Decoupled Constant Primary Systems

The chilled water decoupler is the heart of the primary secondary common pipe arrangement. It lets primary and secondary flows differ without the pumps fighting each other. It carries every litre the chillers produce that the building doesn't use. It also carries every litre the building draws beyond what the chillers supply.

Size the decoupler for at least the design flow of the largest chiller, because the surplus can approach that chiller's full flow at low load. Keep velocity low: the decoupler is commonly the same diameter as the primary header or the largest chiller connection. It should be short, straight and free of restrictions. Some designers also specify a minimum straight length to limit turbulence and unwanted mixing at low decoupler flow.

The decoupler sits in the plant room, between the secondary supply and secondary return connections. Following the direction of primary flow, the secondary supply must leave the primary supply header before the decoupler, and the secondary return must join the primary return after it. If these connections are reversed, return water short-circuits straight back into the supply.

The decoupler only works if its pressure drop is negligible compared with either loop. Valves, strainers or undersized pipe in the decoupler create a pressure difference between the loops, and the primary and secondary pumps start to interact. We keep the decoupler free of anything that adds resistance, apart from a full-bore bidirectional flow meter where specified.

Generally, no. A check valve blocks reverse flow, but it also puts the primary and secondary pumps in series whenever the building needs more flow than the chillers supply. The secondary pumps then pull extra water through the running chillers, and the loops are no longer independent.

Some designers use a check valve deliberately to limit low delta-T effects, but only after checking chiller flow limits and pump interaction. In most plants, fixing the cause of reverse flow is the better solution.

When primary flow exceeds secondary flow, cold supply water flows through the decoupler to the return. The chillers see slightly cooler return water, but the building still receives water at the design supply temperature. A modest surplus is the normal operating state.

When secondary flow exceeds primary flow, warm return water flows backwards through the decoupler into the secondary supply. This is decoupler reverse flow, and it means the chillers are not supplying enough water for the building.

When primary and secondary flows match, decoupler flow is close to zero. That is efficient but leaves no margin: any increase in building flow causes reverse flow. Staging logic therefore aims for a small surplus rather than exactly zero.

Reverse flow blends warm return water into the building supply. Take a plant with 100 m³/h of primary flow at 6.7 °C, 125 m³/h of secondary flow and return water at 11.7 °C. The 25 m³/h shortfall is made up from the return and mixes into the supply, so the building receives water at about 7.7 °C instead of 6.7 °C.

The consequences then feed each other:

• coils lose capacity and dehumidification suffers
• valves open further to compensate
• secondary flow rises
• reverse flow grows worse

This primary secondary flow imbalance is the classic downward spiral of a low delta-T chilled water system.

A straight, low-resistance pipe connects the primary supply and return in the plant room. It is the standard arrangement for chiller plants, and the simplest to design, instrument and maintain.

A hydraulic separator is a large-diameter vessel with four connections, two for each loop, and very low internal velocity. It separates the loops hydraulically and often includes air venting and dirt separation. Hydraulic separators are common in heating systems and compact chilled water plants.

A secondary circuit is connected to a primary loop through two tees placed very close together, commonly no more than about four pipe diameters apart. The pressure drop between the tees is negligible, so the secondary circuit stays hydraulically independent. This arrangement is widely used for heating circuits and zone loops.

In large campuses and district cooling networks, each building can have its own tertiary pumps. These connect to the secondary distribution through a local decoupler or bridge, often with a control valve that limits the flow drawn from the secondary loop to protect return temperature. Each building manages its own flow, and the central plant stays stable.

Each chiller has its own primary chilled water pump, which starts and stops with it. Controls are simple, each chiller gets its design flow, and unequal chillers are easy to accommodate. Standby can be provided per pump or through crossover piping.

Primary pumps discharge into a common header serving all chillers, with a motorized isolation valve on each chiller. Any pump can serve any chiller, and one standby covers all duty pumps. Flow distribution between chillers needs careful design, especially when chiller sizes differ.

