Originally Posted by
PHall
So somebody explain to me why in the first diagram the UAT will suck air and in the following diagrams the center tank will not suck air. No matter what you call the tanks, a tank is a tank and air is thinner than fuel so what will the header tank suck first. Air or fuel??
Air.
I think all the tanks should be in series.
In the first diagram (also pasted below), an "X" connector is used to connect the clunk line from all three tanks to the feed line to the UAT. The pump draws fuel out of the UAT which is replenished by fuel drawn from the three tanks. In theory, fuel is drawn at the same rate from all three tanks. In practice, this will vary depending on the length of tubing between the X-connector and each tank. However, if all three tanks are not the same volume, the smallest tank will empty first and then air will be drawn past the X-connector and into the UAT because, as you said, air is thinner than fuel.
In the second diagram (also pasted below), a T-connector is used to connect the clunk line from two saddle tanks to the vent line of the center tank. The clunk line from the center tank is connected to the UAT. In this arrangement, the pump draws fuel from the UAT which is replenished by fuel from the center tank. The center tank is replenished by fuel arriving from the saddle tanks through the center tank vent line. The saddle tanks which are assumed to be the same size) both contribute fuel equally (provided that the lines between the saddle tanks and the T-connector are the same length) to keep the center tank full. Once the saddle tanks are empty, air is drawn through the saddle tanks and into the center tank which continues to feed fuel to the UAT.
Installing the tanks in series as you suggest will also work. The downside of that is that it can lead to a high vacuum level required to pull the fuel through a single path (electrical analogy: resistance series circuit is Rtotal = R1 + R2 + R3 whereas resistance in a parallel circuit is 1/Rtotal = 1/R1 + 1/R2 + 1/R3).
Regards,
Jim
