It is now following the pump flow-head curve at a much higher pressure. The designer may overlook this condition. Under low flow conditions, the valves may lift up as described earlier. If the selected actuators have inadequate holding power at the higher head shut-off condition, the valves may lift up as previously described. Three-way valves have little use in variable volume pumping systems.
Bypassed water lowers the return water temperature and increases flow rates.
As a result, additional chillers and pumps are brought on-line to provide flow, rather than to control the cooling load. As a rule of thumb, the Hydraulic Institute 4 and ITT Fluid Handling recommend a minimum flow rate of 20 percent of the best efficiency point flow rate. To limit the radial and axial loads and shaft deflection, our recommendation is to maintain a minimum flow rate of 25 percent of the best efficiency point flow rate. When a pump is operating in a low flow condition, the temperature rise occurs in a small volume of water. If an insufficient volume of water flows through a pump, the inefficiency of the pump is converted to heat.