Glycol Cooling System Design, Installation, Maintenance and Repair

A glycol cooling system is a sealed, pressurized circuit that pumps a mixture of water and inhibited ethylene or propylene glycol between a chiller and the equipment it cools, so the fluid can run colder than 32°F or sit in outdoor piping through winter without freezing; Paul Industries designs, installs, pipes, flushes, fills, starts up, maintains and repairs these systems together with their chillers, pumps and heat exchangers. We do this nationwide for process plants, food and beverage facilities, commercial HVAC and data center heat-rejection circuits. Below: how the loop works, how much glycol it needs, how it is designed, piped, installed and flushed, and what goes wrong with it.

What is a glycol system?

A glycol system is a closed hydronic circuit charged with a water and glycol solution instead of plain water, so the fluid stays liquid at temperatures that would freeze water, and so a corrosion inhibitor package travels with it to protect the metals inside. The glycol lowers the freezing point; the inhibitors, a few percent of the concentrate, are what keep carbon steel, copper and brass from corroding.

The term covers three families of equipment. A closed loop glycol system serving a process uses a chiller to cool the solution to a setpoint chosen for the product, which can sit well below the freezing point of water, and pumps it to jackets, plate exchangers, coils and molds. A building HVAC circuit uses glycol mainly so that outdoor coils, rooftop air-cooled chillers and air handlers drawing in winter air cannot freeze. A heat-rejection circuit runs glycol through dry coolers in a yard or on a roof and gives the heat to outdoor air.

How does a glycol cooling system work?

A glycol cooling system works by pumping chilled solution from the chiller’s evaporator to the process load, where it picks up heat and warms by a designed temperature difference, then returning it to the chiller, whose refrigeration circuit removes that heat and rejects it to outdoor air or condenser water. The circuit is sealed, so one charge circulates for years; an expansion tank absorbs the change in volume as the fluid warms and cools, and an air separator strips out the air that comes out of solution.

On a typical industrial installation the chiller’s leaving solution feeds a supply header, branch valves meter flow to each load, the warmed fluid returns through a strainer to a buffer or return tank, and the system pump sends it back through the evaporator. Many plants put a heat exchanger between the glycol and the process so only the chiller side carries glycol. A plate heat exchanger is the usual choice for that break, because its close temperature approach gives up the least capacity.

The chiller follows the standard vapor-compression cycle, and choosing and sizing the machine is covered on our process chillers page and in more depth in the industrial process chillers guide.

Why is glycol used in cooling systems?

Glycol is used in cooling systems because it lowers the freezing point of the circulating fluid, which lets a loop deliver supply temperatures below 32°F and ride out outdoor exposure, power loss or a pump trip in freezing weather without ice splitting tubes, coils or evaporator passages. Water grows about 9 percent in volume as it turns to ice, enough force to rupture copper tube, a brazed plate pack or a chiller barrel, so one frozen coil can cost far more than the glycol that would have prevented it.

Glycol vs water: what the mix costs

Weighed as glycol vs water, the solution carries less heat per gallon, is thicker and takes more pumping power, so a loop should get only as much glycol as its lowest temperature calls for. At 40°F, published data for one inhibited propylene glycol fluid show specific heat dropping from about 1.0 Btu/lb·°F for water to 0.847 at 40 percent glycol by volume, while viscosity climbs from roughly 1.5 centipoise for water to about 9.6, around six times thicker. An inhibited ethylene glycol fluid at the same strength measures about 4.9 centipoise, one reason it moves heat more readily.

Those properties show up in three places. Flow rises: for water, Btu/h equals 500 × gpm × temperature rise, but for 40 percent propylene glycol at 40°F that constant falls to about 449, so a 100-ton load at a 10°F rise needs roughly 267 gpm instead of 240. Film coefficients fall in every exchanger and coil, and fall faster at low temperature where viscosity climbs steeply, so equipment selected on water loses capacity on glycol. Pump head rises with viscosity.

Ethylene vs propylene glycol, and why the inhibitor matters

Between the two glycols, ethylene glycol transfers heat better and stays thinner at any given freeze point, while propylene glycol has far lower acute oral toxicity, so propylene is the normal choice in food, beverage and pharmaceutical plants and in any circuit where a leak could reach product, drinking water or people. Ethylene glycol for chiller loops in closed industrial and commercial service remains a sound choice wherever that exposure is ruled out and efficiency matters, particularly at low temperatures, where the viscosity of propylene glycol rises steeply.

