Author: John Shumbera
Suggested Courses: Agricultural
Engineering, Environmental Engineering
Level: Sophomore, Junior &
Cotton gins produce a medically harmless, but
nuisance dust in their operations. Cotton is grown in the United States in a belt
across the southeastern and southwestern states. Harvesting of this cotton occurs over a
three month period in the fall of each year. What is harvested is cotton lint
attached to cotton seed. There are two types of harvesting methods, a spindle picker
and a stripper harvester. Stripper harvesting is very economical with specific
varieties of cotton plant, but is not suitable for all varieties. One difference is
that the stripper harvester retrieves cotton tufts from the plant with significant
quantities of leaf and stalk matter. Both types of cotton are presented to the gin
where removal of the lint from the seed is accomplished on modern machinery derived from
the cotton gin invented by Eli Whitney. A secondary function of the gin is
partial removal of dust, leaf and stalk matter to the extent possible without damaging the
fiber. This process involves the transportation of cotton lint along with the
foreign matter in an air stream at about 3400 feet per minute (fpm). The cotton
seed is separated from the lint at the gin stand. The cotton lint is separated from
the remaining foreign matter downstream. It is here that a problem exists. The
cotton is emptied into the bale press for compaction and shipment. The dust and
other matter has to be collected. Because of the fineness of the remaining matter,
no economically feasible system presently available can trap 100% of the dust. When
this dust escapes from the gin it falls out in the neighborhood on houses, cars, and the
like. This dust is not medically harmful, but it is a nuisance to those
neighbors. The Environmental Protection Agency (EPA) has responded to complaints
from the neighbors with a regulation on the amount of nuisance dust that can be released
from a gin. You are an engineer working for John's Gin and Warehouse in middle
Texas. The task you have been assigned is to help the gin meet the new regulations
set by the EPA.
Contact with the regulatory commission gives you an
allowable nuisance emission rate of 2.24 lbs. of particulate per bale. A bale is
nominally 500 lbs. of cotton. The ginner has said that the gin operates at 14 bales
per hour. Past experience with the gin stands shows that at this production rate,
the set of gin stands require 100,000 ft3 per minute of air flow. It is
this air stream that must be cleaned to regulatory standards. Average waste for the
two different harvesting methods are:
203 lb. per bale
740 lb. per bale
The devices available to you for separation
of dust from the air stream are a cyclone system or a bag house. In this
application, the cyclone system would consist of a preseparator and 14 cyclone
units. Each cyclone is designed for the particular gin where the system is to be
used. Cyclones have a tapered conical shape (the tip is pointed downward) and
require an entry air velocity of 3400 linear feet per minute. The efficiency of a
cyclone is fixed by the particle sizes of the contaminants in the air stream.
Efficiency, h, is a function of P1, the smallest particle size, and P2,
the average particle size, in mm. The equation is as follows.
This efficiency represents waste exiting from the cyclone
over the waste fed to the cyclone.
III. Numerical Problems
1) Calculate the entry diameter of each cyclone unit.
2) For a smallest particle size of 8.56 mm and an average particle size of 62 mm, what is
the cyclone efficiency?
3) What is the amount of waste exiting from the cyclone for spindle picker feed cotton and
for stripper harvester feed cotton?
IV. Narrative (continued)
The preseparator added in line to this system has the
effect of reducing the quantity of waste fed to the cyclones. For spindle picked
cotton the reduction in waste is a factor of 2.5 and for stripper harvested cotton the
reduction is 7.25. Preseparators cost $0.30/cfm and cyclones cost $1.00/cfm.
As the other alternative to this system, the ginner could
install a bag (filter) house. Essentially, this system has the air flowing into a
sealed chamber whose only exit ports are holes in the floor from which fabric filter bags,
perhaps twenty or thirty, are suspended. Mechanisms knock down dust collected on the
bag walls regularly. The cost of installing a bag house with the associated
machinery is $8.00/cfm. The bag house manufacturer informs you that the bag system
will filter dust emissions to 1.0 lb. per bale for spindle picked cotton and to 1.2 lb.
per bale for stripper harvested cotton.
4) Calculate the emission levels (lb./bale) for the
cyclone system with a preseparator installed.
5) What is the cost of a cyclone system with preseparator
for this gin?
6) What is the cost of a bag house system for the gin?
Discussion with the gin owner reveals that there is only a
small financial margin in a ginning operation. Therefore, it is very important that
costs for the new equipment be kept to a minimum. A gin's profits for a year are on
the order of magnitude of $100,000. The gin owner just heard of a gin across the
state who was fined $100,000 by the EPA for consistent violation of the dust emission
The engineer knows that any nuisance emission causes
problems for the gin's neighbors. His situation is that he has his employer to
consider with the need to be as cost effective in capital expenditures as possible, he has
the regulatory requirements to meet, and he has the public interest in mind with respect
to the nuisance aspects of even small dust discharges.
