SteinarN
27-03-2008, 12:46 PM
The physics behind low pressure gas flow is rather interesting. The resistance of flow in the hoses between the pump and the system does actually increase relatively as the pressure in the system decreases.
We can do a thought experiment:
Lets say we have atmospheric pressure in the system and start to evacuate. In a short time, lets say 10s, the pressure is halved. To halve the pressure once more takes more than 10s. The next halving takes considerably more than 10s. For every halving of the remaining pressure the necessary time increases. It is maybe not easy to understand that without imagine what happens when the system is under an extreme vacuum. Imagine you are a blind gas molecule bouncing accidentally around in the system. At the beginning of the evacuation process you will feel that you have far less molecules at the side of you where it is an outlet to the vacuumpump and you will bounce a longer distance in that direction than you do in the other direction before you bounce into an other molecule. Therefore you will rather quickly bounce yourself toward the outlet of the system. Try to imagine how it will be if the system was under an extreme vacuum and there was only a few gas molecules, lets say 10 molecules, left in the system. You would then be bouncing accidentally around in the system, hitting the walls of the system, but hardly ever hitting an other gas molecule. You would then have very hard to say which way is toward the outlet, and you had to bounce accidentally around for a very long time to recognise that it maybe is slightly fewer molecules in the one direction where the outlet is. Imagine if you was the last gas molecule left in the system. Remember you are blind. How long do you think you have to bounce accidentally around in the system before you find the outlet?. A VERY long time I can assure you!
It is of course not possible, nor necessary to achieve such vacuums we have described here. We don't even reach vacuums as low as tens of billions of molecules left in the system. But to imagine such extreme vacuums is very helpful in order to understand how increased vacuum causes the remaining gas to have harder and harder to find the way out of the system. And that the limiting factor is often not the capacity of the vacuum pump but instead the size and number of the outlets and hoses.
I have a 5 cfm (50Hz) pump. I use that pump on my commercial stationary systems as well as my transport refrigeration systems.
The important thing to be aware of when trying to get a efficient and rapid evacuation is the hoses between the pump and the system. I have made a manifold with four 1/4" shrader connections at the pump. I always use hoses directly from the pump to the system, not through the gauges. It is necessary with at least three 2ft hoses between my 5cfm pump and the system to get a good utilization of this pump. The pump capacity get severely decreased with only two 2ft hoses between the pump and system, and i could then have rather chosen a smaller pump.
We can do a thought experiment:
Lets say we have atmospheric pressure in the system and start to evacuate. In a short time, lets say 10s, the pressure is halved. To halve the pressure once more takes more than 10s. The next halving takes considerably more than 10s. For every halving of the remaining pressure the necessary time increases. It is maybe not easy to understand that without imagine what happens when the system is under an extreme vacuum. Imagine you are a blind gas molecule bouncing accidentally around in the system. At the beginning of the evacuation process you will feel that you have far less molecules at the side of you where it is an outlet to the vacuumpump and you will bounce a longer distance in that direction than you do in the other direction before you bounce into an other molecule. Therefore you will rather quickly bounce yourself toward the outlet of the system. Try to imagine how it will be if the system was under an extreme vacuum and there was only a few gas molecules, lets say 10 molecules, left in the system. You would then be bouncing accidentally around in the system, hitting the walls of the system, but hardly ever hitting an other gas molecule. You would then have very hard to say which way is toward the outlet, and you had to bounce accidentally around for a very long time to recognise that it maybe is slightly fewer molecules in the one direction where the outlet is. Imagine if you was the last gas molecule left in the system. Remember you are blind. How long do you think you have to bounce accidentally around in the system before you find the outlet?. A VERY long time I can assure you!
It is of course not possible, nor necessary to achieve such vacuums we have described here. We don't even reach vacuums as low as tens of billions of molecules left in the system. But to imagine such extreme vacuums is very helpful in order to understand how increased vacuum causes the remaining gas to have harder and harder to find the way out of the system. And that the limiting factor is often not the capacity of the vacuum pump but instead the size and number of the outlets and hoses.
I have a 5 cfm (50Hz) pump. I use that pump on my commercial stationary systems as well as my transport refrigeration systems.
The important thing to be aware of when trying to get a efficient and rapid evacuation is the hoses between the pump and the system. I have made a manifold with four 1/4" shrader connections at the pump. I always use hoses directly from the pump to the system, not through the gauges. It is necessary with at least three 2ft hoses between my 5cfm pump and the system to get a good utilization of this pump. The pump capacity get severely decreased with only two 2ft hoses between the pump and system, and i could then have rather chosen a smaller pump.