Constant speed secondary pumps with three-way valves at the coils create a simple, constant flow secondary loop. They offer none of the energy savings of variable secondary flow. They also force enough chillers to run to match full secondary flow at all times.

A variable speed secondary pump on a VFD is the standard modern choice. It is controlled by chilled water differential pressure control at the index circuit, with a two-way control valve at each coil. Secondary flow follows the load and pumping energy falls sharply at part load. This variable secondary flow system is the heart of a CPVS chilled water system.

Instead of one secondary pump set for the whole site, each zone or building can have its own secondary pumps drawing from the primary header. Each set is sized for its own zone's flow and head. Short, low-head zones are then not over-pressurized just to serve long, high-head ones.

Secondary pumps are normally arranged as N+1, for example two duty and one standby. The standby is included in rotation and starts automatically if a duty pump faults.

A bidirectional flow meter in the decoupler measures both the size and the direction of flow. A chiller and its pump are added when reverse flow persists for a set time. One is removed when the surplus exceeds one chiller's flow, plus a margin, for a set time.

Temperature sensors at the decoupler, or on the secondary supply just downstream of it, reveal flow direction. If the secondary supply temperature rises above the chiller leaving temperature, return water is blending in and another chiller is needed. The method is simple and robust, but slower and less precise than flow measurement.

The building load is calculated from secondary flow and delta-T, and the number of running chillers is matched to it using suitable thresholds. Load-based staging works well while delta-T is healthy. When delta-T is low, flow-based checks must be added, because the plant can run out of flow before it runs out of capacity.

Short cycling is prevented by three measures:

• minimum run and off times
• time delays on staging decisions
• a deadband between stage-up and stage-down thresholds

Stage-up decisions also consider supply temperature, so a brief flow spike doesn't start an unnecessary chiller.

When building delta-T falls below design, secondary flow rises above what the running chillers supply. The plant has to start chillers for flow rather than capacity, and efficiency falls.

Reverse decoupler flow blends return water into the supply. This raises the supply temperature and weakens dehumidification, which in turn drives even more secondary flow.

Primary pumps run at full speed whenever their chiller runs. Their energy falls only when chillers are staged off, not as building load falls.

With low delta-T, the plant often runs one more chiller than the load needs, with every chiller lightly loaded, which raises kW/TR.

Two sets of pumps, a decoupler and extra instrumentation take up more space, cost more and need more maintenance than a primary-only design.

Primary flow is set chiller by chiller. At a 10 °F (about 5.6 °C) delta-T, each TR needs about 2.4 GPM, or roughly 0.55 m³/h. A plant with three 300 TR chillers therefore needs about 720 GPM (164 m³/h) per chiller, or 2,160 GPM (about 491 m³/h) in total.

Secondary design flow comes from the building load and coil delta-T. Many designers set primary flow slightly above secondary design flow, so the decoupler stays in surplus at design conditions.

Primary pumps overcome only the chiller-side losses:

• the evaporator
• primary piping and headers
• valves and strainers
• the decoupler, whose loss should be negligible

Building distribution losses are excluded, so primary head is usually modest and primary pump power stays low.

Secondary pumps overcome the distribution losses. The path starts at the supply-side decoupler connection, runs through the risers and branches to the index coil and its control valve, and ends at the return-side connection. The chillers are excluded, and the DP setpoint at the index circuit forms part of this head.

Delta-T links everything in a decoupled plant. A higher design delta-T reduces primary and secondary flow, pipe sizes and pump power, provided the coils are selected to deliver it. Protecting delta-T in operation matters just as much as designing for it, because low delta-T drives reverse flow and extra chillers.

The decoupler is sized for at least the largest chiller's design flow at low velocity. Headers are also sized for low velocities, so every chiller and pump connected to them sees nearly equal pressure and flow divides evenly.

Primary pumps are usually one per chiller, with standby provision. Secondary pumps are typically N+1: two duty and one standby, or three duty and one standby for larger plants. Critical facilities may also add redundancy to instrumentation and controls.

