Air Conditioner Compressors. The Black “Box”…

Air conditioner compressors are, what you normally see as the Black “Box” in air conditioning units. I call it the Black “Box” because the casing is normally black in colour.

The page about air conditioner parts has explained the principles, most air conditioner compressors use for compressing refrigerant.

The compression may occur due to rapid reduction in volume, positively “sweeping” refrigerant from inlet to outlet port, and by changing kinetic energy into potential energy.

Now it’s time to dig deeper into understanding air conditioner compressors:

I would like to give you an overview of how air conditioner compressors are categorised. These are categorised by,

  • the seal type (e.g. completely sealed, semi-sealed and open type), and

  • the operation type (e.g. piston, scroll, rotary and centrifugal)

Let us also consider the basic characteristics that ALL compressors share.

That set of basic characteristics is broken into,

  • the pressure rating p (in bar or pounds per square inch - psi)

  • the flow rating (in m3s-1 or cubic feet per minute - cfpm), and

  • the speed rating N (in rpm)

These characteristics will be plotted to form a compressor map for a specific compressor.

Performance of manufactured compressors will be tested, to obtain results of the pressure ratio, efficiency, mass flow rate, and surge/maximum pressure ratio limit, against the compressors’ rotational speed. Results will be plotted in a compressor map.

Air conditioner compressor mapA compressor map will consist of a set of speed curves, and a surge/maximum pressure ratio. The y-axis will be the pressure ratio range of the compressor, and the x-axis will be the mass flow rate of the compressed medium.

A contour of compressor efficiency ranges will be plotted within the speed lines. The maximum efficiency of a compressor will be in the centre contour area.

The centre contour will be,

  • away from the surge/maximum pressure ratio line, and

  • away from the maximum achievable flow rate of the compressor, at a given speed

Surge/maximum pressure, maximum flow rate and efficiency:

Any compressor rotating at a specific speed N1 will only be capable of,

  • sustaining a maximum flow rate of 1, and

  • a maximum pressure ratio of p1/p0, where p0 is the pressure of the gas at inlet

Maximum flow rate

The maximum flow rate could not be exceeded simply because the speed is already fixed.

The surge/maximum pressure ratio

Now, imagine if the air conditioning compressor keeps compressing the refrigerant, but the overload protector is malfunctioning and the condensate line is blocked. Pressure in the system will rise.

There will come a point, when the compressor could no longer support the rising pressure and backflow of compressed refrigerant will start to happen.

This is known as the surging/maximum pressure point. Backflow will occur quite violently from this point onwards, and damage to compressors will be significant, due to high vibration. This happens as the force from compression, is lower than force from the downstream high pressure gas.

The only way to get pass this point is by,

  • lowering the downstream gas pressure, or

  • increasing the compression force by increasing the compression speed

However, the latter choice will utilise more shaft power from driver, per compression work. In other words, the efficiency will drop.

Lowering the downstream pressure would be more practical to avoid surge/maximum pressure ratio. The efficiency will increase, as an effect to that. However, lowering the downstream pressure too low will make the compressor inefficient, as the shaft power conversion into compressing the gas will be very low.

Surge occurs only for centrifugal compressors, axial compressors and fans, as backflow may occur between the tip of the blade, and the casing.

Surge can be easily detected by the “low hammering” or pulsating sound. One word. Compressor surging is bad. Ok, that’s four words.

Other tightly sealed compressors will only face the maximum pressure ratio, as the back pressure from the compressed gas, will overcome the driver’s (e.g. an electrical motor) force.

Compressors’ efficiency

Defined as the actual work done to compress gas, per energy input from the driver’s shaft. The driver is usually an electric motor.

Compressors’ efficiency (and that includes air conditioner compressors as well), is proportional to the product of mass flow rate, and pressure ratio.

We have to bear in mind that, the pressure ratio is low when the mass flow rate is high. The compressor will act to transport the gas, rather than compressing it.

On the other hand, the mass flow rate is low when the pressure ratio is high. This makes sense, as the back pressure from the gas will slow down the gas transfer.

Yes, absolutely. The maximum compressor efficiency will be attained at moderate mass flow rates and pressure ratio.

The maximum compressor efficiency is normally around 80%.

The compression power of air conditioner compressors are normally rated in horsepower hp, or kilowatt kW. One horsepower is equivalent to 0.7457 kW.

We have talked about the general features of compressors. It is now time to go a little deeper. You shouldn’t miss this fun.

Sealing of compressors:

There are three types of air conditioner compressors construction. The completely sealed, semi-sealed, and the open type.

Completely sealed compressors are also known as the hermetic type, and the semi-sealed are known as the semi-hermetic type.

What do these suppose to mean? The,

  • hermetic compressors will have direct connection to the motor, and sealed in a welded casing. This type of construction allows no maintenance except in factories with proper repair tools and skill

  • semi-hermetic compressors will have direct connection to the motor. However, the sealing is not a complete one. There will be a provision to dismantle the casing for parts replacement, and maintenance

  • open type, has the compressor and driver separated from each other. The compressor and the driver will be separate entities, connected by a coupling. Sealing of refrigerant gas from atmosphere is achieved by means of labyrinth and mechanical seals. Yes, this compressor construction is suitable for high capacity cooling

Air conditioner compressors construction types

Air conditioner compressors for residential, and for average-large sized office buildings, will normally be of hermetic, and semi-hermetic construction.

