بایگانی ماهانه: آوریل 2020

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How do you check a brake booster for a vacuum leak?

Vacuum boosters require three basic tests:

Booster Function Test: Check pedal feel and vacuum booster function while test-driving the vehicle. With the engine off, apply the brake pedal repeatedly with medium pressure until the booster reserve is depleted. At least two brake applications should have a power-assisted feel before the pedal hardens noticeably. If the pedal feels hard immediately, or after only one brake application, it may indicate a vacuum leak or a low level of engine vacuum. Inspect the vacuum hose to the booster for kinks, cracks or other damage. Check vacuum at idle with a vacuum gauge. vacuum booster To test booster function once the reserve is depleted, hold moderate pressure on the brake pedal and start the engine. If the booster is working properly, the pedal will drop slightly.

Booster Vacuum Supply Test: With the ignition off, pump the brake pedal to deplete the booster reserve. Disconnect the vacuum supply hose from the booster and connect a vacuum gauge to the hose using a cone-shaped adapter. Start the engine and allow it to idle while observing the vacuum gauge. Although the amount of vacuum will vary by application, most will register between 15 inHg and 20 inHg (50 kPa and 70 kPa) at idle. Disconnect the vacuum booster supply hose and check for source vacuum with a gauge. If the reading is low, check to see if the vacuum hose is kinked, clogged or cracked. If the hose is not at fault, suspect an engine mechanical problem such as leaky valves, worn rings, an intake manifold vacuum leak, improper cam timing, etc.

Vacuum Inlet Check Valve Test: To test the vacuum check valve, disconnect the vacuum supply hose from the intake manifold or vacuum pump, and blow into the hose. If air passes through the valve into the booster, the check valve is defective and should be replaced.
Hydro-Boost Power Assist Service

The hydro-boost power assist system performs the same function as the vacuum assist system in helping apply a vehicle’s brakes. The difference is that the hydro-boost system uses hydraulic pressure instead of vacuum to provide power assist for the brake system. By using hydraulic pressure, a greater amount of assist can be provided. The hydro-boost uses pressure from the power steering pump to provide braking boost, and includes a high-pressure accumulator that has enough capacity to provide several power-assisted stops in the event that the power steering pump belt breaks or a hose ruptures. When inspecting the hydro-boost system, the inspection must include checking power steering hoses and pump for leaks, power steering fluid level, and drive belt tension. Hydro-boost operation and accumulator performance must also be tested.

Hydro-Boost Function Test: With the engine off, apply the brake pedal five or more times with medium force to discharge the accumulator. The pedal feel will harden noticeably. Next, apply the brake pedal with medium force and then start the engine. If the booster is working properly, the pedal will drop toward the floor and then push back upward slightly. If the booster passes this test, perform the accumulator test as described in the following section. However, if there is no change in the pedal position or feel, the booster is not working. Check the power steering system to determine whether the problem is in the pump or the booster.

How do you check a brake booster for a vacuum leak?

vacuum booster
Hydro-Boost Accumulator

Similar to the vacuum booster, the hydro-boost is equipped with a backup or reserve in case the source of pressurized fluid is lost. In the event of a loss of pressurized fluid, the accumulator will provide two to three power-assisted stops. Upon the first application of the brakes after an engine stall or loss of power steering, you would find approximately 60% to 75% of the normal assist available. If you were to release and apply the brakes again, you would find approximately 30% to 40% assist, then again approximately 10% to 20% until you depleted all stored reserve assist. Once you have depleted all of the stored pressure, the brakes will no longer have power assist and will be manual in their operation.

Hydro-Boost Accumulator Test: To test the ability of the system to store a short-term high-pressure charge in the accumulator, start the engine and allow it to idle. Charge the accumulator by turning the steering wheel slowly one time from lock to lock. Do not hold the steering at full lock for more than five seconds. Switch the engine off, release the steering wheel, and repeatedly apply the brake pedal with medium force. If the accumulator can hold a charge, a Hydro-Boost I unit will provide two or three power-assisted applications, while a Hydro-Boost II unit will only provide one or two.

To test the ability of the system to store a long-term charge, start the engine and recharge the accumulator as described previously. As the accumulator charges on a Hydro-Boost I system, a slight hissing sound should be heard as fluid rushes through the accumulator-charging orifice. Once the accumulator is charged, switch the engine off and do not apply the pedal for one hour. At the end of the hour, repeatedly apply the brake pedal with medium force.

If the hydro-boost unit fails these tests, it usually means the accumulator of a Hydro-Boost I unit, or the accumulator/power-piston assembly of a Hydro-Boost II unit, is leaking. In either case, the booster must be rebuilt or replaced. However, if a Hydro-Boost I system fails the test but does not make the hissing sound to indicate the accumulator is charging, the fluid in the system is probably contaminated. Simply flushing the hydro-boost system may cure the problem.

