A chilled water system makes cold water at a central chiller plant and pumps it through closed piping to air handlers, coils and process equipment, where it absorbs heat before returning to be chilled again. Paul Industries designs, installs, flushes, commissions and services chilled water plants and distribution piping for industrial plants, commercial buildings and process facilities nationwide. This page explains how the systems work, how they are piped and pumped, and what keeps them efficient for decades.
What is a chilled water system?
It is a hydronic cooling system: instead of running refrigerant to every room or machine, a chiller cools water in one place, and pumps carry that water wherever cooling is needed. The water never leaves the pipes, so the chilled water loop is a closed circuit, and the refrigerant stays inside the chiller. That arrangement is why large buildings, campuses, hospitals, data halls and manufacturing plants use chilled water: one plant serves many loads, capacity can be added by adding chillers, and the cooling medium is cheap, safe water rather than miles of refrigerant piping.
A chilled water air conditioning system serves people: chilled water air handlers and fan coil units pass building air across coils. A process system serves equipment: jacketed vessels, heat exchangers, molding machines, lasers and product coolers. Many plants run both from one chilled water plant, sometimes at different temperatures. For dedicated machine cooling and packaged process units, see process chillers and our industrial process chillers guide.
How does a chilled water system work?
Water is cooled in the chiller’s evaporator, pumped out to the loads, warmed by the heat it picks up, and pumped back to the chiller. Inside the chiller a refrigeration cycle moves that heat from the evaporator to the condenser, and the condenser rejects it to outdoor air or to a cooling tower loop. The traditional rating point, used in AHRI Standard 550/590, is 44 degrees Fahrenheit leaving chilled water and 54 degrees returning, a 10 degree difference.
Chilled water supply and return temperatures set the flow the pumps must move. For water, the heat carried in Btu per hour equals 500 times the flow in gallons per minute times the temperature difference, so one ton of cooling, 12,000 Btu per hour, needs 24 divided by the temperature difference in gallons per minute. A wider difference means less water, smaller pipe and less pump energy.
| Design temperature difference | Flow per ton | Flow for 500 tons | Effect |
|---|---|---|---|
| 10 °F (44/54 °F) | 2.4 gpm | 1,200 gpm | Traditional rating point; largest pipe and pumps |
| 12 °F | 2.0 gpm | 1,000 gpm | Common modern compromise |
| 14 °F | about 1.7 gpm | about 860 gpm | Smaller distribution; coils selected for it |
| 16 °F | 1.5 gpm | 750 gpm | Lowest pumping energy; needs coils and chillers designed for the range |
Chiller lift is the other number worth knowing: the difference between the temperature at which the chiller condenses refrigerant and the temperature at which it evaporates it, roughly tracking condenser water leaving temperature minus chilled water leaving temperature. Every degree of lift costs compressor work, so warmer chilled water and cooler condenser water both save energy.
How do commercial chillers work inside? Most use the vapor-compression cycle: liquid refrigerant boils in the evaporator as it takes heat from the chilled water, a compressor raises the pressure and temperature of the vapor, the condenser rejects that heat, and an expansion device drops the pressure so the cycle repeats. Scroll, screw and centrifugal compressors cover roughly rising ranges of capacity, from small modular units to the largest plant machines.
Chilled water system components
A working system is more than chiller units and pipe. Each component below has a job, and most service problems trace back to one of them being missing, wrongly placed or wrongly sized.
