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Tips, Tools for Pulling an Effective Vacuum

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Tips, Tools for Pulling an Effective Vacuum VOLUME 1, ISSUE 2 YELLOW Article reprinted with ® permission from RSES Journal JACKET UUPDATEPDATE June 2002 Tips, Tools for Pulling An Effective Vacuum Knowing how vacuum tools and equipment work and their advantages can help technicians properly and safely apply them when evacuating refrigerant from a system BY BILL WEST t is important to understand what vacuum is and vacuum. It is not practical to measure a deep vacuum (low how it is measured in order to correctly apply it in absolute pressure) in inches of mercury because the units the hvacr industry. The dictionary defines vacuum are so large; it is measured in microns. There are 1,000 as “a space with nothing in it.” In the real world microns in a millimeter (Torr) and 25,400 microns in 1 inch I there is no such place because molecules of of mercury, so the use of this unit makes the measurement various elements are everywhere. It is the density of of deep vacuum much more precise. these molecules within an enclosed space that we measure as pressure. Applying vacuum to hvacr For our purposes, a vacuum is a space where the den- Refrigeration and air-conditioning equipment must prop- sity of molecules is less than that of the earth’s atmos- erly be evacuated prior to charging to prevent unwanted phere. In other words, a space with a vacuum has less molecules — primarily water vapor — from damaging the than atmospheric pressure. system. Moisture in a system can cause freeze-ups in cap- The inventor of the mercury barometer, Evangelista illary tubing and expansion valves. The natural tendency Torricelli, first measured atmospheric pressure in 1643. A of moisture to cause corrosion is multiplied when it reacts long glass tube with one sealed end was filled and then with certain refrigerants and forms hydrochloric or inverted in a bath of mercury. hydrofluoric acid. Torricelli found that at sea level the earth’s atmosphere Refrigeration oils — especially the newer POE and exerts enough force to support a column of mercury 760 PAG oils — absorb moisture, and the combination forms millimeters (29.921 inches) high at a temperature of 32° what is commonly referred to as sludge. The most practi- F. The resulting empty space at the top of the tube was a cal method for removing these molecules is to connect a (nearly) total vacuum. high vacuum pump to the system and reduce the absolute The unit of measurement for vacuum was named for internal pressure (vacuum) to less than 1,000 microns. the inventor, and one atmosphere of pressure is now Reducing the pressure lowers the boiling point of water referred to as 760 Torr, a measurement of absolute pres- vapor so it can be removed from the system at tempera- sure. If it were possible to achieve a vacuum of zero Torr, tures below 32° F. we would have a perfect vacuum or zero absolute pressure. If a system is wet, reaching this pressure may take When vacuum is measured in inches of mercury it quite some time and require several changes of pump oil must be remembered that atmospheric pressure (gauge due to water vapor contamination. After evacuating, the pressure) is the starting point and that the gauge readings system must be isolated and the pressure must be main- are negative numbers. Thus, -29.921 inches Hg is a perfect tained at less than 2,000 microns for a reasonable length RSES Journal of time. This ensures that there are no leaks or signifi- um gauge should not be used for measurements near cant contamination present. Either of these conditions atmospheric pressure. Knowledge of the limitations of the may result in premature system failure and a warranty gauges available is necessary in order to choose the proper service call. gauge to produce an intended result. The following is a list Understanding the way molecules behave in a vacuum of gauges and their advantages and disadvantages: explains why they are so difficult to remove. Molecules in • Mercury barometers: These are fragile instruments a vacuum do not flow through hoses and orifices the way containing a hazardous substance. Their use has been dis- that they do under positive pressure. At near-atmospheric couraged and mostly discontinued for this reason. With pressure the molecules flow in much the same way they the exception of some very sophisticated laboratory do at positive pressures. gauges such as the McLeod gauge, they do not have the As the pressure is reduced the distance between mole- accuracy required for the measurement of pressures cules increases until, in a below 1,000 microns. deep vacuum, the molecules Knowledge of the limitations of the • Mechanical dial gauges: recoil off of the surfaces at These are useful for rough odd angles, eventually mak- gauges available