OPERATION OF A PORTABLE VACUUM

SYSTEM

Written by Dr. Richard PottsModified by Ruth Dusenbery, Dawn Wisniewski,

and Pranjal PatelUniversity of Michigan-Dearborn 2002

Objective The purpose of this module is to learn about

the proper operation and maintenance of a portable vacuum system.

This type of system is frequently used in advanced laboratory courses and research labs for distillation, sublimation, drying, or manipulation of volatile compounds.

Learning to use and maintain the equipment properly will help prevent damage to expensive equipment.

The Basic Vacuum SystemThe vacuum system is composed of four

main components:

PumpPump

TrapTrap

WorkingWorkingSystemSystem

EquipmentEquipment

Flash Animation OVERVIEWTo start the animation, click on the picture in the PPT presentation To start the animation, click on the picture in the PPT presentation

or on the link provided in the Web version.or on the link provided in the Web version.

PUMPPUMP

WORKINGWORKINGSYSTEMSYSTEM

EQUIPMENTEQUIPMENT

TRAPTRAP

The Pump – 1st Component

The pump is the source of vacuum for the system.

The vacuum is created by the rotor turning clockwise and the spring-loaded vanes opening up a larger volume.

As the vanes rotate, the inlet is closed and the gas is

compressed and ultimately forced through the outlet.

A check valve at the outlet prevents gas from re-entering the pump.

RotorRotorVaneVane

StatorStator

INLETINLETOUTLETOUTLET

Oil Level The contact of the rotor (at the

seal) and the vanes with the stator is made tight by bathing them in oil.

The pump is immersed in oil to

the appropriate level, indicated on the casting around the pump.

If the oil level is too low, the pump may not work efficiently.

If the oil level is too high, oil may spray out of the top of the pump through the vent cap.

ActualActualoil leveloil level

Pump Oil Vapor One factor limiting the vacuum is the vapor pressure of

the pump oil. The lowest pressure that a pump is capable of pulling is the vapor pressure of the pump oil.

The pump oil vapor will fill the vacuum and inhibit the

gas from coming in the inlet. For this reason special oil is used which has a low vapor pressure.

It is extremely important that no substance is pulled through the pump. It may dissolve in or react with the pump oil, producing a solution that has a higher vapor pressure than the original oil.

Contaminated Oil The pump will be capable of producing a pressure

only as low as the vapor pressure of the contaminated pump oil.

Corrosive substances may also cause the metal parts to deteriorate, preventing a gas tight seal.

If contamination of the oil occurs, the oil must be changed. To clean the pump, all the oil must be drained. The pump is then flushed with a flushing oil and refilled with fresh oil.

Pump Performance A label will identify the last time that the pump was

cleaned and indicate the lowest pressure that it was capable of producing after cleaning.

Note that there is no gauge on these systems to measure the actual pressure during laboratory procedures.

The Trap – 2nd ComponentThe trap is placed between the pump and the

working system.

(working system)

The Trap The working system feeds

into the side arm inlet on the trap.

The inner tube of the trap serves as the outlet which is connected to the pump.

The trap is fitted with a large standard taper ground glass joint so that it can be disassembled and readily cleaned.

to PUMPto PUMPWORKINGWORKINGSYSTEMSYSTEM

Grease the Joints The joint should be lubricated with Silicone High

Vacuum Grease. The grease should be lightly and evenly applied. The entire ground glass area of the joint should become transparent.

Wipe off any excess grease.

The Trap and Cold Bath The function of the trap is to protect the pump oil

from contamination. To achieve this, the trap is immersed in a cold bath which is capable of freezing out contaminants.

The low temperature of the cold bath lowers the vapor pressure of the contaminants so they will not move from the trap under the low pressure conditions.

For best protection and most uses, the lowest temperature cold bath available should be used.

Liquid Nitrogen The lowest temperature cold

bath commonly available in the laboratory is liquid nitrogen

(-196C) which, for example, produces a vapor pressure of water of 10-23 torr.

Liquid nitrogen should be used with appropriate safety precautions since it can cause serious “burns” to the skin.

The Dewar

The cold bath is contained in a Dewar (a wide mouthed “thermos” bottle) to minimize heat transfer and extend the life of the bath.

