PN-260-E
February 2001
Production Rates and Coupling Stocks
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1. Production Rates and Coupling Stocks
This note contains an analysis of the production rates, coupling stocks and production
capacities of a production system. In order to illustrate the situation, we use as reference a
simplified version of the case National Cranberry Cooperative.
NCC.- NCC is a Cooperative specialising in the harvesting and processing of cranberries for
subsequent sale. Cranberries can be harvested by two methods: wet or dry. Using the dry
harvesting method, the bushes are shaken and the fruit falls to the ground, from where it is
collected manually. Using the wet harvesting method, the land is flooded with water, so that
when the cranberries fall, they remain floating on the water. The water (with the cranberries)
is then pumped to a truck, where it is separated from the cranberries by a filtration process.
The unit of measure for cranberries is the barrel (bbl). The cranberries are loaded onto trucks
belonging to the Cooperative and taken to the processing plant. In the processing plant, the
trucks unload into temporary holding bins. In the bins, the wet cranberries are stored in a
dedicated area with a storage capacity of 3,200 bbls. The dry berries are stored in another
area with a storage capacity of 4,250 bbl’s.
Trucks arrive from 7 o’clock in the morning to 7 o’clock in the evening, with an average arrival
rate of 20 trucks per hour, each truck transporting a load of 75 bbl’s. The ratio of wet
cranberries to total cranberries is 58% and dry cranberries account for the remaining 42%.
The fruit is used in several types of final product. Cranberries can be sold bulk in tanks for
freezing and subsequent consumption as fruit; they can be sold bulk to liqueur manufacturers;
or they can be packaged fresh, which enables them to be consumed with their aromatic
properties intact up to two months after harvesting. The cranberries harvested using the dry
method are always sent to liqueur manufacturers. The wet cranberries, once dried, are shipped
for consumption as fruit, either immediately (packaged) or later (frozen). The percentage of
frozen and packaged cranberries is the same, 50% of the wet cranberries.
This technical note was prepared by Professor Josep Riverola. February 1990.
Copyright © 1990 IESE. To order copies contact IESE Publishing via www.iesep.com. Alternatively, write to iesep@iesep.com,
send a fax to +34 932 534 343 or call +34 932 534 200.
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Last edited: 7/9/13
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PN-260-E
Production Rates and Coupling Stocks
The production operations are shown in the flowchart in Figure 1. Operations are grouped
according to the machine, or processor, that performs them (the processors are represented as
shaded rectangles in the diagram). The number of processors is indicated at the bottom
of each shaded rectangle. For example, the indication 3 x means that there are three
processors that can perform any of the operations stated in the inserts.
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Thus, the dryer can dry cranberries to be sent to packaging or freezing. The maximum
operating speed in the relevant processor is indicated in the operation insert in bbl’s per
hour. The processor, once started, operates at a speed somewhat lower than the indicated
speed, until it is stopped. In most cases, the processor’s operating speed is the same for all the
different types of cranberry (operations) that may be processed. The exception is the drying
operation, which is performed at different speeds, depending on whether the cranberries are
for packaging (150 bbl’s per hour and dryer) or for freezing (200 bbl’s per hour and dryer).
The plant starts operating at 9.00 a.m.
Figure 1 is a condensed and, to a certain extent, misleading representation of the true nature
of the situation. In practice, and in the case of NCC as well, the company handles a collection
of both finished (or final products) and intermediate products (or components). Finished
products are those that are sold to outside clients. Intermediate products are those that are
used as materials in the manufacture of other products.
The company also handles a collection of processors able to carry out a certain type of more or
less specialized operations. For example, in NCC there are the drying processors (dryers), the
dechaffing processors, the separators, etc. The dryer, for example, can dry (operation) both
berries to be bagged and those that will be packaged. In fact, the dryer is only able to remove
moisture from a fruit. The amount of moisture, and therefore the application of the dryer,
depends on the characteristics of the drying operation applicable to each type of product.
The primary data in all this process are basically of a technical nature and concern the operations
that comprise a product’s production process. These data are the result of the decisions taken
during the SBA Process Engineering activity. In this activity, the technicians decide on the
appropriate production process for obtaining the desired item. For this, they specify the so-called
operations diagram, in which the individual operations the product must undergo are specified.
The choice of individual operations is based on a compromise between ease of use, cost of the
operations available and suitability of the operations for the purpose of the production. This is so
because the company has a series of processors and their possible operations as part of its assets.
Of course, other operations in other processors, purchased or subcontracted, are also possible, but
the cost and ease of use may be very different. All these conditioning factors must be taken into
account by the technician designing the production process.
Once the process design has been completed, another department, often the Methods
Department, evaluates the times required to perform each of the operations in the chart,
considering each operation in isolation from the rest. This evaluation must be based on the
existing processor’s technical features and gives an operation performance time and therefore
a maximum instantaneous operation speed in a particular processor. For example, NCC’s
Methods Department calculated that a dryer could operate at a speed of 200 bbl’s/hour when
drying packaged cranberries or, in other words, that the processor time (quantity of work)
required for 1 bbl was 0.005 hours.
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Production Rates and Coupling Stocks
Figure 1
Process flow chart
1500 bbl/h
POL
Dumping
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Holding Bins
Destoning
1500 bbl/h
Dry cranberries
Wet cranberries
for freeziing
Wet cranberries
for packaging
Dechaffer
Dechaffing
1200 bbl/h
Dechaffing
1200 bbl/h
2x
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PN-260-E
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Separator
Drying
200 bbl/h
Selection
400 bbl/h
Selection
400 bbl/h
Selection
400 bbl/h
Drying
150 bbl/h
3x
Drier
3x
PN-260-E
Production Rates and Coupling Stocks
Figure 2
Process diagrams and resources existing in NCC
a) Process diagrams
Holding
Bins
a) Trucks
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b) Dry
cranberries
c) Wet
cranberries
for freezing
d) Wet
cranberries
for
packaging
Dumping
7-10 m.
Destoning
1500bbl/h
Dechaffing
1500bbl/h
Selection
400bbl/h
Dechaffing
1500bbl/h
Selection
400bbl/h
Dechaffing
1500bbl/h
Drying
150bbl/h
Selection
400bbl/h
b) Resources (processors)
Dumpers
5 units
Dechaffers
3 units
Destoners
3 units
Dryers
3 units
Separators
3 units
It is often possible to carry out the same operation in different processors, which may have the
same or different technical characteristics, and therefore at different processing rates. In these
cases, it is useful to determine the times for each possibility in order to be able to assign
processors in the most suitable manner to overall production requirements when all
the products are handled simultaneously. Figure 2 shows the operation charts for each of NCC’s
products, both intermediate and finished.
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