Field-style article image prepared for primary secondary pumping system.
You’re reviewing a chilled-water system with six air handlers that cycle on and off throughout the day. The chiller manufacturer specifies minimum flow of 240 GPM, but peak building load requires only 180 GPM. A single variable-speed loop would force the chiller into low-delta-T operation or require a bypass valve that wastes pump energy.
A primary-secondary pumping system solves this by splitting the circuit into two hydraulically independent loops connected by a low-resistance bridge pipe. The primary loop serves the chiller at constant flow. The secondary loop serves the building at variable flow. When secondary demand drops below primary flow, excess water circulates back through the common pipe without forcing flow through either loop’s equipment.
This configuration maintains manufacturer-required flow rates on production equipment while allowing distribution pumps to match actual load. It eliminates the bypass-valve energy penalty and prevents pump interaction that causes control instability in single-loop variable-flow systems serving equipment with strict flow limits.
要点
Primary-secondary systems use a common pipe to hydraulically decouple two pumped loops so each can operate at independent flow rates without affecting the other.
The primary loop maintains constant flow through chillers, boilers, or other production equipment that require minimum flow for safe operation.
The secondary loop runs at variable flow to match building load, reducing pump energy compared to constant-volume distribution.
Common-pipe pressure drop must stay below 0.5 feet of water to maintain hydraulic separation; this typically requires pipe diameter at least one size larger than connected headers.
Hydraulic Separation Through the Common Pipe
The common pipe creates a point of equal pressure between the two loops. When both pumps run, flow in the common pipe depends on the difference between primary and secondary flow rates.
If secondary flow exceeds primary flow, water flows from the return header into the common pipe and mixes with primary return water. If primary flow exceeds secondary demand, surplus water bypasses the secondary loop through the common pipe back to the primary return.
The ASHRAE HVAC Systems and Equipment Handbook (https://studylib.net/doc/27428574/ashrae-hvac-systems-and-equipment-2020) specifies that common-pipe velocity should not exceed 2 feet per second to prevent pressure drop that would couple the loops. At higher velocities, the pressure differential across the pipe creates hydraulic interaction where one pump influences flow in the other loop.
Pressure drop across the common pipe must be negligible compared to the primary and secondary loop pressure drops. Most designs target common-pipe pressure drop under 0.1 feet per head to ensure true hydraulic independence.
When to Specify Primary-Secondary Over Variable Primary Flow
Primary-secondary configuration suits systems where production equipment cannot tolerate the full range of building flow variation. Chillers with minimum-flow requirements, condensing boilers with turndown limits, and heat exchangers with velocity-dependent performance all benefit from decoupled loops.
A variable primary flow system (https://assets.danfoss.com/documents/275758/AB361177374144en-000101.pdf) eliminates the primary-secondary split by using variable-speed pumps on a single loop with bypass control or equipment staging. This saves the cost and complexity of additional pumps but requires that all equipment handle the full flow range from minimum to peak.
The crossover point typically falls around 40% of design flow. If building minimum load drops below equipment minimum flow, primary-secondary decoupling prevents flow starvation. If equipment can modulate down to building minimum, variable primary flow offers better efficiency.
Fire-tube boilers and many air-cooled chillers tolerate wide flow variation, making them candidates for variable primary flow. Shell-and-tube chillers and plate-and-frame heat exchangers often require minimum velocity for heat transfer, favoring primary-secondary separation.
Primary Loop Sizing and Control
Primary pumps size to the maximum flow required by production equipment at design conditions. For a chiller rated at 500 tons with 2.4 GPM per ton, primary flow would be 1,200 GPM.
Primary pumps typically run at constant speed because production equipment operates near design flow most of the time. Modulating primary pump speed saves little energy and risks violating equipment minimum-flow requirements during startup or low-load staging.
Some designs use multiple primary pumps matched to staged chillers or boilers. Each primary pump runs at constant speed when its associated equipment operates. This maintains equipment flow while allowing the primary loop total flow to vary with the number of online units.
Temperature sensors in the common pipe provide control feedback. When common-pipe temperature approaches supply temperature, secondary flow exceeds primary flow and additional production capacity should start. When common-pipe temperature approaches return temperature, primary flow exceeds demand and production equipment should stage off.
