Top Tap pressure barrel ---Means you draw the beer off the top of the barrel. The idea is that the beer clears from the top down so the beer is clearer at the top. Works on a float system. There's a float in the barrel with a pipe from the back of the tap to the float. The barrel can go at the back of the shelf---But....You have to use gas to get the beer out, run out of gas, run out of beer! Also you can just put a piece of wood under the front of bottom tap barrel so it slopes backwards. You just take it out when the beer gets near the bottom of the barrel!
Bottom Tap Pressure Barrel---You can still get beer out if you run out of gas! Hooray! But... Draws beer off bottom of barrel..Just put a book under the front of the barrel so the sediment runs to the back. Take the book out when the beer gets near the bottom of the barrel. If you run out of gas you can just loosen the cap and let a bit of air in. We'd go for a bottom tap beer barrel every time. Big disadvantage is that it has to hang over the edge of a shelf so you can get the beer out of the tap!
Why do you need Gas in your home brew beer barrel? - When you drink the beer it causes a vacuum which sucks the gas out of the beer, so,the beer goes flat. You put a bit of gas in to create top pressure which means the beer stays nice and gassed. You're not injecting gas in the beer, just keeping the beer fresh. Without gas you have to let air in so the beer will come out!
Letting air in makes the beer go off very quickly-- In about a week usually.
So, We have two types of gas system.
Soda Syphon Bulb/Pin Valve Uses the small 8gm Co2 bulbs --- Easy to use, just screw a small gas bulb on to the barrel when you need it. There's a small adaptor to hold the gas bulb.Throw away the bulb after it's empty. Cheap easy and simple. The valve has a pin inside which pieces the gas bulb. If you look inside the top of the valve you should see it. If your barrel leaks you'll go through a lot of gas, but the system is very good.
S30 Valve and S30 refillable cylinder-- Add just the amount of gas you need and take the cylinder off- cheap to run once you buy it. You have to return the empty cylinder to us for refil which is a nuisance. The gas is cheap to buy and because the cylinder reseals you put in just what you need so you don't over gas the beer. It's not very convenient unless you can get into the shop.