Either glycol has to be an industrially inhibited formulation made for HVAC and process circuits. Fluid makers’ engineering specifications call for phosphate-based inhibitor packages, exclude formulations containing silicates, and require the fluid to pass ASTM D1384 corrosion testing at less than 0.5 mils per year of penetration on every system metal. Uninhibited glycol is worse than plain water: one water-treatment bulletin found an uninhibited ethylene glycol solution about 4.5 times as corrosive to carbon steel as water, because glycol oxidizes over time into organic acids.

How much glycol in a chilled water system or process loop?

The right concentration is the lowest one whose freeze point sits at least 5°F (3°C) below the coldest temperature any part of the circuit will reach, whether that is the chiller’s leaving fluid, an outdoor coil on the coldest night or a run of pipe through an unheated space. Going higher wastes heat transfer and pump energy; going lower risks a frozen evaporator.

The table lists freeze points from fluid makers’ engineering data for two inhibited HVAC fluids, one ethylene-based and one propylene-based, by volume percent of concentrate, with specific heat and viscosity at 40°F to show what each step costs. Figures vary slightly by product, so design with the supplier’s own table.

Freeze point and burst point are two different limits. Below the freeze point, ice crystals begin to form and the fluid turns to slush; burst protection means that slush stays soft enough for pipes and coils to survive, provided the expansion tank has room, even though the loop can no longer be pumped. For 0°F, published figures call for roughly 23 percent ethylene glycol by volume for burst protection against roughly 35 percent for full freeze protection, and roughly 25 against 38 percent for propylene glycol. A loop that must keep running in winter needs the freeze-protection figure.

A chiller adds a further margin. Refrigerant in the evaporator boils roughly 10°F or more below the leaving fluid setpoint, so the film of solution against the tubes or plates is colder than the bulk temperature on the controller. Chiller manufacturers state the freeze point their machine needs relative to its leaving setpoint, and on low-temperature work that figure, rather than the outdoor design low, sets the concentration. Two floor rules apply as well. Below roughly 25 percent concentrate a mix invites bacterial growth and holds too thin an inhibitor reserve, so suppliers treat that as a practical minimum even where freeze protection would permit less. And the water used to cut the concentrate should be distilled or deionized, with chloride and sulfate each under 25 ppm and calcium plus magnesium under 50 ppm, or the charge should be bought premixed.

Glycol chiller system design

Glycol chiller system design starts from the heat load and the coldest supply temperature the process needs, then fixes the flow and temperature rise, the glycol type and concentration, the loop volume and the redundancy the plant can justify, in that order. The concentration has to be settled before the chiller, pumps and exchangers are chosen, because each of them is rated differently on glycol than on water.

Chillers, plate exchangers and coils should all be rated by their manufacturers at the actual glycol, strength and temperatures; a capacity quoted on water at standard rating conditions overstates what the equipment delivers on glycol.

Loop volume deserves attention on process work. Chiller and tank vendors commonly cite about 3 to 5 gallons of system volume per ton for comfort cooling and 6 to 10 gallons per ton where temperature stability is critical, with a buffer tank making up the difference when piping and equipment hold too little. The chiller maker’s minimum volume or loop time governs. Plant layouts with primary and secondary pumping are explained on our chilled water systems page.

Glycol piping: materials, joints and layout

Glycol piping is usually carbon steel, type L copper or stainless steel, with fusion-welded thermoplastics used on low-temperature circuits where the pipe maker lists them for the fluid, temperature and pressure. Galvanized pipe is the one common material to leave out, because glycol suppliers warn that zinc reacts with the inhibitor package.

Piping for glycol systems is designed to ASME B31.9 for building services, or to ASME B31.3 where the owner or the process calls for process piping rules, and follows the same layout basics as any closed circuit, with a few additions specific to glycol. Cold lines need closed-cell insulation with a continuous vapor retarder, because a supply below the dew point drips and ices otherwise. Every high point needs a vent and every low point a valved, capped drain, so glycol can be drained into containers for reuse rather than to a floor drain. A direct city-water makeup connection has no place on the loop, since it dilutes the charge without anyone noticing, and fluid makers’ specifications say so. Our pipe fabrication shop builds headers and spools ahead of the field work.