Never begin any work on a hydro-boost system until you have discharged the dangerously high pressure stored in the accumulator by pumping the brake pedal numerous times with the engine off.

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Why does a vacuum pump need oil?

What Is Vacuum Pump Oil?

If you use a vacuum pump, you need to familiarize yourself with its oil. Each pump type has its own requirements for oil, and the oil needs to be inspected and periodically replaced. These oils come in hydrocarbon, silicone and other varieties specially formulated for vacuum applications.
Description

Vacuum pump oil serves as a mechanical lubricant and a medium for trapping gas molecules. It’s chemically stable, unreactive to most gases and materials, and has a low-vapor pressure.
Vapor Pressure

All substances will boil or otherwise shed molecules into a vacuum. Over time, a pressure will build up, called vapor pressure, contaminating the vacuum. Some substances, like water, boil a lot into a vacuum, others, like glass, boil very little. A clean vacuum system needs all parts, including the oil, to have vapor pressures of 10^-5 torr or lower.
Mechanical Pump

Why does a vacuum pump need oil?

A mechanical vacuum pump has valves and rotary parts designed to pump from atmospheric pressures and below. Mechanical pumps use a hydrocarbon oil to lubricate the parts and seal the vacuum.
Diffusion Pump

A diffusion vacuum pump collects gas molecules in a heated oil spray. It is meant to pump from low pressures only. This vacuum pump uses a silicone, hydrocarbon or perfluorinated polyether (PFPE) oil, depending on the application.
Lifetime

The useful lifetime of a vacuum pump’s oil depends on the kind of oil, how often it’s used, and the contaminants that result from use. A mechanical pump has an inspection window to check the oil’s condition. If it’s dark-brown, the oil needs to be replaced.

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What is a gas ballast on a vacuum pump?

When atmospheric air (or a gas) is used as a source for a vacuum system, it will however “pure” it may appear to be invariably contain some vapour. As the pressure drops this vapour will condense out and unless vented from the system form a contaminant which will prevent the pump from achieving its optimum vacuum pressure. In addition, this condensate can enter the pump’s oil-seal where as a contaminant, it can have a further detrimental effect.

In simple terms, a gas ballast valve incorporated into the system will allow a small portion of the compressed gas (containing this detrimental condensate) to be expelled without impacting upon the overall performance of the pump.
What is the theory of gas ballast?

The theory of gas ballast (and indeed their operation) is simple, however, the rationale behind their inclusion in vacuum systems is still shrouded in much confusion and misinformation, and as a result the whole subject is frequently considered somewhat esoteric. Therefore, their inclusion in a vacuum system is all too frequently blindly followed with little thought being given to the simple – but effective – science behind them.

Originally developed in 1935, gas ballast is nevertheless still considered a significant addition to a mechanical oil sealed vacuum pump. When used in this way, the gas ballast helps reduce the amount of vapour condensation (and thus contamination) of the pump’s sealing-oil. By reducing any potential condensation in the sealing oil, the pump can achieve its vapour duty at near full-specification, ensuring the speedy attainment of its ultimate pressure level.
Usually in vacuum work, the gas stream being evacuated from the vacuum chamber will contain water vapour, solvent vapour and/or other unwelcomed contaminants. These contaminants usually occur because they have been “converted” (under vacuum pressure) from liquid molecules to gas molecules prior to (i) their flowing back to the pump where they are “converted back” from a gas to a liquid contaminant within the pump oil, or (ii) exiting the pump itself.

Click here to learn about the different types of vacuum pump technologies.

What is a gas ballast on a vacuum pump?

All gasses have the ability to form vapours at temperatures and pressure that depend upon their physical -properties. In simple terms, gas ballasting is the practice whereby air or another gas that is admitted into a vacuum pump, such that vapours can be expelled before they can condense or contaminate the system and/or pollute the sealing oil. Gas ballasting is achieved in the following way: if the pump is “working” a gas that would naturally condense in the pump, the gas ballasting will enable a “pre-outlet” valve to open before the vapour has had a chance to condense, with the result that this condensable-vapour is discharged (to the atmosphere) as in gas phase.
A pump that has been subject to condensed vapour can be “cleaned-up” by using gas ballast. This is achieved in the following way: close the pump inlet port and allow the pump to run with the gas ballast valve in the open position which will help purge the pump. Continue this purging for several hours or overnight (depending on the size of the system and the degree of contamination).

However, if the pump is “smoking” or emitting a visible mist in large quantities, it is very possible that you could return to find a room full of oil-fog. Venting into an extraction hood or using capture filters will help to eliminate this problem. Nevertheless, even if the purging vapours are not actually visible, it should be remembered that in exiting the system, they may also be entering the working environment where depending upon their content and concentration they may pose a health and/or a fire hazard. The use of a gas-ballast oil return kit overcomes this issue.

Gas ballasting which is associated with oil-sealed rotary vane and dry scroll pumps, is frequently employed in freeze drying, rotary evaporation, distillation and gel drying.

