Constant Primary – Variable Secondary Systems

(Design, Installation, Commissioning, Retrofit & Lifecycle Support for Chilled Water Pumping Systems)

Constant Primary – Variable Secondary Systems – Chilled Water Plant Engineering & Execution

Compress India delivers end-to-end engineering and execution for constant primary – variable secondary systems in chilled water plants. From hydraulic calculations and pump selection to plant room piping, controls, TAB, retrofits, and AMC, we take complete responsibility for how your pumping system performs. The result is a plant where chillers run steadily, every zone gets adequate chilled water, and secondary pumping energy drops with every hour of part-load operation.

The constant primary variable secondary system is one of the most trusted pumping designs in the HVAC industry. It pairs constant speed primary pumps on the chiller side with variable speed secondary pumps on the building side, separated by a decoupler. This gives chillers the steady flow they need while letting the building draw only as much chilled water as it actually uses.

Whether you are building a new chilled water plant, expanding an existing one, or correcting a system that suffers from low delta T and high power bills, Compress India's HVAC execution team designs and delivers a constant primary variable secondary pumping system that works as intended from day one.

What Is a Constant Primary – Variable Secondary System?

What is constant primary variable secondary system design? It is a chilled water pumping arrangement with three defining features:

Constant flow in the primary loop: Constant speed pumps push a fixed flow of water through each operating chiller.
Variable flow in the secondary loop: Secondary pumps with VFD adjust their speed to match building demand.
A decoupler (common pipe): A short, low-resistance pipe separates the two loops so they can run independently.

Often called a CPVS system in HVAC, this is the classic constant flow primary variable flow secondary design. A typical constant primary variable secondary system diagram shows chillers and their primary pumps on one side, the decoupler in the middle, and secondary pumps feeding AHUs and FCUs fitted with two-way valves on the other side.

What Is a Primary-Secondary Chilled Water Pumping System?

A primary-secondary system splits the chilled water circuit into two hydraulically independent loops:

The production loop (primary), where the chiller primary pump moves water through the evaporators.
The distribution loop (secondary), where the secondary pump in chiller plant rooms sends water out to the building.

Each primary pump and secondary pump in chiller plants is sized only for the pressure losses in its own loop. This is why primary and secondary chilled water pumps are smaller, more efficient, and easier to control than a single pump set trying to serve both chillers and the entire building.

Why Use Constant Flow in the Primary Loop and Variable Flow in the Secondary Loop?

Why constant primary variable secondary system is used comes down to two different needs:

Chillers prefer steady flow. Constant evaporator flow keeps refrigerant-side control stable, avoids low-flow trips, and protects against freezing.
Buildings need varying flow. Cooling load changes with occupancy, weather, and time of day. Pumping full flow at all times wastes energy.

Constant flow on the chiller side and variable flow on the building side gives you the best of both worlds: stable, protected chillers and a distribution system that saves energy at part load, which is where most plants operate for most of the year.

How a Constant Primary – Variable Secondary System Works

How does constant primary variable secondary work in practice? The system runs through six coordinated stages.

1. Constant Speed Primary Pumps Serving the Chillers

Constant speed primary pumps run at fixed speed whenever their chiller is operating. They only need enough head to push water through the evaporator, primary piping, valves, and fittings up to the decoupler.

2. Constant Flow Through Chiller Evaporators

Each running chiller receives its full design flow at all times. The chiller's capacity control handles changes in load by adjusting refrigerant flow, not water flow.

3. Decoupler / Common Pipe Between Primary and Secondary Loops

The common pipe connects primary supply and primary return. Any difference between primary and secondary flow passes through it, allowing each loop to operate at its own flow without interfering with the other.

4. Variable Speed Secondary Pumps Serving the Load

Variable speed secondary pumps draw chilled water from the primary supply header. A differential pressure sensor in the distribution system tells the VFDs how fast to run.

5. Two-Way Control Valves at AHUs & FCUs

Each coil has a two way valve. As a zone's cooling demand drops, its valve closes, reducing flow. The secondary pumps sense the rise in differential pressure and slow down, cutting energy use.

