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Cottonseed Storage

Cottonseed Storage

cottonseed storage index - Cottonseed Storage

M. Herbert Willcut, Mississippi Cooperative Extension Service
William D. Mayfield, USDA Extension Service, Memphis, TN
Thomas D. Valco, Cotton Incorporated, Raleigh, NC.

For every bale of cotton ginned approximately 800 pounds of seed must be handled from beneath the gin stands and placed either in a temporary or long-term storage facility. For long-term storage, aeration is necessary to reduce seed temperature and moisture, and minimize mold growth and insect activity within the seed.

This paper briefly describes structural and aeration requirements to maintain seed quality during storage. A complete discussion of design criteria is reported in the USDA Marketing Research Report No. 1020, “Aeration of Cottonseed in Storage” (Smith, 1975).

Characteristics of Cottonseed

Product Bulk Weight
(lb/bu)		 Specific Count
(seed/lb)
Density
(lb/ft3)		 Volume
(ft3/ton)
Whole Seed
Loose on Conveyor 20 100
<24 ft deep 25 80 32 1,800-2,400
24-50 ft deep 27 75
>50ft deep 30 70
Machine Delinted 35 57 44 2,400-3,200
Acid Delinted 34-37 54 42-46 4,800-5,600
Meal (extracted) 38 53
Hulls 12 167
Oil 57 35
Table 1. Basic properties of cottonseed and cottonseed products.

The bulk density of gin-run cottonseed averages about 25 lb/ft3 and requires about 80 ft3 of storage space per ton. Cottonseed is hygroscopic and therefore absorbs moisture from or gives up moisture to the surrounding air. Storage temperatures below 60°F and 10% moisture content wet basis (mc w.b.) provide best storage conditions.

Whole, fuzzy cottonseed has some unique characteristics which make it difficult to handle with common grain handling facilities. Unlike grain, cottonseed has a variable angle of repose. The angle of repose when an unrestricted pile of cottonseed is formed is about 45°. However, after the seeds have settled, they will bridge — an indication that the angle of repose is greater than 90°.

Cottonseed may be handled by belts, screw conveyors, or pneumatics. Pneumatics is the most effective way to load seed into storage facilities (Shaw & Franks, 1963). The basic handling and storage characteristics of whole cottonseed and cottonseed products are shown in Table 1.

Aeration System Design

Long-term cottonseed storage facilities must be equipped with an aeration system. A properly designed aeration system must have: 1) aeration ducts to distribute air through the cottonseed, 2) properly sized supply pipes to transport the air, and 3) fans to supply the required volume of air for a given static pressure or resistance. Factors that influence the design and selection of aeration equipment are: 1) the size and shape of storage facility, 2) airflow rate per ton of cottonseed, and 3) the maximum depth of stored cottonseed.

A safe airflow rate for cottonseed in flat storage is 10 cubic feet per minute per ton (cfm/ton). In areas with cooler and dryer climatic conditions, lower capacity aeration systems (5 cfm/ton) can be used.

Buildings

Many types of buildings can be used to store cottonseed at gins. Most gins select common clear-span metal frame buildings, but quonset huts and Muskogee houses are also used. Metal frame buildings have a fairly low cost for 1,000-5,000 ton capacities. This type of construction is efficient and has other uses when not used for seed storage.

Quonset huts rely on formed panels for structural integrity and have no steel framework to support the outside skin. When constructed as a true quonset hut, the floor space is not efficiently used. Four to twelve feet vertical concrete wall/foundations are sometimes used to increase the storage volume and minimize wall damage from loading equipment. A quonset hut should be filled uniformly on each side to avoid building deflection.

Muskogee houses generally have roof slopes of 45° and a high ridge line to maximize storage volume while minimizing wall forces of seed “flowing” against the walls. The roof slope and height also require a greater steel superstructure. Muskogee houses are normally used for volumes greater than 6,000 tons. Seed depths of 40-75 ft and lower air flow rates are found in most Muskogee houses.

Cottonseed storage facilities should have moisture-proof concrete floors. Floor loads from seed alone will be approximately 500lb/ft2 for a 20 foot seed depth. Also, floors should have sufficient concrete and reinforcement to support a 60,000 lb truck and loader.

Buildings used to store cottonseed must be designed to withstand the lateral forces exerted by the cottonseed as it is loaded and unloaded. For vertical sided buildings, wall forces depend on seed depth and bulk density. Maximum force and wall failure usually occur at 10 to 20% of wall height above the floor. Forces will be higher during filling because the seed has not yet settled.

Several side and end wall failures have recently occurred in cottonseed storage facilities. Walls of existing buildings converted to cottonseed storage should be analyzed and strengthened as needed before filling with seed. Lining the interior walls with ¾-inch plywood helps prevent damage to the outside building walls, facilitates clean out, increases wall strength, and reduces settling pressure on otherwise exposed framework. The top of this inside wall should be sealed to prevent seed and lint from accumulating between the walls.

