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Example 2: Standby Warehouse
In This Section
Moisture damage in a standby or storage warehouse can be avoided by surrounding the machinery, equipment, or material with dry air.
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Physical Facts
- Area to be conditioned - 210' x 176' X 45' = 1,663,200 cubic feet
- Outside design condition - 95° F db; 77 F wb
- Controlled space requirement* - 85°F db; 40 percent rh
- No physical openings nor appreciable amount of door openings or closings specified
- No people working in the area
- Construction - 8" masonry.
Problem
To determine the size of the dehumidifier required to maintain standby conditions.
Assumptions
- All physical cracks are sealed and the floor properly vapor-proofed.
- If the room is completely vapor-proofed, use Table 4 on page 8.
- Two coats of vapor barrier paint has been applied externally for metal clad construction.*
* External application is recommended because:
Outside walls are usually easier to access than inside walls for paint application.
Coating the outside walls discourages water permeation into the wall and thus minimizes water accumulation in the wall structure itself.
Space Moisture Loads to be Computed
- Permeation load
- Moisture load.
The Permeation Load is the only moisture load involved in this example.
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= Grains per hour. |
| Where: | ||
| V | = 210 × 176 × 45 = 1,663,200 ft³ | |
| C | = 14 = constant | |
| G | = 58 grs/lb. Outside design web bulb of 77°F gives 130 grs/lb. Controlled space requirement of 85°F db, 40% rh yields 72 Grs/lb from a standard Psychrometric chart. Therefore, 130 – 72 = 58. | |
| F1 | = 1.54 From Table II – Factor for moisture difference of 63 gr/lb | |
| F2 | = 0.24 from Table III – extrapolated as straight line for a volume of 1,663,200 cubic feet. | |
| F3 | = 1.0 From Table IV – Factor for 8” masonry | |
| F4 | = .75 From Table IV – Factor for 2 coats paint | |
1,663,200 |
× 58 × 1.54 × 0.24 × 1.0 × .75 | = 1,910,019 grs/hr |
Refer to schematic below which shows the load requirements and drying system.
X = C × |
gr/hr |
÷ (S – G) |
| Where: | ||
| X | = cfm Delivery air rate from dryer to space | |
| C | = 14 = constant | |
| S | = 72 = grs/lb moisture requirement of controlled space. | |
| G | = grs/lb of air leaving dryer. |
Enter curve at 72° “Inlet Moisture Condition”. Interpolate “Inlet Air Temperature Curve” between 75° and 95° and find “leaving Moisture” at 33 grs/lb.
X = 14 × |
1,910,019 |
÷ (72 – 33) |
X = 11,427 cfm
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Therefore, 11,427 cfm of air (33 gr/lb) from the dehumidifier is needed to maintain a grain level of 72 gr/lb.
In a building of this size and shape, air distribution ducts are practical for effectively spreading the air so it can return to a common point and re-enter the dehumidifier.
Recommendation
Use one Bry-Air VFB-150 Dehumidifier at 12,500 CFM in this standby warehouse example with a fan sized to handle the necessary static pressure of the duct system.
Bry-Air Dehumidifier Calculation Sheet
Project: Example II – Standby Warehouse
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