A $19 desktop constant head tank

I have been planning on doing some pipe flow demonstrations in class using straws and rubber connectors. However, to do that I need a constant head tank that will drive the flow. Ideally the entire system would sit on a desktop so that no special mounting would need to be brought into the classroom. I am still working on the pipe flow demonstration but I thought that the budget desktop constant head tank design may be helpful to share.

Equipment

You will need:

  1. A small aquarium pump $8 
  2. A funnel $1
  3. 2 plastic tubs with straight sides (I used plastic shoe boxes) $4
  4. A small tube of silicone sealant $3
  5. A binder clip $1
  6. A short length of tubing to connect to the pump $1
  7. A hard plastic straw $1

20180530_140256

Figure 1: Materials needed for construction (binder clip missing).

Design

The basic idea is that there is a lower reservoir tank that feeds the upper tank via a pump. The upper tank contains a constant height weir overflow, with return to the reservoir tank, and an outlet below the overflow that will have a constant head. Provided the weir length on the overflow is large and the flow rate through the pump is substantially larger than the flow rate out of the constant head outlet then there will always be water flowing over the weir and the head over the weir will be relatively constant. In this budget design the reservoir and upper tank are plastic shoe boxes and the weir overflow is a funnel. Water is pumped using an aquarium pump from the lower to the upper tank and returns through the funnel to the lower tank. The upper tank rests on the lower so that the entire system can sit on a desk.

CH Tank

Construction

  1. Drill a hole in the center of the base of one of the shoe boxes with a diameter equal to that of the middle of the funnel neck.
  2. Drill a hole in the side of the same shoe box with a diameter a fraction smaller than the straw.
  3. Push the straw through the side hole (it should be a tight fit) and then seal around the hole with the silicone sealant on both sides
  4. Place the funnel inside the same shoe box with the neck protruding through the hole and seal around the funnel neck on both sides of the hole. The top of the funnel should be below the rim of the box so that water will flow into the funnel before it overflows out of the box.
  5. Attach the tubing to the aquarium pump and place it in the second shoe box.
  6. Attach the binder clip to the box with the funnel and use it as a mount for the tubing such that the tubing is pointed into the box but not into the funnel
  7. Place the box containing the funnel on top of the box containing the pump with the funnel outlet draining into the lower box.

Figure 2: (a) fully assembled constant head tank system. The yellow straw is the constant head outlet. (b) close up of the upper tank showing the inflow tube mounted (from the pump) through the binder clip and the funnel overflow back into the lower reservoir tank. (c) alternate view of the entire system.

Operation

  1. Block the end of the outlet straw or connect it to the test rig to be used.
  2. Fill the lower box until it is almost overflowing and the upper box until it is about to overflow into the funnel.. This is most easily done by pouring water into the upper tank and allowing it to overflow through the funnel into the lower tank.
  3. Turn on the pump. The water will be pumped into the upper box and drain through the funnel back into the lower box. The head in the upper box will remain essentially constant provided there is water overflowing into the funnel.

Comments

The $19 budget is approximate. You will use only a fraction of the $3 tube of sealant and may need to buy a box of binder clips to get the one you want. The whole thing takes about 15 minutes to assemble provided you have an electric drill with the appropriate drill bits for cutting the holes.

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Pump efficiency calculations

Just a quick note on an extension of the pump performance curves demonstration that was suggested by Ed Maurer of SCU. The initial demonstration had students calculate the head rise – flow rate relationship for a cheap aquarium pump. Ed extended this with the addition of an inline power meter (such as the Kill A Watt meter). Simply record the power consumed (P in watts) for each flow rate (Q) and pump head rise (Ep=height over which water is pumped plus tubing and exit head loss) . The efficiency can then be calculated as

η=γQEp/P.

You can then plot the pump efficiency against flow rate on the same plot as the head rise vs flow rate. When Ed Maurer did this he reported very low efficiency peaking around 20%. This is not terribly surprising as they are cheap pumps (typically only cost around $20).


An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

Video of Pipe networks and head loss

Here is a video of the “Pipe networks and head loss” demonstration. The full video is here. Sorry that it is sideways.

Video Feb 10, 9 51 27 AM 00_00_00-00_00_10

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

Pipe networks and head loss

I find that students often find pipe networks difficult because they have difficulty visualizing that the head change along two different paths that are connected at each end must be the same. One way to demonstrate this is to drain a tank through two different length pipes. You can do this with the same equipment as the siphoning experiment though you will need an extra length of tube. I use a tank with pneumatic tubing connectors in it so that you don’t have to hold the tubes at both ends.

Equipment

  1. A large tank and a table to put it on
  2. Two tubes or pipes of substantially different (known) lengths
  3. Two measuring cups
  4. A stop watch.

Photo Feb 10, 9 45 05 AM

In the example in the photos there is a press-to-connect fitting in the base of the tank which I connect to a T-junction and then have the two pipes connected to that.

Photo Feb 10, 9 45 12 AM

Demonstration

  1. Fill the tank with water and flood the tubes.
  2. Place the measuring cups on the floor so that there is a substantial head difference between the top of the water in the tank and the top of the measuring cups
  3. Lower the outlet of the tubes to the top of each measuring cup and start the water flowing
  4. Record the time taken for each cup to fill. The longer tube should take a longer time

Analysis

The analysis is very similar to the siphoning demonstration so I will only summarize it here.

The total head (H) is the distance from the water surface in the tank to the top of the measuring cups. The head is balanced by the entrance head loss (hen), the head loss in the junction (hj), the head loss in the pipe (hp1 or hp2) and the exit loss from each pipe (hex1 and hex2). That is,

H = hen +hj + hp1 + hex1 = hen +hj + hp2 + hex2.

The first two loss terms cancel. Using the Darcy Weisbach equation we can re-write the remaining terms as

U12/2g (1+f1L1/d1)=U22/2g(1+f2L2/d2)

The ratio of the flow rates is given by

Q2/Q1=((1+f1L1/d1)/(1+f2L2/d2))1/2

Here is where it gets messy and, as a result, you won’t get a nice neat ratio of flow rates. In the example that I show in the video the tubes are 1/4” in diameter and 256” and 64” long respectively. Therefore, if the exit velocity head is negligible, the flow rate in the long tube should be roughly half that in the shorter tube. In the test I ran the ratio was 2.5:1. There are various explanations for this difference. First, the exit velocity head may not have been negligible (though accounting for that would have reduced the predicted flow rate ratio). Second, the friction factors for the tubes would be different due to the different Reynolds number (though again, accounting for that would have reduced the predicted flow rate ratio). The most likely explanation for the difference between the predicted and measured flow rate ratio is that the longer tube was curled up in a series of loops whereas the shorter tube was straight. There are additional measurement errors in the tube lengths, and the time for each cup to fill. For example, the times we measured were 25 seconds and 63. It does not take much of an error to get a substantial change in ratio. For example, there is a parallax error in reading the water level in the cups, and There can also be a delay between the person watching the cup calling out ans the timer registering the time. If the recorded times were each out by say plus or minus 3 seconds then the flow rate ratio would vary from  2.1 to 3.  Clearly the demonstration provides a great opportunity to talk about experimental error and uncertainty.

Thank you to Kate and Austin for helping out with the video.

An index of all the demonstrations posted on this blog can be found on my website here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.