6. Chiller & Pump Staging Based on Load

The plant controller monitors decoupler flow, supply temperature, and load. When the building needs more water than the running chillers supply, another chiller and its chiller primary pump start. When surplus flow circulates through the decoupler, a chiller and pump are shut down.

Hydraulic Decoupling in Constant Primary – Variable Secondary Systems

Role of the Decoupler Line

The decoupler line in chilled water system design is the heart of the CPVS arrangement. Because it has almost no pressure drop, it prevents primary and secondary pumps from working in series. Without it, secondary pumps would pull water through chillers at varying rates, defeating the purpose of constant evaporator flow.

Decoupler Sizing and Location

Sized for at least the flow of the largest primary pump
Kept short with minimal fittings and very low pressure drop
Located between the primary and secondary connections, with enough spacing from tees to avoid turbulence and unintended mixing
Fitted with a flow meter and temperature sensors to guide chiller staging

Supply and Return Flow Direction in the Decoupler

Supply to return (surplus flow): Primary flow exceeds secondary demand. Excess chilled water returns directly to the chillers, lowering their return temperature.
Return to supply (deficit flow): Secondary demand exceeds primary flow. Warm return water mixes with supply water, raising the temperature delivered to the building.
Near-zero flow: The ideal balance, where chiller output closely matches building demand.

Main Components of a Constant Primary – Variable Secondary System

Chillers (centrifugal, screw, or scroll)
Constant speed primary pumps, dedicated or headered
Decoupler / common pipe with flow and temperature sensing
Variable speed secondary pumps with VFDs
Primary and secondary supply and return headers
Differential pressure sensors at index circuits
Two-way control valves at AHUs and FCUs
Chiller flow switches
Isolation valves, check valves, strainers, balancing valves, and flexible connectors
Expansion tank, air separator, and chemical treatment
Chiller plant manager / BMS controller
Insulated chilled water piping

Primary Pump Arrangements in Constant Primary Systems

Dedicated Primary Pumps (One Pump per Chiller)

Each chiller is piped directly to its own pump. The chiller and pump start and stop together as one unit, making control simple and flow to each chiller predictable, even when chillers differ in size.

Headered Primary Pumps

All primary pumps discharge into a common header serving every chiller. Any pump can support any chiller, improving redundancy. Each chiller then needs a motorized isolation valve, and balancing flow between chillers requires careful design.

Comparison Table – Dedicated vs Headered Primary Pumps

Parameter	Dedicated Primary Pumps	Headered Primary Pumps
Pump–chiller link	Fixed, one to one	Flexible, any to any
Control logic	Simple	Moderate
Chiller isolation valves	Generally not needed	Motorized valves needed
Unequal chiller capacities	Easy to handle	Harder to balance
Standby arrangement	Per pump or shared	Built into header
Best suited for	Mixed chiller sizes, simple operation	Equal chillers, high redundancy

Types of Variable Secondary Pump Configurations

Common Secondary Pumps (Central Distribution)

A set of parallel VFD pumps in the plant room serves the full building through a single distribution network. This is the most common constant primary variable secondary pumping layout.

Zone / Distributed Secondary Pumps

Separate pump sets serve different zones, towers, or wings, each sized for its own head. Low-head zones no longer pay the energy penalty of the most demanding zone.

Building-Wise Secondary Pumps for Campuses

Each building on a campus has its own secondary pumps drawing from a central primary loop, allowing phased construction and independent control.

Primary–Secondary–Tertiary Pumping Systems

Large campuses and district cooling plants add tertiary pumps at each building, connected through local decouplers or heat exchangers.

Standby Secondary Pumps (N+1 Arrangement)

At least one standby pump is provided beyond peak requirement. Critical facilities often choose N+2 or 2N for higher resilience.

Variable Speed (VFD) Secondary Pump Control

Differential Pressure Control of Secondary Pumps

A differential pressure sensor measures the pressure difference between supply and return at a chosen point. The VFD adjusts pump speed to hold this value at its set point as valves open and close.