Find out more about the topic of vacuum measurement by downloading our eBook below.

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How do you filter vacuum pump oil?

Vacuum Pump Oil
Why use dedicated filtration for your vacuum pump oil?

Vacuum pump oil filtration is inexpensive compared to the cost of repairing or replacing a vacuum pump. Vacuum pump oil has no additives, and you do not have to add additive packages to your vacuum pump oil after filtering. Additives reduce the performance of the vacuum pump by raising vapor pressures and absorbing moisture.

How do you filter vacuum pump oil?

Additionally, during the pump down process, contaminants will enter the air chamber and mix with air, causing the oil to oxidize and form sludge. Using the proper fluid, and filtering that fluid on a regular basis with dedicated filtration, will provide you the lowest possible pressure while minimizing costly repairs.
Are you experiencing any of the following problems caused by dirty vacuum pump oil?

Dropping vacuum pump efficiency
Sludge build-up in your vacuum pump oil
Improper oil in your vacuum pump
Decreased vapor pressure
Excess moisture
Pump failure

Solution: COMO Filtration Systems

Particulate contamination does not mean that the vacuum pump oil must be disposed of and replaced. COMO Multi-Pass Filtration removes the particulate from the vacuum pump oil, thus extending the life of the oil and the vacuum pump.

COMO Multi-Pass Depth Filtration removes both fine and gross contamination, which will reduce sludge build-up and impurity contamination while increasing the vacuum pump’s efficiency.

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Is vacuum pump oil the same as compressor oil?

Difference of vacuum pump oil, compressor oil and refrigeration oil

Vacuum pump (mainly mechanical vacuum pump), compressor (air compressor, various gas compressors) and refrigerator have special requirements for lubricating oil. Vacuum pump oil is used in mechanical vacuum pump of vacuum system. Compressor oil is mainly used in the lubrication of cylinder and piston friction parts of gas compressor, exhauster and piston pump, as well as inlet and outlet valves. When it is used in automatic oil supply lubrication system, it also lubricates the main bearing, coupling rod bearing, crosshead, sliding plate, etc. of compressor. Low viscosity oil is also used in each Type rotary compressor. The refrigeration oil is mainly used in the compressor for compressing the refrigerant.

The three have different uses, but they have many common points. For example, the three should not only play the role of lubrication at the cylinder, piston, valve, bearing, sliding plate and other parts, but also play the role of sealing to prevent the gas channeling between the piston and the cylinder wall, and also have the chemical stability that does not work with the compressed gas.
Vacuum pump oil is a special lubricating oil for mechanical vacuum pump. Although there are different types of pumps, the requirements for oil quality are the same. When the vacuum pump is running, the oil in the pump cavity plays a sealing role, which makes the pump obtain the necessary vacuum degree. On the other hand, the oil in the pump cavity is affected by friction, temperature and pressure reduction. The components with high vapor pressure evaporate to the vacuum space pollution vacuum system, which limits the improvement of vacuum degree. In addition to proper viscosity, high viscosity index, good oil-water separation and excellent thermal oxidation stability, the main quality requirements of vacuum pump oil are low saturated vapor pressure and extremely strong limit to ensure high vacuum degree. Therefore, in the production of vacuum pump oil, in addition to selecting the appropriate type of base oil, a single narrow fraction oil or pure chemicals should be used instead of several fractions as much as possible.

Is vacuum pump oil the same as compressor oil?

For reciprocating compressor, because the discharge temperature of compressed gas is higher than that of suction, it needs to have better viscosity temperature performance, appropriate viscosity and better anti-oxidation safety. Generally, it is made of deeply refined base oil (preferably naphthenic base oil) and additives with anti-oxidation corrosion and anti-wear performance (generally, pour point depressant and tackifier are not allowed). For rotary compressor oil, because its working condition and lubrication mode are different from reciprocating compressor, it puts forward more stringent requirements for lubricating oil quality. In addition to lubricating, cooling and sealing parts, rotary compressor oil also plays a prominent role in cooling compressed gas. In the use process, oil mist is separated from gas after mechanical collision and absorption medium, The oil is easy to be polluted and aged by repeated circulation. At the same time, in order to overcome the rotating centrifugal force, the oil needs better adhesion, and the bearing does not need too much viscosity lubricating oil. Therefore, the rotary air compressor oil is generally produced by deep refining narrow fraction base oil and adding compound antioxidant, antirust, antiwear, demulsification and defoaming additives.

For refrigeration oil, in addition to meeting the above basic requirements, it is more necessary for the oil to have excellent low-temperature performance and good separation with refrigerant or water. Therefore, the general refrigeration engine oil is produced with the deep refined low freezing point naphthenic base oil. It is worth noting that when SO2 is used as the refrigeration engine oil, the deep refined lubricating oil (such as white oil) that does not contain aromatics and thiooxygenates should be selected, otherwise, colloidal precipitation will be generated, which will affect the operation of the refrigerator. When chloroethane is used as refrigerant, oil will be dissolved, and glycerin and castor oil are generally used for lubrication; when ammonia or carbon dioxide is used as refrigerant, lubricating oil without fat oil shall be used, because fat oil is easy to emulsify, which is not conducive to water separation and ice formation, and the thermal conductivity of fat oil is small, so as to reduce the refrigeration efficiency.

