Vacuum Techniques
I extracted and translated a part of a document writed by a local Hvac engineer ( Ing. Ricardo J. J. Sardañons ) , I know my translation is not the best at all, it tooks me some time with the aid (well, not much aid, I had to rewrite a lot...) of the altavista translator
It would be a second part, if there is anyone that can help translating or wants the original doc, send me a pm.
I hope it helps
The Vacuum pump:
In order to know what capacity the pump must have, we must know previously what capacity in TR (tons of refrigeration) or Kcal /h has the system that we are installing or repairing.
In order to be able to choose a suitable pump, we must consider that by each cubical foot per minute or each 28.56 liters per minute that the pump has (capacity), we can use it to evacuate or dehydrate systems with capacity up to 6 TR (18,000 Kcal/h or 72000 BTU/h).
An approximate rule will be: With 1 cfm or 28.56 l/min, we can evacuate equipment up to 6 TR For example, with a pump of 4cfm we can evacuate equipment up to 24TR.
The Vacuum pumps that have oil for their lubrication require oil change, not only in acordance to their use, but also to the degree of contamination in the circuit to dehydrate. It is advisable to change the oil frequently to avoid the diminish of the efficiency of the pump, take in account that the oil is contaminated by the water steam that is eliminated from the circuit. It is advisable that if the circuit to dehydrate contains much humidity, at the time that the pump stops after finishing the dehydration, we change its oil and replace it by new one of the same characteristics that the manufacturer advises, doing this, we will avoid the humidity that remains within the pump affect its mechanical parts and diminish its efficiency.
When we begun the vacuum in a circuit, never let the pump work if the pressure in the circuit to evacuate is superior than the atmospheric pressure. The atmospheric pressure is 760 mm Hg (atmosphere) = 14.7 Lbs/p2, this pressure is what we support daily on our body. The air which we breathed (78% of Nitrogen + 21% Oxygen + 1% of other gases) surrounds our Planet and by action of the gravity it stays attracted to the Earth up to 960 km at the level of the Sea. If we took a square lead column of 1" by side and a height of 960 km that contained air within, we would have a 14.7 pressure lbs/p2. When a volume of water within a container submissive to the heat of a burner arrives to 100°C will begin boiling whenever it is put under atmospheric pressure. (760 mm Hg).
If with the same container is put at the top of a mountain, and repeated this same test, the water will begin boiling at an inferior temperature than 100°C . This is due to that the atmospheric pressure at a greater height diminishes, and therefore the boiling is made at a smaller temperature. Similarly it happens with a pump applied to a circuit of refrigeration, the smaller the pressure, the smaller the temperature the water (humidity) starts boiling within the circuit. It is then easy to understand, that working at a very low pressure and favored as well by the room temperature at which the parts of the system are put (pipe, condenser, evaporator, compressor), the micro drops of water within the circuit transform into steam and this will be extracted by the pump.
The pump must be taken care of and be maintained to assure that the awaited degree of vacuum is obtained, for that reason it is important to recommend the following:
*The pumps in general must have a manual valve or a solenoid that assures the interrupt of the work of vacuum before proceding to stop it, to not lose the vacuum obtained until that moment, avoiding in addition that the oil of the pump can enter to the system by the low pressure at which it is, an interruption of electrical energy also must be consider, treating that a solenoid valve (normally closed) act to protect the vacuum obtained until that moment. The last generation pumps had incorporated valves to interrupt the evacuation process.
*We must control and to make the manufacturer control the state of the pump to know if problems of mechanical type do not exist that have diminished their efficiency, this happens generally when the pumps have a very frequent use, and the oil change is not made frecuently.
*If we have a pump that has "gas balast" this valve allows that drier atmospheric air be mixed with saturated air extracted from the first stage of the pump, facilitating the expel of the humidity and increasing the efficiency of the pump.
The pipes and components that tie the pump with the equipment:
Although the capacity of the pump is an important factor for the evacuation of an equipment according to the volume to evacuate, the time and effectiveness of the vacuum depends on the restrictions that are in the evacuation way. For example, is very common the use of of ¼ hoses, flare, manifolds and valves that are already installed in the same circuit to be evacuated, these will generate a lot of restriction when doing use of these elements, although is of practice to use them since they offer many advantages in the maneuvers of the coolant, not always is recommendable its application in the vacuum technique.
With Valve, Manifold and pipe of ¼ ", we arrive at a vacuum of 100 microns in 121 minutes, using the pump in its total capacity. If the valve cores are retired , the time of vacuum diminishes 56 minutes, so: 121- 56 = 65 minutes.
If we retire the Manifold and the valve cores, and conect the pump with the system using a 3/8" hose, the vacuum is obtained within 5 minutes.
If we increase the diameter of the hose from 1/4"to 1/2", the vacuum time is reduced 8 times. If a 2 meters hose is cuted to 1 meter, the time reduces to half.
Now if we analyze that if a connected hose of 1/2" is replaced it by one of 1/4" in our pump, and this one has a capacity of 5cfm, this reduction in the diameter of the pipe reduces the capacity of the pump in a 75%, being its final capacity 1.25 cfm.
Concluding, it is deduced that to obtain an efficient vacuum, we must consider the following :
*To have a direct connection between the pump and the system and the possible amplest section with safe connections that do not have losses.
*The distance that separates the pump with the system must be the shortest possible.
*We must replace the valve cores transitorily while the vacuum is made to eliminate this severe restriction.
*The procedure of extraction of the cores is easily made with a valve Core remover like the one on this picture.
Sorry and I hope you guys can understand something
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Originally Posted by pythagoras
I forgot "you must" 
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