DP Sensor Location – Index Circuit / Remote Point

The sensor should sit across the index circuit, the hydraulically most remote or demanding coil. A sensor placed in the plant room forces pumps to run faster than necessary and wastes energy.

Differential Pressure Setpoint Reset

Instead of a fixed set point, a smart constant primary variable secondary control sequence lowers the DP set point when all control valves are partly closed and raises it only when a valve approaches fully open. This trim-and-respond approach squeezes out additional pumping savings without starving any zone.

Parallel Secondary Pump Staging with VFDs

Parallel pumps run at matching speeds. An additional pump starts when the running pumps approach maximum speed and stops when fewer pumps can serve the load efficiently near their best efficiency point.

Two-Way Valves vs Three-Way Valves in Variable Secondary Systems

A two way valve chilled water system is essential for variable secondary pumping. Here is why:

Parameter	Two-Way Valves	Three-Way Valves
Flow through system	Varies with load	Stays nearly constant
Return water temperature	Stays high	Diluted by bypassed supply water
Effect on delta T	Maintains design delta T	Causes low delta T
VFD pump savings	Full savings possible	Little or no savings
Suitability for CPVS	Required	Not recommended

Three-way valves bypass unused chilled water back to the return, which keeps flow high and return temperature low. This defeats the purpose of variable speed secondary pumps and is a leading cause of low delta T syndrome. Compress India designs with two-way valves and provides a controlled minimum-flow arrangement where needed for pump protection.

Benefits of a Constant Primary – Variable Secondary System

Constant Evaporator Flow & Chiller Protection

Chillers always receive design flow, preventing freeze-ups, low-flow trips, and unstable operation.

Variable Flow to Load

The building gets only the chilled water it needs at any moment, improving coil control and comfort.

Reduced Secondary Pumping Energy

Pump power falls roughly with the cube of speed, so VFD secondary pumps cut energy significantly during part-load hours.

Simple and Stable Chiller Operation

With constant water flow, chiller controls work in their most stable and predictable mode.

Easy Chiller–Pump Staging

Starting and stopping chillers with their primary pumps is straightforward and robust.

Zone-Wise Pumping Flexibility

Secondary pumps can be arranged by zone or building, matching head to actual need.

Proven and Widely Used Design

CPVS has decades of successful use across commercial, healthcare, and industrial facilities. Operators understand it, spare parts are standard, and maintenance teams can manage it confidently.

Limitations & Design Challenges

Constant primary variable secondary advantages and disadvantages should both be considered. The main challenges include:

Low Delta-T Syndrome

When return water comes back colder than designed, the building draws more flow than the chillers produce, triggering reverse decoupler flow and extra chiller operation.

Reverse Flow in the Decoupler

Warm return water enters the supply, raising supply temperature. Coils then demand even more flow, making the problem worse.

Constant Primary Pumping Energy

Primary pumps run at full speed whenever a chiller is on, regardless of load, so primary pumping energy is not reduced at part load.

Extra Chillers Running at Part Load

To meet flow rather than cooling demand, plants with low delta T often run more chillers than necessary, each at poor efficiency.

Higher Pump Count and Plant Room Space

Two pump sets mean more equipment, piping, and floor space compared with variable primary designs.

Compress India's design, controls, and commissioning expertise minimizes each of these issues.

Constant Primary – Variable Secondary vs Variable Primary Flow (VPF) System

The constant primary variable secondary vs variable primary flow comparison is one of the most common design questions. The key difference between constant primary and variable primary lies in how chiller flow is handled:

Parameter	Constant Primary – Variable Secondary	Variable Primary Flow
Pump sets	Two	One
Chiller evaporator flow	Constant	Varies within chiller limits
Hydraulic separation	Decoupler	Minimum flow bypass
Primary pump energy	Constant	Reduced at part load
Control complexity	Lower	Higher
Chiller compatibility	Any chiller	Chillers rated for variable flow
Operator skill needed	Moderate	Higher
First cost	Higher	Lower

CPVS is simpler, more forgiving, and compatible with virtually any chiller. VPF can save more energy and space but demands capable chillers, precise flow measurement, and skilled operation.