Aeration Ducts

Aeration ducts distribute air through the seed mass. In some storage facilities, the ducts are poured into the concrete floor and covered with a perforated plate. In others, perforated, half-round or round ducts are placed on top of the floor. Various types of homemade ducts are being successfully used for cottonseed aeration. Corrugated culvert material can be cut into half-circles with legs welded on to hold the half culvert above the floor, creating a pathway for the air. Wood framed ducts with hardware cloth to retain the seed can also be used.

Duct Diameter (in) Surface Area1(ff2/lin.ft)
16 2.09
18 2.36
20 2.62
22 2.88
24 3.14
26 3.40
28 3.66
30 3.93
32 4.19
34 4.45
36 4.71
38 4.97
40 5.23
42 5.50
44 5.76
46 6.02
48 6.28
50 6.54
1. Double surface area for round duct.
Table 2. Surface area of corrugated half-round ductling.

Two important considerations for cottonseed aeration duct design are the duct open surface area and the duct cross-sectional area.

Open surface area is defined as the open area along the duct surface through which air can pass. It is generally described as a percent of the total area. Ducts should have at least 10% total open surface area; 15% is better. Pressure losses can be held to a minimum by limiting the velocity of the air into the duct (face velocity) from 10 to 15 ft/min.

The air velocity within an aeration duct should range from 1,500 to 2,000 ft/min. This will minimize static pressure losses and fan power requirements. Air velocities should be kept below 2,500 ft/min to avoid transporting seed within the duct.

Ducts should be evenly spaced on the floor of a seedhouse to keep the airflow path uniform within the seed mass. Duct spacing is determined by the depth of seed and the airflow rate used. For depths less than 20 ft a spacing no greater than the seed depth is preferred and should not exceed 1 1/2 times the cottonseed depth (Smith, 1975).

Ducts installed across the width of a rectangular building allow aeration to be started as soon as the first duct is covered with seed. The airflow can be concentrated in selected areas to remove hot spots that sometimes develop. For extra wide storage buildings, a central manifold pipe can be used to reduce duct length.

Manifold and Supply Pipes

An air manifold and supply lines distribute air between the fan and the aeration ducts. Air velocities up to 3,000 ft/min are used. Pipe diameter or cross-sectional area for the designed airflow rate can be determined from Table 3 or by the equation A = cfm / v.

Slide values should be installed between the manifold and supply line. This allows air to be directed to specific ducts during filling or when hot spots are detected.

In some instances, individual fans are attached to a single duct and manifold pipes are not needed. This design uses multiple small fans rather than a single large fan with manifold pipes. The advantages of a single duct fan system include energy reduction, higher efficiency fans, less conveyance losses and lower electrical service requirements.

Fans and Motors

Pipe Diameter (in) Cross1Area (ft2) Airflow (cfm)2
@1500ft/min @2000ft/min @3000ft/min
5 0.14 210 280 420
6 0.20 300 400 600
7 0.27 405 540 805
8 0.35 525 700 1050
9 0.44 660 880 1320
10 0.55 825 1100 1650
11 0.66 990 1320 1880
12 0.79 1185 1580 2370
13 0.92 1380 1840 2760
14 1.07 1605 2140 3210
15 1.23 1845 2460 3690
16 1.40 2100 2800 4200
17 1.58 2370 3160 4740
18 1.77 2655 3540 5310
19 1.97 2955 3940 5910
20 2.18 3270 4360 6540
21 2.40 3600 4800 7200
22 2.64 3960 5280 7920
23 2.88 4320 5760 8640
24 3.14 4710 6280 9420
25 3.41 5115 3820 10230
26 3.69 5535 7380 11070
27 3.97 5955 7940 11901
28 4.27 6405 8540 12810
29 4.58 6870 9160 13740
30 4.91 7365 9820 14730
1. Cross sectional area of inside air duct, not necessarily circular.
2. Airflow through duct in cfm = A x v
Table 3. Airflow rates for selected air velocities and duct across sectional areas.

Most aeration systems are designed with downward airflow through the cottonseed. This helps minimize moisture condensation and accumulation in the top layers of cottonseed, which can occur when warm, moist air moves upward into the cold top layers of seed. In addition, temperature and odor of the exhaust air from the fan can give an indication of cottonseed condition.

To select a fan, the required volume of air to aerate the number of tons of seed must be known as well as the static pressure. Static pressure or resistance to air flow is measured in inches of water and depends on the depth and density of the seed. Static pressures for different airflow rates and cottonseed depths are shown in Table 3. Knowing airflow and static pressure, a fan and motor can be selected from a supplier’s catalog to meet the design specifications.

