h1 class="product-title">MoistAirTab™ Example 7 - Solids Drying
Example 7
A continuous hot air drying system is employed to remove 3,500 lb of moisture per hour from the solid stock being dried. The system consists of a low-head fan, a steam-heated air heater, and a rotary drum dryer. The ambient air, under the design condition of 68°F, 760 mmHg and a relative humidity of 40%, is pressurized by an axial fan unit to 775 mmHg. The fan unit has an adiabatic efficicency of 70%. The steam heater would preheat the to 175°F with a pressure drop of 10 mm Hg; The air emerges from the drier at 95°F, a pressure of 755 mm Hg and a relative humidity of 90%. Seek to define all process units involved in terms of energy requirement, sizing, and flow rates.
Solution
The solution is presented below as a table generated from Excel. The gray shaded areas represent the input data.
| State Point | Unit | 1 | 2 | 3 | 4 | Overall |
| Location | | Ambient Air | Fan Outlet | Heater Outlet | Dryer Outlet | Summary |
| Pressure | mm Hg | 760 | 775 | 765 | 755 | |
| | psia | 14.70 | 14.99 | 14.79 | 14.60 | |
| Dry bulb temperature | oF | 68.00 | 72.22 | 175.00 | 95.00 | |
| Relative humidity | % | 40.00% | | | 90.00% | |
| Humidity ratio | lbw/lbdry air | 0.00582 | 0.00582 | 0.00582 | 0.03311 | |
| | | | | | | |
| Total moisture removed | lbw/hr | | | | | 3500 |
| Moisture removal capacity | lbw/lbdry air | | | | | 0.02729 |
| Air flowrate (dry basis) | lbdry air/hr | | | | | 128,250 |
| | | | | | | |
| Entropy of moist air | Btu/(lbdry air.°F) | 0.0302 | 0.0308 | 0.0747 | 0.0994 | |
| Enthalpy of moist air | Btu/lbdry air | 15.00 | 16.02 | 41.03 | 51.64 | |
| | | | | | | |
| Adiabtic efficiency | % | | 70% | | | |
| Entropy (Ideal) of moist air | Btu/(lbdry air.°F) | | 0.03024 | | | |
| Enthalpy (Ideal) of moist air | Btu/lbdry air | | 15.71 | | | |
| Enthalpy Difference (Ideal) | Btu/lbdry air | | 0.72 | | | |
| Enthalpy Difference (Real) | Btu/lbdry air | | 1.02 | | | |
| Enthalpy of moist air (calc.) | Btu/lbdry air | | 16.02 | | | |
| | | | | | | |
| Energy Input | Btu/lbdry air | | 1.02 | | | 131,077 |
| Motor | kW/lbdry air | | 0.0003 | | | 38 |
| | | | | | | |
| Specific volume of moist air | ft3/lbdry air | 13.42 | 13.27 | 16.04 | 14.82 | |
| Volumetric flow rate | ACFH | 1,721,300 | 1,701,496 | 2,057,650 | 1,900,480 | |
| | ACFM | 28,688 | 28,358 | 34,294 | 31,675 | |