Leak Detection, Recovery, Evacuation and Charging Four basic service procedures used to repair and maintain a mechanical refrigeration system are leak detection, evacuation, recovery, and refrigerant charging. Some of these procedures need to be performed to conform to requirements of the Clean Air Act. Refrigerant leaks can affect system performance and the escape of refrigerant into the atmosphere can damage the environment. Refrigerant leaks can be detected using different techniques and different leak detection devices. Electronic leak detectors use a probe that is moved about 1 inch per second over the suspected area to detect leaks. An air filter in the tip of the probe must be replaced periodically. Air mixed with refrigerant is drawn over the detecting element. A leak is indicated by a flashing light, audible signal, or both. Refrigerant leaking under pressure, or air entering a system under vacuum, makes ultrasonic sounds that can be picked up by an ultrasonic leak detector. A leak can be found by injecting a small amount of a fluorescent solution into the refrigeration system. Running the compressor disperses the solution. An ultraviolet light is then scanned over the refrigeration system. A bright yellow-green glow from the fluorescent solution indicates a leak. A leak detecting solution can be applied to areas where leaks are suspected. Bubbles indicates a leak. Small leaks may take several minutes for bubbles to form. Leak testing can be done on a fully operational system, or a system with a partial charge or no charge. An operating system can be checked for leaks by looking for visual indications such as oil leaks and listening for escaping refrigerant. Systems without a charge can be checked for leaks by first pressurizing the system to about 10 psi with refrigerant HCFC-22. Increase the test pressure by backing up the refrigerant pressure with regulated nitrogen. Do not exceed the equipment manufacturer s recommended test pressure (about 125 psig maximum) and use an electronic leak detector to pinpoint the leak. A system with a partial charge may have adequate pressure for leak testing. If not, treat it as a system without charge and follow the procedures for a system without charge. When disassembling a brazed joint to make a repair, first make sure there is no pressure in the system.
Flux the joint, apply heat, and take the joint apart. After the joint has cooled, remove the flux residue with a wet cloth and warm water. As part of routine service procedures, refrigerant may be recovered, recycled, or reclaimed to prevent its release into the atmosphere. Recovery is the removal and temporary storage of a unit s refrigerant. Recovery does not clean or filter the refrigerant. When a refrigerant is recycled after recovery, acid, moisture, and other contaminants are removed. Recycling does not restore the refrigerant to the cleanliness level of new refrigerant. Reclamation restores contaminated refrigerant to the cleanliness standards of new refrigerant and is done in a special refrigerant reprocessing facility. The evacuation standards for refrigerant recovery machines vary according to the quantity and type of refrigerant to be recovered. Recovery and recycle units must be certified by the EPA and must be able to recover refrigerant to a specified level. Many different recovery machines are on the market. Select a machine to meet your needs and follow the equipment manufacturer s operating instructions. Most recovery machines can recover liquid or vapor. If the refrigerant is contaminated, install a filterdrier between the unit and recovery machine to prevent contamination of the recovery machine. Some recovery machines have the ability to recycle the refrigerant to remove moisture and acid. Recycling machines cannot process the refrigerant to the cleanliness level of new refrigerant. When recovering or recycling different refrigerants with the same machine, always drain and replace the oil in the recovery machine compressor. Certain refrigerants such as HFC-410A require dedicated recovery machines. Air and moisture contamination of a refrigeration system can lead to compressor failure. Air takes up space needed for condensing and makes the compressor work harder. Under the heat of compression, moisture in the system will react with refrigerant to form harmful acids. Air and moisture can be removed from the system by evacuation. A system evacuation should be done whenever the system has been opened to the atmosphere or if tests show the presence of acid or moisture in the system. Evacuation of a system requires a good vacuum pump and an accurate vacuum indicator. A vacuum pump creates a low pressure. The air and water vapor contained inside the refrigeration system flow into this low pressure.
