Can't understand why bypass valves are still installed when the pumps are vfd driven when differential drives the pump speed. Failsafe is never an engineering concern so I've ruled that out right away
Why are pressure differentia/ bypass valves still installed on vfd pumps??
dave1234
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So really it can be boils down to redundancy I.e failsafe which I ruled out initially right away. Bypass janitor accidentally bumps the selector switch to bypass (assuming it's a TRUE bypass drive with a starter) bypass loop can handle it. And another failsafe- minimum flow if drives are at minimum speed but chillers still thirsty
Not thirsty ,freeze protection. Chillers have minimum flows to keep from fouling tubes and freezing tubes. If water flow stops chiller will trip on flow switch, hopefully. When it restarts more power drawn and wear and tear on starting components.
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lets say you have an office that "is always hot" even on cold days. it is a 1 ton load. cycling the chiller causes the loop to get warm. if you leave the chiller running and open a modulating bypass valve to maintain minimum water flow, then the entire loop stays cold and the "always hot" office still gets it proper cooling. it is an inefficient way to run a plant but it is also cheaper than adding a variety of different size chillers to meet such varied loads.
what he said.
also, if you have a basic single loop, then your bypass needs to be sized to maintain the minimum flow requirements for the chiller.
if you have a primary / secondary system, the secondary bypass (installed at the end of the loop) only needs to be sized to keep the loop temp. down. If all demand ends, the secondary loop remains cold. if you didn't have that bypass on this type of system, the loop would warm up with zero demand. With a call for cooling, there would be a delay before the space receives cooling.
also, if you have a basic single loop, then your bypass needs to be sized to maintain the minimum flow requirements for the chiller.
if you have a primary / secondary system, the secondary bypass (installed at the end of the loop) only needs to be sized to keep the loop temp. down. If all demand ends, the secondary loop remains cold. if you didn't have that bypass on this type of system, the loop would warm up with zero demand. With a call for cooling, there would be a delay before the space receives cooling.
Could also be a Variable Primary system where all system flow passes through the chiller or boiler. This eliminates the need for separate Primary and Secondary pumps. The bypass maintains minimum flow requirements through the chiller or boiler while being able to satisfy the system's full-load requirements.
Side-note: Variable Primary is often done poorly, and causes random low evap pressure (on chillers), high Delta T (on boilers) or flow switch trips due to the bypass not opening quickly enough when demand drops quickly.
Side-note: Variable Primary is often done poorly, and causes random low evap pressure (on chillers), high Delta T (on boilers) or flow switch trips due to the bypass not opening quickly enough when demand drops quickly.
I learned a ton when I posted the following question a few years back that I think may be relevant to your post. Learned a ton from the guys here.
https://hvac-talk.com/vbb/showthread.php?1925331-Variable-Primary-Chiller-Piping
https://hvac-talk.com/vbb/showthread.php?1925331-Variable-Primary-Chiller-Piping
cool ill check it out thanks
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