This article on aircon refrigeration is written mainly for aircon servicing technicians or someone who has interest in the technicality of air conditioning. Many refrigeration problems can be due to lack of airflow, either from dirty filters or dirty coils, as noted above. If the evaporator coil is freezing up you would check the filter first, indoor coil second then the fan, these are easy preliminary checks to be made. Another cause of low airflow can be a belt /pulley problem, if your equipment has this sort of motor / fan drive, as the belt may be slipping and reducing the fan air output. This is easily fixed by tightening the belt, but you should check your manual, as over tightening may reduce the life of the bearings.
Low refrigerant charge is a common cause of coil freezing as the pressure of the refrigerant in the system determines the temperature, and a lower pressure corresponds to a lower temperature. Over the years you may expect that some refrigerant is lost through joints and fittings, particularly with an older system. Also, most equipment is designed to be used in hot weather. If the outside temperature is not high, say below 65ºF, but your system still needs cooling because of high internal heat loads, you may find that you get freezing, and this is a result of the equipment manufacturer’s decisions for design of the unit. This condition can be overcome with various fan or refrigerant controls.
Recharging Your System
Recharging the refrigeration circuit on your system is one of the trickiest mechanical operations to perform because there are so many variables to be taken into account. I don’t tell you that to scare you I just want you to keep in mind that you need to be looking at more than one or two things when you perform this task. You must know these things before you begin!!! The indoor and outdoor coils Must Be Clean, (air is your heat transfer media so limited airflow means limited transfer).
The outdoor temperature where your outdoor unit is takes the reading where the air enters your outdoor coil. The indoor temperature of the air as it enters your indoor coil (if attic or crawl space or outside air is infiltrating your ductwork a reading at the filter will not be accurate), you really need to know the dry bulb and wet bulb temperature but many of you will not have a pshycrometer to determine wet bulb. The suction or low side pressure of the system, the liquid or high side pressure of the system, the suction temperature and the liquid temperature, these readings can be gathered through the use of a set of hvac-r gauges and an inexpensive surface probe thermometer.
To properly charge the refrigeration circuit you will have to become familiar with a few terms, two of these are super heat and sub cooling. Super heat is heat added to a vapor after it has boiled, if the steam coming off your boiling pot is 220 degrees and the boiling point of the water is 212 degrees then that steam has 8 degrees of super heat or additional heat added to the vapor. Sub cooling is heat taken away from a liquid after it has condensed, if that steam at 220 degrees was allowed to cool and condense back into water and the water was further cooled to 198 degrees that liquid would have 14 degrees of sub-cooling. These two terms are used as guide posts for charging your ac system the reason for their importance is that you cannot see what is happening inside the refrigeration circuit and the ratios of liquid and vapor are critical to the long term health and efficient operation of your equipment.
Information courtesy from Mike Newberry from the Aircon Repair