatic saturation temperature and the wet-bulb temperature do not coincide. Thus the significance of the adiabatic cooling lines on the psychrometric chart is very much restricted. They no longer represent the changes which take place in a gas as it is humidified by contact with liquid initially at the adiabatic saturation temperature of the gas, but simply give the compositions of all gases with the same adiabatic saturation temperature. 

From the humidity chart, Figure 13.4 in Volume 1, air at 294 K and of 40 per cent relative humidity has a humidity of 0.006 kg/kg. This remains unchanged on heating to 366 K. At the dryer inlet, the wet bulb temperature of the air is 306 K. In the dryer, the cooling takes place along the adiabatic cooling line until 60 per cent relative humidity is reached. 

Derive" the equations for saturated volume, humid heat and the adiabatic cooling lines for the psychrometric chart. 

Find the adiabatic-saturation temperature. Find the adiabatic-cooling line (these are the straight lines having negative slope) that passes through point A, interpolating a line if necessary, and read the abscissa of the point (point B) where this line intersects the 100 percent humidity line. This abscissa is the adiabatic-saturation temperature. In the present case, it is 80°F (300 K). 

When both wet- and dry-bulb temperatures have been found, the humidity is read from the psychrometric chart in the following way. The point on the saturation curve corresponding to the wet-bulb temperature is found first. An adiabatic cooling line is then interpolated and followed until the coordinate corresponding to the dry-bulb temperature is reached. The humidity is read from the other axis. 

Define humidity, humid heat, humid volume, dry-bulb temperature, wet-bulb temperature, humidity chart, moist volume, and adiabatic cooling line. 

Explain and show by use of equations why the slope of the wet-bulb lines are the same as the adiabatic cooling lines for water-air mixtures. 

The equation, when plotted on the humidity chart, yields what is known as an adiabatic cooling line. We take the equilibrium temperature of the water, Ts, as a reference temperature rather than 0 C or 32 F. Do you see why We ignore the small amount of makeup water or assume that it enters at Ts. The energy balance is... 

The adiabatic cooling lines are lines of almost constant enthalpy for the entering air-water mixture, and you can use them as such without much error (1 or 2%). However, if you want to correct a saturated enthalpy value for the deviation which exists for a less-than-saturated air-water vapor mixture, you can employ the enthalpy deviation lines which appear on the chart and which can be used as illustrated in the examples below. Any process that is not a wet-bulb process or an adiabatic process with recirculated water can be treated by the usual material and energy balances, taking the basic data for the calculation from the humidity charts. If there is any increase or decrease in the moisture content of the air in a psychrometric process, the small enthalpy effect of the moisture added to the air or lost by the air may be included in the energy balance for the process to make it more exact as illustrated in Examples 4.47 and 4.49. 

A diagram will help explain the various properties obtained from the humidity chart. See Fig. E4.46. You can find the location of point A for 90°F DB (dry bulb) and 70°F WB (wet bulb) by following a vertical line at Tdb = 90°F until it crosses the wet-bulb line for 70°F. This wetbulb line can be located by searching along the 100% humidity line until the saturation temperature of 70 F is reached, or, alternatively, by proceeding up a vertical line at 70 F until it intersects the 100% humidity line. From the wet-bulb temperature of 70°F, follow the adiabatic cooling line (which is the same as the wet-bulb temperature line on the humidity chart) to the right until it intersects the 90 F DB line. Now that point A has been fixed, you can read the other properties of the moist air from the chart. 

The whole process is assumed to be adiabatic, and, as shown in Fig. E4.48b, takes place between points A and B along the adiabatic cooling line. The wet-bulb temperature remains constant at 22°C. Humidity va]hes are... 

The air supply for a dryer has a dry-bulb temperature of 32 C and a wet-bulb temperature of 25.5°C. It is heated to 90 C by coils and blown into the dryer. In the dryer, it cools along an adiabatic cooling line as it picks up moisture from the dehydrating material and leaves the dryer fully saturated. 

When the wet-bulb temperature is 20 C, the wet-bulb and adiabatic cooling lines very nearly coincide. 

The dry bulb temperature (DBT) is the temperature of air measured by a thermometer freely exposed to the air but shielded from radiation and moisture. The wet-bulb temperature (0w) is the temperature a parcel of air would have if it were cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat being supplied by the parcel. In other words, wet bulb temperature is the temperature reached by water surface if the air is passed over it. Wet bulb temperature is a function of dry bulb temperature and humidity. The chart shows dry bulb temperature on the x-axis and moisture content on the y-axis. Any point below the saturation line represents air that is unsaturated, therefore, the chart has relative humidity cxirves going up to 100% relative humidity. Wet bulb temperature lines are constant enthalpy or adiabatic cooling lines. The change in composition of... 

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