A tank manufacturer was building specially designed tanks for their customer. These tanks were to hold cryogenic materials, so they had to create an outer shell to hold a special type of insulation material. This shell maintained a 1/2 inch (13 mm) gap around the tank. The overall dimensions of the cylindrical tanks were 10 feet (3 meters) in diameter and 43 feet (13 meters) in length. The internal volume of the gapped area was 56.1 cubic feet (1560 liters). With this type of insulation, it had to be under a negative pressure, or vacuum, to help facilitate the insulation properties. So, when the tanks were completed, they had to check for leaks between the shell and the tank. The requirement was to draw a vacuum to 21” Hg and to hold it at that level for 30 minutes. As the tank manufacturer researched vacuum pumps to test their design, they came across EXAIR.
Being that they were not familiar with EXAIR E-Vacs, they wondered how they worked. I explained that the E-Vac uses compressed air to create a vacuum by a venturi method. It can reach vacuum levels up to 27” Hg in a very compact and lightweight design. Being that they do not have any moving parts, they are very durable and long lasting in systems with on/off cycling or continuous running. The tank manufacturer was very intrigued by this concept as they had electric vacuum pumps fail in the past.
He wondered about the evacuation time to get to 21” Hg of vacuum. The idea for the leak test was to reach the vacuum level and turn a valve off to isolate the area. From there, they would watch a gage to see if they were losing vacuum. If so, then they would have to find and fix the leak and recheck. If the vacuum pressure held, then they could fill the area between the shell and the tank with insulation material; use the E-Vac to put it under vacuum; and cap. Because the volume was large and time was a concern, I suggested the model 840060M. This had the highest vacuum flow rate and can reach a vacuum level of 25” Hg.
To help explain a little better about vacuums, when you are near atmospheric pressure, you have the highest air flow rates. As the vacuum levels rise, less air is present to be drawn out. When you reach the vacuum pump capacity or complete vacuum, the flow rate is zero. You can notice this with your vacuum at home. As you turn the vacuum on, the air is rushing in (the highest flow rate/very low vacuum pressure). As you cover it with your hand, the vacuum pressure increases and the flow rate decreases to zero (no flow rate/highest vacuum pressure). To figure the amount of time to reach a certain vacuum level, we have to take into consideration the different flow rates as the vacuum level continues to increase. The equation that we use is below:
Equation 1: t = V * ln(p0 / p1) / q
t = evacuation time (min)
V = enclosed volume (ft^3)
p0 = atmospheric pressure (“Hg)
p1 = end vacuum pressure (“Hg)
q = flow rate of vacuum pump (SCFM)
With the performance data of the model 840060M E-Vac, we can start to calculate the time to reach 21” Hg vacuum. A couple of details are required to make the equation work properly. Just as a note, the end vacuum pressure, p1, has to be converted to an absolute pressure. This will equate to 29.92” Hg – 21” Hg = 8.92” Hg (absolute).
Here are the details for Equation 1:
V = 56.1 ft^3 (above)
p0 = 29.92” Hg (absolute start)
p1 = 8.92” Hg (absolute target)
q = 70 SCFM (cataloged at 80 psig)
t = V * ln(p0 / p1) / q
t = 56.1 ft^3 * ln(29.92” Hg/8.92” Hg) / 70 SCFM
t = 0.97 minutes
For this application, the model 840060M worked great for both procedures. With less than 1 minute to get to the desired set point, it didn’t hinder production for leak checks or to vacuum set the insulation. If you have a timing sequence with vacuum chambers or “pick and place” systems, you can use this equation to find the best E to meet your goal. If you need any additional help, you can always contact an Application Engineer at EXAIR.