three way valve - mixing & diverting type

8/12/2019 Three Way Valve - Mixing & Diverting type

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Learn More About HVAC Three-Way Valves

There are two types of three-way valves used in the HVAC industry: Mixing Valves and Diverting Valves. In

order to prevent any misunderstanding due to terminology, we will consider mixing valves to have two inlets

and one outlet, and diverting valves to have one inlet and two outlets.

Figure 1.

Many people will call all three-way valves mixing valves. Three-way valves can also be referred to asbypass valves, constant flow valves, and many other terms.

Note: Improper use of one for the other will cause chattering, water hammer, vibration, and damageto the system.

Mixing valves are more commonly used in the HVAC field. Mixing valves make good modulatingvalves although they can be used like two-position valves, taking the full flow from one or the otherinlet to the common outlet.

Diverting valves are usually used as two-position. The flow is totally diverted either one way or theother. Generally speaking, diverting valves do not make good modulating valves, although somevalve manufacturers are putting characterized plugs into three-way diverting valves so they can beused to modulate. Valve manufacturers normally specify in their catalogs if a valve is for mixing ordiverting service.

Once the determination has been made as to what three-way valve youre dealing with, mixing ordiverting, modulating or two-position, the selection should proceed much like two-way valves. Find

the CV factor. As before, you need to know full flow rate and P.

Three-way valves are used in many closed-system applications. Examples include:

1. Flow temperature variation

2. Flow volume variation

3. Primary / secondary pumping systems


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4. Two / four pipe distribution systems

There are no rule of thumb ways todetermine flow rate or available pressure for a three-wayvalve. All the specifications must be known to determine the flow rate for a three-way valve.

Figure 2.

Figure 2 shows a three-way valve varying the temperature of the flow. Notice the amount of water to

the system (shown here as a coil) doesnt change. In this case, a low P is desired. Use 20% of the

available pressure. In this example, 20 psi is available. 4 psi would be the P to use to find a CV.


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Figure 3.

In Figure 3, we are varying the amount of flow through the coil. In this application, a high P acrossthe valve is desirable. Use 50% of available pressure, 5 psi minimum if possible. In the example, 18

psi is available, so 9 psi is the P to use to find the CV. If the available pressure fell below 10 psi,

say 8 psi, use 5 psi as P.

As in two-way valves, if the three-way valve selected is less than line size, dont forget the FPfactor. Resize the valve applying the FP factor to find the new CV.

For three-way valves used in chilled water-hot water, summer-winter changeover, mixing two-position

or diverting, use a line size valve. This is a low P application. Full flow is desired.

To find the static pressure a valve must be rated for, the following formula is used:

Static Pressure Rating (in psig) = [(HFP + HT) + (HP - HF)] / 2.31

Where HFP = Fill pressure at low point of system in feet of water.

HT = Distance of valve above low point of system.

HP = Total pump head in feet of water.

And HF = Friction loss in piping between valve and pump in feet of water.

Unfortunately, not all of the information may be known to solve the Static Head Pressure Rating(SHPR). A method can be used to determine an approximation of the SHPR. Take the fill pressureand add the pump head pressure of the largest pump in the system. Make sure the valve bodystat ic pressure rat ing equals or exceeds that sum .You do need those two pieces of information.


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Close-off pressure ratings for three-way valves in a closed loop must equal or exceed the totalpressure difference that can occur across either port when that port is closed.

Figure 4.

In Figure 4, the maximum pressure the valve would have to close-off against would be equal to the

sum of the pressure drops in the coil, coil pumping legs, and the valve with full flow from B toAB. This is because when there is no flow through the bypass, X to A, the pressures at X and A are

the same. The maximum P the valve must close off against is equal only to the P from X towhichever circuit (A or B) has the highest resistance to maximum flow plus the pressure drop throughthe valve.


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Figure 6.

When the condenser is below the level of the cooling tower a bypass using a two-way valve isrecommended.

The P from A to B at full flow should equal the head C-D. See Figure 7.

Figure 7.


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Source:

http://www.industrialcontrolsonline.com/training/online/learn-more-about-hvac-three-way-valves

three way valve - mixing & diverting type

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