Glycol system components and the glycol feeder system

Besides the chiller and the loads, a working circuit needs circulating pumps, an expansion tank, an air separator, strainers or side-stream filtration, isolation and balancing valves, temperature and pressure instruments, a fill connection, and a glycol feeder system that keeps it pressurized with premixed solution.

Pumps are often mounted with their valves, strainers and controls on a shop-built frame, delivered as one of our pump skids. The expansion tank is sized for the solution’s own expansion across its full temperature range, from fill temperature to the warmest condition the circuit can reach when idle. The air separator belongs where fluid is warmest and pressure lowest, usually on the return ahead of the pump suction, because that is where dissolved air leaves solution.

A closed loop glycol cooling system has to stay full, pressurized and free of air, and that is the job of the feeder. A glycol feeder system is a small packaged unit with a tank of premixed solution, a pump, a pressure switch and a check valve, piped to the circuit in place of a city-water makeup. When pressure falls it pumps in solution at the correct strength, and its low-level alarm turns a slow leak into a work order instead of a diluted, half-protected circuit. Isolation exchangers need service access; gasket replacement, cleaning and leak repair on them are part of our heat exchanger repair work.

HVAC glycol systems vs industrial process glycol systems

HVAC glycol systems mainly use glycol as freeze protection at ordinary chilled water temperatures, while process systems use it to reach supply temperatures below freezing and to hold them tightly, and that difference changes concentration, controls, redundancy and materials.

An industrial glycol cooling system usually serves equipment that loses product, quality or uptime the moment cooling stops: extruders, molds, reactors, jacketed vessels, product chillers and compressors. In food and beverage plants, propylene glycol, stainless piping near product, sanitary-grade exchangers and documented fluid changes are typical requirements; our food and beverage page covers the wider plant scope, and the control of cellar tank jackets has its own page on tank jacket and glycol cooling.

On data center sites, glycol usually lives in the outdoor heat-rejection circuit: dry coolers or free-cooling coils carry glycol through the winter, and a plate exchanger hands the heat over from the indoor water side so the larger indoor volume needs no glycol. The plant-level picture is on our data center cooling systems page, and fluid strength for dry coolers is in our dry cooler guide.

Glycol refrigeration system design for low-temperature process loads

A glycol refrigeration system is a secondary-coolant arrangement in which a refrigeration machine chills glycol well below freezing and the glycol, rather than refrigerant, is piped out to the cold loads, which keeps the refrigerant charge inside the machine and lets the plant run ordinary hydronic piping to the floor.

At these temperatures concentration comes from the chiller’s requirement, not the weather. If a chiller manufacturer asks for a freeze point 15°F below a 15°F leaving setpoint, the circuit needs protection to 0°F, which the table places between 30 and 40 percent for either glycol. Viscosity then dominates the hydraulics: propylene glycol at 50 percent is already around 14 centipoise at 40°F and far thicker below freezing, so pumps, branch sizes and exchanger selections are checked at the coldest operating temperature, and pumps are often given extra head. Vapor sealing has to be continuous, because a gap lets ice build up under the insulation.

Glycol system installation, step by step

Glycol system installation runs from survey and submittals through setting equipment, fabricating and installing piping, pressure testing, cleaning and flushing, filling with premixed solution, venting, verifying concentration, and starting up and commissioning the chiller and controls.

Test water left in low points dilutes the charge, so it is drained completely or the test is run with premixed solution. Our hydrostatic testing crews run and document the pressure test.

How to flush a glycol system

To flush a glycol system, isolate or bypass the components that trap debris, circulate a cleaning solution through the piping at good velocity, drain it completely, rinse with clean water until the rinse runs clear and close to neutral, then fill straight away with premixed glycol so the cleaned metal is never left wet and unprotected.

For new or lightly corroded circuits, fluid makers recommend cleaning with a 1 to 2 percent trisodium phosphate solution before charging; heavily corroded circuits need a more aggressive industrial cleaning followed by repair of whatever the cleaning exposes. Plate exchangers, control valves and flow meters are bypassed or protected by temporary strainers during cleaning. Old fluid goes to drums for recycling or permitted disposal.

A glycol flush is also the fix when the wrong fluid has gone in. Ethylene and propylene fluids carry different inhibitor packages and read on different refractometer scales, so a mixed charge cannot be tested accurately and is better drained, flushed and replaced than adjusted.

Glycol system maintenance

Glycol system maintenance comes down to keeping concentration, inhibitor reserve and cleanliness within limits, keeping air out and pressure up, and keeping pumps, strainers and heat exchangers in condition; a practical schedule pairs quarterly field checks with a laboratory analysis at least once a year.

