3 way valve - Valve engineering

Respectully disagree with cliff. 3-way ball valves are not universally wonderful, but no choice is. However they can be useful in temp-control bypass applications such as this.

Regardless of the type of valve you choose, you have the choice on a bypass temp control application to use the valve for diverting (installed upstream of the HX, or mixing (installed downstream of the HX.

The leg that goes thru the HX has more pressure drop due to the toruous path thru the fins, coils, fittings, etc.
It is frequently useful to install a balancing valve in the bypass leg, throttled down so that causes the same resistance as the heat exchanger. Usually the balancing valve is manual, tweaked until the system stabilizes, and then locked in position.

Another pitfall if temperature control is if the desired temperature is really close to either the outlet temperature of the Heat Exchanger or the bypass temperature. if that is the case you wind up controlling at one of the extremes of the valve's span and like any control loop that gives really unstable results.

So if, for example, the fluid coming down the pipe is at 50 degrees. and the fluid coming out of the HX is at 150 degrees, and you want to control at 100 degrees, a three-way valve will work pretty well.

If you try to control the same system so its outlet temp setpoint is at 148 degrees, then you just have a trickle coming thru the bypass and a tiny control signal change gives you a drastic change in outlet temp. In this case it would be beneficial to install a relatively large valve on the 150 degree leg, a much smaller valve on the bypass leg which operates in the reverse direction of the other valve, and operate them with the same control signal.

Back to the ball valves: A 90-degree 3-way ball valve has linear characteristic for the middle 60% of its stroke. With approprite selection of the other components in its system(valve size, actuator, positioner, zero-lash coupling) it can do a pretty good job. 3-way globe valves are generally just two unbalanced valve plugs in a common body, contoured to be linear throughout their stroke. So on the sucky control scale they are only marginally better than possible with a ball valve, harder to plumb, bigger for a given pipe size, and massively more expensive. Big advantage with a globe valve is that it is more likely to be able to tolerate higher temperatures, but unlikely to shut off either port completely if desired. (Metal seats) Ball valves (usually) have polymeric seats.

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Ball valves are a popular choice for industrial applications due to their simple design, reliable shutoff, and ease of operation. But beyond the basic on/off functionality, ball valves come in various configurations to handle different flow control needs. 

This article explores the three main types of ball valves based on the number of ports they offer: 2-way, 3-way, and 4-way.

2-Way Ball Valves: Simple On/Off Control

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• The workhorse of ball valves, the two-way option features two ports: an inlet and an outlet.
• When the ball is aligned with the flow path, fluid passes freely. A quarter turn rotation positions the ball perpendicular to the flow path, creating a positive shut-off.
• Two-way ball valves are ideal for isolating sections of pipe for maintenance or regulating flow in simple on/off applications.

3-Way Ball Valves: Diverting and Mixing Flow

• 3-way ball valves introduce a third port, allowing for diversion or mixing of fluids.
• The ball's rotation determines which ports are connected for flow.
• Common applications include diverting flow between two pipes, mixing hot and cold water in plumbing systems, or controlling flow direction in instruments.

4-Way Ball Valves: Advanced Flow Control

• 4-way ball valves offer the most complex flow control with four ports.
• The ball's rotation can connect any combination of inlet and outlet ports, enabling tasks like: 

• Diverting flow to multiple destinations

• Filling and draining tanks

• Reversing flow direction in a system

By understanding the capabilities of two-way, three-way, and four-way ball valves, you can select the right option for your specific flow control requirements.

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