Tank Considerations: Pressure vs Vacuum.

[eddycurr]

How can I determine whether a commercially-available 10 gal portable AIR PRESSURE tank (Max 135 PSI) can safely be used as a VACUUM ACCUMULATOR tank for deep vacuum (eg: 1000 microns/0.13 kPa/29.88 HG)?

I have a 1.5 CFM two-stage HVAC vacuum pump I intend to use for degassing RTV mold making/casting materials. The material vendor recommends using a 8 CFM pump for this application. The issue is one of having sufficient capacity to draw enough vacuum quickly enough to degass the materials within their working time, ie: before the materials set up such that they no longer flow adequately. My budget does not permit changing up to a higher capacity pump at this time.

The thought occurs that by using a tank to accumulate vacuum, choosing the smallest suitable vacuum chamber and taking steps to further reduce any voids with packing, then my 1.5 CFM pump could be serviceable for the task.

Even if considerations preclude drawing such a tank down to a deep vacuum (eg: 1000 microns/0.13 kPa/29.88 HG), providing a vacuum relief valve set to a safe working value and creating a partial vacuum would be helpful by speeding up initial evacuation of the chamber. With the chamber stabilized at the tank's partial vacuum, a valve to the tank can be closed, leaving the pump to take the chamber down to the desired deep vacuum.

[eddycurr]

In the absence of having all the necessary components assembled together for testing, perhaps I am overthinking the matter.

Using ET's "Vacuum - Evacuation Time Calculator" and entering the following values, I receive an answer of 7.41 seconds

• Vacuum Chamber Volume: 2 US gal / 0.00757 m³
• Pump Flow Rate: 1.5 CFM / 0.000707921175 m3/s
• Initial Pressure: 14.76 in HG / 500 mbar
• Final Pressure: 29.53 in HG / 1000 mbar

Is this result correct or have I made an error in my conversions and/or the use of the calculator?

Even if drawing the chamber down from atmospheric to 29+ in HG required 2 min, this would be adequate for the application.

[Hudson]

You'd think it would work just fine as the external pressure is only 1/10 of the rated internal pressure capacity.

[eddycurr]

Thank you.

Now that I am in possession of what passes for a commercial vacuum chamber these days (a deep-drawn stainless chef's stock pot, a 3/4" acrylic disk for a top, a gauge and some brass valves/fittings), the 10 gal pressure tank appears more than substantial enough by comparison. As you suggest.

The brief literature that accompanies the 'vacuum chamber' cautions that the device is tested to 29 HG and warns about drawing down below 29 out of concern that the chamber will 'distort'. Their recommendation is to attend while pulling a vacuum, observe when the needle stops moving on the gauge and turn off the pump at this point, rather than allowing the pump to evacuate the chamber further. Another recommendation is not to leave the chamber under vacuum for extended periods, graphically likening this to "holding a grenade after pulling the pin".

I believe the vendor is alluding to the limitations of a gauge to report deep vacuum levels. It is not uncommon for pumps to be able to pull down below levels that are difficult to detect on a analog Bourdon tube gauge. Just because the needle on the gauge stops moving, this is not necessarily indicative that the pump has ceased to further reduce the pressure in the chamber below the reading on the gauge. Apparently digital gauges are required to sense pressures accurately at deep vacuum levels, their expense is something of a factor for more casual users.

A consideration regarding leaving a chamber pressurized is that pressure is relative. Changing atmospheric conditions can influence the differential between the negative pressure within the chamber vs the positive pressure outside the chamber.