Constant Primary – Variable Secondary vs Variable Primary with Secondary Loop

A variable primary with secondary loop system adds VFDs to the primary pumps, so both loops vary flow. This reduces primary pumping energy and helps limit surplus decoupler flow, but requires chillers that tolerate variable flow and more advanced control sequences. CPVS keeps the primary side simple and stable, which many facilities prefer for reliability and ease of operation.

Constant Primary – Variable Secondary vs Constant Primary Only System

A constant primary only system uses one set of constant speed pumps to circulate water through chillers and the entire building, with three-way valves at coils. It is simple but pumps full flow even at light load and tends to suffer from low delta T. Moving to CPVS adds variable speed secondary pumps and two-way valves, cutting distribution pumping energy and improving plant performance.

Constant Primary – Variable Secondary Systems for HVAC

A constant primary variable secondary in HVAC design is the backbone of central air conditioning for offices, malls, hotels, and hospitals. The primary pump and secondary pump in HVAC systems work together to keep chillers stable while AHUs and FCUs receive precise, demand-based flow. With proper DP control and two-way valves, the system delivers consistent comfort and measurable energy savings across the year.

Constant Primary – Variable Secondary Systems for Industrial & Process Cooling

Industrial facilities rely on stable cooling for machines, reactors, compressors, and clean rooms. A constant primary variable secondary chilled water system supplies separate secondary loops for process and comfort loads, each with its own pressure and flow. Constant chiller flow keeps production cooling reliable, while variable secondary pumps adjust to shift patterns and changing process demand.

Applications of Constant Primary – Variable Secondary Systems

Commercial Buildings & Office Complexes

Changing occupancy and floor-wise loads make variable secondary pumping a natural fit.

Shopping Malls

Retail zones, food courts, and multiplexes each get flow matched to their operating hours and loads.

Hospitals

Stable chiller operation and N+1 or N+2 pump redundancy support critical care areas.

Data Centers

Constant chiller flow and redundant secondary pumps provide dependable 24x7 cooling for IT loads.

Pharmaceutical Facilities

Dedicated secondary loops hold tight temperature conditions in clean rooms and production areas.

Manufacturing Plants

Process and comfort cooling loops operate independently for consistent production.

Hotels

Guest rooms, banquet halls, and kitchens receive efficient, demand-based cooling.

Airports & Metro Systems

Distributed secondary pumps serve widely spread terminals and stations efficiently.

IT Parks & Campuses

Building-wise secondary pumps support phased development and tenant-wise operation.

District Cooling Plants

Primary–secondary–tertiary arrangements distribute chilled water to multiple buildings over long distances.

Constant Primary – Variable Secondary Systems for Centrifugal, Screw & Multiple-Chiller Plants

CPVS works with centrifugal, screw, and scroll chillers alike, since each chiller always sees its design flow. In multiple-chiller plants with unequal capacities, dedicated primary pumps make it easy to deliver the right flow to each machine. Compress India integrates pumping with chiller sequencing so the most efficient chiller combination runs at every load.

System Design & Sizing

Chilled Water Flow Calculation (GPM / m³/hr per TR)

Imperial: Flow (GPM) = TR × 24 ÷ ΔT (°F). At a 10°F delta T, this equals 2.4 GPM per TR.
Metric: Flow (m³/hr) = kW × 0.86 ÷ ΔT (°C). At a 5.5°C delta T, this is about 0.55 m³/hr per TR.

Primary Pump Head Calculation

Primary head includes evaporator pressure drop, primary piping friction, valves, strainers, and fittings up to the decoupler, plus a safety margin. It is usually modest.

Secondary Pump Head Calculation

Secondary head is calculated along the index circuit: distribution piping, fittings, coil, two-way valve, balancing devices, and a margin. Zone pumps are calculated separately for each zone.

Chilled Water Delta-T Selection

A higher design delta T reduces flow, pipe sizes, and pump power. Coils and valves must be selected to achieve and hold it in operation.

Decoupler and Header Sizing

Decouplers are sized for at least the largest primary pump flow with minimal pressure drop. Headers are sized for low velocity and even flow distribution.

