Temperature, dry-bulb

The dry-bulb temperature, denoted is what is usually meant by air temperature. It is measured with a normal thermometer. 

---

Dry-bulb Temperature (DB) (Abscissa). DBT is the temperature of a gas or mixture of gases iadicated by an accurate thermometer after... 

Process Requirements. Typical inside dry-bulb temperatures and relative humidities used for preparing, processing, and manufacturing various products, and for storing both raw and finished goods, are Hsted in Table 1. In some instances, the conditions have been compromised for the sake of worker comfort and do not represent the optimum for the product. In others, the conditions Hsted have no effect on the product or process other than to increase worker efficiency. 

Effective temperature (ET ) is a single number representing those combinations of temperature and humidity which are equivalent in terms of comfort. It is defined as the dry-bulb temperature of the environment at 50% relative humidity. Standard effective temperature loci for normally clothed, sedentary persons are plotted on Eigure 3. The sensation of comfort depends in part upon the wetness of one s skin. Thus, as a person becomes more active the effective temperature lines become more hori2ontal and the influence of relative humidity is more pronounced. 

Dehumidification. Dehumidification may be accompHshed in several ways (see Drying). Moderate changes in humidity can be made by exposing the air stream to a surface whose temperature is below the dew point of the air. The air is cooled and releases a portion of its moisture. Closed cycle air conditioning systems normally effect dehumidification also. The cooled air may require reheating to attain the desired dry-bulb temperature if there is insufficient sensible load in the space. 

Hours in kiln Dry-bulb temperature, °C Wet-bulb temperature, °C Relative humidity, %... 

De.sign dry-bulb temperature. The typically selected value is the temperature which is equaled or exceedea 2 /4 percent of the time during the warmest consecutive 4 months. Since air temperatures at industrial sites are frequently higher than those used for these weather-data reports, it is good practice to add 1 to 3°C (2 to 6°F) to the tabulated value. 

Approach temperature. The approach temperature, which is the difference between the process-fluid outlet temperature and the design dry-bulb air temperature, has a practical minimum of 8 to 14°C (15 to 25°F). When a lower process-fluid outlet teiTperature is required, an air-humidification chamber can be providea to reduce the inlet air temperature toward the wet-bulb temperature. A 5.6°C (10°F) approach is feasible. Since typical summer wet-bulb design temperatures in the United States are 8.3°C (15°F) lower than diy-bulb temperatures, the outlet process-fliiid temperature can be 3°C (5°F) below the dry-bulb temperature. 

When the mean annual temperature is 16.7°C (30°F) lower than the design dry-bulb temperature and when both fans in a bay have automatically controllable pitch of fan blades, annual power required has been found to be 22, 36, and 54 percent respectively of that needed at the design condition for three process services [Frank L. Rubin, Tower Requirements Are Lower for Air-Cooled Heat Exchangers with AV Fans, Oil Gas J., 165-167 (Oct. 11, 1982)]. Alternatively, when fans have two-speed motors, these dehver one-half of the design flow of air at half speed and use only one-eighth of the power of the full-speed condition. 

Wet-bulb temperature is the dynamic equilibrium temperature attained by a water surface when the rate of heat transfer to the surface by convection equals the rate of mass transfer away from the surface. At equilibrium, if neghgible change in the dry-bulb temperature is assumed, a heat balance on the surface is... 

Example 2 Determination of Moist Air Properties Find the properties of moist air when the dry-bulb temperature is 80 F and the wet-bulb temperature is 67 F. 

Example 3 Air Heating Air is heated by a steam coil from 30 F dry-bulb temperature and 80 percent relative humidity to 75 F dry-bulb temperature. Find the relative humidity, wet-bulb temperature, and dew point of the heated air. Determine the quantity of heat added per pound of dry air. 

Example 4 Evaporative Cooling Air at 95 F dry-bulb temperature and 70 F wet-bulb temperature contacts a water spray, where its relative humidity is increased to 90 percent. The spray water is recirculated makeup water enters at 70 F. Determine exit dry-bulb temperature, wet-bulb temperature, change in enthalpy of the air, and quantity of moisture added per pound of dry air. 