| Component | What it does | Installation point that matters |
|---|---|---|
| Chillers | Cool the water by refrigeration | Service clearance for tube pulls, vibration isolation, flow switches, isolation valves |
| Chiller pumps | Move water through chillers and loads | Straight suction piping, suction diffuser or strainer, check valve, alignment |
| Air handlers and coils | Transfer heat from air or process to the water | Two-way control valves, balancing, condensate drainage |
| Expansion tank | Absorbs the water’s change in volume and holds system pressure | Connected near pump suction; precharge set to fill pressure |
| Air separator | Removes entrained air from the loop | In the return to the chiller, where water is warmest, ahead of the pumps |
| Buffer tank | Adds loop volume so chillers do not short cycle | Sized to the chiller maker’s minimum volume |
| Makeup, fill and relief | Replaces lost water and protects against overpressure | Backflow preventer, meter, relief valves piped to drain |
| Strainers and side-stream filter | Keep debris out of coils and exchangers | Accessible for cleaning, with pressure gauges across them |
A chilled water system expansion tank is needed even in a loop that never sees heat, because water expands as it warms from fill temperature toward ambient during shutdowns, and a closed loop with nowhere for that volume to go will lift relief valves. The expansion tank connects near the pump suction, the point of no pressure change, so the pumps cannot pull the system below its fill pressure. The air separator in chilled water system return piping catches air where its solubility is lowest, and an automatic air vent valve for chilled water system high points releases what collects there. A buffer tank for chilled water system loops with small volume is common in process plants: chiller makers typically ask for about 3 to 5 gallons per ton for comfort cooling and 6 to 10 gallons per ton where temperature stability is critical. Every chilled water system pressure relief valve must be sized to the code and piped to a safe drain. Most expansion and buffer tanks are pressure vessels built to ASME Boiler and Pressure Vessel Code Section VIII.
Control valves deserve their own note. A 3 way valve chilled water system sends unused water around the coil and back to the plant, which keeps flow constant but lowers the return temperature. Two-way valves throttle flow to the load, letting variable speed pumps slow down and keeping return water warm, which is why modern systems use two-way valves almost everywhere except at the ends of long runs.
Condenser water vs chilled water
Chilled water is the cold, closed loop that serves the loads; condenser water is the warm loop that carries the chiller’s rejected heat to a cooling tower. Both are water, but they behave differently and are treated differently.
| Factor | Chilled water | Condenser water |
|---|---|---|
| Typical design temperatures | 44 °F supply, 54 °F return at the rating point; often warmer today | 85 °F entering the chiller, about 95 °F leaving at the rating point |
| Flow at rating point | 2.4 gpm per ton | 3.0 gpm per ton |
| Loop type | Closed, no evaporation | Open at the tower, constant evaporation and blowdown |
| Main water quality risk | Corrosion, sludge, oxygen from makeup | Scale, corrosion, biological growth including Legionella |
| Insulation | Required, with vapor retarder | Usually not, except freeze protection outdoors |
A water cooled chilled water system uses that condenser loop and a tower; an air-cooled system rejects heat through fans on the chiller and has no condenser water at all. Water-cooled plants are more efficient and dominate large installations, from a 1000 ton chiller in a campus plant to multiple machines in a manufacturing plant; air-cooled packaged chiller installations are simpler and suit smaller loads or sites where tower water is a burden. Air cooled chiller units sit outdoors on a pad or roof, so they need clear space for airflow around the coils, and their efficiency drops on the hottest days, when the condensing temperature follows the outdoor air. Most commercial chillers and industrial chiller systems are electric chiller designs driven by motors, with absorption machines used where waste heat or steam is available. A plant’s broader cooling water system may also include once-through or tower water serving process equipment directly. Tower treatment is covered on our cooling tower water treatment page and tower upkeep on cooling tower cleaning.
Primary and secondary pumping and variable primary flow
Pumping arrangements decide how a plant handles changing loads. In a primary and secondary chilled water system, constant-speed primary pumps hold steady flow through each chiller, while variable-speed secondary pumps deliver only the flow the loads ask for. The two circuits are joined by a short common pipe, shown as the decoupler in chilled water system drawings, which lets the primary and secondary flows differ without the pumps fighting each other. When secondary flow exceeds primary flow, warm return water mixes backward through the decoupler into the supply, the classic sign of too few chillers running or low return temperature.
A variable primary flow chilled water system removes the primary pumps: one set of variable-speed pumps moves water through both chillers and loads, and a bypass valve keeps each chiller above its minimum evaporator flow. It saves pump energy and plant space, but the controls must respect the chiller maker’s minimum flow and maximum rate of flow change, and operators need to understand the sequence. Primary-secondary remains common in retrofits and in plants with many small chillers; variable primary is common in new plants with capable controls.
