Forced carbonation - the act of forcing CO2 into solution as opposed to allowing the yeast to natural carbonate the beer has several advantages. For me, the biggest advantage is speed. You can have your beer carbonated and ready to drink within 24 hours. In addition, the level of carbonation is much more controllable than depending upon natural carbonation. You can calculate exactly how much pressure you need to get a specific level of carbonation based on the temperature of your beer.
There are three basic approaches I know of to force carbonation of beer in kegs. I have used all with success and each has its advantages. The first and slowest approach is to put the keg under the pressure you have calculated based on the amount of carbonation you desire. For me, this is ~15 psi at ~42 degrees F. Simply place the kegs under this level of pressure and wait about a week for the kegs to slowly carbonate. I often force carbonate 1 keg using the next method below and let the other two slowly carbonate while I serve the first. This method has the advantage of physical effort and lack of issues with over/unstable carbonation encountered with the more aggressive methods described below.
The second method is the quickest approach and gives you beer ready to drink within about 24 hours. Although quick, this approach produces a carbonated beer that seems to me to be "unstable" and requires a lot of finesse during serving. The beer does seem to "settle" over time and become easier to serve. Due to my impatience, this is the method I have used the most. To carbonate a keg using this method, crank the CO2 pressure up to 20-25 psi and attach to the keg you want to carbonate. Next, sit down and place the keg across your legs with the carbonation "in" on the high side of the keg (this is important so that beer does not enter your gas lines). Begin shaking the keg and/or rocking the keg side to side across your legs. As teh top of the keg goes below the level of the bottom of the keg you should be able to hear CO2 bubbling in. Continue to agitate the beer - which allows the CO2 to enter solution - for 4 minutes. Yes - 4 minutes is sufficient and necessary. Now, incubate the keg overnight at 20-25 psi. The next day, you can reduce the pressure and serve. The beer will produce a lot of foam until the carbonation has settled. A word of caution - once you have shaken the keg, be careful not to release or reduce the pressure to that keg (can happen when you attach an additional keg to the system). If pressure is released, the foam you just produced by shaking the keg will be pulled into your gas lines.
The third approach is the one I am currently using and may be a sign that I have mellowed somewhat with age. This approach is a compromise between the two approaches above and yields carbonated beer ready to drink in ~3 days. To carbonate beer with this method, start by placing the kegs under 30 psi for 24 hours (no shaking is necessary). Next, reduce the pressure to 20 psi for another 24 hours. On the third day, reduce the pressure to 10 psi and enjoy. This last method, although slower than the second, seems to produce a more "stable" carbonation which is easier to serve, yet is quicker than the first approach.