Secondary Loop Design for Variable Flow
Secondary pumps size to peak building load plus distribution losses. For the same 500-ton chiller with 15% distribution losses, secondary design flow would be 1,380 GPM.
Variable-frequency drives on secondary pumps allow flow to track building load as terminal units modulate. Differential pressure sensors at the farthest zone maintain minimum pressure while reducing pump speed during low-load periods.
Secondary pump energy savings typically range from 30% to 50% compared to constant-volume secondary pumps with three-way valves. The savings come from reducing flow at partial load rather than bypassing flow through pressure-regulating valves.
Two-way control valves at terminal units are required for variable secondary flow. Three-way valves bypass water and prevent flow reduction, eliminating the variable-flow energy benefit.
Common Applications and Installation Details
Chilled-water systems in buildings with variable occupancy benefit most from primary-secondary decoupling. Central plants serving multiple buildings often use primary-secondary-tertiary configurations where each building has its own tertiary loop.
Condenser-water systems can use primary-secondary separation when cooling towers have minimum-flow requirements or when tower staging doesn’t match chiller staging. The hydronic design manual (https://online.flippingbook.com/view/507435777/25/) notes that condenser-water applications need larger common pipes because of higher flow rates relative to temperature rise.
Hot-water heating systems use primary-secondary configuration when mixing valves or outdoor reset control in the secondary loop create supply temperatures different from boiler supply. The primary loop runs at boiler design temperature while the secondary loop modulates between supply and return.
Common-pipe location should be close to the primary pump discharge and secondary pump suction to minimize pressure drop in connecting headers. Vertical installations should have the common pipe at the high point of both loops to prevent air entrainment.
Efficiency Trade-offs and Design Limits
Primary-secondary systems add pump and piping first cost compared to single-loop designs. Two sets of pumps, additional pipe, and more complex control sequences increase installation cost by 15% to 25% for typical chilled-water plants.
Operating cost comparison depends on primary loop control. Constant-speed primary pumps consume full-load power regardless of building demand. Variable primary flow systems can reduce total pumping energy but require more sophisticated controls to maintain equipment minimum flow through bypass or staging logic.
The patent literature (https://patents.google.com/patent/US7017606B1/en) describes control improvements that allow primary pumps to vary speed while monitoring flow at each production unit. These hybrid configurations capture some variable-primary-flow efficiency while maintaining equipment protection through local flow monitoring.
System head calculations must account for both loops independently. Primary head equals production equipment pressure drop plus primary piping losses. Secondary head equals terminal unit pressure drop, secondary piping losses, and control valve authority. Common-pipe head should not appear in either calculation because it’s shared between loops.
よくある質問
Can I convert an existing constant-volume system to primary-secondary configuration?
Conversion requires adding a second pump set, installing the common pipe between loops, and replacing three-way control valves with two-way valves in the secondary loop. The existing constant-volume pumps can become either primary or secondary pumps depending on their head characteristics and flow capacity relative to production equipment requirements.
What happens if both pumps shut off simultaneously?
The system reaches equilibrium with no flow in either loop. Restart sequencing should bring the primary pump online first to establish production-equipment flow before the secondary pump starts. Most control systems interlock the secondary pump to prevent operation unless the primary pump is running.
Why not use a single variable-speed pump with a bypass valve?
Bypass valves waste pump energy by throttling flow that must still be pumped at full head. A typical bypass valve in wide-open position still dissipates 5 to 10 feet of head. Primary-secondary separation eliminates this pressure drop because excess primary flow recirculates through the common pipe at near-zero head loss, and secondary flow reduces through pump speed reduction rather than valve throttling.
How do I size the common pipe diameter?
Common-pipe diameter should equal or exceed the smaller of the two connected headers. Conservative sizing uses the larger header diameter minus one pipe size. Velocity below 2 feet per second ensures pressure drop stays under 0.1 feet per 10 feet of pipe length, maintaining hydraulic separation across typical common-pipe runs of 5 to 15 feet.
結論
Specify primary-secondary pumping when production equipment minimum-flow requirements conflict with variable secondary demand below 40% of design load. Size the primary loop for equipment flow at constant speed, the secondary loop for building peak load with variable-frequency drives, and the common pipe for negligible pressure drop at the higher of the two flow rates. Confirm that space and budget accommodate the additional pump set and piping before committing to primary-secondary configuration over variable primary flow alternatives.