How To: Select Nitrogen Cylinders to Pre-Charge a Bladder Tank
At times, an expansion tank pre-charge must be increased using compressed air. Wessels recommends dry, oil-free air like the compressed air used in our factory, which is taken to below -50°F and processed through a dryer. Another option is to use compressed nitrogen because it is an inert gas that avoids oxidation and moisture content that can corrode the system. Nitrogen also provides more consistent pressure and is readily available. But how many cylinders are needed to pre-charge a bladder tank to the desired pressure? There are several factors that need to be considered: the size of the compressed gas cylinders, the size of the expansion tank accepting the nitrogen, and the pressure that is needed for the pre-charge.
Additionally, we need to think in terms of stored potential energy. With the product pV (where p is absolute pressure and V is volume) equal to potential energy in joules (ft-lbf), we can calculate the energy in a cylinder of gas and compare it to the potential energy requirement of the bladder tank.
To make the units more conducive to gallons and psia, let’s use pV in terms of GAP (gallons times absolute pressure in psia). Gas cylinders are rated in cubic feet. This is actually the cubic feet of gas at 150 psig (164.7 psia) that, when released, would make up that amount of volume in cubic feet at 0 psia. In terms of potential energy in terms of GAP, the amount of energy in each cylinder is equal to 100 times the “rated” cubic feet of the cylinder. A 20 cubic foot cylinder charged to 2,200 psig yields a potential energy of 20 x 100 = 2,000 GAP.
Some common size cylinders whereas the rating in Cubic Feet based on 150 psig:
Be aware that the rating for the cylinders is referenced above 0 psia. If charging a tank to a pressure greater than 0 psia, which of course is any expansion tank, the potential energy will be somewhat less, but this will yield a close approximation. For example, if the tank at the job site is 250 gallons and needs to be charged from 40 psig to 90 psig, this requires the potential energy to be increased:
250 gallons × (90-40) psia=12,500 GAP
Bottle “H” is rated for 244 Cu.Ft. or 24,400 GAP above 0 psig. To recalculate the potential energy above 90 psig, a simple ratio should be used:
The 14.7 factor to convert psig to psia drops out of the equation and yields:
For the above example, the resulting cylinder potential energy above the 90-psig set point for the expansion tank calculates thusly:
The potential energy above 90 psig for the “J” 330 cu.ft. is 23,402 GAP and is large enough to handle the tank in this example. An alternative is to take two (2) “Q” tanks which also have the necessary GAP requirement for the example expansion tank.
An excel spreadsheet is available to download for evaluating most potential jobs.
Written by:
Sam Fuller
Technical Sales Engineer
Wessels Company
Every pressure vessel manufactured by Wessels Company is equipped with a metal data tag affixed to its front side, positioned below the smart bracket system. This data tag serves as a quick reference for essential vessel information.
Additionally, the tag includes the manufacturing year, affirming its origin in the United States. H.D. denotes Head Dimension, indicating the size of the vessel’s top, while SH signifies the shell thickness or steel gauge employed in its construction.
GREENWOOD, IND. – Wessels Company is thrilled to announce the debut of its latest product enhancements at the AHR Expo in Chicago, showcased at booth 6321. Among the highlights is the new Drum Mounted Glycol Makeup Package, known as DM-GMP, featuring enhanced mobility and refill capabilities. The system ingeniously mounts atop a standard drum containing glycol or glycol mixture, streamlining the refill and installation process.
In addition to these advancements, the industry-leading smart bracket technology has undergone a redesign to be more compact and resilient. The WessView® bladder monitor, air pressure gauge, and Schrader valve have all been optimized to snugly fit the exterior of the tank with a single connection. The ASME tank and smart system seamlessly integrate all essential technologies into a convenient location. The new smart bracket will become a standard feature on all bladder-style tanks later this year.
We happily introduce the newest smiling face to the Wessels family, Allyson McQueen! She joined our engineering department at the beginning of November as a Process Engineer. She loves learning new things, which will surely benefit her in her new role on our engineering team. Allyson is an avid sports fan and has many teams she roots for including the Florida Gators, Atlanta Braves, Miami Dolphins, and the L.A. Lakers. She is a dog lover and has a large German Shepherd who she loves to spoil. In her free time, she can be found training large dogs and playing darts around the country in tournaments.