For circuits over 250 gallons, fluid makers’ specifications call for a yearly manufacturer analysis; the trend across years tells more than any single result. Rising iron or copper means metal loss; falling reserve alkalinity means the inhibitors are being used up. We fold these checks into preventive maintenance visits.

How often should glycol be replaced?

Glycol is replaced on condition, not on a calendar: a well-maintained, inhibited charge in a sealed circuit can run for many years, and it is due for replacement when analysis shows reserve alkalinity spent, pH fallen well below the new-fluid range, rising dissolved metals, heavy contamination or biological growth. One maker’s propylene glycol fluid, for comparison, reads pH 9.5 to 10.5 new at 50 percent strength. A fluid caught early can often be re-inhibited instead.

Glycol system leak, foaming and loss of concentration: diagnosis and repair

A glycol system leak shows first as a feeder tank that empties faster than usual or a loop pressure that keeps falling, and most other glycol problems, foaming, dark or sour fluid, lost concentration and lost capacity, trace back to air entering the circuit, the wrong water going in, or inhibitors running out.

Glycol system repair

Glycol system repair covers whatever the diagnosis turns up: replacing pump seals, bearings or whole pumps, repairing or re-gasketing heat exchangers, cutting out and replacing corroded or leaking pipe sections, replacing expansion tanks and feeders, and draining, flushing and recharging the fluid. When the fault sits inside the refrigeration machine, glycol chiller work goes through our chiller repair team, who treat the chiller and its glycol circuit as one system. When a loop goes down during production, our emergency repair service can get crews moving on it.

Glycol cooling system services from Paul Industries

Paul Industries engineers glycol circuits to the plant’s loads and temperatures or builds to the owner’s engineered design, sets chillers, pumps, tanks and feeders, fabricates and installs carbon steel, copper and stainless glycol piping, builds pump skids in our shop, and installs plate and shell-and-tube heat exchangers for glycol isolation. We pressure test, clean, flush, fill and mix to the target concentration, start up the system and support commissioning, then keep it running with fluid testing, preventive maintenance, repair, chiller service and emergency response. The same crews handle the chilled water and process piping around it, nationwide. Request a quote for a new glycol system, an expansion, a fluid changeover or a repair.

Standards referenced: ASME B31.9 · ASME B31.3 · ASTM D1384

Frequently asked questions

Why use glycol instead of water in a process cooling loop?

Water alone freezes at 32 degrees Fahrenheit, and many process setpoints and every outdoor exposure in a cold climate sit at or below that. Glycol lets the loop run colder and survive a winter stop, and its inhibitor package protects the metals, which plain water would slowly corrode. The cost is extra pumping power and lower heat transfer, so the mix is kept no stronger than it needs to be.

Can ethylene glycol and propylene glycol be mixed in the same loop?

They are better kept apart. The two fluids use different inhibitor packages, and refractometers read them on separate scales, so a mixed charge cannot be tested reliably for freeze point or inhibitor level. If the wrong fluid has been added, draining, flushing and recharging with one fluid is the dependable fix.

How is glycol concentration measured in the field?

A refractometer with the correct scale for ethylene or propylene glycol gives a freeze point reading from a few drops of fluid in under a minute. Samples are taken from a flowing line after the loop has circulated, not from a stagnant tank. A laboratory analysis once a year confirms the field readings and adds pH, reserve alkalinity, inhibitor and metals results.

Can an existing water-only loop be converted to glycol?

Yes, provided it is checked first. Pumps, coils, plate exchangers and the chiller must be rerated at the new concentration because flow needs and heat transfer change, the expansion tank must be sized for the solution, any galvanized pipe must be replaced, and city-water makeup is swapped for a feeder. The loop is then cleaned, flushed and filled with premixed solution.

Does adding glycol reduce chiller capacity?

It does, because the solution has lower specific heat and higher viscosity than water, which lowers the heat transfer coefficient in the evaporator. The reduction grows with concentration and with colder setpoints. The chiller manufacturer supplies the capacity and pressure drop at the exact glycol, strength and temperatures, and that rating is the one to plan around.

How is the expansion tank sized for a glycol circuit?

From the total system volume and the expansion of the actual glycol solution between its fill temperature and the warmest temperature the loop can reach, including when it sits idle in a warm room. A tank sized from water tables, or on the running temperature alone, can be too small and push fluid out through the relief valve.