Aeration Examples

To better understand cottonseed aeration systems, the following typical systems are considered:

Depth of Cottonseed (ft)
Airflow Rate (cfm/ton)
Static Pressure (in H20)
Power Required (hp/100 tons)
10
5.0
1.0
0.1
7.5
1.1
0.2
10.0
1.6
0.4
12
5.0
1.0
0.1
7.5
1.6
0.4
10.0
2.3
0.7
14
5.0
1.3
0.2
7.5
2.1
0.6
10.0
3.1
1.2
16
5.0
1.7
0.2
7.5
2.8
0.6
10.0
4.0
1.2
18
5.0
2.2
0.3
7.5
3.5
0.8
10.0
5.0
1.5
20
5.0
2.7
0.4
7.5
4.5
0.9
10.0
6.3
1.7
22
5.0
3.3
0.5
7.5
5.3
1.1
10.0
7.7
1.9
24
5.0
4.1
0.6
7.5
6.4
1.4
10.0
9.5
2.5
26
5.0
4.7
0.6
7.5
7.6
1.6
10.0
11.6
2.9
28
5.0
5.5
0.8
7.5
9.0
1.7
10.0
13.5
3.4
Table 4. Airflow resistance and fan power requirements for cottonseed at different storage levels and airflow rates.

Example 1. Design a storage and aeration facility for 1,200 tons of cottonseed. The cottonseed will be carried into storage with an existing small-pipe seed-handling system and unloaded with a front-end loader:

I. Building
Volume required for 1,200 tons of cottonseed is 96,000 ft3. The storage structure could be 60 ft wide and 100 ft long with an eve clearance of 18 ft to accommodate the front-end loader. It will be filled to a depth of 16 ft. The building should be constructed 120 ft long to provide access for unloading without using a bulkhead door.

II. Aeration System
System capacity = 1,200 tons of cottonseed. Design airflow rate = 10 cfm/ton.

Total air volume needed: 1,200 tons x 10 cfm/ton = 12,000 cfm.

Duct layout as shown in Figure 1 with 5 rows spaced 20 ft apart, 50 ft long (Maximum duct spacing: 1.5 x 16 ft = 24 ft).

Duct Design: Each duct carries 1/5 of the total air or 2,400 cfm per duct. Total surface area needed for a 10 ft/min face velocity is 240 ft2 (2,400 cfm/ 10 ft/min = 240 ft2) or 240 ft2/ 50 ft = 4.8 ft2/linear ft. From Table 2 select a 36-inch diameter half-round or 18-inch diameter round duct.

Supply pipes: Each supply pipe carries 1/5 of the total volume of air or 2,400 cfm. From Table 3 select an 18-inch diameter supply pipe that will carry 2,651 cfm at a velocity of 1,500 ft/min.

Manifold pipes: The manifold pipe carries air for two supply pipes or 4,800 cfm. From Table 3 determine that a 21-inch diameter manifold will handle 4,800 cfm at 2,000 ft/min. Because larger velocities can be tolerated for short distances and odd sized pipe is not common, a 20-inch diameter manifold could be used.

Fan and motor: At a cottonseed depth of 16 ft and an airflow rate of 10 cfm/ton the static pressure to be overcome by the fan is 4.0 inches of water and 15 horsepower requirements (Table 4).

From a manufacturer’s catalog, select a centrifugal fan with a 15 horsepower motor which would deliver 12,500 cfm at 4 inches static pressure. An alternative would be to use five small fans and motors capable of 2400 cfm per fan. This would require 3 hp motors which could be operated with 220 volt single phase power and eliminate the manifold pipe.

Example 2. Design a storage and aeration system for 2,000 tons of cottonseed. The cottonseed will be carried into storage with an existing small-pipe handling system and unloaded with a front-end loader.

I. Building
The storage structure would be 60 ft wide and 160 ft long with 18 ft eve clearance. It will be filled to an average cottonseed depth of 17 ft. The building should be constructed 180 ft long to allow access for unloading.

II. Aeration System
System capacity = 2,000 ton. Design airflow rate = 10 cfm/ton.

Total air volume required is 2,000 ton x 10 cfm/ton = 20,000 cfm.

Duct layout is shown in Figure 2 with 850 ft rows spaced 20 ft apart.

Duct Design: Each duct carries 1/8 of the total air or 2,500 cfm per duct. Total surface area needed for a 10 ft/min face velocity is 240 ft2 (2,500 cfm/ 10 ft/min = 250 ft2) or 250 ft2/ 50 ft = 5.0 ft2 linear ft. From Table 2 select a 38-inch diameter half-round or 20-inch diameter round duct is needed.

Supply pipes: Each supply pipe carries 1/8 of the total air volume (1/8 of 20,000 cfm = 2,500 cfm. From Table 3 select an 18-inch diameter supply pipe.