Solution. Figure 12-8 shows the path on a psychrometric chart. The leaving dry-bulb temperature is obtained directly from Fig. 12-2 as 72.2 F. Since the spray water enters at the wet-bulb temperature of 70 F and there is no heat added to or removed from it, this is by definition an adiabatic process and there will be no change in wet-bulb temperature. The only change in enthalpy is that from the heat content of the makeup water. This can be demonstrated as follows ... 

Example 5 Cooling and Dehumidification Find the cooling load per pound of dry air resulting from infiltration of room air at 80 F dry-bulb temperature and 67 F wet-bulb temperature into a cooler maintained at. 30 F dry-bulb and 28 F wet-bulb temperature, where moisture freezes on the coil, which is maintained at 20 F. 

Example 8 Determination of Air Properties For a barometric pressure of 25.92 inHg (Ap = —4), a dry-bulb temperature of 90 F, and a wet-bulb temperature of 70 F determine the following absolute humidity, enthalpy, dew point, relative humidity, and specific volume. 

I. The wet-bulb or saturation temperature line gives the maximum weight of water vapor that I kg of dry air can cariy at the intersecting dry-bulb temperature shown on the abscissa at saturation humidity. The partial pressure of water in air equals the water-vapor pressure at that temperature. The saturation humidity is defined by... 

A given humidity chart is precise only at the pressure for which it is evaluated. Most air-water-vapor charts are based on a pressure of 1 atm. Humidities read from these charts for given values of wet- and diy-bulb temperature apply only at an atmospheric pressure of 760 mmHg. If the total pressure is different from 760 mmHg, the humidity at a given wet-bulb and dry-bulb temperature must be corrected according to the following relationship. 

Station pressure, inches of mercury, four digits Dry bulb temperature, °F, three digits"... 

Figure 1, taken from the GPSA Engineering Data Book, gives design wet and dry bulb temperatures for the continental United States. 

The dry bulb temperatures shown will be exceeded not more than 5% o1 the 12 hours during the middle of the day in June to September inclusive, of a normal summer. 

The calculations are made as follows. The exchanger is divided into small increments to allow numerical integrations. A tube wall temperature is first calculated and then QAV. The gas temperature and composition from an increment can then be calculated. If the gas composition is above saturation for the temperature, any excess condensation can occur as a fog. This allows the degree of fogging tendency to be quantified. Whenever possible, experimental data should be used to determine the ratio of heat transfer to m.ass transfer coefficients. This can be done with a simple wet and dry bulb temperature measurement using the components involved. 

Pb Barometric pressure, mmHg Pc Critical pressure of water, 166,818mmHg Pd Saturation pressure of w-ater at the dry-bulb temperature, mm Fig... 

Tc Critical temperature of water = 1,165.67°R td Di-y-bulb temperature, °F Td Dry-bulb temperature, °R U Wet-bulb temperature, °F T Dry-bulb temperature, °R... 

From the psyehrometrie chart, for a dry bulb temperature of 90°F with a relative humidity of 95%,... 

A comparison of wet and dry bulb readings allows the relative humidity to be determined from a psychrometric chart. The wet bulb temperature is always lower than the dry bulb value except when the air is already saturated with water - 100% relative humidity. This is when the wet and dry bulb temperatures are the same. Tlie air will no longer accept water and the lack of evaporation does not allow the wetted bulb to reject heat into the air by evaporation. This situation would be... 

Ambient Dry-Bulb Temperature External outdoor temperature as indicated by a dry-bulb thermometer and expressed in degrees Fahrenheit. 

Does the employer regularly monitor heat eonditions (i.e., dry bulb or adjusted dry bulb temperatures) to determine the risk of heat stress and to establish appropriate work/rest regimens (Note Wet bulb globe temperature is not the most appropriate measure of environmental heat eonditions when employees are wearing vapor impermeable proteetive elothing.)... 

By cooling a certain surface so cold that water starts condensing on it and measuring that temperature, the dewpoint can be measured. Combining this with the measurement of the dry bulb temperature, the state of air can be defined. 

---