Transcript Chapter 1. Introduction - Florida Gulf Coast University

Chapter 5. Capacity and Bottlenecks
Outline
Bottlenecks, near bottlenecks, and non bottlenecks
Identifying Bottlenecks
Working with bottlenecks
Theory of constraints basics
Modeling a simplified process: Queuing Theory
Tradeoffs of queuing theory
Characteristics of a queuing system
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Bottlenecks, Near Bottlenecks, Non Bottlenecks
Bottleneck: a resource which has a demand requirement
greater than capacity. Planned utilization > 100%
Given process variability, even if utilization is less than 100%, a
resource/activity can become a process bottleneck.
 Near bottleneck: a resource with a planned utilization of
close to 100%, but where there are other resources with a
higher utilization
Non bottlenecks: a resource with a planned utilization <<
capacity
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Working with Bottlenecks
Recognizing bottlenecks is important in order to minimize
their effect.
Critical message is: time lost in the bottleneck is lost
production time and thus lost output (profits?). Time lost on
a near bottleneck or on a non bottleneck has (in most cases)
no effect on the output.
Operations improvement should then focus on
the improvement of capacity on the bottlenecks, and
maintaining the bottleneck operating at all times.
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Working with bottlenecks: Theory of
Constraints
Basic Steps
1. Identify the system constraints. (No improvement is possible
unless the constraint or weakest link is found.)
2. Decide how to exploit the system constraint. (Make the
constraints as effective as possible.)
3. Subordinate everything else to that decision. (Align every other
part of the system to support the constraints even if this
reduces the efficiency of non-constraint resources.)
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Working with bottlenecks: Theory of
Constraints
Basic Steps
4. Elevate the system constraints. (If output is still inadequate,
acquire more of this resource so it no longer becomes a
constraint.)
5. If, in the previous steps, the constraints have been broken, go
back to step 1, but do not let the inertia become the system
constraint. (After this constraint problem is solved, go back to
the beginning and start over.)
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Working with bottlenecks: Theory of
Constraints
Operating System called: Buffer, Rope , Drum
Buffer: inventory in front of the bottleneck, Rope: signal sent
upstream to get additional inventory, Drum: refers to the
bottleneck that maintains the production rhythm
product
Start
Activity 1
Make
Q
Activity 2
Hold area (WIP)
Activity 3
The Bottleneck
Information on
the current
quantity in hold
area
Yes
If quantity in
hold area < min R
No
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Activity 4
Customer
End
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Queuing Theory and its tradeoffs
the tradeoffs costs
1600
High1400
1200
1000
Cost
Body of knowledge
that analyzes
waiting lines
QT answers question
related to the size of
the line (queue) time
waiting in line, and
the tradeoffs
between service and
waiting costs
Cost of
Service
Cost of
Waiting
Total
Cost
800
600
400
200
Low
0
1
Low
2
3
4
High5
Capacity
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Characteristics of a Queuing System
A process model that is condensed into four elements:
arrival area (of a single type of entity)
a waiting area
an activity (the transformation process)
a departure area
Entity Arrives
Waiting area
Server
(activity)
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Entity Departs
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Characteristics of a Queuing System
Entity Arrivals
Modeled as a Poisson
distribution with parameter

Waiting Line
Organized by FCFS
Customers always joins
the line and never leaves
the line
Line has infinite space
Only one line
Arrival is modeled by a rate:
5 people/minute, 11 forms/
hour
Infinite population of
customers
Each arrival is of a single
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entity.
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Characteristics of a Queuing System
Server
One entity (customer) at a time
When there are more than one server, they work in parallel
Service time is a rate: 6 people/minute, 12 forms/ hour. The
inverse is the service time (10 seconds/person)
Service time is either constant or variable. If variable, it is
modeled as an exponential distribution with parameter 
Departure
Once the service is completed entities leave the system.
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Queuing Theory: Line behavior
To evaluate system alternatives we need two cost
measures:
Cost of service for each option
Cost of waiting per customer per time unit (difficult to quantify)
Performance of a system
Wq = average time in the queue (waiting)
Ws = average time in the system (waiting and in service)
Lq = average length of the queue
Ls = average total number of entities in the system
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Calculating the measures of performance
Three models. Each uses some different equations (Will use
spreadsheets for the calculations)
Model 1 - constant service times and a single server
Model 2 - exponential service times and a single server
Model 3 - exponential service times and multiple servers
Total Costs = Cost of Service + Cost of Waiting
Cost of Waiting = Number of customers through system * time
waiting per customer * cost per waiting time (for a specified
time, I.e. day, month, year)
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