As the vacuum pump lowers pressure in the system, the boiling point of any water in the system is lowered, allowing that water to boil off. For example, at a vacuum of 25,400 microns, water boils at around 80 F. A good vacuum pump is able to produce a vacuum of 500 microns or below. The time it takes to achieve a vacuum depends on the size of the system and the pumping capacity of the vacuum pump. For example, a 6 to12 cfm pump is recommended for systems up to 30 tons. Change the oil in the vacuum pump every 10 hours of operation and always change the oil immediately after pumping down a wet or contaminated system. Always use an electronic vacuum gauge capable of measuring to at least 500 microns or below to achieve a vacuum deep enough to dehydrate the system. The lower the micron reading displayed on the gauge, the closer the vacuum is to a perfect vacuum. A standard gauge manifold or a special high-capacity evacuation gauge manifold can be used for evacuation. The high-capacity gauge manifold is designed to evacuate quicker and easier than the standard set. The condition of the system determines the evacuation method. The deep evacuation method is used in most cases. The triple evacuation method is used on wet or contaminated systems. The deep vacuum method requires that the system be pulled down to a vacuum of 500 microns or below. After 500 microns is achieved, the pump is isolated and the vacuum gauge is observed. If the gauge moves to between 1000 and 2000 microns and stabilizes, the system is leak-free but still contains moisture. If the gauge shows a pressure rise and the pressure continues to rise without leveling off, a leak exists. A wet or contaminated system requires the triple evacuation method in which the system is evacuated to 1000 microns three times. After the first and second evacuations, the system is purged with dry, regulated nitrogen to help absorb moisture. Always use regulated nitrogen to pressurize the system. Never use oxygen or compressed air because an explosion will result. After the final evacuation to 1000 microns, the system is charged. Refrigerant can be charged into the system by weight, by superheat or subcooling, or by using pressure charts.
Refrigerant can be dispensed from disposable or refillable cylinders in a liquid or vapor state. Refillable cylinders have valves that dispense liquid or vapor refrigerant. The position of a disposable cylinder --upright or inverted -- determines the state of the refrigerant dispensed. Various refrigerant charging scales are available to charge a system by weight. Many are electronic and offer very good accuracy. Charging cylinders use a calibrated shroud that gives the service technician a visual indication of how much charge has been dispensed. Some recovery/recycle units can be used to charge refrigerant. Follow the manufacturer s instruction for liquid or vapor charging. A sight glass in the liquid line can be used to charge systems with a TXV metering device. With a correct charge, the sight glass should be clear with no bubbles at a 130 F condensing temperature. The desired 130 F condensing temperature can be obtained by blocking the condenser coil. The correct charge for a unit can be found in the manufacturer s service literature or on the equipment nameplate. Split systems may require extra charge if longer than normal line sets or accessories such as liquid line filter-driers are installed. The amount of extra charge required can be found in the manufacturer s literature. Charging by weight is a precise method for charging the entire quantity of refrigerant when the weight is known. Liquid or vapor can be charged that way. Liquid charging by weight is the fastest way to charge an empty system. To prevent compressor damage, liquid is charged into the high side of the system through the high-side service valve with the compressor off. Vapor charging by weight is normally done to top off a system when the complete liquid charge cannot be dispensed. With over 50% of the charge in the system, the compressor can be turned on and the remaining charge dispensed through the low-pressure side of the system. The superheat charging method is used to check or adjust the charge of an operating system that uses a fixed-orifice metering device.
The first step is to find the required superheat for the current outdoor air dry-bulb temperature and indoor air wet-bulb and dry-bulb temperatures. This is done using a table supplied by the equipment manufacturer. The required superheat and the measured suction pressure can be used with another table to determine what the correct suction line temperature for a correctly charged system should be. Some manufacturers supply this same information on a slide rule-type superheat charging calculator. The subcooling charging method is used to check or adjust the charge of an operating system with a TXV metering device. The subcooling value for a piece of equipment can be found on the unit nameplate. The method uses the temperature of the subcooled liquid as a means of determining if the correct amount of liquid refrigerant is being fed to the TXV. Actual subcooling is obtained by subtracting the measured liquid line temperature from the saturated liquid line temperature. Some manufacturers supply a slide rule-type subcooling calculator to use when charging with this method. Pressure charts can determine if equipment is charged properly by comparing temperatures and pressures on a chart. If the measured discharge pressure is too high, remove excess refrigerant using a recovery unit to lower the discharge pressure. A zoetrope is a blend of two or more refrigerants in which each component of the blend retains its individual characteristics. As a result, the liquid saturation temperature for a zoetrope is different from the vapor saturation temperature for a given pressure. Each refrigerant in a blend retains its own chemical properties, resulting in different leak rates that alter the properties of the original blend. This is called fractionation. Temperature glide is the range of temperatures in which a zoetrope refrigerant will evaporate and condense for a given pressure. Leaks will alter the original blend by fractionation. This requires the recovery of any remaining refrigerant and a recharge with new refrigerant after the leak is repaired. Zeotrope refrigerants should always be charged in the liquid state to avoid fractionation. If vapor charging into an operating system is necessary, charge with liquid fed through a metering orifice attached to the low-pressure side of the system.
Due to the temperature glide, superheat and subcooling must be calculated differently when charging a system that uses a zoetrope.