The manifold pipe carries air for two supply pipes or 5,000 cfm. From Table 3 determine that a 22-inch pipe will carry 5,280 cfm at 2,000 ft/min. Because larger velocities can be tolerated for a short distance, a 20-inch manifold would be acceptable.

Fans and motors: At a cottonseed depth of 17 ft and an airflow rate of 10 cfm/ton (Table 4), the static pressure requirement is 4.5 inches of water (interpolate between 16- and 18-ft depths).

From a manufacturer’s catalog, select a centrifugal fan with a 10-horsepower motor that would deliver 10,000 cfm at 4.5 inches of water, static pressure. Two fans and motors would be required for this system.

aeration ducts 1200 tons - Cottonseed Storage
Figure 1. Aeration duct layout for 1,200 tons of stored cottonseed.
aeration ducts 2000 tons - Cottonseed Storage
Figure 2. Aeration duct layout for 2,000 tons of stored cottonseed.

Loading and Unloading

The lobed blower normally used to handle cottonseed from the seed auger from inside the gin to the temporary house can usually be used to transport seed to a storage building. These high pressure, low volume blowers can move cottonseed for long distances if they are properly sized. Valves located inside the seed house and movable pipe and seed deflectors can direct seed to the desired locations and effectively fill storage facilities with minimum management and cost.

Removing the cotton from the storage facility can be accomplished in several ways. A bucket loader dumping directly into a truck is the most common method at gins. Others use a lobed blower system to load cottonseed into rail cars, trailers or the temporary overhead house for staging shipments in open top trailers. Another effective method of elevating cottonseed is an old fashioned drag-chain conveyor or belt conveyor.

Emergency Storage

Ginners have successfully stored dry seed in all types of buildings for short periods. Seed cotton modules or cotton bales stacked along the edges to form walls in machinery sheds have been used to store cottonseed. Old gin buildings can also be used. Module builders have been tried, but the sides of the cottonseed modules will slough unless they are restrained. A module builder can be lined with a plastic bag which is supported with straps in the center as well as around the circumference and used to store dry seed for short times. These bags cost about $100 and store about 25 tons. Storage in modules requires significant labor to fill and to load the seed from the module. A module of cottonseed is too heavy for a module truck or trailer to move. Total costs would be about $10 per ton. Since aeration is impossible and the seeds are compacted, wet seed would be damaged quickly in a module.

Managing Aeration Systems

Cooling cottonseed as it comes from the gin, often at temperatures above 100°F, and preventing hot spots and moisture buildup requires good aeration management. Aeration to remove ginning heat usually is started as soon as the first lots are in storage. Although there are times when very little cooling occurs, experience has shown that seed quality can be maintained by running the aeration fans continuously until the facility is filled and conditioned.

Being hygroscopic, cottonseed will absorb moisture from or give up moisture to its surrounding air. Once gin heat has been removed, aeration fans should not be operated during high humidity periods of rain or fog. Ideally, cottonseed in storage should be cooled to 50° to 60°F by selecting cool dry days to run the fans.

Usually no additional aeration is necessary once the seed has been cooled to the desired temperature. Even so, seed temperatures should be monitored throughout the storage period, since hot spots occasionally develop. Many seed storage facilities are equipped with temperature monitoring systems. Seed temperature monitoring can also be accomplished by placing a thermometer in the exhausting aeration airflow and/or by hand inserting thermocouple probes attached to steel rods or electrical conduit.

References

Shaw, Scott C. & Gerald N. Franks. 1963. Cottonseed handling at gins. USDA Prod. Res. Rpt. No. 66. Washington, D.C. 23 pp.
Smith, Lloyd L. 1975. Aeration of cottonseed in storage. USDA, Marketing Res. Report No. 1020. Washington, D.C. 41 pp.

For further information please contact:

Herbert Willcut, Mississippi Cooperative Extension Service, 601 -325-3103 William D. Mayfield, USDA ES, Memphis, TN 901-766-7690 Thomas D. Valco, Cotton Incorporated, Cary, NC 919-678-2368

This publication was printed by Cotton Incorporated and is available free of charge from Cotton Incorporated, 6399 Weston Parkway; Cary, NC 27513.

The statements, recommendations and suggestions contained herein are based on experiments and information believed to be reliable only with regard to the products and/or processes involved at the time. No guarantee is made of their accuracy, however, and the information is given without warranty as to its accuracy or reproducibility either express or implied, and does not authorize use of the information for purposes of advertisement or product endorsement or certification. Likewise, no statement contained herein shall be construed as a permission or recommendation for the use of any information or product that may infringe any existing patents. The use of trade names does not constitute endorsement of any product mentioned, nor is permission granted to use the name Cotton Incorporated or any of its trademarks in conjunction with the products involved.

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