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Exhaust System Analysi

May 8, 2000

 

 Summary

This report is an analysis of an exhaust system on a plastic extruder.  This report discusses the present problems with the existing hood.  It briefly discusses the airflow around the hood and its vicinity.  This report briefly details the problems with problems caused by the hood mounting.

It is concluded that the exhaust system is not safe or efficient.  Firstly, the fume hood is incapable of capturing all of the fumes.  Secondly, the beam that supports the hood, hinders the daily operation, by making it more difficult and time consuming to make adjustments. 

It is recommended that a new exhausts system be implemented.  The fume hood design discussed in this report is a suitable choice.  In order to mount the fume hood, it is also recommended that one of the two support methods, discussed in this report, be implemented. 

It further recommended that a study on the effects of the cooling jets be conducted.  This study can lead into other ways to improve the capture of the existing hood.

 Velcro Introduction

Velcro is a global company with fourteen locations around the world.  Velcro fabricates a wide range of hook and loop products to meet specific customer applications in the automotive, consumer, and industrial markets.  Velcro Canada Inc is one of the fourteen companies that make up the “The Velcro Companies”.  It is located in Brampton, Ontario, and is a QS9000 certified company.  Velcro Canada manufactures a wide range of hook and loop products for the automotive markets. 

Introduction

The Objective of this report is to analyze a fume hood on a plastic extruder.  This report contains no prototyping, detailed calculations, and detailed drawings

Modern industry uses a wide range chemical compounds, many of which are highly toxic.  The use of such compounds may release gasses, and vapours in to the work environment, where it can accumulate into concentrations that are hazardous to the health of workers.  In order to maintain a work area that contains clean and uncontaminated air, an effective exhaust system must be in place.

There are two types of exhaust systems, general and local.  General exhaust systems are typically used in large areas to control temperature, or to remove contaminants.  Local exhaust ventilation are used to capture contaminates at or near its source.  In most cases local exhaust systems are preferred because it is more efficient [1]. 

Local exhaust systems are comprised of four basic elements: the hood, the ducting, the air cleaner, and the fan.  The hood collects the contaminants through an air stream traveling upwards.  The ducting system then transports the contaminated air to the air cleaner.  In the air cleaner the contaminant is removed from the air stream.  The fan controls the flow of the air stream, and ejects the air into the atmosphere [1].

Elastomer Background

The elastomer is a machine used at Velcro Canada to manufacture a specific type of hook and loop product.  The elastomer continuously extrudes a layer of plastic, which is 12” wide by 1/8” thick, and applies it onto another process

Figure 1 is a non-detailed diagram of the elastomer.

(1)    Screen:  The screen, filters impurities from the extruding plastic.  It is changed regularly, even on the run.

(2)    Dye Head:  The plastic is extruded through the dye head.  It helps maintains the thickness and the dimensions of the extruded plastic.  Adjustments are usually made to the dye head while the plastic is being extruded.  The dye head is 20” wide.

(3)    Rollers:  The rollers guide the extruded plastic, and flatten the plastic to a uniform thickness.

(4)    Calendar Stack:  The calendar stack holds all of the rollers.

(5)   Air Jets:  The air jets blow a rapid stream of air on to the oncoming plastic to cool it down.

(6)   Fume hood:  The fume hood captures the fumes released as the plastic extrudes.

(7)    Fume hood support bar:  The support bar holds the fume hood in position.

(8)    Elastomer body:  Contains the inner workings of the elastomer

Local Exhaust

The elastomer is fitted with a local exhaust system.  The existing fume hood is made out of sheet metal and it has a hood opening which is 13” x 10”.  This fume hood is positioned 13” above the source of the fumes, which is where the plastic extrudes out of the dye head.  Figure 1 shows that the fume hood is not centered exactly over the source.  The hood is connected to a 14” diameter duct.  The duct travels 6’ vertically and elbows and travels a further 12’ horizontally into a fan.  The fan has two user settings, a low (1500 cubic feet per minute) and a high (3000 cubic feet per minute).  The fan is usually run on the higher setting. Minute

 The fume hood is mounted on a support beam.   The beam is connected to the base of the machine and extends to the tip of the dye.  The beam is 13” above the tip of the dye. 

Elastomer Hazards

As the elastomer extrudes the plastic, it releases hydrogen chloride gas, which is toxic.

Most of the fumes are released at the tip of the dye head as the plastic comes out of the extruder.  Fumes are also released from within the plastic as it moves down the line.  Hydrogen chloride gas has a threshold limit value-ceiling (TLV-C) of 5 parts per million.  This value indicates the maximum concentration of an airborne substance that workers can be exposed to without developing adverse health effects [1].

Tests on the elastomer have shown unsafe operations.  Readings on the concentration of hydrogen chloride gas have averaged about 4.8 parts per million.  The readings reached as high as 7 parts per million in certain areas.  Although the average reading is below the TLV-C limit, this condition is unsafe.  Velcro Canada requires all operations to have a high safety factor.  The current condition of the elastomer does not fall under this safety factor.  In order for it to be deem safe under Velcro, the concentrations of hydrogen chloride should at no time exceed 5 parts per million, and have an average of approximately 4.5 parts per million.

