Vacuum Pumps Explained - Basic working principle HVAC

Hey there guys, Paul here

from theengineeringmindset.com

In this video we're going to look at how vacuum pumps work,

the main parts, and why we use them.

Vacuum pumps are used extensively by air conditioning

and refrigeration engineers to remove air

and non-condensables, such as water, from a system.

We need to remove these with a vacuum pump

because they cause the refrigeration system

to operate inefficiently and they can also corrode

the internal parts.

This procedure is carried out before a new system

is charged with refrigerant or when an existing system

has undergone some repairs,

or the refrigerant has already been recovered.

In either case, there's a chance that air or moisture

have contaminated the inside of the system.

On a typical air conditioning system,

you'll see the vacuum pump is connected

via manifold across the high and low pressure side

of the system.

A better way to do this is to remove the manifold completely

and connect the vacuum pump to the suction line

with a pressure gauge connected to the liquid line.

We connect the pressure gauge here

because it's the furthest point in the system

so you get a true reading.

Now I've teamed up with my friend Brian

over at HVAC school for this video and he's going to

run you through how to actually connect a vacuum pump

to a real world system.

He's also got lots of great technical tips to help

build your knowledge and skills.

So do check that video out, link is down below.

If we take a standard vacuum pump,

which looks something like this,

then we have the electrical motor on the back

and the compressor on the front.

On the top we have a handle and on the bottom

we have a support base.

We then have an inlet, which connects to the system

to remove the air.

And then above the compressor section,

we have the exhaust or the outlet.

On the front of the compressor section we find

an oil sight level glass and we can tell how much oil

is in the chamber as well as it's condition.

As we take the unit apart we can see we have a fan

in protective casing mounted at the back of the motor.

Then inside the motor we have the stator, which holds

the copper coils but we're going to look at this part

in detail just shortly.

Concentric to this, we have the rotor and the shaft

which drives the compressor.

At the front we have the compression chamber.

These are the two stage compression version,

which allows us to pull a deeper vacuum.

So we, therefore, have two compression chambers.

Inside the chambers are the compressor rotors

and the vanes which move the air out of the system.

On top of the compression chamber is a reed valve

which vents the exhausts.

When we remove the fan's protective casing,

we see that the fan is connected to the shaft,

which runs it through the pump.

The fan is used to cool down the electrical motor

and it will blow ambient air over the casing

to dissipate this.

The fins on the casing increase the surface area

of the casing which allows us to remove more unwanted heat.

Inside the motor, we have the stator, which is wound

with copper coils.

When an electrical current flows through the copper coils,

it will generate a magnetic field.

The rotor is affected by this magnetic field

and this forces it to rotate.

The rotor is connected to the shaft

and the shaft runs along the length of the pump

from the fan all the way up to the compressor.

This way, when the rotor rotates, so will the compressor.

And that's what we're going to use to create

the vacuum effect and evacuate the air from the system.

Just to note, that when we think of a vacuum,

we usually think of a sucking force.

That's not actually the case and we're going to see why

just shortly.

If we look inside the compressor, we can see we have

the inlet, which is connected to the system we're evacuating

Then we have the outlet and the reed valve,

which vents the air and moisture out which is being

evacuated from the system.

In the centre, we have the compression rotor

and the compression chamber.

Notice the rotor is eccentrically mounted

inside the chamber.

This means it isn't perfectly central.

That's a key feature which we'll see in detail shortly.

The shaft connects to the rotor and will cause it to rotate.

Mounted inside the rotor are two spring loaded vanes.

The springs always trying to push the vanes outwards

but they're held in place by the compression chamber walls.

Therefore, the tips of the vanes are always in contact

with the wall and there's a thin layer of oil

which helps to form a seal between the two.

When the rotor rotates, the springs continue to push

the vanes outwards so the vanes will follow the contour

of the compression chamber.

When the pump starts the rotor is going to move across

the inlet and expose an error inside the compression chamber

This error will be at a lower pressure compared to the

pressure inside the system.

So the air and moisture inside the refrigeration system

is going to rush in to try to fill this empty region.

So then why does it do this?

Well pressure always flows from high to low.

So if we connected, for example, two balloons

of different pressures then the gases will move

from the high pressure side into the low pressure side

until both are of equal pressure.

In this example, the low pressure side was a vacuum

but it didn't suck the gases in.

The high pressure side pushed it's way in.

That's the vacuum effect.

Gases want to equalize and will flow from a high pressure

to a low pressure.

Therefore, we use a vacuum pump to create a region

of lower pressure so that the unwanted gases

inside a refrigeration system will rush out of the system

to try to fill this lower pressure region.

So in our scenario the connection hose

and the new low pressure error

within the compression chamber have become an extension

to the refrigeration system.

So the gases in the system are going to rush in

to try to fill this and try and make that the same pressure

between the two.

But it's actually a trap.

Because as the rotor continues to rotate,

the second vane sweeps in and traps that volume of gas

in the chamber between the two vanes.

The other vane then passes across the inlet

and creates another low pressure region.

So more gas rushes in from the system

to try and fill this void.

As the compressor rotates, the volume of the chamber

is going to start to decrease.

That's why the rotor isn't perfectly centered

so that we can vary the volume of the trapped gases.

This decrease in volume is going to compress the gases

into a tighter space.

That would increase the pressure and the temperature.

It continues to rotate into a smaller volume

until the pressure becomes high enough that it forces

the reed valve of the exhaust to open

and the gases are then discharged.

The compressor continues to rotate,

and as it does so, the next batch of gases

is poured into the system and the cycle continues.

Most vacuum pumps will be two stage,

which means there are two compression chambers

linked in series.

The exhaust from the first compressor links directly

into the inlet of the second chamber.

This design allows the pump to achieve a deeper vacuum.

When we have a single compressor, the outlet is

pushing against atmospheric pressure.

But with the two stage design, the outlet is

pushing against a much lower pressure because it's simply

the inlet of a second rotating compressor

and the low pressure region it's created during that rotation.

As the vacuum pump continues to run, it will eventually

pull the gases out of the closed system,

which will reduce the pressure down below the pressure

of the atmosphere, which is surrounding the outside

of the system.

As the pressure reduces, any moisture in the system

will become easier to boil and evaporate.

We can add a little heat with a heat lamp or heat gun

to help it vaporize and that way we can extract it

from the system.

Okay that's it for this video but to continue your learning

then check out one of the videos on screen now

and I'll catch you there for the next lesson.

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