Farm Progress

Conserve fuel energy needed to dry cotton during ginning

April 21, 2006

7 Min Read

The first seed cotton dryer was developed by U.S. government researchers at Stoneville, Miss., and installed at a gin in Louisiana about 1926.

The first dryer was constructed of wood and insulated pipes. Steam coils heated the drying air.

Another version was built of metal and covered with a layer of insulating board.

Although dryers are used in virtually every U.S. gin today, very few are insulated. The dryers enabled gins to process cotton but at an added expense.

Since drying is energy intensive, the costs have increased over time. In 1979, A. C. Griffin Jr. surveyed 230 Mid-South gins and found that 312 cubic feet of natural gas was used per bale of cotton ginned.

In a similar study done in 1987, I reported that gins used 248 cubic feet of natural gas per bale of cotton.

Valco surveyed 176 gins in 2001 and reported that gins paid about $1.26 per bale for drying fuel. In 2004, fuel costs averaged $1.96 per bale.

Larry Stalcup reported that dryer fuel costs varied from less than $2 to $10.45 per bale in Texas in 2000.

The cost of drying cotton is rapidly escalating due to the cost of fuel. The price of natural gas increased from 95 cents per 1,000 cubic feet in 1975 to $3.28 in 2001. In July 2005, the futures price for natural gas for November 2005 was $8.55, a 261 percent increase over 2001 prices but similar to 2004 prices.

Unfortunately, hurricanes Katrina and Rita created enormous problems with natural gas and caused further price increases. In fact, prices peaked at over $14 in October 2005 before dropping to less than $12 in November.

Without the impact of the hurricanes, natural gas prices would have increased 261 percent from 2001 to 2005, and gins likely would have paid about $3.29 per bale for drying in 2005. The hurricanes likely caused fuel costs to exceed $5.00 per bale for the 2005 crop that was ginned in October and November.

Unusually dry weather during harvest helped lower fuel costs because of the reduced need for drying. Regardless, energy continues to be a major issue.

With the rapidly escalating costs of energy, proper management as well as improvement of drying systems to minimize fuel usage deserves serious attention. Gins can take a number of positive actions to conserve fuel as described in the following paragraphs.

  1. Insulate the drying system. About 30 percent of the heat lost in drying systems by radiation and convection can be retained by covering the hot-air pipes and perhaps the dryer with thermal insulation, according to A.C. Griffin Jr. and R.E. Childers.

    Insulation is relatively inexpensive and can be installed by gin personnel although commercial installation is available.

    Money spent on dryer fuel can be reduced about 30 percent by proper insulation. In 2004, savings would have been 59 cents per bale; however, if dryer fuel costs $3.29 per bale as estimated for 2005, then insulating could save nearly $1 per bale. For areas where the fuel prices were spiked, savings would have been $1.50 per bale in 2005.

    Insulating a typical two-stage drying system would likely cost $10,000 to $15,000 for material and labor, depending on pipe length, number of elbows, dryer design, labor cost, etc. The cost should be paid back after 10,000 to 15,000 bales are ginned.

    Note that insulation should be approved by the Underwriters Laboratory for extended use at temperatures above 1000 degrees F.

    Roy Childers of Lubbock, Texas, successfully used insulation approved for extended use at temperatures above 500 degrees F.

    Highest temperatures occur close to the burner and decrease rapidly with distance. Insulating the hot-air ducts/pipes between the burner and the dryer provide the greatest benefit while insulating the dryer and ducts downstream are of less benefit.

    External protective jackets can be used to protect the insulation. Also, double-walled insulated pipes and elbows can be used.

  2. Avoid unnecessary drying. Use only the amount of drying required to lower the fiber moisture content to 5 to 6 percent. Excessive heating of fiber above about 220 degrees Fahrenheit causes excessive fiber breakage and fiber should never be heated above 350 degrees Fahrenheit.