Number of Secondary Pumps & Redundancy Selection

The number of pumps depends on peak flow, part-load profile, zones, and redundancy. Two to four duty pumps plus standby is common, keeping each pump near its best efficiency point.

Pump Selection Criteria

End-Suction vs Split-Case Pumps

End-suction pumps are compact and economical for small to medium flows. Split-case pumps suit high flows with excellent efficiency and serviceability.

Inline Pumps

Inline pumps save space and simplify piping, especially for zone secondary pumps and smaller plants.

Pump Efficiency and Best Efficiency Point (BEP)

Selecting pumps near BEP reduces energy use, vibration, seal wear, and noise.

Motor & VFD Selection

High-efficiency motors paired with correctly rated VFDs, harmonic filters where needed, and proper cabling ensure long, trouble-free service.

Piping Layout & Plant Room Arrangement

Primary Loop Piping

Primary piping is kept short and compact, with each pump close to its chiller and the decoupler placed cleanly between primary and secondary connections.

Secondary Loop & Zone Piping

Secondary headers are laid out for easy branching to zones, future connections, and flow measurement.

Valves, Strainers, Check Valves & Flexible Connections

Each pump gets isolation valves, a suction strainer, a check valve, and flexible connectors. Pressure gauges and test points support commissioning and diagnostics.

Insulation of Chilled Water Piping

Closed-cell insulation with a vapour barrier and protective cladding prevents heat gain and condensation.

Controls, Automation & BMS Integration

Chiller–Primary Pump Interlocking

Each chiller starts only after its primary pump runs and flow is proven by a flow switch.

Chiller Staging Based on Decoupler Flow

A chiller is added when deficit (return-to-supply) flow or supply temperature rise persists beyond a set time. A chiller is removed when surplus flow through the decoupler exceeds the flow of one chiller by a safe margin.

Secondary Pump Sequencing Logic

Secondary pumps are staged and speed-controlled on DP feedback, with automatic duty rotation and standby changeover on fault.

Decoupler Flow and Temperature Monitoring

Continuous monitoring of decoupler flow direction, flow rate, and temperatures reveals staging problems and early signs of low delta T.

Plant Manager & BMS Integration

Compress India integrates the plant with a chiller plant manager and your BMS for real-time monitoring, alarms, trends, and energy reports.

Energy Efficiency & Power Consumption

Pumping Energy Calculation

Pump power (kW) = Flow (m³/hr) × Head (m) × Specific Gravity ÷ (367 × Pump Efficiency × Motor Efficiency). Calculating this for both primary and secondary pumps at different loads shows where energy is being lost.

Plant Optimization Strategies

Move DP sensors to index circuits
Apply DP set point reset
Replace remaining three-way valves with two-way valves
Clean coils and fix faulty control valves to restore delta T
Stage chillers on decoupler flow and temperature
Right-size or trim oversized pumps
Reset chilled water supply temperature based on load

Installation of Constant Primary – Variable Secondary Systems

Our execution teams manage foundations and inertia bases, pump alignment, header and loop piping, decoupler fabrication, valves and accessories, insulation, electrical and VFD wiring, and controls. Every installation follows approved drawings and quality checkpoints for smooth, quiet operation.

Testing, Balancing & Commissioning (TAB)

Compress India performs flushing and cleaning, pressure testing, pump performance verification, coil flow balancing, DP sensor calibration and set point tuning, decoupler flow checks, and full chiller and pump staging tests. Detailed TAB reports give you a reliable baseline for future operation.

Maintenance & AMC for Chilled Water Pumping Systems

Our AMC plans include:

Vibration, bearing, and mechanical seal checks
Strainer cleaning and coupling alignment
Motor and VFD inspections
DP sensor, flow meter, and temperature sensor calibration
Two-way valve and actuator checks
Water treatment monitoring
Periodic energy and performance reviews

Common Problems & Troubleshooting

Low Delta-T

Dirty coils, leaking or oversized valves, leftover three-way valves, and incorrect set points are common causes. We identify root causes and restore design delta T.