The Dewar should have a plastic net or adhesive tape cover. This safety feature minimizes flying glass from the implosion that occurs if the Dewar is broken.

Cold Bath A liquid nitrogen cold bath may last from 2 to 8 hours.

The level of the cold bath on the trap should be as high as possible to provide a maximum area of glass surface for freezing out contaminants.

The level of the cold bath should not include the standard taper joint, since the low temperature will stiffen the grease on the joint and make it difficult to disassemble.

Cold Bath - Level The level of the cold bath should be

checked periodically while the system is in use to insure proper trapping.

The level should be

kept within 5 cm of the optimum level.

Add coolant when

necessary to maintain the level.

Working System – 3rd Component

The working system is between the trap and the equipment to be evacuated.

This system consists of thick wall rubber vacuum tubing and stopcocks. The tubing provides flexibility in attaching the equipment.

Standard wall rubber tubing is not used since it will collapse under the force of atmospheric pressure when a vacuum is pulled inside.

The thick wall tubing should make gas tight seals to all glass and should not be old or cracked.

Main Valve

A stopcock on the Main Valve adjacent to the trap allows control of the vacuum to the equipment.

The stopcock should be

lubricated with Silicone High Vacuum Grease.

If the stopcock is hard to

turn, it should be taken apart, cleaned with hexane or dichloromethane, and re-greased.

Stopcock on the Main Valve

To turn on the vacuum to the working system, rotate the stopcock plug until the hollow hole in the plug lines up with the side arm.

The handle on the plug always lines up parallel with the side arm when it is on.

The 2-Way ValveThis stopcock can be used to evacuate two different lines,

one at a time, or to admit air into the system through one line.

The upper path is open, the lower path is closed.

The lower path is open, the upper path is closed.

Both paths are closed

A T-Shaped Connector

A glass T-shaped connector allows application of a vacuum simultaneously to two pieces of equipment.

If only one is needed, the other can be shut off by means of a stopcock between the equipment and the T connector.

Equipment – 4th Component

The equipment to be evacuated is attached to the working system with the thick wall rubber tubing.

The equipment to be used depends on the laboratory operation to be performed.

Several Types of Operations

Several operations commonly encountered are:

Distillation of high-boiling liquids under reduced pressure.

Drying solids in a vacuum.

Sublimation of a solid under vacuum for purification.

Procedure for Starting the System

1. Attach a clean, dry trap tube.2. Place the Dewar around trap.3. Close the stopcock on the Main Valve and the 2-way

Valve to the equipment.4. Turn on the pump by plugging in the power cord.5. Fill the Dewar with liquid nitrogen.6. Attach the equipment and open 2-way Valve to the

equipment.

The procedure for starting the system to produce a vacuumconsists of six main steps, that are explained in more detail in the following slides.

Step 1: Attach a clean, dry trap tube.

Remove the trap tube from storage.

Check to make sure the tube is clean and dry. Wipe any remaining grease off the ground glass joint with a paper towel.

Grease the ground glass joint. Replace the cap on the tube to keep dirt from getting mixed with the grease.

Fit the tube into the trap. Once the vacuum is applied, the air pressure will hold the tube in place. The grease will generally suffice to hold the tube on until the Dewar can be placed around it.

Step 2: Place Dewar around trap.

Place the trap inside the Dewar by gently sliding the Dewar up on the trap until it touches the bottom of the trap.

Place the Dewar and trap into

the special container. The Dewar will be resting on a support beam.

Carefully attach the clamp around the trap, so that it is now suspended inside the Dewar.

Step 3: Close the stopcock on the Main Valve and the 2-way Valve.

Close the Main Valve by turning the handle perpendicular to the side arm.

Close the 2-way Valve by turning the handle perpendicular to tubing on both sides.

Step 4: Turn on the pump. Plug the pump into the wall socket to turn the system on.

When first turned on, the pump should make a loud noise while it is pulling the air from the system, and then it should become quieter as the maximum vacuum is reached.

If no noise at all is made when the switch is thrown, unplug the pump and check to see that the motor, belt, and pulley wheel turn freely. If they do not, the motor could be burned out by leaving the pump on.

