I see, I see...

Soda water:

First of all, soda water is nothing more than regular tap water with CO2 gas dissolved in it. When CO2 gas enters water, it becomes carbonic acid. I'm not going to give you a chemistry lesson here; just understand that soda water is water with CO2 dissolved in it.

Now think about popping a can of soda open. What happens? It fizzes. That should tell you something. It tells you that CO2 doesn't like to be dissolved in water. As soon as it can, the CO2 exits the water causing fizz. Well, how can we make the CO2 enter the water? There are two ways:

Lower the temperature of the water
Increase the pressure of the CO2 gas

Well, with number one we have a problem because we'd have to keep a huge tank of really really cold water on hand. That isn't going to work too well for high-volume production (remember, that's what these machines are made for.) So instead, we will just increase the pressure of the CO2 gas. At room temperature, we only need about 100 psi (pounds per square inch, which is a measure of air pressure).

But that presents another problem! Most city water systems only provide water at about 60 psi. And worse yet, small community stations may only provide 40-50 psi, and a single home well with a pump may get as low as 30 psi. When we turn on the gas at 100 psi, it is just going to push the water back into the pipes! That is no good. So the solution is the carbonator!

Carbonator - This device has a tank where the CO2 gas and water are mixed. But it also has a booster pump attached to it. This pump pressurizes the water up above the pressure of the CO2 gas, causing the two to mix together to form soda water. The carbonator can run as often and as long as we need it, so we don't have to keep tons of extra water on hand, and it can pressurize the incoming water so we don't push CO2 gas back into the water supply. Perfect!

McCann's E3000 carbonator

The tank is on the left, pump and motor on the right. The large red device on the top of the tank with a cord coming out of it is the trigger that tells the pump when to turn on and off. Typically, the pump only runs for about 30 seconds every X glasses of soda, where X depends on how big of a tank you have.

But what about the flavor?!

In the old days, you would mix up a bunch of items into a flavoring solution. Then you'd add sugar and a certain amount of your flavoring to soda water to make a nice drink. But ever since the drug store soda fountain, people have combined these processes into one, making use of syrup.

Syrup, for soda, is mostly sugar with the flavoring already added. It is also kept to a specific consistency, so you can always know to mix X amount of syrup with Y amount of water. Typically, this is something like 4 parts water to 1 parts syrup, or 5 parts water to 1 part syrup.

So you have the syrup, and you have the carbonated water. You could just put a spray nozzle on the end of your carbonator. Then measure teaspoons of syrup into a cup and fill with soda water. But that wouldn't work very well for two reasons. First, it is a royal pain and let's face it: we are lazy. Perhaps more importantly, soda water must be kept cold! Remember from above that we had to increase the pressure from 14.7 psi to 100 psi to get the soda water to mix correctly. What happens if we expose the soda to air (14.7 psi, the air pressure at sea level)? It immediately lets all the CO2 escape! BUT if we dispense the water cold into a glass full of ice, we can keep the soda fizzy for a lot longer.

A soda fountain might look like this:

The blue section on top would be the ice bin. Then the yellow is the solid block of aluminum (coldplate) through which tubes (green) have been bored. Tubing connects to the cold plate at the back and front. Thus, when someone vends soda, the water and syrup have passed through a solid block of aluminum that is in direct contact with large quantities of ice! For the slow class, that means the water and syrup should be quite cold when dispensed.

All that remains is to mix the soda, and for that we use a fountain head:

The soda (blue) and syrup (red) both pass through the standard mounting point into the actual fountain head assembly. That assembly is covered by a grey or black plastic housing, which also has a detachable front cover (solid grey part, to the right).

Two solenoids control the flow of the soda. The teal one controls the soda water, and is set further back. It also has a screw that adjusts the flow rate. The one up front controls the syrup, with a screw to control the rate. As you can sorta see from my crude MSpaint diagram, the syrup and soda water don't mix until they are falling into the glass.

Now you understand the theory. Time to go on a spending spree!