Problems with Existing Exhaust System

Observations and chemical tests of the fume hood, while under operation, proves that the fume hood is inefficient in capturing all the fumes.   The fume hood’s inability to capture all the fumes causes fumes to spread and disperse in to the surrounding air.  It is this spread of the fumes that causes the safety hazard.

The existing fume hood is mounted onto the body of the elastomer, as shown in figure1.    This mounting present a new set of problems that deal with operation.  

(1) While the plastic is being extruded, the screen needs to be changed occasionally.   This operation requires the screen to be lifted vertically before the change can be made.  The low clearance of the support bar makes it difficult for the operators to change the screen. 

(2)  The components of the elastomer are cleaned once in two week.  In order for it to be clean, the components must be dismantled.  The low clearance of the support bar makes it difficult for the operators and consumes a lot of time.

Existing Air Flow Analysis

The capture and control of contaminates is achieved by the inward airflow created by the exhaust hood.  Airflow towards the hood must be high to maintain control of the contaminants until it reaches the hood.  External airflow may disrupt the airflow towards the hood [1].  The external airflow that affects the elastomer is: 

  1. Thermal air currents:  The dye head on the extruder operates at a high temperature.  This creates a thermal air current that travels upwards from the face of the dye.
  1. Material Motion:  The plastic is extruded at approximately ½” per second.  This motion causes small air currents that flow with the material.
  1. Motion of Machinery:  As the plastic enters the calendar stack, it encounters many rollers.  These rollers cause air currents in the direction they turn in. 
  1. Movement of the operator:  As the operator moves alongside the machine making adjustments he creates air currents. 
  1. Room air currents:  The air surrounding the machine is moderately still.  Air currents are caused by the movement of people, and by the heating/cooling system of the plant is negligible
  1. Rapid air movements:  The two air jets that cool the incoming material causes air currents that have a considerable affect on the intake of the fume hood. These two air jets blow a high-speed stream of air onto the extruded material in order to cool it down.  The air jets are located approximately 5” away from the dye head.  As the air jets cool the incoming material it stirs up the fumes and causes it to spread randomly.    Since these jets are close to the dye head, it causes a complicated air current that spreads the fumes.

Capture velocity is the minimum velocity necessary to capture and transport the contaminants into the hood.  Table 1 shows the range of capture velocities for different conditions.  The fumes released by the elastomer are in an area of rapid air motion [1].  The capture velocity required for this condition is in the range of 200 to 500 fpm. 

Calculations on the airflow conducted on the existing elastomer show that the capture velocity is approximately 262.92 fpm.  Since the present capture velocity is not capable of collection all the fumes, the capture velocity must be increased.  

The design aspects of the fume hood that causes great inefficiencies are:

(1)   The fume hood is located relatively far (13”) from the source of the fumes. This placement allows more fumes to disperse in to the surroundings.

(2)   As seen in figure 1 the fume hood is not centered above the source of the fumes.  A larger portion of the hood is above the dye, which releases no fumes.  Since the center of a hood face has the highest capture velocity, this improper positioning reduces the amount of fumes captured.

(3)   The fume hood has a relatively large hood face area.  Because capture velocity is inversely related to the hood face area, this hood is inefficient.   Furthermore a large hood face allows unnecessary air into the hood.

Solutions

In order for the hydrogen chloride gas be captured efficiently a new fume hood must be designed.

Design Criteria

·         The fume hood must be able to capture all the fumes safely and efficiently

·         The fume hood must not slowdown the operators during their everyday operations

o       Operators must be able to adjust the dye on the run while the hood is down

o       The operators should be able to make screen changes easily

o       The Cleaning of the dye and unit should not be hindered by any part of the fume hood

Table 1:  Range of Capture Velocities

Condition of Dispersion of Contaminant

Example

Capture velocity (fpm)

Released with practically no velocity into quiet air

Evaporation from tanks; degreasing;

50 –100

Released at low velocity into moderately still air

Spray booths; intermittent container filling; low speed conveyor transfers; welding; plating; pickling

100 –200

 

Active generation into zone of rapid air motion

Spray painting in shallow booth; barrel filling; conveyor loading; crushers

200 –500

Released at high initial velocity into zone at very rapid air motion

Grinding; abrasive blasting; tumbling

50 –100

In each category, a range of capture velocity is shown.  The proper choice of values depends on several factors. 

Lower End of Range

  1. Room air currents minimal or favorable to capture
  2. Contaminants of low toxicity
  3. Intermittent, low production
  4. Large hood - large air mass in motion

Upper range

  1. Disturbing room currents
  2. Contaminants of high toxicity
  3. High production, heavy use
  4. Small hood –local control only
 

Design Constraints

  • The fan must be run at 1500cfm, (3000 cfm is reserved for emergencies)
  • The fume hood must not be too heavy, so users can have access.
  • The operation of the fume hood must be kept simple
  • Improvements should not cost too high

 

Fume Hood Design

Figure 2 shows a design of a fume hood to replace the existing one.  This hood is design to capture all the fumes safely and efficiently.  The new fume hood has a hood area of 0.84 sq ft.