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What is the working principle of vacuum pump?

Vacuum pumps are categorized by their operating pressure range and as such are classified as primary pumps, booster pumps or secondary pumps. Within each pressure range are several different pump types, each employing a different technology, and each with some unique advantages in regard to pressure capacity, flow rate, cost and maintenance requirements.

Regardless of their design, the basic principle of operation is the same. The vacuum pump functions by removing the molecules of air and other gases from the vacuum chamber (or from the outlet side of a higher vacuum pump if connected in series). While the pressure in the chamber is reduced, removing additional molecules becomes exponentially harder to remove. As a result, an industrial vacuum system (Fig. 1) must be able to operate over a portion of an extraordinarily large pressure range, typically varying from 1 to 10-6 Torr of pressure. In research and scientific applications, this is extended to 10-9 Torr or lower. In order to accomplish this, several different styles of pumps are used in a typical system, each covering a portion of the pressure range, and operating in series at times.

Vacuum systems are placed into the following broad-based grouping of pressure ranges:

Rough/Low Vacuum: > Atmosphere to 1 Torr
Medium Vacuum: 1 Torr to 10-3 Torr
High Vacuum: 10-3 Torr to 10-7 Torr
Ultra-High Vacuum: 10-7 Torr to 10-11 Torr
Extreme High Vacuum: < 10-11 Torr

The different types of pumps for these vacuum ranges can then be divided into the following:

Primary (Backing) Pumps: Rough and low vacuum pressure ranges.
Booster Pumps: Rough and low vacuum pressure ranges.
Secondary (High Vacuum) Pumps: High, very high and ultra-high vacuum pressure ranges.
Terminology

The two technologies used by vacuum pumps are gas transfer and gas capture (Fig. 2).
Transfer pumps operate by transferring the gas molecules by either momentum exchange (kinetic action) or positive displacement. The same number of gas molecules are discharged from the pump as enter it and the gas is slightly above atmospheric pressure when expelled. The ratio of the exhaust pressure (outlet) to the lowest pressure obtained (inlet) is referred to as the compression ratio.

Kinetic transfer pumps work on the principle of momentum transfer, directing gas towards the pump outlet to provide increased probability of a molecule moving towards the outlet using high-speed blades or introduced vapor. Kinetic pumps do not typically have sealed volumes but can achieve high compression ratios at low pressures.

Positive displacement transfer pumps work by mechanically trapping a volume of gas and moving it through the pump. They are often designed in multiple stages on a common drive shaft. The isolated volume is compressed to a smaller volume at a higher pressure, and finally, the compressed gas is expelled to atmosphere (or to the next pump). It is common for two transfer pumps to be used in series to provide a higher vacuum and flow rate. For example, a turbomolecular (Kinetic) pump can be purchased in series with a scroll (Positive displacement) pump as a packaged system.
Capture pumps operate by capturing the gas molecules on surfaces within the vacuum system. Capture pumps operate at lower flow rates than transfer pumps but can provide ultra-high vacuum, down to 10-12 Torr, and generate an oil-free vacuum. Capture pumps operate using cryogenic condensation, ionic reaction, or chemical reaction and have no moving parts.
Types of Pumps – An Overview
The different pump technologies are considered either wet or dry type pumps, depending on whether or not the gas is exposed to oil or water during the pumping process. Wet pump designs use oil or water for lubrication and/or sealing and this fluid can contaminate the swept (pumped) gas. Dry pumps have no fluid in the swept volume and rely on tight clearances between the rotating and static parts of the pump, dry polymer (PTFE) seals, or a diaphragm to separate the pumping mechanism from the swept gas. Although dry pumps may use oil or grease in the pump gears and bearings, it is sealed from the swept gas. Dry pumps reduce the risk of system contamination and oil disposal compared to wet pumps. Vacuum systems are not easily converted from wet to dry by simply changing the pump from a wet to a dry style. The chamber and piping can be contaminated by the wet pump and must be thoroughly cleaned or replaced, otherwise, they will contaminate the gas during future operation.
Following is an introduction to the most commonly used vacuum pump types by function.

