What is the use of by pass line in chilled water system and how can we design the bypass line in chilled water pipe?

Hi Rizwan,

Though your question is not clear at many instance, I would like to answer on the basis of what I interpret from your question.

Bypass line in chilled water system is used in variable frequency drive(VFD) system. When the difference between return water temperature and supply water temperature becomes less(Close to the required supply water temperature) then the motorized valve installed on the return chilled water piping tends to close. At this time pressure sensor installed on both chilled water supply and return line senes the pressure difference of both the pipes and lower the frequency of chilled water pumps which results in power saving. So at this point of time bypass line plays a prominent role to circulate the water without entering into the Air handling unit.

Bypass line is mostly consider THREE Way valve.(constant chilled water flow system is consider THREE way valve)

A three way mixing valve in one position, the valve is open from Port A to AB, with Port B closed off. In that position, all the flow is through the load. As the valve moves from the A-AB position to the B-AB position, some of the water bypasses the load by flowing through the bypass line, thus decreasing flow through the load. At the end of the stroke, Port A is closed, and all of the fluid flows from B to AB with no flow through the load.(REFER ASHRAE Hydronic Heating and Cooling System Design Chapter).

Commonly termed decoupler is an open bypass line before secondary variable pump suction to bypass back to chiller on the NO DEMAND for cooling. With return temperature back to chiller below the delta T requiref will cycle chiller to Standby mode or staging.

For Maintenance purpose we can use the bypass line in chilled water system

bypass pipe is for minimum chiller flow in variable speed system 

size of bypass is the same as the size of one chiller piping