Pat, the pressure is the pressure and there are only two of them; static and dynamic - standing still and moving. No matter the volume of the tank, pressure is pressure and it won't increase because of a larger tank being installed. Plus, your problem is lack of pressure and flow. So putting a big bulge (a tank) in the plumbing ins't going to get more pressure or flow; just a larger volume of low pressure slow/low flow water. The capative air precharge pressure isn't going to increase pressure, it equalizes to your main line water pressure when you shut off the shower.
Your pressure (to the tank) is controlled by the pressure switch settings you have the well pump pressure switch set for, or whatever the city pressure is at your house. Since you want to boost pressure, IIRC, you have city water, a pressure tank is not going to increase your pressure (even if you had a well); or prevent pressure fluctuation.
You need a booster pump but, if the pump wants more water than the city supplies, you must go with an atmospheric storage tank to provide flooded suction (more volume) to the pump or it won't solve your problem while it causes another problem; cavitation damage of the pump, along with less money in your wallet.
The best choice for a pump is a correctly sized for your house shallow well single line/jet pump. They require a pressure tank. That setup, either a pressure tank or atmospheric tank and pump, will come with a corresponding pressure fluctuation. The only solution is a variable speed pump or CSV or Smart Tee (uses a type of CSV not as good as a CSV).
A CSV will prevent the pressure fluctuation in your house. Although I have them available to me, I do not sell them but Bob does. I turned him on to them... I know Bob, and he may, if you talk nice to him, guarantee no noticeable fluctuation in water pressure past the CSV. I could sell you one with that guarantee... but I prefer not to sell pumps, tanks etc. and concentrate solely on water treatment; which usually has me overworked without taking on more.
CSVs cost way less than variable speed pumps and their controllers and don't require anything other than simple plumbing to install one in your plumbing. And to go variable speed, you then must use an atmospheric type tank which opens the door to water quality issues and increased maintenance to deal with them and equipment in some cases. Which all equates to more money to simply be able to power wash yer butt in the shower.... [img]/forums/images/icons/wink.gif[/img]
Gary Slusser
Quality Water Associates