PRIMARY (BACKING) PUMPS

Oil Sealed Rotary Vane Pump (Wet, Positive Displacement)

In the rotary vane pump, the gas enters the inlet port and is trapped by an eccentrically mounted rotor which compresses the gas and transfers it to the exhaust valve (Fig. 3). The valve is spring-loaded and allows the gas to discharge when atmospheric pressure is exceeded. Oil is used to seal and cool the vanes. The pressure achievable with a rotary pump is determined by the number of stages used and their tolerances. A two-stage design can provide a pressure of 1×10-3 mbar. It has a pumping speed of 0.7 to 275 m3/h (0.4 to 162 ft3/min).
Liquid Ring Pump (Wet, Positive Displacement)
The liquid ring pump (Fig. 4) compresses the gas by rotating a vaned impeller located eccentrically within the pump housing. Liquid is fed into the pump and, by centrifugal acceleration, forms a moving cylindrical ring against the inside of the casing. This liquid ring creates a series of seals in the space between the impeller vanes, which form compression chambers. The eccentricity between the impeller’s axis of rotation and the pump housing results in a cyclic variation of the volume enclosed by the vanes and the ring, which compresses the gas and discharges it through a port in the end of the housing. This pump has a simple, robust design as the shaft and impeller are the only moving parts. It is very tolerant of process upsets and features a large capacity range. It can provide a pressure of 30 mbar using 15°C (59° F) water, and lower pressures are possible with other liquids. It has a pumping speed range of 25 to 30,000 m3/h (15 to 17,700 ft3/min).
Diaphram Pump (Dry, Positive Displacement)

What is the working principle of vacuum pump?

A diaphragm is rapidly flexed by a rod riding on a cam rotated by a motor, causing gas transfer in one valve and out the other. It is compact and low maintenance. The lifetime of the diaphragms and valves is typically over 10,000 operating hours. The diaphragm pump (Fig. 5) is used for backing small compound turbo-molecular pumps in clean, high vacuum applications. It is a small capacity pump widely used in R & D labs for sample preparation. A typical ultimate pressure of 5 x 10-8 mbar can be achieved when using the diaphragm pump to back a compound turbo-molecular pump. It has a pumping speed range of 0.6 to 10 m3/h (0.35 to 5.9 ft3/min).
Scroll Pump (Dry, Positive Displacement)

The scroll pump (Fig. 6) uses two scrolls that do not rotate, but where the inner one orbits and traps a volume of gas and compresses it in an ever decreasing volume; compressing it until it reaches a minimum volume and maximum pressure at the spirals’ center, where the outlet is located. A spiral polymer (PTFE) tip seal provides axial sealing between the two scrolls without the use of a lubricant in the swept gas stream. A typical ultimate pressure of 1 x 10-2 mbar can be achieved. It has a pumping speed range of 5.0 to 46 m3/h (3.0 to 27 ft3/min).

BOOSTER PUMPS

Roots Pump (Dry, Positive Displacement)

The Roots pump (Fig. 7) is primarily used as a vacuum booster and is designed to remove large volumes of gas. Two lobes mesh without touching and counter-rotate to continuously transfer the gas in one direction through the pump. It boosts the performance of a primary/backing pump, increasing the pumping speed by approximately 7:1 and improves ultimate pressure by approximately 10:1. Roots pumps can have two or more lobes. A typical ultimate pressure of < 10-3 Torr can be achieved (in combination with primary pumps). It can achieve pumping speeds in the order of 100,000 m3/h (58,860 ft3/min).

Claw Pump (Dry, Positive Displacement)

The claw pump (Fig. 8) features two counter-rotating claws and operates similarly to the Roots pump, except that the gas is transferred axially, rather than top-to-bottom. It is frequently used in combination with a Roots pump, which is a Roots-claw primary pump combination in which there are a series of Roots and claw stages on a common shaft. It is designed for harsh industrial environments and provides a high flow rate. A typical ultimate pressure of 1 x 10-3 mbar can be achieved. It has a pumping speed range of 100 to 800 m3/h (59 to 472 ft3/min).

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How many microns should a vacuum pump pull down to?

Pull a proper vacuum to make a system clean, dry, and tight

Pulling a vacuum on a system is important to make the system operate properly and efficiently, and to help ensure a long service life. Yet many technicians are never fully schooled on how to pull a proper vacuum.
Pulling a vacuum on a system is important to make the system operate properly and efficiently, and to help ensure a long service life. Yet many technicians are never fully schooled on how to pull a proper vacuum.

Years ago, Harold G. Saunders and Emmit C. Williams wrote a little book called Review of Vacuum for Service Engineers. It’s hard to find, but if you can get your hands on a copy of this book, you might just realize how little you know about this common and yet often incorrectly performed procedure.

Many people say, “Pull a vacuum to 500 microns and you’re good,” but when I started studying how vacuum works in a system, I realized that there was a lot more involved to get the system to 500 microns, or lower, and to hold that level. The placement of the gauge, the size of the hoses, and the size of the pump are all important factors. As an example, a small pump with bigger hoses on it will get the job done faster than having a large pump with small hoses.

Here are a few rule-of-thumb tips:

How many microns should a vacuum pump pull down to?

