Valves

A hopeless case

1. It is considered rude to type in all caps.

2. Have you done a search, maybe starting with Wiki?

3. Do you want us to cut your head open and pour some knowledge in there?

Well said.

When you are installing a control valve I assume you are controlling water flow and attempting to control temperature. If I am correct, witha 2-way you you lower or stop the flow which would increase the head and make te valve not linear thus loosing accuracy and possibly allowing you medium to warm or cool. With 3-way you maintain a constant flow thereby eliminating a change in pressure or temperature and maintain better accuracy. With a 3-way I find I can control my target setpoint within hundredth of a degree or % rh. If you are as inexperienced as we are led to assume, remember to size your valves by Cv with a 4-7 # pd. There are numerous books and articles that will give this in greater detail. Don't be put off by the other resposes. Most of the guys on here are great but they won't let you slip by on basics like this. You need to make the effort to gather some basic engineering knowledge and demonstate some basic discipline before you try to ask them to do your homework

Well said.

I obtained the following information from Google search .It took me only few seconds.

2-way valves are pretty simple and straight-forward. A 2-way valve is any type of valve with two ports: an inlet and an outlet port, typically labeled "A" and "AB" respectively. 2-way valves are used in many applications, from basic on/off to more complex variable flow applications with pumps and VFDs. The type of valve you need for an application depends on the amount of flow, the degree of control, shut-off, and pressure drops over the valve.

3-way valves have, yes, three ports, labeled "A", "B", and "AB". Port "AB" is common to the "A" or "B" port. 3-way valves are commonly found in constant flow/volume pumping systems and can be either mixing or diverting valves. 3-way valves can be piped in the supply or return. If in the supply, then a diverting valve is used. If piped in the return, a mixing valve is used. Ball valves can be piped to be mixing or diverting, but globe valves require different bodies for mixing or diverting.

Mixing applications have the 3-way valve configured with two inputs from the supply piping and one output to the return piping, thus mixing together two inputs before sending it out. Mixing valves are most commonly used with modulating control but can be on/off.

Diverting applications have the 3-way valve configured with one input from the supply side and two outputs to the return piping. In general, diverting valves are more expensive than mixing valves.

http://www.controltrends.org/2010/01/2-way-and-3-way-valves/

Mrswamy gave you the best answer. To simplify , a 2-way valve consider as a flow on/off valve, a 3-way both as mixing valve (hot water/steam 1 port, cold water 2nd port = mixed water 3rd port) or a diverting valve ( used for heating with fluid like hotwater/thermal oil). Acts as a 2-way to send flow to a radiator/heat exchanger and when the temperature controller signals the 3rd ports diverts the flow from/off the radiator. Yes , I can understand. You need to study more