Excess Decoupler Flow

Large surplus flow usually means too many chillers are running. Improved staging logic resolves it.

Reverse Decoupler Flow and Supply Temperature Rise

Deficit flow signals insufficient chillers online or low delta T in the building. We correct staging and fix the underlying hydraulic issues.

Pump Cavitation & Vibration

Low suction pressure, clogged strainers, trapped air, misalignment, or operation far from BEP can cause damage. We diagnose and correct each cause.

Insufficient Flow at Remote Zones

Often caused by a plant-room DP sensor, low set points, or imbalance. Sensor relocation and rebalancing restore flow.

VFD Hunting / Unstable DP Control

Aggressive PID tuning or poorly located sensors make pumps hunt. Proper tuning stabilizes operation.

Retrofitting & Upgrading Existing Chilled Water Systems

Converting Constant Primary–Constant Secondary to Variable Secondary

Adding VFDs, two-way valves, and DP control transforms an older constant flow plant into an efficient CPVS system.

Adding VFDs to Existing Secondary Pumps

VFD retrofits are one of the fastest-payback energy upgrades available for chilled water plants.

Replacing Three-Way Valves with Two-Way Valves

This change restores delta T, reduces flow, and enables real VFD savings.

Converting to Variable Primary Flow

Where chillers support variable flow, we can convert your CPVS plant to VPF or variable primary with secondary loop for additional savings.

Constant Primary – Variable Secondary System Cost in India

The cost of a constant primary variable secondary system depends on plant capacity, number of chillers and zones, pump type, VFDs, controls, and redundancy. Compress India provides transparent, itemized quotations so you can plan your investment with confidence.

Factors Affecting Project Cost

Total cooling capacity (TR) and flow rates
Number of chillers and primary pumps
Number of secondary zones and pumps
Pump type and brand
VFDs, DP sensors, and controls
BMS integration
Pipe sizes, lengths, and insulation
Redundancy level
New installation or retrofit in a running plant
Site access and project timeline

Information Required for a Quotation

Building or process type
Cooling load (TR) and chiller details
Existing or proposed layout drawings
Number of zones or buildings
Design supply and return temperatures
Valve type at coils (two-way or three-way)
Redundancy requirements
Scope: design, supply, installation, commissioning, retrofit, or AMC
Location and timeline

Constant Primary – Variable Secondary System Contractor in Mumbai

Compress India is an experienced constant primary – variable secondary system contractor in Mumbai, serving commercial towers, hospitals, hotels, malls, data centers, and factories. Our local teams offer prompt site surveys, fast mobilization, and dependable after-sales support.

Chilled Water Pumping Solutions in Mumbai, Navi Mumbai, Thane & Maharashtra

We deliver primary and secondary chilled water pumps, VFD retrofits, and complete plant room execution across Mumbai, Navi Mumbai, Thane, Pune, Nashik, Aurangabad, Nagpur, and other cities in Maharashtra.

Chilled Water Pumping System Solutions Across India

Compress India supports chilled water projects across India, from new plant installations to energy retrofits and long-term AMC for commercial, institutional, and industrial clients.

Why Choose Compress India for Constant Primary – Variable Secondary Systems?

Hydraulic Design Expertise

Precise flow, head, decoupler, and pipe sizing for reliable performance.

Complete Chiller Plant Integration

Chillers, pumps, cooling towers, and controls designed to work as one.

Pump & VFD Selection Support

Equipment selected near BEP for efficiency and long life.

Control Sequence Engineering

Staging, DP reset, and pump sequencing logic written and tuned for your plant.

Installation & Commissioning

Skilled teams install, test, balance, and commission to documented standards.

Retrofit & Energy Optimization

Practical upgrades that cut energy use and fix chronic problems.

AMC & Lifecycle Support

Planned maintenance and quick response to keep your plant running.

How to Select the Right Chilled Water Pumping Arrangement

Do your chillers support variable evaporator flow? If not, CPVS is the natural choice.
How skilled is your operations team? CPVS is simpler to operate than variable primary designs.
Are chillers of different sizes? Dedicated primary pumps handle this easily.
Do zones need different pressures? Zone secondary pumps can save energy.
What redundancy do you need? Critical facilities should plan N+1 or better.
Is expansion planned? Building-wise secondary pumps simplify phased growth.

