By Kevin Roberts
Anyone who has performed automotive service for any amount of time has learned the principle of balance. Fuel must be balanced with air in the gasoline engine combustion chamber. Brake-pad-to-rotor coefficient of friction must be balanced with vehicle weight and tire design. Piston ring friction must be balanced to prevent excess heat production and to keep engine oil out of the combustion chamber.
How Is Balance Relevant To The HVAC System?
The short version is that the amount of refrigerant must be balanced with the interior volume of the system. If you find yourself asking, “Why does that matter,” You are missing an idea that will prevent you from making the most common mistake found in HVAC service.
The long version is that balance is not only between the internal volume and the amount of refrigerant. The size and capability of each component must be balanced against the size and capability of every other component. An oversized condenser is a waste of material and space. An undersized condenser will be unable to remove the amount of heat that needs to be removed. This highlights the difference between An HVAC system and the atmosphere. The weight of the atmosphere provides a mostly fixed pressure worldwide. For the geeks out there, this is known as an isobaric system. Iso=same, baric=pressure. This causes any “boiling water” analogy to “boiling refrigerant” analogy to break down. An HVAC system provides a fixed volume. This is known as an isochoric system If you heat a balloon, it expands. If a hose balloons like tires occasionally do, that could disrupt the balance between volume and refrigerant.
If the refrigerant charge is either too low or too high, the balance is disrupted and you either frost the evaporator or see the high side pressure spike. Because this is a balancing act, changing the interior volume will accomplish the same result as the wrong amount of refrigerant. This mistake is more common than overcharging or undercharging the system.
How Do We As Techs, Mistakenly Change the Interior Volume of the System?
We need to understand that some common words might have different meanings to different techs. This may come as a surprise to people who know me; I’m a stickler for definitions and the proper use of terminology. But in this case, think about the word “empty”. If you are diagnosing a “cranks, no start” condition of an admin Tahoe, you need to know that there’s fuel in the tank. If there isn’t, it’s empty. In the HVAC world, if you are recharging a system after a repair, you must first make certain the system is empty. What is empty to a drivability tech is not the same as empty to an HVAC tech. The empty fuel tank is actually quite full. It is full of air. To empty an HVAC system, you must remove the air, or it is not empty. So, what does this have to do with balance?
To come full circle, we need to understand another word: condense. To condense is to turn a vapor into a liquid. That, oddly enough, is what happens in a condenser. Refrigerant is condensable. That means when you compress it as a vapor and thereby raise both its temperature and heat content and then pass it through a condenser that is subject to airflow around the tubes, you remove that heat and cause it to condense. If you can’t get it to condense, you either aren’t getting proper airflow to remove the heat, or you have something in the system that is not a refrigerant. The most common “something”, in this case, is air. Air in the system is known as a non-condensable gas. That means that pressurizing it and cooling it cannot condense it at the pressures and temperatures found in an automotive HVAC system. The upshot of this is that any residual air in the system effectively upsets the balance between refrigerant charge and system interior volume. Residual air takes up space, making the interior volume of the system too small for the amount of refrigerant. The most common mistake made in most HVAC systems is not properly evacuating the air. This leads to rapid compressor cycling, high pressure readings, and ineffective cooling.
Yes, it might be boring to wait for the vacuum pump to evacuate sufficient air molecules from the far end of the system, but this is how you prevent the vehicle from returning for an HVAC repair shortly after an HVAC repair.
Finally, what does sufficient mean? Do we evacuate all the air? Unfortunately, no. Could we? Not practically. Once the air molecules are far enough apart, the pressure is so low there is nothing to force those air molecules out of the service ports. Molecules that linger in the far reaches of the system just sit there. What we need is a system that is designed to work with whatever amount of air that we cannot get out. This usually works out to 99.93% of the air molecules. How many molecules are left? Well, that depends on…you guessed it, the interior volume of the system. But a cubic foot of volume with 99.93% evacuation still has 4.66 X 1020 molecules of air. That’s 466 quintillion molecules compared to 7.6 X 1023 or 760 septillion molecules at sea level pressure. What do we call this level of evacuation? 500 microns. What’s a micron? That we will cover next time.