Design Features

  • The length of this hood face is slightly greater than the length of the dye head.  This allow the hood to capture fumes released from any part of the dye, and to capture the fumes released from underneath the extrude material.
  • The hood is flanged on the length closer to the dye.  The flange prevents fumes from spreading outside the range of capture.  Temperature fluctuations on the dye head can change the properties of the material being extrude.  Cool air drawn over the dye can cause these fluctuations.  The flange restricts drawing in air from over the dye.  The flange also increse the capture velocity, which will aid in capturing the chaotic fumes cause by the air jets.  .  To maximize the capture velocity, and still allow operator eases, the hood should be located 4.25” from tip of the dye.  This creates a capture velocity of 1008fpm, which will capture all the fumes.

Support Design

Name

Design 1:  Lever Support

Design 2:  Vertical Slide Support

Figure

3

4

Description

The fume hood is mounted on to a lever system.  This enables the operator to swing a lever and bring down the fume hood for working, or bring up the fume hood for clearance.

This design is similar to the existing design.  A horizontal support bar is attached from the body of the elastomer.  The hood is attached to the end, and can be slid vertically.  This allows the operator to raise the hood on demand..

Mechanics

The fume hood is mounted onto a lever.  The lever is connected to the body of the elastomer.  The pivot for the lever is strategically placed, to minimize verticle travel, and to prevent it from striking the rollers.

Attached to the lever is a counter weight, which allows the operators to move the fume hood with less effort.

The lever is designed in a particular shape to minimize the vertical displacement.  This is important because of the amount ducting available.

The fume hood is mounted on a vertical track that allows it to be slid vertically to sets positions.  To ease the load of the operator a spring is attached to the hood and support.

Advantages

With the hood in its upward position the area above the dye is clear.

The operators can change the screen filters with no obstruction

The cleaning of the dye can be accomplished without any vertical obstructions

Simple and easy design to construct
 

Inexpensive
 

Changes to the screen can be made easily
 

Disadvantage

Expensive

Because this system is made up of many individual components which are irregular in shape.

 

 

Does not provide complete clearance during cleaning

 

Conclusions

This report concludes that the existing exhaust system on the elastomer is not safe or efficient.  Firstly, the fume hood is incapable of removing all the toxic fumes because of its design and positioning.  Secondly, the beam that supports the hood, hinders the daily operation, by making it more difficult and time consuming to make screen changes.

Recommendations

It is recommended that a new exhausts system be implemented.  The fume hood design discussed in this report is a suitable for the elastomer, since it is capable of capturing all the fumes.  In order to mount the fume hood, it is recommended that one of the two support methods, discussed in this report, be implemented.  However, the vertical-slide support method is more feasible, because it is more cost effective.

It further recommended that a study on the effects of the cooling jets be conducted.  This study can lead into other ways to improve the capture of the existing hood.

References

[1] J.M.Kane: “Design of Exhaust Systems.”  John Wiley and Sons, 1967


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Appendix A: Calculations


 

Calculations

 

Q  - Airflow into suction point (cfm) 

V – Capture velocity at distance X (fpm)     

X – distance from hood opening to source (ft)

A – Area of hood opening (sq ft)   

 

Existing hood data

 

Type of hood:  Plain opening

Fan capacity:  3000 cubic feet per minute (cfm)

Length of hood face: 13 inches

Width of hood face:  10 inches

Distance from fume source to hood face:  13 inches

 

Hood face area   =

                           =  0.90 sq ft

 

Velocity at Entrance of Hood =

                                                = 3333.33 fpm

 

Aspect Ratio =

                     =

                     = 0.77

 

Q = V (10X2 + A) is the equation to calculate the air flow for a plain opening hood with an aspect ratio greater than 0.2

 

 

Q = 3333.33 fpm

V =?

X = 1.08 ft

A = 0.90 sq ft

 

Q = V (10X2 + A)

 

 

Calculations for New Hood

 

Type of hood:  Plain opening/ flanged opening *

Fan capacity:  1500 cubic feet per minute (cfm)

Length of hood face: 21 inches

Width of hood face:  5.75 inches

Distance from fume source to hood face:  4.25 inches

 

*The characteristic of the proposed hood design falls under two categories, a plain opening hood, and a flanged opening hood.  The actual capture velocity is apporximatly the average of the two.

 

Area

21” x 5.75” = 120.75”

                   = 0.84’

21” x 5.75” = 120.75”

                   = 0.84’

Aspect Ratio (Width/length)

 = 0.27

 = 0.27

Type of equation

Velocity at entrance of hood (Q)

 

= 1785.71fpm

 

= 1785.71fpm

Capture Velocity

 

 

Average of capture velocities =  = 1008.88 fpm