In HVAC chilled water system why we can't use 2 way valve and 3 way valve in the same system??

Dears

We cant use2 and3 way together because2 way are use with pumps having VFD and3 ways are used with constant speed pump.

In a complex system where multi secondary circuits are there with constant and variable flow then we can use2 and3 way in different circuit but from same chiller but those will be completely different secondary circuit not linked with each other.

-2 way valve used with variable speed bump 

-3 way valve used with constant speed bump

 

Actually the usual is to use two way valves in variable flow systems which use VFD pumps, while3 way valves are used incase of constant flow systems. However you can use two way in systems with constant flow and provide a common bypass with two way valve between supply and return....Also3 way valves can be used with2 way valves in variable flow systems to maintain minimum flow and keep chilled water circulation.

because2 way need variable speed pump and3 way doesn't,,,,you can use them together but with the special pumps put that's extra cost

Its depend on the Pump speed,If  pump working with constant speed then we will use3 way valve and if pump working with variable speed than we will use2 way.

 

Regards

Shafique Ahmad 

HVAC Engineer

Using2-way &3-way valve at the same time in a system is a big contradiction in aspect of control, economy, feasibility & optimum operation. However, the purpose of having either of these system is not achieve smooth recirculation of an closed loop chilled water system.

A)3-way modulating valve (either diverting or mixing) is usually seen in low capacity/size system where constant speed primary pumps are employed for chilled water recirculation. In this case, the3-way valve's ability to have proportion of mixing or diverting chilled water is witnessed through adequate communication with BMS/BAS having responsive field sensors such as thermostat.

B) A2-way valve is generally seen in a system with primary-secondary pumping scheme. Such system are big in size & the investment of employing primary-secondary system seems feasible. This contributes to the low cost of operation to a great extent. A combination of solenoid valves & de-coupler header in communication through BMS/BAS plays a major role to have an energy efficient system.

Hello Senior Mechanical Engineer,

Where did you see 'bump' in the chilled water system?

 

Three way control valve is used for constant flow systems while a two way control valve is used for a variable flow system. A two way control valve is more sensitive to high differential pressure and it is harder to close off against line pressure compared to a three way control valve of the same size. This is because a two way valve creates a dead head condition while a three way valve merely shifts pressure from one place to another Therefore, the actuater for a three way valve does not have to be as powerful for the same size valve. For chilled or hot water the equal percentage valve is recommended and is most effective operating at50% to100%

 

Use of2-way and3-way valves in the same system is not economical as with2-way valves, Variable Speed Pump is used having the VFD and in3-way valve system there is constant speed pump because variation of flow is achieved by bypassing water through3-way valve.

1. cost

2. space

3. unpractical