Moving on, to the advantages and disadvantages of these two sealing types.

AdvantagesHigher efficiency, as the possibility of compression leakage is very smallPossible to do maintenance and parts replacement, without having to change the complete unit
DisadvantagesConsider buying a new air conditioning compressor, if you note signs of trouble Lower compression efficiency. Leakage from compressor body is highly possible

Sealing topic is done, and you’re already prepared to go a little more into air conditioner compressors. Great!

Operation types of air conditioner compressors:

The list is almost non-exhaustive. We have centrifugal, rotary, screw type, roots blower type, sliding vane type, plunger type, ejector type, liquid-ring type, axial type, swash plate type and gear lobe type.

Talking about all of those will be quite interesting indeed, but let’s keep our topic to the most commonly used compressors in air conditioning cycle. The piston and the scroll compressor.

It begins now...

The piston or reciprocating compressor

It’s the oldest type of compressor, going back as far as 1600’s. Very robust and simple in construction, able to deliver very high compression pressures. Just to illustrate, a reciprocating compressor with 40 mm cylinder length, is able to deliver about 12 barG (gauge) of pressure (or 150 psiG or 120 metres of water height).

This type of compressor is constructed using,

  • piston head – functions as the gas compressing “agent” by continuously reducing the cylinder volume

  • piston rings – functions as the sealant between the piston head, and the cylinder, to prevent gas leakage from the compression chamber

  • crank shaft – a shaft that enables the reciprocating motion of the piston

  • piston rod – the connecting piece between the piston head, and the crankshaft

  • spring loaded suction and discharge valves – separates low pressure side and high pressure side from the compression chamber. Enables positive displacement of gases, by correct opening and closing of the valves. Suction valve will open as the piston moves away from the valves, and discharge valve will open as the piston moves towards the valves. The valves will otherwise, be in closed position

  • and, compressor’s cylinder block – functions as the housing for the compressor parts

Piston type air conditioner compressors. Working principle

Those are the basics. However, modifications are carried out in air conditioner compressors of this type, to have two pistons, and oriented horizontally, to improve the compression efficiency.

Piston air conditioner compressors. Cutaway. Courtesy:


Identifying piston air conditioner compressors by the squarish and squashed construction. Courtesy: KK
Coursety: KK
Identifying a reciprocating compressor is quite easy, even without opening its casing. The casing is almost square in shape, with similar height of its width.

Most approved refrigerants are compatible with this type of compressor. Those are, R-22, R-134a, R-404A, R-407C and R-507.

Robust and powerful as it may be, volumetric efficiency became a concern, as the valve opening do not happen as soon as the piston is on suction stroke. In addition to that, failure rate of reciprocating compressors is quite high due to mechanical stresses from the alternating motion.

Fast forward to three hundred years later, we have the first

Scroll compressor

It was invented in the beginning of 1900’s. As usual, initial inventions need improvements, but all credits to Léon Creux who created it, so we can work on improvements. It was optimised and improved around late 60’s.

Scroll type compressors, able to achieve the flow rate, and outlet pressure, similar to reciprocating compressors, at a smaller size, and better efficiency.

However, cooling for scrolls are quite difficult compared to piston air conditioner compressors. This is the reason for its performance drop against piston type, at higher compression ratios.

Nevertheless, most air conditioning applications require scroll compressors to be used against piston type, due to the advantages.

Scroll air conditioner compressors operating principle

This type of compressor is constructed using,

  • a stationary scroll and an orbiting scroll – the orbiting spiral scroll will orbit around the stationary scroll, thus continuously and progressively trapping gas and directing towards the centre of the scrolls, to be discharged pass the discharge check valve, and towards discharge line

  • crank shaft – used for creating the orbiting motion. This shaft is equipped with counter weights to equalise the centripetal force due to eccentric shaft rotation

  • casing – to ensure that the discharge part is separated from the suction part, and motor winding is separated from the refrigerant

Scroll air conditioner compressors. Cutaway view. Courtesy: Sanyo
Courtesy: Sanyo
Sealing between high pressure and low pressure side of the scrolls are helped by the downward force to the stationary scroll, by the discharge gas pressure.

This type of air conditioner compressor, could handle similar refrigerants to piston compressors.

Differentiating scroll and rotary air conditioner compressors. Courtesy: Sanyo &
Courtesy: Sanyo &
Identifying this type of compressor is not that easy though. Yes it is normally a vertical cylinder, but rotary compressors share the same shape.

But if you look closely, scroll compressors have a "cap" like top cover, and the discharge line is on top of the casing. Whereas discharge line for rotary compressors is on the side, slightly lower than the top part.

Supporting parts for air conditioner compressors:

Air conditioner compressors are supported by important parts such as,

  • bearings

  • spiral grooved shaft for lubrication oil distribution, and

  • strainers to prevent solid ingress into the compression chamber

What manufacturers normally practice?

Designing air conditioner compressors is a tedious process, as there is the need to balance with the heat extraction requirement at the evaporator.

Manufacturers will normally have a standard list of common compressor sizes and ratings, to be matched with a suitable evaporator and condenser.

The compressor type and size selection, will be made based upon the refrigerant used, and the desired heat removal at the evaporator.

We sure have covered the basics of air conditioner compressors.

Top of Air Conditioner Compressors Page

Back to Air Conditioner Parts