The use of the vacuum pump may be the single most important development in refrigeration and air-conditioning servicing. The purpose of a vacuum pump is to remove the undesirable materials that create pressure in a re- refrigeration system. These include:
- Moisture
- Air (oxygen)
- Hydrochloric acid
In addition, there are other materials that will vaporize at low micron range. These, along with a wide variety of solid materials, are pulled into the vacuum pump in the same way a vacuum cleaner sucks up dirt.
Evacuation is being routinely performed on almost every service call on which recharging is required.
NOTE: It is no longer permitted to sim- ply add refrigerant to the system with one end open for evacuation into the atmosphere. This shortcut was a favorite of many service technicians over the years since it was quick and the refrigerant was inexpensive.
Vacuum levels formerly unheard of for field evacuations are being accomplished daily by service per- sons who are knowledgeable regarding vacuum equipment. These service persons have found through experience that the two-stage pump is much better than the single-stage pump for deep evacuations. See Figs. 1-53 and 1-54. It was devised as a laboratory instrument and with minor alterations; it has been adapted to the refrigeration field. It is the proper tool for vacuum evacuations in the field. The latest in vacuum pumps is shown in Appendix 4.
Fig. 1-53 Single-stage portable vacuum pump. (Thermal Engineering)
Fig. 1-54 Dual or two-stage portable vacuum pump. (Thermal Engineering)
To understand the advantages of a two-stage pump over a single-stage pump refer to Fig. 1-55. This shows the interior of a two-stage vacuum pump. This is a simplified version of a vacuum stage. It is built on the principle of a Wankel engine.
There is a stationary chamber with an eccentric rotor revolving inside. The sliding vanes pull gases through the intake. They compress them and force them into the atmosphere through the exhaust. The vanes create a vacuum section and a pressure section inside the pump. The seal between the vacuum and the pressure sections is made by the vacuum pump oil. These seals are the critical factor in the depth that a vacuum pump can pull. If the seals leak, the pump will not be able to draw a deep vacuum. Consequently, less gas can be processed. A pump with high leakage across the seal will be able to pull a deep vacuum on a small system, but the leakage will decrease the pumping speed (cfm) in the
Fig. 1-55 Two-stage vacuum pump showing seals and intake, exhaust and vacuum section. (Thermal Engineering)
deep vacuum region. Long pull-down times will result. There are three oil seals in a single-stage vacuum pump. Each seal must hold against a high pressure on one side and a deep vacuum on the other side. This places a great deal of strain on the oil seal. A two-stage vacuum pump cuts the pressure strain on the oil seal in half. Such a pump uses two chambers instead of one to evacuate a system. The first chamber is called the deep vacuum chamber. It pulls in the vacuum gases from the deep vacuum and exhausts them into the second chamber at a moderate vacuum. The second chamber, or stage, brings in these gases at a moderate vacuum and exhausts them into the atmosphere. By doing this, the work of a single chamber is split between two chambers. This, in turn, cuts in half the strain on each oil seal, which reduces the leakage up to 90 percent.
A two-stage vacuum pump is more effective than a single-stage vacuum pump. For example, a single- stage vacuum pump rated for 1.5 cfm capacity will take one and one-half hours to evacuate one drop of water. A two-stage vacuum pump with the same rating will evacuate the drop in 12 min.
For evacuation of a 5-ton system saturated with moisture, a minimum of 15 h evacuation time is required in using a single-stage vacuum pump. A two- stage pump with the same cfm rating could do the job in as little as 2 h.
Another advantage of the two-stage pump is reliability. As you can see, if the oil seal is to be effective, the tolerances in these vacuum pumps must be very close between rotor and stator. If the tolerances are not correct, the oil seal will not be effective. Slippage of tolerance due to wear is the major cause of vacuum pump failure. With a single-stage pump, when the tolerance is in the stage slips, the pump loses effective-
ness. With a two-stage pump, if one stage loses tolerance, the other one will still pull the vacuum of a single-stage pump.
Larger cfm, two-stage vacuum pumps are preferred to the single-stage vacuum pumps. The cost difference between the two is not great. In addition, the time saved by using the two-stage pump is evident on the first evacuation.
