Variable Primary Chilled Water Minimum Flow Bypass

The bypass exists to protect the MACHINE, not to control temperature, and that distinction explains everything about how it behaves. Below the evaporator's minimum flow the water in the tubes goes laminar, heat transfer collapses, leaving-temperature control becomes unstable, and the low-temperature safety trips -- and on some machines repeated low-flow operation damages tubes. So the bypass opens whenever system demand falls below what the running chillers require, and the pump moves water in a circle to keep the machines happy. The interaction with STAGING is the part that catches plants out, and it is arithmetic rather than judgment: two chillers running have twice the minimum flow of one. A plant that stages up at low load can find itself with a system flow far below the combined minimum and a bypass valve wide open, burning pumping energy to circulate water that does no work at all. That is an argument for staging down promptly, and it means the staging logic and the bypass logic have to be designed together rather than separately -- so what staging one machine off would do to the bypass is computed here beside the current condition. Sizing the valve is the straightforward part with one counter-intuitive turn. It must pass the largest bypass flow, which occurs at minimum system load with the maximum number of machines that could be running -- and at that condition the pumps are near shutoff, so the differential is HIGH and the required Cv is SMALLER than a sizing at design differential would suggest. Sizing on the design differential oversizes the valve, and an oversized bypass valve controls badly at the small openings it actually spends its life at. Minimum flow fractions are ENTERED from the chiller manufacturer, because they vary by machine and by evaporator design. This does not model the control sequence, the sensor location the bypass modulates from, the check valve and flow measurement a variable primary plant needs, or the transient during a stage change, which is when low-flow trips actually happen. The chiller manufacturer's minimum flow data and the plant's designer govern.

minimum per machine = design evaporator flow x the minimum fraction; combined minimum = that times the machines running; bypass = combined minimum minus system flow when the system falls below it; valve Cv = bypass gpm / sqrt(differential psi) at the differential the pumps develop at that low flow.

The variable primary chilled water minimum-flow bypass as central-plant practice writes it. The evaporator minimum fraction is ENTERED from the chiller manufacturer, because it varies by machine and evaporator design. It does not model the control sequence, the sensor location the bypass modulates from, the check valve and flow measurement a variable primary plant needs, or the transient during a stage change, which is when low-flow trips actually happen.

One flow balance and one valve-sizing relation.

Estimate. AHJ and licensed professional govern.

Field names used by the API: machine_design_gpm, minimum_flow_fraction, machines_running, system_flow_gpm, bypass_differential_psi, design_differential_psi, minimum_per_machine_gpm, combined_minimum_gpm, bypass_gpm, required_cv, cv_at_design_differential

• The minimum fraction is entered it varies by machine and evaporator designthe chiller manufacturer
• No control sequence the sensor location and the modulation are the designer'sthe plant designer
• Steady state the stage-change transient is when low-flow trips happenthe controls engineer