How should used glycol be disposed of?

Drain it into drums or totes and send it to a glycol recycler or a permitted waste handler under local and state rules. Used fluid can pick up metals and other contaminants, so it should never go to a storm drain or floor drain, and many sewer authorities restrict or require permits for any discharge.

Which glycol should a food plant use?

Inhibited propylene glycol, because its acute oral toxicity is far lower than ethylene glycol's and fluid makers recommend it wherever incidental contact with food or drinking water is possible. Plants should confirm the specific formulation's suitability and listings with the supplier and keep the fluid's data on file for audits.

What makes glycol turn dark or smell sour?

Oxidation. Air drawn into the loop, high temperatures and spent inhibitors let the glycol break down into organic acids, which darken the fluid, give it a vinegar-like smell and attack metals. A lab analysis confirms it, and once the reserve alkalinity is gone the usual remedy is to flush the loop and recharge it.

How long does a glycol system installation take?

It depends on the size of the system, how much piping is prefabricated in the shop, equipment lead times, and whether tie-ins must wait for planned shutdowns in an operating plant. Shop-built pump skids and pipe spools shorten the field schedule, and the cleaning, flushing, filling and startup steps should be planned in from the beginning rather than squeezed at the end.

Should a glycol cooling system have side-stream filtration?

On most process loops it pays for itself. A small filter carrying a portion of the flow continuously removes rust, scale and debris that would otherwise settle in plate exchangers, small passages and control valves. It is especially useful after a repair or on older steel piping, and filter changes become a routine maintenance item.

Why does loop pressure change with temperature?

The solution expands as it warms and contracts as it cools, so pressure rises when a loop warms during a shutdown and falls as it is pulled down to setpoint. A correctly sized and precharged expansion tank keeps the swing within limits; repeated relief valve discharges on warm-up or a sharp loss of pressure on cooldown point to a tank problem.

Can plastic pipe be used for glycol?

Fusion-welded thermoplastics such as polypropylene and high-density polyethylene are used on cold circuits where the pipe maker lists them for the glycol, temperature and pressure. Designers allow for the pressure derating with temperature, closer support spacing and larger thermal movement than metal pipe, and insulate the lines like any cold service.

What does a glycol makeup unit do?

It holds premixed solution in a tank and pumps it into the loop whenever pressure drops below its setting, replacing what a small leak or air removal has taken out at the right concentration. A low-level alarm on the tank signals a leak early, and the unit removes the temptation to top off with plain water.

How is a new load added to an existing glycol system?

Check the chiller, pump and piping capacity against the new peak, confirm the concentration still suits the new user's lowest temperature, then fabricate the branch, isolate or hot-cut the header during a planned window, test, flush and fill the new section with premixed solution, and rebalance the loop so existing users keep their flow.

How cold can a glycol circuit be run?

Fluid makers' data show freeze protection to about minus 60 degrees Fahrenheit at 60 percent glycol, but practical supply temperatures are set by the chiller's rated range, the margin it requires below setpoint, and how thick the solution becomes at the cold end. Very low setpoints call for chillers, pumps and exchangers selected specifically for that duty.

What should a glycol system turnover package include?

As-built drawings, pressure test records, the cleaning and flushing record, the volume and type of fluid charged with the first lab analysis, equipment startup reports, control setpoints and alarm settings, balancing data, feeder and expansion tank settings, and the recommended testing and maintenance schedule.

How do you find a leak in a glycol system?

Start with the feeder log to measure the loss rate, then walk the loop looking for the fluid's dye at pump seals, valve stems, threaded joints and coil headers. If nothing shows outside, isolate sections and watch pressure, and check heat exchangers and chiller barrels for internal leaks by testing the fluid on the other side for glycol.

How does a glycol chiller differ from a standard water chiller?

The refrigeration machine is similar, but a glycol chiller is built and controlled to run below the freezing point of water, with freeze protection settings, evaporator selection and capacity ratings matched to the solution. A standard water chiller is set up for leaving temperatures above freezing, and running glycol through one requires the manufacturer's glycol rating and control changes.

What drives the cost of a glycol cooling system?

Cooling capacity and supply temperature, the chiller type, how far the loads are from the plant, piping material, redundancy, the volume of glycol, insulation, building access for rigging, and whether the work happens during production. Accurate load data and a clear supply temperature requirement do more to control cost than any other input.

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