Chilled water system design
Design starts with the loads, their diversity and their temperature needs, then sets supply temperature, temperature difference, pumping arrangement, chiller count and redundancy. A chilled water system diagram shows the plant as the ASHRAE and manufacturer literature does: chillers, pumps, decoupler or bypass, expansion and air removal, and the distribution to coils with their control valves.
Several choices drive the outcome. Higher supply temperatures improve chiller efficiency when the coils can use them. A larger temperature difference shrinks pipe and pumps. Chiller sizing should cover the low-load months as well as the peak, which is why plants often mix machine sizes. On a campus or a multi-building site, a central chilled water plant serves every building from one plant room, so loads that peak at different times share the same chillers and one crew maintains them. A glycol chilled water system adds inhibited propylene or ethylene glycol wherever piping runs outdoors or fluid temperatures approach freezing, accepting lower heat transfer and higher pumping energy in exchange for protection. Plant rooms housing chillers follow ASHRAE Standard 15 for refrigerant safety, and building systems must meet the efficiency requirements of ASHRAE Standard 90.1.
The most common performance failure is low delta T syndrome: return water comes back too cold, so the plant runs more chillers and pumps than the load needs. The usual causes are three-way valves, oversized or leaking control valves, dirty coils, coil setpoints lower than design, and decoupler flow in the wrong direction. Good design, and good balancing at startup, prevents most of it.
Chilled water piping: materials, sizing and insulation
Chilled water piping material is chosen for size, pressure, installed cost and corrosion behavior. Schedule 40 carbon steel, welded or roll grooved, dominates mains and risers; copper type L is common in smaller branches; stainless steel is used where water quality or process requirements demand it; and polypropylene or high-density polyethylene appears in some systems where the design allows. Building systems are typically installed to ASME B31.9, with ASME B31.3 applied where a plant specifies process piping. Chiller piping at the machine needs flexible connections, isolation valves, strainers, flow switches, thermometer and gauge connections, and removable spool pieces so the tube bundles can be cleaned.
Chilled water piping sizing follows long-standing ASHRAE practice: limit velocity to about 4 feet per second in pipe 2 inches and smaller to control noise and erosion, and size larger pipe for a friction loss of about 1 to 4 feet of head per 100 feet, with 2.5 feet a common target. Velocity that is too low lets air and dirt settle, so minimum velocities matter too. Our pipe schedule chart gives inside diameters and wall thickness for each size.
Chilled water piping insulation is not optional. The pipe runs below the dew point of most indoor air, so any break in the vapor retarder lets humid air reach the cold surface, condense, soak the insulation and corrode the pipe underneath. Closed-cell insulation or fiberglass with a continuous vapor retarder jacket, sealed at every seam, fitting, valve and hanger, is the standard, with insulated pipe supports so hangers do not bridge the insulation. Thickness is set by the energy code, which in ASHRAE Standard 90.1 depends on pipe size and fluid temperature. Underground chilled water piping between buildings usually uses pre-insulated, jacketed pipe with leak detection, since repairs mean excavation.
Chilled water system installation
Installation runs in a sequence that matters as much as the parts. Chillers and pumps are rigged and set, aligned and isolated; piping is fabricated, often in shop spools, and installed with supports and anchors; then the system is pressure tested before insulation goes on. ASME B31.9 requires a hydrostatic test of at least 1.5 times the design pressure, held at least 10 minutes, with any component that cannot take that pressure isolated. Rigging and alignment are covered on millwright and rigging services, and general piping practice on process piping.
Chilled water system flushing comes next and is the step most often shortened. Construction debris, mill scale, cutting oil and weld slag have to come out before water reaches chiller tubes, coil passages and control valves. The loop is flushed at velocity with coils and chillers bypassed, chemically cleaned where specified, then refilled and treated. BSRIA BG 29 is a widely used guide to pre-commission cleaning of pipework systems.
Chilled water system testing and commissioning follows: confirming pump rotation and flows, stroking every valve, verifying sensors, running each control sequence and proving chiller staging and alarms. How to balance chilled water system flows is part of it: a balancing contractor measures and sets flow at each coil to the design value, and with two-way valves and variable speed pumps that usually means proving each coil gets design flow at its valve fully open. The process is documented as described in ASHRAE Guideline 0. We support that work through commissioning and validation.