Use a high-accuracy digital gauge with 1.0 or .1 micron resolution down to or below 50 microns.
Remove the valve cores with valve core removers rated at less than 30 microns of vacuum. Use the core removers as your blank off valves to keep the hoses out of the system during rise testing.
Use two 3/8-in. or ½-in. vacuum rated hoses, or even just one ½-in. vacuum rated hose (depending on the size of the system), from the core remover(s) straight to the vacuum pump.
Put your vacuum gauge as far away from the pump as you can possibly get it. The farther away from the pump your vacuum gauge is, the more accurate your measurement of the vacuum in a system will be and the more you will be able to tell about contaminants and leaks in the system during evacuation.

Remember, vacuum moves in a wave and seeks its own level, just like pressure above atmospheric level does. We do not see the wave as much as we do with higher pressure, but pressure drops occur through a lineset or a coil, and the lowest pressure in a system during evacuation is at the pump.

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How long can you run a vacuum pump?

This is an interesting question. The supplier states that a minimum of 10 minutes is necessary for a vacuum to be drawn, but as a footnote adds that if the gauge indicates that the vacuum is being lost, then water may be present in the system or a leak could exist. This is basically correct, provided the following facts are considered.

Normal compound analog manifold gauges used by installers cannot accurately measure vacuum. A vacuum gauge (typically a micron gauge) is required. There are more advanced manifold gauge sets that are able to measure vacuum accurately; however, I doubt that an installer would be using high-end, expensive electronic sets.

In an ideal world, the installer would be using new tubing that has the ends sealed to prevent the ingress of contaminants, including water. Their tooling, and in particular the gauge hoses, are kept sealed when not in use, and during the installation, care is taken to prevent contaminants/water from entering the pipework. The only job left for the vacuum pump to take care of, is the removal of air entering the pipework. Perhaps then a minimum of 10 minutes would suffice, provided the level of vacuum attained is 500 microns or less.

Having said that, after the 10 minutes of evacuation and with the vacuum pump turned off, the vacuum would have to remain stable at 500 microns or less for between 10 and 20 minutes.

Again, a vacuum analyser would have to be used, as the normal manifold gauges are not accurate at high levels of vacuum.

So, the bottom line is: A deep vacuum level of 500 microns or less must be reached and remain stable for the system to be declared free of moisture (water) and non-condensables (air).

I would like to expand on this with a more complete explanation.
Purpose of the vacuum

How long can you run a vacuum pump?

It is most undesirable to have any ‘foreign’ gas present in a refrigeration system. The most likely foreign gas is air (which is a mixture of gases, to be exact). Air is non-condensable in terms of the working pressures and temperatures of a refrigeration system. When various gases share a space, all their pressures add together to make the total pressure of the volume. This means that the pressure of any air (or other ‘non-condensable’ that may be present) is added to the working discharge pressure of the refrigerant. This means more power is used; refrigerant discharge pressure rises higher than necessary, and less refrigeration work is done. Temperature, particularly of the oil and at the discharge valves, builds up excessively.
But even more harmful in the system is moisture. This may enter as atmospheric humidity, or in many repair cases, a chilled water or condenser water tube could have fractured, and water could have entered by that route. Moisture may be present in two forms in the system:

Visible moisture (water)
Invisible moisture (water vapour)

Refrigerant oil is extremely hygroscopic (that means it readily absorbs water from the atmospheric humidity). Even if great care is taken, moisture can enter the system in the oil that is supplied ready charged in some compressors or is charged into the compressor before final evacuation.

Moisture reacts in the most damaging way with refrigerant oils and also with refrigerant itself, particularly if the system runs hot. This causes chemical reactions between refrigerant, oil, and the water, building up some extremely powerful acids, including hydrofluoric acid, which dissolves glass. These acids attack and corrode system metals, creating foulants, which add to the sludges that form in the oil, seriously harming the compressor lubrication. The compressor can literally be torn apart.

A good vacuum goes nearly all the way towards removing all the moisture from the system, particularly if the system is kept warm during evacuation. As we lower the pressure in the system, we lower the boiling point of the water in the system. At sea-level, water boils at 100°C. The barometric pressure at sea-level is 101kPa. We know that lowering the pressure lowers the boiling point of a substance. If we lower the pressure from 101kPa to 1kPa, we lower the boiling point of water from 100°C to 7°C. If the plant is exposed to an ambient temperature of 20°C, then there is sufficient heat to boil off the water. As we wish to remove the water from the system as quickly as possible and considering the pressure drop over long pipe runs, it is desirable to draw a vacuum of 500 microns.

Table 1 shows the boiling point of water at some low and very low pressures.

Absolute pressure

Boiling point (°C)

500 microns

-24°

5 000 microns

0°

1kPa

7°

2kPa

18°

3kPa

24°

Note: Normal analog gauges are not accurate enough on the vacuum scale to read 1kPa difference.
Deep vacuum versus triple evacuation

A normal system is treated by pulling a deep vacuum down to 500 microns. The vacuum is then observed for a period of time. If the vacuum remains at 500 microns, the system is deemed to be leak and moisture free. This is called ‘deep evacuation method’.