Our engineers evaluate each factor and recommend the arrangement that balances reliability, energy, and cost.

Get a Constant Primary – Variable Secondary Solution from Compress India

Whether you are designing a new plant, upgrading an old constant flow system, or fixing low delta T and high energy use, Compress India delivers a constant primary – variable secondary system tailored to your facility.

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Frequently Asked Questions

Q1: What is a constant primary – variable secondary system?

It is a chilled water pumping system where constant speed primary pumps serve the chillers, variable speed secondary pumps serve the building, and a decoupler separates the two loops.

Q2: How does a constant primary – variable secondary chilled water system work?

Primary pumps deliver constant flow through chillers, secondary pumps adjust speed based on differential pressure, and the decoupler absorbs any difference between the two flows.

Q3: Why is constant flow maintained in the primary loop?

Constant evaporator flow keeps chiller operation stable and protects against low-flow trips and freezing.

Q4: Why do secondary pumps run at variable speed?

To match building demand and save energy, since pump power falls sharply as speed reduces.

Q5: What is a decoupler line and why is it required?

It is a low-pressure-drop common pipe that hydraulically separates the primary and secondary loops so both pump sets can run independently.

Q6: Why are two-way valves used in variable secondary systems?

Two-way valves reduce flow as load drops, allowing VFD pumps to slow down and keeping return water temperature high.

Q7: How are secondary pumps controlled using differential pressure?

A DP sensor measures supply-return pressure difference, and the VFD adjusts pump speed to maintain the set point.

Q8: Where should the differential pressure sensor be installed?

Across the index circuit, the hydraulically most remote or demanding coil, rather than in the plant room.

Q9: What is low delta-T syndrome in constant primary – variable secondary systems?

It occurs when return water is cooler than design, causing the building to draw more flow than the chillers supply, leading to reverse decoupler flow and extra chillers running.

Q10: What happens when flow reverses in the decoupler?

Warm return water mixes into supply, raising supply temperature and reducing cooling at coils.

Q11: What is the difference between constant primary – variable secondary and variable primary flow?

CPVS keeps chiller flow constant with two pump sets. VPF varies chiller flow with a single pump set and a minimum flow bypass.

Q12: Which is better: constant primary – variable secondary or variable primary flow?

CPVS is simpler and works with any chiller. VPF can save more energy but needs compatible chillers and advanced controls. The best option depends on your plant.

Q13: Is a constant primary – variable secondary system suitable for data centers and hospitals?

Yes. Its stable chiller operation and easy redundancy make it a strong fit for critical facilities.

Q14: How do you calculate chilled water flow and pump head for this system?

Flow (GPM) = TR × 24 ÷ ΔT (°F). Primary head covers the chiller-side loop; secondary head covers the index circuit of the distribution system.

Q15: How much energy can VFD secondary pumps save?

Savings depend on the load profile, but because power varies roughly with the cube of speed, even moderate speed reductions produce large energy savings.

Q16: Can an existing constant flow system be converted to variable secondary?

Yes. Adding VFDs, DP control, and replacing three-way valves with two-way valves converts it to CPVS.

Q17: Can a constant primary – variable secondary system be converted to variable primary flow?

Yes, if chillers support variable flow. The conversion adds primary pump VFDs, flow measurement, a minimum flow bypass, and upgraded controls.

Q18: What maintenance does a constant primary – variable secondary system need?

Pump and motor checks, VFD inspection, sensor calibration, valve checks, strainer cleaning, water treatment, and periodic performance reviews.

Q19: What information is needed for a constant primary – variable secondary system quotation?

Cooling load, chiller details, layout drawings, zones, design temperatures, valve types, redundancy needs, scope, location, and timeline.

Q20: Does Compress India design and install constant primary – variable secondary systems?

Yes. Compress India offers complete design, supply, installation, commissioning, retrofit, and AMC for constant primary – variable secondary systems across Mumbai, Maharashtra, and India.