Chilled water system treatment
Treatment protects a closed loop from corrosion, sludge and biological growth. Because the loop does not evaporate, it should need very little makeup, and a closed loop chilled water system that keeps calling for water has a leak that is bringing in fresh oxygen and minerals. Treatment combines a corrosion inhibitor matched to the metals in the loop, side-stream filtration to remove suspended iron and debris, regular testing of inhibitor level, pH and bacteria, and metered makeup so losses are noticed. We install and service the treatment equipment, feeders, filters and meters; chemical programs come from a water treatment supplier. See our plant water treatment systems.
Chilled water system maintenance and troubleshooting
Maintenance protects both efficiency and reliability, and most of it is routine.
| Interval | Task | Why it matters |
|---|---|---|
| Daily or by monitoring | Log supply and return temperatures, delta T, pressures, chiller alarms | Trends catch fouling and control faults early |
| Monthly | Check strainers, makeup meter, expansion tank pressure, pump seals | Debris, leaks and pressure loss show up here first |
| Quarterly | Test water treatment, inspect insulation and vapor retarder, exercise valves | Corrosion and condensation damage are slow and silent |
| Annually | Chiller service, condenser tube cleaning, pump alignment, control calibration, relief valve inspection | Restores heat transfer and confirms safety devices |
When something goes wrong, chilled water system troubleshooting usually starts from one of a few symptoms.
| Symptom | Likely causes | Fix |
|---|---|---|
| Supply water too warm | Load above capacity, chiller faulted or limited, condenser water too warm | Stage another chiller, clear the fault, service the tower or condenser |
| Low delta T | Three-way valves, dirty coils, low setpoints, overpumping | Valve conversion, coil cleaning, reset setpoints, pump control |
| Chiller short cycling | Loop volume too small, oversized chiller at low load | Add a buffer tank, revise staging |
| Air noise and air-bound coils | Low fill pressure, failed vents, separator in the wrong place | Restore pressure, repair vents, relocate separator |
| Frequent makeup | Leaks, relief valves lifting, failed expansion tank | Leak search, tank precharge check or replacement |
| Sweating pipe or wet insulation | Broken vapor retarder | Remove wet insulation, reseal the retarder system |
Routine work runs best under a preventive maintenance plan; breakdowns are handled by our emergency repair service.
Chilled water system cost per ton
There is no reliable single figure per ton, because the chiller is often a minority of the installed cost. The main drivers are capacity and redundancy, air-cooled or water-cooled heat rejection, the length and size of distribution piping, insulation, the number of coils and connections, controls, the electrical work by others, and whether the work is a new building or a retrofit in an operating plant. Retrofit tie-ins, rigging access and outage windows often decide the budget more than tonnage does.
What Paul Industries does
Paul Industries installs and services chilled water systems: chiller setting and piping, pump installation and alignment, plant and distribution piping in carbon steel, copper and stainless, plate heat exchangers and economizers, expansion, air removal and buffer tanks, glycol systems, insulation coordination, pressure testing, flushing and cleaning, commissioning support, treatment equipment, preventive maintenance and emergency repair. Related pages cover plant utilities, heat exchangers and data center cooling. We do not manufacture chillers and we are not an electrical contractor. Request a quote for chilled water installation or service.
Standards referenced: ASME B31.9 · ASME B31.3 · AHRI 550/590 · ASHRAE Standard 90.1 · ASHRAE Standard 15 · ASHRAE Guideline 0 · ASHRAE Handbook Fundamentals · BSRIA BG 29 · ASME BPVC Section VIII
Frequently asked questions
What is chilled water?
Water cooled by a chiller, usually to the 40s or 50s Fahrenheit, and circulated in a closed loop to absorb heat from air handlers, coils or process equipment. It is not drinking water; it is treated with corrosion inhibitors and kept separate from potable supplies.
What temperature is chilled water?
The traditional design is 44 degrees Fahrenheit supply and 54 degrees return. Many modern systems supply somewhat warmer water with a wider temperature difference to save energy, while some process systems run colder with glycol. The right value is set by the loads, especially coil dehumidification needs.