is necessary in measurements of vacuum ing their way through the only. The accuracy of com- vacuum pump. At pressures order to choose the proper gauge to pound dial gauges on below 2,000 microns most of charging manifolds used by the molecules are attached produce an intended result refrigeration technicians is to the surface in layers. It at best +/-1 percent of the may require some time for them to detach from the sur- full-scale reading. faces and make their way to the pump. On a low-side gauge with a maximum pressure of 120 When a system is isolated from the vacuum pump, the psi this translates to an accuracy of +/-1.2 psi or 62,000 molecules continue to detach from the surfaces and start microns. Even when using a large dial gauge that is grad- to bounce around in the void left by the molecules that uated for vacuum only, +/-1 percent accuracy would be were removed. This results in an increase in pressure (rise accurate only to +/-7,600 microns. in the micron reading). A fast rate of rise must be inter- • Electronic pressure transducers: These rely on the preted as either a leak or as a contaminant (usually mois- deflection of a metal ture) in the system. diaphragm as it acts on a If a leak is suspected, find the leak using standard leak piezoelectric crystal to pro- detection equipment and procedures and fix it. If mois- duce a signal that is propor- ture in the system is suspected, continue to evacuate and tional to the pressure acting isolate and recheck to see if the system will maintain the on it. With the exception of vacuum. Continue to evacuate until readings are acceptable. the extremely expensive capacitance manometer, Types of gauges vacuum transducers also Various types of gauges are used to measure vacuum at tend to have limited accura- different ranges and each type has its place in the hvacr cy due to the lack of sensi- industry. tivity. Common • Barometers, mechani- Constant current or voltage is applied to thermistor-style sense dictates cal gauges and transduc- electronic vacuum gauges, that a tape ers: These measure pres- like the one pictured above, measure sure that directly acts on while evacuating refrigerant. should not be the sensing device, which used to then registers a change in measure the reading. At pressures of less than 1,000 microns the highly accu- density of molecules is quite low and not enough force is rate exerted on the sensing device to cause mechanical ele- machined Mechanical dial gauges are used for rough ments to respond. What is needed at this low pressure is a parts, and a measurements of pressure while evacuating method to indirectly measure the density of molecules, micrometer refrigerant from a system. and this is exactly what electronic vacuum gauges are should not designed to do. be used to measure a football field. • Electronic vacuum gauges: Most of these gauges avail- That same logic tells us that a dial gauge should not be able to the refrigeration trade operate on the principle of ther- used for measuring deep vacuum and an electronic vacu- mal conductivity. The short explanation is this: when an ele- RSES Journal ment within an ent temperature is necessary. Automated assembly tech- enclosed housing niques will produce sensors with very consistent charac- is heated, mole- teristics, so they usually can be interchanged without cules that collide recalibrating the instrument. with the element A type of electronic carry the heat gauge that does not away to the sen- rely on thermal con- sor housing. The ductivity uses a absolute pressure quartz crystal that is (vacuum) can be electrically stimulated determined from to oscillate at a given the rate of heat The response of a thermocouple gauge frequency. The fre- loss. Thermal like the plug-in style gauge above is quency changes that conductivity extremely quick. occur as molecules gauges generally are removed from the are either thermistor gauges or thermocouple gauges. environment is moni- • Thermistors: They are thermally-sensitive resistors. tored and then dis- A vacuum pump is connected to an Either a constant current or a constant voltage is applied played as an indica- air-conditioning system during to the thermistor. As heat is removed, the resistance of the tion of pressure. refrigerant evacuation. Both a heated element changes and this change is relayed to the • Thermal conduc- mechanical-dial and thermocouple- user as a reading – usually in microns. In the past these tivity gauges: These style gauge are used together to gauges required the user to manually adjust the unit to tend to be less accu- optimize measurement accuracy. compensate for ambient temperature. rate at pressures Modern thermistor gauges use two thermistors, one of greater than 25,000 microns, although still useful for giv- which helps to com- ing a rough indication of the evacuation process.
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