System Evacuation, Leak Detection, and Recharging CHAPTER 16 Objectives After studying this chapter, you will be able to: • Evaluate the reason for performing a proper evacuation. • Recognize the factors involved in a time- efficient evacuation. • Describe and demonstrate the two main methods of evacuation. • Identify and correct contamination problems. • Identify and describe the different types of refrigerant leak detection. • Determine the proper system pressure needed to check for leaks. • Detect refrigerant leaks using approved methods. • Use refrigerant cylinders properly. • Predict and correct low-side pressures. • Recognize and eliminate causes of high head pressure. • Use electronic scales and graduated cylinders for charging. Important Terms acid air space balance point black oxides charging contaminants deep vacuum disposable cylinder electronic leak detector electronic scale graduated cylinder halide torch head pressure heat load hydrostatic expansion inert liquid charging low-side test pressure nonflammable permeation pressure control red iron oxide setpoint sludge soluble standing pressure test temperature difference (td) thermostat triple evacuation ultrasonic leak detector vacuum pump vapor charging 16.1 System Evacuation For a refrigeration or air conditioning system to operate at its peak efficiency, the refrigerant needs to be pure, clean, and dry. This is done by accomplishing the following important objectives before any refrigerant is even charged into the system: • Install all components of the system tightly • Locate and repair any system leaks • Perform a proper evacuation on the system to remove any moisture, air, or contaminants. Moisture, air, and contaminants are major causes of problems in compression refrigeration systems. These systems are designed to operate with a single pure refrigerant. Excess moisture in a refrigeration system can cause ice to form, restricting refrigerant metering devices and preventing proper refrigerant flow. Moisture also causes rusting, corrosion, refrigerant decomposition, oil sludge, and system deterioration. Air in the refrigeration system causes high head pressure and high operating temperatures. Sludge, acid, corro- sion, and other contaminants result in various system problems. Acid, for example, eats the insulation off motor windings, causing a burnout. 281 Copyright Goodheart-Willcox Co., Inc. Ch16.indd 281 10/24/2014 3:40:15 PM 16.1.1 How Much Moisture Is Safe? The answer is NONE. While all refrigerants might safely tolerate a small amount of moisture, no one knows for sure how much moisture is “safe.” So the general agreement is the need to remove as much moisture as possible. Water is more soluble (easily dissolved) in some refrigerants than in others. Any excess water will exist as a separate liquid and cause considerable damage in a system. If the temperature is low enough, the water will freeze. 16.1.2 Moisture and Air Removal Moisture and air are the primary enemies of a refrigeration system. Proper evacuation and the use of driers are necessary for trouble-free operation. Evacuation is performed using a special pump called a vacuum pump. As it operates, it creates a very low pressure and removes all moisture and contaminants from a refrigeration system. Thorough evacuation of a system should be performed whenever it has been contaminated or exposed for prolonged periods to atmospheric air. Blowing the system out with an inert gas (nitrogen or carbon dioxide) might remove most of the air from the system, but then the inert gas must be removed as well. Blowing a system out does not remove moisture and air from trapped areas. While filter-driers remove small amounts of moisture and contaminants, they should not be relied on alone. Nothing is better than pulling a proper evacuation on the system. 16.1.3 Evacuation Factors Before evacuation, a system’s internal pressure must be reduced to atmospheric (0 psig). The vacuum pumps used to evacuate systems are not designed to handle pressures above atmospheric. Proper evacuation of a system is performed with a two-stage, rotary-type vacuum pump. The vacuum pump draws all vapor (gas) out of the system, reducing its internal pressure to a very low vacuum. The vacuum causes any moisture to boil, allowing it to be removed as a gas. To make sure that as much air and moisture has been removed from the system that can be removed, the system needs to be pulled down to a vacuum level of 500 microns. Microns are a very small measurement. See the values below for comparisons to other units of pressure. • 500 microns = 0.5 mm Hg • 25.4 mm Hg = 1 in Hg • 1 in Hg = 25 400 microns • 1 psia = 2 in Hg • 1 psia = 51 715 microns • 0 psig/14.7 psia (atmospheric pressure) = 760 209.4 microns As shown in the relationship of pressure units above, a micron is extremely small compared even to psi. A system that is at atmospheric pres- sure (0 psig or 14.7 psia) contains 760 209.4 microns of pressure. To pull a vacuum on a system that is at atmospheric pressure, the vacuum pump must remove 759 709.4 microns! While a proper evacuation of a refrigeration or air conditioning system is one of the most important steps to having a clean and dry system, it is usually the one process that is not properly performed. Many technicians when asked why they do not perform a proper evacuation, their answer will almost always be “TIME. It just takes too much time, and we need to get to the next job.” The time required for a vacuum pump to remove moisture and air from a system depends on several factors: • Size of the system. • Amount of moisture present in the system. • Capacity of the vacuum pump being used. • Size and length of the connecting lines. • Condition of the vacuum pump oil. It is quite obvious that the larger the system, the longer it will take to achieve a proper evacua- tion. With the proper equipment and conditions, even a large system can be pulled down in a reasonable amount of time. Note that time must be allowed for the pump to pull a vacuum in all parts of the system and for moisture to work its way out of the oil in the compressor crankcase. A large quantity of moisture in a system cannot boil into a vapor immediately upon lowering the pressure. Similarly, an open pan of water does not immediately flash into a vapor upon reaching the boiling temperature of 212°F or 100°C. The boiling 282 Heating and Cooling Essentials Copyright Goodheart-Willcox Co., Inc. Ch16.indd 282 10/24/2014 3:40:16 PM Important Terms list the key terms to be learned in the chapter. Review this list after completing the chapter to be sure you know the definition and context of each term. Objectives clearly identify the knowledge and skills to be obtained when the chapter is completed. Text narrative has been updated to include the latest HVACR practices information. Features of the Textbook vi Copyright Goodheart-Willcox Co., Inc.