End-suction pumps suit small to medium primary and secondary duties at low cost. Split-case pumps suit larger secondary duties with high flow, offering high efficiency and easy access to the rotating assembly.

Vertical inline pumps save floor space and suit primary duty in compact plant rooms, where they can be mounted directly in the chiller piping.

A constant speed primary pump runs at one duty point, so it is selected at or very near BEP. Variable speed secondary pumps are selected with the design point slightly to the right of BEP, so they stay efficient as speed falls at part load.

IE3 or IE4 motors reduce losses on both loops. Secondary pump VFDs are sized on motor full-load current, rated for variable torque duty, and specified with harmonic mitigation and BMS communication.

The primary loop runs from the primary pumps through the chillers to the supply header, then from the return header back to the pumps. The layout keeps evaporator connections straightforward and each chiller easy to isolate.

The decoupler is a straight, unrestricted pipe between the supply and return headers. In the direction of primary flow, the secondary supply connects before it and the secondary return after it. A bidirectional flow meter is installed with the straight runs it needs.

Secondary pumps draw from the supply header and discharge into risers and zone mains. Where zones have their own pumps, each set gets its own suction and discharge connections, check valves and isolation valves.

Every pump gets isolation valves, a strainer or suction diffuser, a non-slam check valve on the discharge and flexible connections. Chillers get isolation valves, which are motorized in headered designs.

All primary, secondary and decoupler piping, valves and fittings need insulation with a continuous vapour barrier. In Mumbai's humidity, any gap quickly leads to condensation, corrosion and lost cooling.

A bidirectional electromagnetic or ultrasonic flow meter measures both the size and direction of decoupler flow. It must read accurately at low flows, because the most important staging decisions happen near zero flow.

Sensors at four points reveal blending, delta-T and load:

• chiller leaving water
• secondary supply, downstream of the decoupler
• secondary return
• primary return, after the decoupler

Immersion sensors in thermowells give the accuracy these calculations need.

A DP transmitter at or near the index circuit controls secondary pump speed. Large or branched systems may use several sensors, with control based on whichever is furthest below its setpoint.

Temperature sensors must sit where the water is well mixed, and the flow meter needs its required straight runs. Every sensor is calibrated at commissioning and checked during maintenance, because small errors can trigger wrong staging decisions.

Each primary pump starts before its chiller and proves flow before the compressor is allowed to run. After the chiller stops, the pump runs on for a set time. A pump failure trips the chiller and starts the standby pump.

Staging logic combines decoupler flow, secondary supply temperature and calculated load. It adds a chiller on sustained reverse flow or a rise in supply temperature. It removes one when surplus flow exceeds a chiller's flow plus a margin. Time delays apply to every decision.

Secondary pumps run on DP control with matched speeds, stage on speed or flow thresholds, and rotate lead and lag by run hours. A failed pump is skipped and the standby starts automatically.

Trends of decoupler flow and temperature show how often the plant runs in reverse flow, how large the surplus is, and how well chiller staging matches the load. They are the single most useful diagnostic in a decoupled plant.

The chiller plant manager coordinates chillers, primary pumps, secondary pumps, cooling towers and valves, and connects to the BMS over BACnet or Modbus. Operators get live plant graphics, trends, alarms and energy reports.

Pump power is calculated as:

Pump power (kW) = Flow (m³/h) × Head (m) ÷ (367 × pump-motor efficiency)

In the 3 × 300 TR example:

• Primary: each constant speed primary pump moves 164 m³/h at 15 m head with 72% efficiency, drawing about 9.3 kW whenever its chiller runs.
• Secondary: at design, 491 m³/h at 30 m head with 75% efficiency needs about 53.5 kW. This falls steeply at part load under VFD control.