In the case of a wet system, that is to say a system with considerable moisture present, the ‘triple evacuation’ method should be used.

This process involves firstly evacuating the system to a ‘reasonable’ vacuum (that is, 5 000 microns). This vacuum is then broken with dry nitrogen. Nitrogen is supplied technically dry when this is what is ordered. The previously evacuated system is pressurised to about 60 or 70kPa (gauge) with dry nitrogen and allowed to stand in this state for an hour, after which it is re-evacuated.

The dry nitrogen will absorb water by evaporation, just as dry air will. This is sometimes referred to as a ‘blotting’ process.

Then, the nitrogen pressure is released and the vacuum pump is restarted. There is a strong flow of this now moist nitrogen towards the pump suction connection.

This process is repeated a second time but drawing a vacuum of 1 000 microns to get a further handle on moisture that may be loitering in the system.

After this, it should be possible to get down to the desired 500-micron vacuum.
Triple evacuation system

Vacuum gauges are used to measure any pressure below atmospheric pressure. Many gauges are available. Vacuum may be expressed in kPa vac and not –kPa. The vacuum scale on the compound gauge is not sufficiently accurate when working with dehydrating vacuums. A micron meter or similar gauge should be used.

Vacuum readings are normally in microns or mbar, and consist of the following:
Single-stage versus two-stage vacuum pumps

Vacuum pumps are normally of the rotary type and are offered in single-stage and two-stage configurations. A two-stage pump is simply two single-stage pumps constructed in series.

A single-stage pump is not recommended for refrigeration work. The vacuum pump is connected to the system through the charging manifold.

Standard reciprocating compressors do not achieve a high enough vacuum to dehydrate a system. The rotary vacuum pump is the most effective method of drawing a vacuum on a system.

A 1.5cfm vacuum pump is recommended for small units, 5–8cfm pump for commercial refrigeration, and larger 15–25cfm pumps for industrial applications.

A single-stage pump is not recommended for refrigeration work.

Testing of the vacuum pump

Before using a vacuum pump, check the safety aspects, the electrical cord, and the suitability of the pump for use. Check the oil level and colour of the oil (the oil should be clear, not black or milky white). Test the capability of the pump, using a micron meter. A vacuum of at least 500 microns is necessary. If it does not achieve 500 microns, replace oil and retest. If after oil replacement has being carried out and the pump still fails to achieve the required vacuum, the pump should be serviced.

The vacuum pump’s oil should be changed regularly and especially after drawing vacuum on a contaminated system.
Drawing vacuum

It is best to purge a system through with nitrogen prior to drawing a vacuum for best results, as nitrogen absorbs moisture better than air.

In order to draw a dehydrating vacuum:

Test your vacuum pump and take note of the vacuum attained.
The pump should achieve a vacuum of 500 microns or better.
Make sure that the system is at 0kPa pressure before connecting the vacuum pump.
Connect vacuum pump.
Connect micron meter.
Open all valves between pump and system.
Run pump until a vacuum of 500 microns or lower has been reached.
Make a note of the reading.
Close valves and stop vacuum pump.
After two hours, the reading should read the same.

Vacuum pump oil

The oil in the pump is very prone to picking up moisture that is being drawn from the system. If the oil becomes saturated, it will be impossible to achieve a good vacuum. It is good practice to ensure that the vacuum pump is capable of drawing 500 microns before each use. Vacuum pumps use specialised vacuum pump oil. Consult with the vacuum pump manufacturer for the correct oil.
Assessment of the vacuum

Once the required 500-micron vacuum has been achieved, manifold gauges closed, and the pump has

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Does pulling a vacuum remove oil?

Setting up Your Vacuum Pump

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Fill the pump with vacuum oil. Before you use your vacuum pump, make sure it is full of clean vacuum pump oil. Unscrew the oil fill cap, typically located on the top of the pump, and look on the interior edge of the opening for the fill line. Fill the opening with oil until it reaches that line. Then, replace the oil fill cap.[1] Be sure to only use oil meant for vacuum pumps. Using other mechanical oils could impact the quality and performance of your vacuum.
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Attach your gauges to the ports. You will need a gauge set that connects to both your vacuum and your pressure ports on your AC system. The blue gauge and hose should connect to the low-pressure service port. The red gauge and hose will connect to the high-pressure port. The yellow hose in the middle should connect your gauges to your vacuum.[2] Make sure the gauges and gauge hoses are connected tightly. Loose seals can compromise your vacuum.
In your car, your high-pressure port will generally be physically higher than the low-pressure port.[3] Image titled Use a Vacuum Pump Step 3
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Open your manifold valves. Once you have your gauges in place, you will need to use the valves on your AC system that opens and closes the service port to the refrigerant lines. With the valves closed, your gauges should have little to no pressure reading.[4]