If any problem occurs, notify the instructor.

Step 5: Fill Dewar with liquid nitrogen.

Slowly add liquid nitrogen to the Dewar around the trap. The Dewar is full when rapid boiling has ceased and the liquid is about 3 cm below the top of the Dewar.

The ground joint should not be in theliquid nitrogen because the grease will freeze and make the removal of the tube difficult when the pump is shut down.

The trap is cooled after the system is closed and evacuated to avoid condensing liquid oxygen from the air in the tube. Liquid oxygen represents an explosion hazard.

Step 6: Attach equipment and open 2-way Valve to equipment.

Attach the rubber tubing to the equipment to be evacuated. Open the 2-way Valve between the trap and the equipment by lining up the hole in the plug with the side arm that leads to the equipment.

Open the stopcock slowly so a sudden evacuation does not cause problems with the equipment or blow contaminants through the trap into the pump.

The system is now set for continuous operation.

Procedure for Maintaining a Vacuum

In general, keep listening to the sound of the pump. If the pump makes a louder sound, indicating that it is pulling air, check all of the connections for a leak and check the oil level in the pump.

Maintain the liquid nitrogen level in the Dewar.

Check the trap periodically to be sure that it hasn’t become plugged with condensed material.

Maintaining the vacuum in the system requires only a few precautionary procedures:

Maintaining the Liquid Nitrogen Level in the Dewar.

Every hour check the liquid level in the Dewar.

If it has dropped 5 cm or more below the optimum level (due to evaporation) add more liquid nitrogen.

If large amounts of material are being condensed out, the liquid level should be checked more frequently.

Checking for a Plugged Trap. When the trap is not plugged, the pump is actively

evacuating the equipment (a state of dynamic vacuum) and the operation is carried out effectively.

When the trap is plugged, the equipment will remain under vacuum, but the pump will not be actively evacuating the equipment (a state of a static vacuum).

Procedure for Shutting Down the System

1. Close the stopcock on the Main Valve.2. Open the 2-way Valve to the atmosphere and

disconnect the equipment.3. Open the stopcock on the Main Valve.4. Turn off the pump.5. Remove the cold bath.6. Remove the trap tube.7. Empty and store the Dewar.

The procedure for shutting down the system consists of seven main steps (steps 4-6 are done rapidly).

Step 1: Close the stopcock on the Main Valve.

Turn the handle on the plug perpendicular to the side arm on the stopcock.

Step 2: Disconnect the equipment.

Slowly open the stopcock on the 2-way Valve (or the glass T connector) to bleed air into the system until the equipment is at atmospheric pressure.

Disconnect the rubber tubing to the equipment.

Step 3: Open the stopcock on the Main Valve.

Turn the handle so the hole in the plug lines up with the side arm. This will admit air to the pump line, and the sound of the pump should change.

Steps 4-6 should then be done quickly to prevent the air from condensing in the trap, and so that any dangerous substance in the trap does not evaporate into the room.

Step 4: Turn off the pump.Step 5: Remove the cold bath.

Turn off the pump by unplugging it from the wall socket. Air was admitted to the pump before the pump was turned off to eliminate the possibility of the vacuum in the system pulling the pump oil into the system.

Remove the liquid nitrogen cold bath from the trap by removing the Dewar.

Step 6: Remove the trap tube.

Disconnect the tube. By supporting the trap on both sides of the ground joint with both hands, apply a force (not too great) and separate the tube.

Clean the tube. If any dangerous substances are contained in the tube, immediately transfer it to a hood. Once the tube has reached room temp, dispose of the trapped substance properly. Clean and dry the tube.

Store the tube. Return it to its storage hole in the top of the cart.

Step 7: Empty and store the Dewar.

Empty the liquid nitrogen from the Dewar into a proper container and return the Dewar to its storage cylinder on the cart.

The system is now shut down and in proper order for the next use.

This concludes the module on the Operation of a PortableThis concludes the module on the Operation of a PortableVacuum System. Obtain a post test from an assistant in theVacuum System. Obtain a post test from an assistant in theScience Learning Center.Science Learning Center.

You may use your handout with notes during the post test.You may use your handout with notes during the post test.