Example 3: Product Drying

Here we have a room used to remove water vapor from such products as cattle feed mixes, nylon or rayon cord for tires, raw plastic material, granular chemicals, raw paper stock, cardboard stock for coatings, or other similar products.

In this example, the room is used for drying cattle feed mixes, which are contained on drying carts that stand in the room until the specified level of dryness is attained.

Space condition requirements and product movement rate are determined by the manufacturer.

Physical Facts

1. Drying room size - 40' x 65' x 16'
2. Outside design condition - 93° F db; 73° F wb
3. Controlled space requirement - 95° F; 15 percent rh (36 gr/lb)
4. One double door: (a) 6' x 7' (b) Opens at 2/hr
5. There are no other openings.
6. There are no workers in room except to bring mix in and out
7. Product movement rate -1500 Ib/hr (i.e., carts with trays of mix are moved into the drying
8. room at the rate of 1500 Ib/hr)
9. Product enters room at 8 percent moisture and leaves at 4 percent moisture. 9. Drying room wall construction - 8" masonry.
10. 350 cfm outside air required by manufacturer.

Problem

To determine the size of the dehumidifier.

Assumptions

1. All physical cracks are sealed.
2. The double door is weather stripped.
3. Two coats of vapor barrier paint have been applied to the wall and ceiling construction of the drying room; the floor is suitably protected against vapor permeation.

Moisture Loads to be Computed

1. Product load
2. Permeation load
3. Door load.

Product Load

Since the product will lose 4 percent moisture (by weight) and there are 1500 pounds of product each hour:
1500 lb./hr × (8 % - 4%)  = 60 lb./hr water removal

Since one pound of water equals 7000 grains, then:
60 × 7000 = 420,000 gr/hr product load.

Note that the time needed to reduce the material to a 4 percent moisture level would have to be given or experimentally determined. These data would determine the amount of material to process and the size of the drying chamber needed.

= grains per hour

Door Load

Total Moisture Load

            420,000 gr/hr     Product Load
            170,742 gr/hr     Permeation Load
            1,839 gr/hr         Door Load     
            592,581 gr/hr     Total Moisture Load

The 350 cfm outside air requirement will be considered at a later stage in the calculation.

Proceed with calculation as follows:

Recommendation

The Bry-Air VFB-75 Dehumidifier, rated at 7500 cfm, should be adequate. However, the first step should be to determine if this Dehumidifier has enough capacity to handle the 350 cfm outside air in addition to the moisture load in the drying room.

If the dryer has a delivery rate of 7500 cfm and 350 cfm of outside air is to be introduced, there remains 7150 cfm from the conditioned space. Tabulate this air mixture as follows:

350 cfm × 113 gr/lb. =   39,550
7150 cfm × 36 gr/lb. =   257,400
7500                             296,950

Reference to Chart 1, Appendix 4, shows that air entering the dryer at 39.5 gr/lb. would leave at 17 gr/lb.

The computed moisture load is 592,581 gr/hr. Therefore, the VFB-75 is adequate to handle the moisture load.

Bry-Air Dehumidifier Calculation Sheet

Project: Example III – Product Drying

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