VI. Ethical Problems
7) Which design should you recommend to the gin owner?
8) Where do your primary responsibilities lie as an
9) If you were told that continuing complaints to the EPA
by persons affected by the nuisance dust was already bringing hearings on a new stricter
standard and that a new standard will likely follow, will that change your decision?
1)The total volume flow rate of air, Q, is 100,000
cfm. The volume per cyclone is 100,000 / 14 = 7143 cfm. Assuming uniform
velocity distribution across the entry diameter, the entry cross sectional area, A, and
velocity, V are related simply by the formula:
Thus, A = 7143 / 3400 = 2.1 ft2,
the diameter is:
2) Solving the equation given for efficiency,
P1 / P2 = 8.56 / 62 = 0.138. This squared is 0.019. The
efficiency is 1 / 1.019 = 98%.
3) Cyclone efficiency yields 2% waste exiting
from the system. The waste values per bale for the two harvesting methods is:
0.02 X 203 lb. / bale = 4.06 lb. / bale
0.02 X 740 lb. / bale = 14.8 lb. /bale ( stripper)
4) Reducing in quantity of dust fed reduces
the quantity of dust exiting the system. The assumption is that the cyclone
efficiency is constant over total dust volume. This is generally not true, at least
in the extremes. The given values for dust reduction by the preseparator yields the
5) The preseparator component of cost is
$0.30, and the cyclone component is $1.00 per cfm of air processed. The total cfm of
process air is 100,000 cfm. The total cost is $130,000.
6) The bag house system costs $8.00 per cfm
giving a total cost of $800,000 for the installation.
7) The gin owner may not be able to stay in
business and purchase the bag house. Nevertheless, the bag house is the clear
choice when the ethical consideration of the effect of dust on the gin's neighbors is
taken into consideration. Your approach is to see first if there is an alternative
that may achieve the dust level of a bag house at lower cost. One such consideration
might be that of having two preseparators before the cyclone.
8) Your responsibilities are divided between
what you owe to your employer and what you owe to the public and its well being.
Your task when facing ethical choices is to work diligently to see that an engineering
solution may resolve an impasse, but if not then to present the sides involved the
information they need and to assist them with a mutually agreeable and responsible
9) The engineer will not make the decision to
purchase a bag house. That is done by the gin owner. Nevertheless, the
engineer has the responsibility to press the case strongly for the solution which gives
immediate and near term relief from the problem. Then the engineer must present the
case for a long-term solution. In fact, the engineer may have a responsibility to
look not just to the next generation of regulations, but to the generation after
that. Here the question becomes one of not making a choice between the two
alternatives, but finding yet another and better alternative.
VIII. Additional Ethical Problems and
1. List 2 conflicting obligations faced by
Hint: Refer to your code of ethics.
While you have an obligation to protect the
health and welfare of the public, engineers also have an obligation to be loyal and
faithful agents to their employers. Here the issue revolves around the obligations
to the neighbors to the gin the obligations to the gin owner. One can argue that the
dust is harmless,since it does not produce any danger to the public. The engineer
then must provide the best dust abatement equipment that he can while working under the
economic constraints of the ginner. One can also argue that in this situation that
the dust does create enough of a nuisance that the neighbors should not be subjected to
it. In this case, the engineer should do everything possible to protect the public,
regardless what the ginner wants.
2. Based on your previous calculations and
the data provided, show how the two options (the bag house and the cyclone) meet each
Both designs meet the obligation to protect
the public. However, the bag filter seems to be a better system than a cyclone by
itself. But, can be seen from the calculations, adding a preseparator to the
cyclone, is the better choice.
Both designs fail to meet the obligation to the employer. The bag house is clearly
to expensive for the ginner to purchase. The cyclone design would be the best choice to
recommend. It is both economically feasible for the ginner and it reduces the dust
admissions to a minimum.
3. A creative-middle-way solution is an
alternative solution to a problem with conflicting issues. This solution usually
will be able to meet both obligations in conflict better that the original
solution(s). From your calculations and the data presented in the case, list an
option that would be a creative-middle-way solution for the engineer.
Here the students can come up with several
ideas, including newer technology and designs. The obvious creative-middle-way solution
would be to use the cyclone design with more than one preseparator.
4. Of the three choices the engineer
has (the bag house, cyclone, and the creative-middle-way solution) which one is the best?
Here the creative-middle-way solution should
be the best choice, although it doesn't have to be. This choice will better meet the
conflicting issues in the case. Engineers should strive to meet the conflicting
issues in all situations that they face, though this is not always possible. In any
event, engineers should always act in a professional way to ensure that they are upholding
the ethics and integrity of their profession.