What is the difference between dx system and chilled water system?
A DX, or direct expansion, system cools air directly with refrigerant in the coil, piped from a compressor and condenser. A chilled water system cools water at a central chiller and pumps the water to the coils. DX suits smaller buildings and individual zones; chilled water suits large buildings, campuses and plants because one plant serves many loads and refrigerant stays in the equipment room.
Why do chilled water piping systems need to be vapor sealed?
Because the pipe is colder than the dew point of the surrounding air. Without a continuous vapor retarder, moisture migrates into the insulation, condenses on the pipe, destroys the insulation value and corrodes the steel. Every seam, fitting and hanger has to be sealed.
How much glycol in a chilled water system?
Enough for the lowest temperature the fluid or piping will see, and no more, because glycol lowers heat transfer and raises pumping energy. Glycol makers publish concentration tables: by Dow's typical data, about 30 percent propylene glycol by volume protects against freezing to roughly 10 degrees Fahrenheit. Choose a protection point at least 5 degrees below the lowest expected temperature and use inhibited industrial glycol, never automotive antifreeze.
How to fill a chilled water system without trapping air?
Fill slowly from the lowest point with high-point vents open, close each vent as solid water appears, bring the system to its fill pressure, then run the pumps to sweep remaining air to the separator. Glycol systems are premixed and pumped in so the concentration is right from the start.
How much pressure in chilled water system piping is normal?
The cold fill pressure has to cover the static height of the system, about 0.433 psi per foot of water, plus a margin so the highest point stays positive. Operating pressure then adds pump head. Components must be rated for the highest pressure at the lowest point of the system, which in tall buildings can be substantial.
Why would chillers of unequal size be installed in a chilled water plant?
To match the load profile. Many plants run far below peak for most of the year, so a smaller chiller can carry low loads efficiently without short cycling, while the large machines handle summer peaks. Unequal sizes also give finer staging steps.
What are chilled water systems used for?
Comfort cooling of large buildings and campuses, cooling of process equipment and products in manufacturing, cooling of data halls and laboratories, and dehumidification. Food, pharmaceutical, plastics and chemical plants rely on them for both process and space cooling.
Why is my chilled water system not cooling?
Check whether supply water is cold at the plant first. If it is warm, the chiller is limited, faulted or short of condenser capacity. If it is cold at the plant but not at the load, look for air-bound coils, closed or failed valves, clogged strainers or insufficient flow. Low delta T at the plant points to bypassing and coil problems.
Who services chillers?
Chiller manufacturers' service organizations and qualified refrigeration technicians handle refrigerant circuits and controls under warranty. Mechanical contractors like Paul Industries service the rest of the system: pumps, piping, heat exchangers, valves, tanks, flushing and treatment equipment, and we coordinate with the chiller service provider.
Does a chilled water system need an expansion tank?
Yes. Water expands as it warms, even in a cold loop that warms during shutdowns, and a closed system without room for that expansion will lift relief valves and lose water. The tank also sets the system's reference pressure.
Can a chilled water system be expanded later?
Usually, if the original design left valved connections, space for another chiller and pump, and headers sized for the future flow. Adding load without checking pump head, pipe capacity and chiller capacity is how plants end up with low delta T and warm spaces.
How long does chilled water pipe last?
Properly treated and insulated steel piping commonly lasts for decades. Failures usually come from corrosion under wet insulation, poor water treatment, or oxygen brought in by constant makeup, all of which are preventable.
What is a decoupler bypass flow reversal?
In a primary-secondary plant, when the secondary loads draw more water than the primary pumps supply, warm return water flows backward through the common pipe into the supply. Supply temperature rises, and the fix is staging another chiller or correcting the low delta T causing it.
Related: Plant Utilities: Steam, Heating, Chilled Water & Ice Builders · Process Chiller Installation & Service · Industrial Process Chillers: How They Work & How to Choose · Data Centers · Data Center Cooling Systems · Plate Heat Exchanger · Request a quote · Hydrostatic Testing of Piping, Vessels and Tanks