Plant kW/TR is calculated as:

Plant kW/TR = Total plant power (chillers, primary and secondary pumps, condenser pumps and cooling tower fans) ÷ Cooling delivered (TR)

The pumping share alone shows why secondary control matters. Take the example plant at 450 TR:

• VFD secondary pumps with two-way valves: two chillers and two primary pumps carry the load. Pumping power is about 18.6 kW primary plus roughly 16 kW secondary, around 35 kW in total, or 0.077 kW/TR.
• Constant speed secondary pumps with three-way valves: secondary flow stays at 491 m³/h, so all three primary pumps must run to avoid reverse flow. Pumping power stays near 81 kW, or 0.18 kW/TR, and three chillers run lightly loaded instead of two.

Secondary power at half flow is taken as about 30% of design, allowing for the DP control head.

• Reset the secondary DP setpoint based on valve positions
• Stage chillers on decoupler flow and temperature, not load alone
• Protect delta-T with two-way valves or PICVs, clean coils and correct setpoints
• Reset chilled water supply temperature where dehumidification allows
• Trim primary pumps to each chiller's exact design flow
• Optimize condenser water and cooling tower operation alongside the chilled water side

We confirm that the decoupler flow meter reads correctly in both directions, and that decoupler flow equals the difference between primary and secondary flows. We also check that the temperature sensors respond correctly to surplus and reverse flow.

Each chiller's primary flow is set to its design value and verified against evaporator pressure drop. Secondary flows are balanced to terminal design flows, and the secondary DP setpoint is established at the index circuit.

We test every part of the staging sequence:

• stage-up on reverse flow and on supply temperature rise
• stage-down on surplus flow
• time delays and minimum run times
• pump failure responses
• restart after a power failure

A large, constant surplus usually means too many chillers are running, staging thresholds are wrong, or primary pumps are delivering more than design flow. Retuning the staging and trimming the primary pumps fixes it.

Reverse flow happens when secondary flow exceeds primary flow. The usual causes are low delta-T, too few chillers running, or secondary pumps overpumping because of high DP setpoints. The fixes are to raise delta-T, stage chillers correctly and reset the DP setpoint.

Low delta-T usually comes from one or more of:

• three-way valves still in the secondary loop
• oversized or leaking two-way valves
• fouled coils
• low supply air setpoints
• uncontrolled bypasses

PICVs, coil cleaning, valve audits and setpoint reviews raise delta-T.

Short cycling comes from staging thresholds set too close together, missing time delays, or staging on a noisy flow signal. Deadbands, delays and signal filtering stop it.

Remote zones usually starve because of a low DP setpoint, a DP sensor in the wrong place, balancing problems or trapped air. Relocating sensors, raising or resetting the setpoint and rebalancing restore flow.

Low suction pressure, blocked strainers, air in the system, or secondary pumps running far out on their curves can all cause cavitation. Checking pressurization, cleaning strainers, venting and correcting pump operating points resolve most cases.

VFDs on the secondary pumps, together with a DP sensor at the index circuit, let secondary flow follow the load. This is usually the highest-value upgrade in a decoupled plant.

Converting the terminal valves makes the secondary loop truly variable flow. PICVs go further, adding automatic balancing and delta-T protection that directly reduce reverse flow.

Many older plants stage chillers manually or on return temperature alone. Adding a bidirectional decoupler flow meter, temperature sensors and staging logic matches chiller operation to actual demand.

Where the chillers can handle variable evaporator flow, the plant can be converted to VPF. The conversion adds VFDs to the pumps, a bypass valve with flow measurement and new controls, and often removes one pump set. We confirm chiller flow limits and rate-of-change tolerances first.

Running a decoupled plant with constant speed secondary pumps? Ask Compress India for an upgrade assessment.

• Plant capacity, number of chillers and chiller types
• Dedicated or headered primary pumps
• Number and size of secondary pumps, and the zone arrangement
• VFDs, harmonic mitigation and panel arrangement
• Decoupler, headers and plant room piping
• Flow meters, temperature sensors and DP sensors
• Two-way valves or PICVs at the terminals
• Chiller plant manager and BMS integration
• Retrofit constraints, shutdown windows and phasing
• Testing, balancing, commissioning and AMC scope