Moisture Content

The moisture content scales are usually located on the right side of the chart. To locate a value for moisture, move horizontal to the right from the state point, Figure 23.3.

A psychrometric chart from the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) or similar group will contain all of seven of these scales. Figure 23.4 is a low precision chart that can be used for learning about psychrometrics and solving the sample problems, but an ASHRAE chart should be used if possible.

up from and wet but they

15 20

Figure 23.2. Enthalpy and specific volume scales.

FIGURE 23.3. Relative humidity and humidity ratio scales.

115 120

115 120

115 120

Dry buib temperature (°F)

Figure 23.4. Complete psychrometric chart.

115 120

Dry buib temperature (°F)

Figure 23.4. Complete psychrometric chart.

Problem: What are the physical properties of air if the dry-bulb temperature is 75°F, and the wet-bulb temperature is 60°F?

Solution: Using a psychrometric chart, Figure 23.5: [Note: not all psychrometric charts are the same. There will be small differences in the readings from different charts produced by different organizations.]

1. The first step is to find the state point. Locate the vertical line that represents the 75°F dry-bulb temperature. Then, locate the slanting wet-bulb line that represents 60°F wet-bulb. The intersection of these two lines is the state point.

2. The state point is just above the 40% rh line. Therefore the value of the relative humidity is between 40% and 50%. Because there are no other lines between 40% and 50%, the relative humidity must be estimated or interpolated. This point is estimated as 43% rh.

3. To find the dew point temperature, move horizontally to the left from the state point to the curved wet-bulb and dew point scale. The reading is slightly less than 50°F.

4. To find the moisture content move horizontally to the right from the state point to the vertical scale. You should read 0.0078 lb of water per pound of dry air.

5. The total heat is found by moving from the state point along the enthalpy line up and to the left (parallel to the dashed lines) until the total heat (BTU/lb) scale of air is reached. You should read 27 BTU/lb of air.

6. The specific volume is determined by locating and identifying the specific volume lines (ft3/lb of dry air) that fall on either side of the state point, which in this case are 13.6 and 13.7. Estimating the state point distance between these lines gives an approximate value of 13.6 ft3/lb.

Psychrometric charts are also used to explain the changes in the properties of air when it is cooled, heated, had water added or water removed.

Problem: What are the values for the physical properties of the air in the previous problem (75°Fdb and 60°Fwb) after the air is heated to 90°Fdb?

Solution: The first step is to determine the direction and the distance to move from the first state point to the second state point. In this problem, because heat is added and the moisture content is not changed, the second state point is located to the right of the first along the horizontal moisture line. Move right until this line intersects with the vertical 90°Fdb line, and then read the values for the characteristics at the second state point, Figure 23.6. The values are:

Specific volume = 14.05 ft3/lb Moisture = 0.0076 lb/lb Relative humidity = 27% Wet-bulb = 65°F

Compare these results with those for the previous problem. When you understand the principles of air illustrated by a psychrometric chart, you should be able to

95 100 105w110 115 120

95 100 105w110 115 120

50 55 60 651S 70 75 80 8520 90 95 100 1052s110 115 120 Dry bulb temperature (°F)

FIGURE 23.5. Psychrometric chart for problem one.

50 55 60 651S 70 75 80 8520 90 95 100 1052s110 115 120 Dry bulb temperature (°F)

era ps TJ

re R

FIGURE 23.5. Psychrometric chart for problem one.

80 8S20 90 95 100 10Sis110 115 120 Dry bulb temperature (°F)

FIGURE 23.6. Psychrometric chart for problem two.

80 8S20 90 95 100 10Sis110 115 120 Dry bulb temperature (°F)

FIGURE 23.6. Psychrometric chart for problem two.

explain why the values for some of the characteristics changed, and why some did not.

0 0

Post a comment