In HVAC/R, the compressor is often referred to as the “heart” of the refrigeration circuit. Indeed, like the heart, a compressor is primarily a pumping device. There is also a historical reason for this, and one that will probably not change over time: the compressor is both the most expensive component and the one that consumes most of the energy in the majority of HVAC/R applications.
It is also true that the other components in the refrigeration circuit are essential. An inefficient heat exchanger wastes the work done by the compressor. An expansion valve can irreparably damage it in just a few minutes if it allows excessive flow of refrigerant into the compressor in the liquid state. Yet even the fans, water pumps and crankcase heaters have a role. And I’m certain, as I myself often see, that the manufacturers of each of these components would highlight the vital importance of their product in the context of the application.
Regardless of individual opinions, it is undeniable that a refrigeration circuit represents a sort of organism in which interaction between its components can become quite complex. This is why drawing a parallel with the human body can help provide some interesting analysis.
HVAC/R applications work like the human body
I should start by saying: nothing described below is intended to put a refrigeration circuit on a par with a human being. Even though it may seem obvious, I wanted to clarify this from the outset, so as to not offend the sensitivities or ethical principles of my readers.
So let’s begin with a description of basic operation:
The heart pumps the
blood that circulates in the
arteries of the human body, so that this can
transfer oxygen and other nutrients to the other parts of the body, for example, the
muscles of the hands, allowing interaction with the outside world, for example,
picking up a glass.
The compressor pumps the
refrigerant that circulates in the
piping of the refrigerant circuit, so that this can
transfer energy to the other parts of the circuit, for example, the
heat exchangers, allowing interaction with the outside world, for example,
air-conditioning a room.
A small cut on the body may be irritating and typically heals on its own, thanks to the proper functioning of the blood clotting system. A crack in a refrigerant circuit, on the other hand, caused for example by vibrations, causes a slow leakage of refrigerant. The cooling unit will slowly decline in efficiency until it stops working. In this case, there is no self-repair system.
For refrigerant leakages, we also need to take into consideration the negative impacts these cause, such as, in no particular order: toxicity, explosiveness, asphyxia, hot and cold scalding that can cause major injuries, and environmental damage. As we also know, for many of the refrigerants commonly used in HVAC/R, a leak into the environment contributes to the hole in the ozone layer and the greenhouse effect.
Therefore, we shouldn’t be surprised if such a common event - neglected for decades - is the subject of international agreements, directives and regulations for use and phase-out, and has a growing impact on the HVAC/R market.
A refrigeration circuit has a degree of complexity that depends on the non-linearity of its different chemical and physical processes. This feature makes modelling and troubleshooting circuits quite difficult, even with today’s computational power.
As a consequence, failure diagnoses, to draw one final parallel with the human body, are anything but trivial. Furthermore, as mentioned in the introduction, they not only concern the compressor. Indeed, they are often related to components that are decidedly less expensive and less complicated.
Here are some examples.
Do heat pumps need doctors?
The liquid receiver is a passive element that does not absorb nor yield energy. It is used to ensure subcooling of the liquid entering the expansion devices. Under-sizing the receiver leads to malfunctions that are not immediately evident.
This occurs on air-water heat pumps, where stoppages may occur due to the high pressure limits being reached in low load conditions, at low capacity or with compressor operation at part load. The difficulty in diagnosis is due to the normal association of high pressure = maximum capacity. Once the refrigerant charge has been calibrated for maximum capacity conditions, the engineer can consider installation and maintenance to be complete.
However, it is precisely at low capacity when the volume imbalance between the air-cooled evaporator and the water-cooled condenser is such as to require greater storage of refrigerant elsewhere in the circuit. Under these conditions, the unit “uses less refrigerant”, which must of course accumulate somewhere else in the circuit. If the liquid receiver volume is insufficient, refrigerant accumulates in the condenser, fills it to the limit and causes excess pressure.
Another passive and low-cost component capable of generating “unexpected” malfunctions is the non-return valve installed on the compressor discharge. In multi-compressor configurations (tandem or trio), a malfunction or even absence of such valve does not cause any problems in the short term.
However, as operating hours accumulate, an imbalance in the lubrication system can occur, leading to the breakdown of one of the compressors due to a lack of oil.
How does this happen?
The oil contained in each compressor comes out of the discharge pipe, entrained by the gas. This phenomenon cannot be avoided. In the absence of a non-return valve, or in the event of leakages, oil can slowly flow into a compressor that is currently not running. In fact, units may often work for hours or days at reduced capacity. A compressor that is off does not generate pressure and as a result “takes in” all of the oil that is inadvertently provided by the compressors that are running, due to malfunction of the valve, and these compressors gradually run out of oil.
In this case too, diagnosis requires knowledge of good design criteria and careful analysis of the circuit, also because the oil that is either present or absent in a compressor is typically not visible.
In summary, then, I have described two cases of malfunctions at medium or low capacity of the HVAC/R application. This demonstrates the fact that the worst conditions do not always correspond to maximum load.
Power is nothing without control
As an expert in the control of refrigeration circuits, it is obvious that I should also focus on the importance of the control electronics, which can simply be referred to as the brain of the refrigeration circuit. However, to confirm what I said above, it is impossible to state that any one component is more important than another.
When diagnosing more complex HVAC/R applications, it is useful to have sophisticated electronic control and protection systems capable of interpreting alarms and providing diagnostic information.
One example involves the alarms relating to the compressor speed drives or inverters. A typical alarm is “high current”, i.e. a safety threshold is reached beyond which the speed drive can no longer control the compressor, thus shutting it down. An immediate diagnosis would focus on the drive itself as the cause, with the solution being its replacement. When the possible explanation is under-sizing compared to the compressor operating conditions, the drive is often replaced with a more powerful version, capable of delivering more current.
The result? The compressor starts again, but after a few hours/days it stops, this time probably forever.
How did this happen?
The initial overcurrent alarm was a symptom, and not the cause. The real cause may have been absence of oil in the compressor or a malfunction of the expansion device, with consequent presence of liquid refrigerant in the compressor. This caused the increase in current and the drive, which is the compressor’s control and protection system, signalled a fault and stopped the compressor.
Failure to fully analyse the cause of the alarm led to breakage of the compressor, which otherwise could certainly have been avoided.
Electronics capable of signalling a “high current alarm - possible insufficient oil/presence of liquid in the compressor” could have saved the heart of the unit, which has been irreparably damaged.
And in the near future, electronics will not be separated from connectivity and the intelligent use of IoT, both for HVAC/R and Healthcare applications. So, the brain of the refrigeration circuit will become more and more evolved, and air-conditioners will become more and more intelligent.
IoT: data processing in a world of connected machines