Vacuum Pump Maintenance
The purpose of vacuum pump oil is to lubricate the pump and act as a seal. To perform this function the oil must have:
- A low vapor pressure that does not materially in- crease up to 125°F (51.7°C)
- A viscosity sufficiently low for use at 60°F (15.6°C) yet constant up to 125°F (51.7°C)
These requirements are easily met by using a low vapor pressure, paraffin-based oil having a viscosity of approximately 300 SSU (shearing stress units) at 100°F (37.8°C) and a viscosity index in the range of 95 to 100. This type of uninhibited oil is readily obtainable. It is the material provided by virtually all sellers of vacuum pump oil to the refrigeration trade.
Vacuum Pump Oil Problems
The oils used in vacuum pumps are designed to lubricate and seal. Many of the oils available for other jobs are not designed to clean as they lubricate. Neither are they de- signed to keep in suspension the solids freed by the cleaning action of the oil. In addition, the oil is not usually heavily inhibited against the action of oxygen. Therefore, the vacuum pump must be run with flushing oil periodically to clean it. Otherwise, its efficiency will be reduced. The use of flushing oils is recommended by pump manufacturers.
If hydrochloric acid has been pulled into the pump, water, solids, and oil will bond together to form sludge or slime that may be acidic. The oil also may deteriorate due to oxidation (action on the oil by oxygen in air pulled through the pump). This results in a pump that will not pull a proper vacuum, may wear excessively, seriously corrode, or rust internally.
Operating Instructions
Use vacuum-pump oil in the pump when new. After 5 to 10 h of running time, change the oil. Make sure all of the original oil is removed from the pump. Thereafter, change the oil after every 30 h of operation when the oil becomes dark due to suspended solids drawn into the pump. Such maintenance will ensure peak efficiency in the pump operation.
If the pump has been operated for a considerable time on regular pump oil, drain the oil and replace with dual-purpose vacuum-pump oil. Drain the oil and re- place with dual purpose after 10 h of operation. The oil will probably be quite dark due to sludge removed from the pump. Operate the second charge of oil for 10 h and drain again. The second charge of oil may still be dark. However, it will probably be lighter in color than the oil drained after the first 10 h.
Change the oil at 30-h intervals. After that, change the oil before such intervals if it becomes dark due to suspended solids pulled into the pump. Be sure to change the oil every 30 h thereafter to keep the vacuum pump in peak condition.
Evacuating a System
How long should it take? Some techniques of evacuation will clean refrigeration and air-conditioning system to a degree never reached before. Properly used, a good vacuum pump will eliminate 99.99 percent of the air and virtually all of the moisture in a system. There is no firm answer regarding the time it will take a pump to accomplish this level of cleanliness. The time required for evacuation depends on many things. Some factors that must be considered are:
- The size of the vacuum pump
- The type of vacuum pump—single or two-stage
- The size of the hose connections
- The size of the system
- The contamination in the system
- The application for the system
Evacuations sometimes take fifteen minutes. Then, again, they may take weeks. The only way to know when evacuation is complete is to take micron vacuum readings, using a good electronic vacuum gage. A number of electronic meters are available. See Figs. 1-56 and 1-57.
Evacuating down to 29 in. eliminates 97 percent of all air. Moisture removal, however, does not begin until a vacuum below 29 in. is reached. This is the micron level of vacuum. It can be measured only with an electronic vacuum gage. Dehydration of system does not certainly begin until the vacuum gage reads below 5000 microns. If the system will not pump down to this level, something is wrong. There may be a leak in the vacuum connections. The vacuum-pump oil may be contaminated. There may be a leak in the system. Vacuum gage readings between 500 and 1000 microns assure that dehydration is proceeding. When all moisture is removed, the micron gage will pull down below 1000 microns.
Fig. 1-56 This vacuum check gage is designed to be as handy as a charging manifold. (Thermal Engineering)
Pulling a system down below 1000 microns is not a perfect test for cleanliness. If the vacuum pump is too large for the system, it may pull down this level be- fore all of the moisture is removed. Another test is preferred. Once the system is pulled down below 1000 microns it will not go any further. The system should be valved off from the vacuum pump and the pump turned off. If the vacuum in the system does not rise over 2000 microns in the next 5 min, evacuation has been completed. If it goes over this level, either the moisture is not completely removed or the system has a slight leak. To find which, reevacuate the system to its lowest level. Valve it off again and shut off the vac- uums pump. If the vacuum leaks back to the same level as before, there is a leak in the system. If, the rise is much slower than before, small amounts of moisture are probably left in the system. Reevacuate until the vacuum will hold.
