We have been selling an arch press for several years now, and as a part of the sales process, we try to help our customers set up the new system. This may include things like vacuum pumps, plumbing, ovens, and most commonly, either a surge tank or a water trap.
There are quite a few resources that sell surge tanks, but I have never found one that I like, is inexpensive enough, and still meets my needs. Most of the commercially available tanks cost several hundred dollars. They are usually made of steel and sit horizontally on the floor, which takes up valuable floor space. The reason most are designed this way is because they are actually pressure vessels designed to hold compressed air, and as such need to be American Society for Testing and Materials (ASTM)-certified to ensure that they don’t explode. This is important if you are actually holding pressurized air, but since we are holding negative pressure in our tank, there is no chance of it exploding. Because of this, we can use a much lighter material to get the job done. I have devised a design that I recommend for people who have more time than money. It’s easy to assemble, lasts forever, and works like a charm.
The first question you have to ask is, “Do I need a surge tank or a water trap?” Technically, this design is both, but the engineering constraints are different depending on your needs. If you need a water trap, then you don’t have to do any math. Just figure out what kind of materials you have and how much room you have, and get on with it. If you need a surge tank, then you will need to do some basic calculations to figure out what size it needs to be.
Figure 1. The complete parts list includes the following:
- A length of PVC pipe sufficient to achieve the predetermined volume.
- Two PVC caps (one for each end).
- Two ¼-inch barbed hose nipples with threaded ends.
- One ¼-inch petcock valve.
- Teflon® tape.
- PVC glue.
Figure 2. Measure the length of PVC pipe you want to use.
Figure 3. Cut the PVC pipe as squarely as you can with a saw.
Figure 4. Once the primary tube is cut to length, drill a hole for the vacuum inlet nipple. I drill it about one third of the length down from the top. For a ¼-inch nipple, I drill a 29/64-inch hole.
Figure 5. Using the same drill bit, I drill a hole in each of the end caps.
For a surge tank, I usually calculate about one half cubic foot per manifold. This number is variable, and you may find that another formula works better for you based on the cubic foot per minute (CFM) output of your pump and the type of device you normally thermoform. Once you figure out what size you need, the calculation is simple: V=pi X r2 X h, or volume equals 3.14 times the radius of the tube squared, times the length (height) of the tube. If you’re as bad at math as I am, you can find lots of volume calculators on the web. Just punch in the known variables and they will do the math for you.
Basically, what we are going to make is a tube with a vacuum inlet at the side, a vacuum outlet at the top, and a drain at the bottom to allow the tube to be emptied easily. To make the tube, I usually recommend you start with materials you have lying around. If you have 3-inch PVC, use that; if you have 4-inch PVC, use that. The size doesn’t really matter as much as the volume does. If you have to buy material, get whatever size looks like it will work in the space you have available. For this example, I used 3-inch PVC because that was what I had. The total cost to fabricate this tank is usually less than $25.
Tony Wickman, CTPO, is the CEO of Freedom Fabrication, Havana, Florida. He can be reached at
Figure 7. Once the holes are tapped, seal the threads with Teflon tape, and install the two nipples: one in the top end cap and one in the side of the tube. Insert the petcock valve in the bottom end cap.
Figure 6. Once all the holes are drilled, tap each hole with a ¼-inch national pipe thread (NPT) tap. These taps are designed with a tapered thread to ensure a tight fit.
Figure 8. With all of the components installed, it’s time to assemble the unit. Clean the ends of the tube and inside of the caps with acetone to prime the glue surfaces. Then paint the tube ends and mating surfaces of the caps with PVC cement, and slide the three parts together making sure the ends are on as tightly as you can compress them by hand.
Figure 9. Once the PVC cement hardens completely, the tank is ready to install. We usually use zip ties to secure the tube to one of the bench legs near our vacuum-forming station. This will hold it in place well but will also allow us to remove it easily if we want to clean it or change its location.
