Metric Problems

Some of the SI units for psychrometrics are the same and some are different. Refer to Table 23.4 and Figure 23.11 before attempting to complete a sample problem.

Table 23.4. Comparison of customary and SI psychrometric terms.

Term

Conventional

Relative humidity Volume Dry bulb Wet bulb Enthalpy Dew point Moisture content Time

Ft3/lb

lb H2O/lb dry air hour

kg H2O/kg dry air hour

55 90 äs 100 10560n0 115 120

55 90 äs 100 10560n0 115 120

Temperature #2 Temperature #1

FIGURE 23.10. Solution for evaporating cooling problem.

Temperature #2 Temperature #1

FIGURE 23.10. Solution for evaporating cooling problem.

FIGURE 23.11. Metric psychrometric chart.

IZO 125 130 140

IZO 125 130 140

1S sa

ffi*

1S sa

Volume fchVäcg Dry Air)

Figure 23.12. Psychrometric chart for di'ying problem.

Table 23.5. Information for drying problem.

Properties

Initial

Final

Difference

Moisture (kg H2O/kg of air) Volume (m3/kg dry air)

0.0108 0.014

0.087

0.0032

Problem: Determine the size of fan (m3/hr) that will be required to naturally air dry 100.0 kg of product in 8.0 hr that must have 23.5 kg of water removed. The outside air conditions are 30°Cdb and 20°Cwb and the air is 85% saturated when it leaves the dryer.

Solution: Fans are sized by air moving capacity (m3/hr). The amount of air (m3) is determined by the amount of water that must be removed and the amount of water that can be adsorbed by each cubic meter of air. The first step is to develop a table of pertinent information, Table 23.5 and Figure 23.12.