    During periods of low relative humidity, cotton often arrives at the gin at or below the desired fiber moisture content. When this occurs, the dryers should be bypassed. If dryers cannot be bypassed, the burners should be turned off.

    Effective moisture and temperature sensors and controllers are essential to obtain proper dryer control so that unnecessary drying is avoided.

    Some ginners have a tendency to apply the same drying temperature to all cotton regardless of its moisture content. Accurate sensors to measure the moisture before and after drying and to control the dryers are absolutely necessary.

    Moisture measurement and control systems such as those developed by the USDA and marketed as part of IntelliGin by Uster Technologies controls drying and reduces fuel costs about 50 percent — nearly $1 per bale in 2004 and perhaps $2.50 in 2005. The key to that system is accurate measurement of the fiber moisture.

  3. Use properly designed dryers. Burners should be positioned close to the hot air and seed cotton mixpoint to avoid unnecessary loss of heat. Substantial drying occurs in the duct before the seed cotton reaches the actual dryer.

    Dryers should provide adequate retention time to allow sufficient drying. Burner capacity must be sized with the desired cotton flowrate, air flowrate, and moisture removal in mind. Improperly designed drying systems waste energy and cause inadequate or excessive drying.

  4. Adjust the burner flame. The air/fuel mixture should be adjusted until the flame at the end of the burner is bright blue. A yellow flame indicates incomplete combustion, which wastes fuel.

    Another frequent problem is that of long flames that extend far enough to set fire to the cotton. Installing an air-diffusing flame holder that has been properly designed for the specific burner can often control such flames.

  5. Maintain proper burner control. Many gins have burners with antiquated and unsafe control systems. For protection of both people and equipment, burners should be equipped with control systems that meet all current construction and safety codes, and obsolete on/off temperature controls should be discarded.

    A maintenance program must be rigidly followed to keep the controls in perfect condition.

    Not only does a modern control system yield obvious safety advantages, it also gives the drying system much more rapid and accurate response capability. If the system is able to change temperatures rapidly in very small increments, fuel can be conserved by using only what is required at a given moment.

    Some older burner control valves lack adequate sensitivity to precisely control gas flow and should be replaced.

    Among the control system components needed to achieve uniform drying with maximum fuel savings are double safety shutoff valves between the gas supply and burner, an automatic shutoff mechanism for flame failure, automatic ignition, an automatic purging device, a modulating valve that will hold the temperature within 10 degrees Fahrenheit of the set point, a temperature-limiting device at the mixpoint of cotton and heated air, and moisture sensors before and after drying.

    A limiting thermocouple must be installed within 10 feet of the cotton-air mixpoint to prevent temperatures from exceeding 350 degrees Fahrenheit. This temperature-limiting device can prevent unnecessary fiber damage.

    The temperature-sensing device for the controls should be installed at the first turn on the top shelf of a tower dryer.

  6. Use heat recovery devices. Warm exhaust air is normally emitted into the atmosphere. Research at the Southwestern Cotton Ginning Research Laboratory suggests that the warm exhaust air from the first stage of drying can be fed into the intake of the second stage of drying, but air-cleaning devices should be used to remove leaf trash, lint fly, dust, etc., from the air to avoid re-entrainment of the debris with the cotton.

This process of reusing the air can result in fuel savings of 20 percent.

Warm-air exhaust lines can act as heat exchangers. A heat exchanger can be as simple as two concentric pipes in which warm air is exhausted through the inner pipe and fresh air is passed in the opposite direction through the outer pipe. The fresh intake air does not make physical contact with the exhaust air but is warmed by it.

A fuel savings of 3 percent to 10 percent is possible from this warming effect.

Another study showed that significant energy savings (about 15 percent) could be obtained by locating heating system air intakes near the roof level to recover some heat loss from drying systems and motor cooling.

W. Stanley Anthony recently retired as Supervisory Agricultural Engineer and Research Leader, Agricultural Research Service, USDA, U. S. Cotton Ginning Research Unit, Stoneville, Miss., and now serves as a consultant to the cotton industry.

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