Part 2
Pulling the Vacuum

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Start your pump. After you are sure everything has been securely connected, use the switch device on your vacuum pump to turn it on. You should be able to hear the vacuum running once it is switched on.[5] If you are trying to start the pump in cold weather, open the intake ports until the pump reaches normal running speed. Then, close it off again.[6] Image titled Use a Vacuum Pump Step 5
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Open the side gauge valve. Once your vacuum is on, you will need to open the gauge valves, located on the side of each gauge. This allows the vacuum to start pulling air out of the system.[7] If you are unsure which way you need to turn your valves to open them, check the manual that came with your gauges or vacuum.
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Allow the vacuum to run for 15-30 minutes. Let your vacuum run at full operating speed for at least 15 minutes and up to 30 to completely pull the air out of your AC system. The exact amount of time you will need to let your vacuum run will vary based on your manufacturer’s recommendations, so check any operational manuals you may have for both your AC system and vacuum pump.[8] Generally, you want to let your vacuum run for at least long enough to get the measurement below 1,000 microns. If you can, try to get it down to 500 microns.

Does pulling a vacuum remove oil?

Part 3
Closing Your Vacuum

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Close the low-side valve and let the vacuum hold for 15 minutes. After you have let your vacuum run for a sufficient amount of time, close the valve that connects to the low-side gauge. Let your pump hold the vacuum for 15 minutes.[9] If it’s not able to hold for that long, you likely have a leak and may need to replace components on your vacuum pump.
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Shut off the vacuum pump. When you are satisfied with how long your system held the vacuum, shut the pump off using the same switch mechanism you used to turn it on. Let your vacuum disengage completely before you disconnect the system.[10] Image titled Use a Vacuum Pump Step 9
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Disconnect your vacuum. Once your vacuum has fully disengaged, you can disconnect the hose leading to the pump. Your AC system should be fully evacuated at this point, and ready to be recharged or repaired

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How does a HVAC vacuum pump work?

Air Conditioning Vacuum Pump

Air conditioning vacuum pump is used to remove unwanted air and water vapor from the air conditioning system when it is under service. When the air conditioner needs to be repaired, the first step taken is usually to recover the refrigerant from the system for reuse later.

Once the recovery job has been done, you can now proceed to do the necessary troubleshooting and repair work. The next step is to check and make sure that the system is leak free else steps will have to be taken to rectify this.

The evacuation process is the next step in which the vacuum pump is used to remove the air and moisture from the refrigerant system. Once this is done, the charging process can be carried out before doing a full test on the system.

The Necessity of Evacuation

In a refrigerant system, only the refrigerant and oil should be circulating. During servicing or after many years of operation, the air may enter the system. The air from the atmosphere that enters the system include oxygen, nitrogen and moisture. These unwanted components will cause:

Rise in head pressure causing higher discharge temperatures and compression ratios. Efficiency of the system is reduced.

Acids are produced in the refrigerant causing chemical reaction resulting in electroplating and damage to the motor insulation. Once the insulation in the compressor motor breaks down, short circuit will happen hence damaging the compressor. The compressor is one of the highest equipment cost in the system. Acids also cause corrosion to the metal parts over time.

Sludge is formed by a combination of oil, acid and moisture in the system. It will cause improper operation to the filter drier, strainers and expansion device over a period of time.

Air Conditioning Vacuum Pump Operation

In order to remove all the unwanted moisture and gases from the refrigerant system, a state of near vacuum has to be achieved in which the pressure in the system is forced to go below the atmospheric pressure. The absolute value of the atmospheric pressure at sea level is given by 29.92 in. Hg or 14.696 psia or 759.999 mm Hg or 759,999 microns.

In the typical system, the vacuum pump is required to create a vacuum state of about 300 to 500 microns. An electronic vacuum gauge is needed to measure the level of vacuum in the system. Check the specifications of the vacuum pump and make sure that it is able to achieve the vacuum state as required by the manufacturers of the equipment.

How does a HVAC vacuum pump work?

A 2-Stage Vacuum Pump from Robinair

Rotary compressors are used in the pumps and the better ones that are able to produce the lowest vacuums are the 2-stage rotary vacuum pumps. However, they are more costly but their abilities to remove moisture effectively is one of the main reasons why many technicians prefer to purchase this model.

As the system enters low vacuum state, the moisture will begin to boil and becomes vapor enabling its removal from the system easily.

Proper oil needs to be used when operating the air conditioning vacuum pump. Use only the recommended oil to enable the pump to achieve the specified vacuum state.

Take note of the followings when selecting air conditioning vacuum pump.

Sight glass oil to see the level of oil before operating.
Anti-suckback feature that will prevent the oil from the pump from flowing into the refrigerant system in the event of power failure. The oil in the refrigerant is different from the ones in the pump.
CFM rating. The higher the cfm, the faster the evacuation process is.
One-Stage or Two-Stage design.
The lowest vacuum level that the pump can achieve.
Intake fittings.
Weight.

Popular brands in the market include Robinair, Yellow Jacket, Inficon and Arksen.

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