How Glycol Chillers Work

The loop

Four components, in a circle

1. A refrigerated reservoir. A tank of propylene glycol and water with a refrigeration circuit chilling it, typically held somewhere around 28 to 30°F -- below freezing for water, which is the whole reason for the glycol.
2. A pump. Usually submersible, sitting in the reservoir, pushing cold fluid out to the vessel.
3. A jacket or a coil. Either a channel built into the fermenter wall, or a stainless coil suspended in the beer. This is where heat transfers from the beer into the glycol.
4. A return line carrying the now-warmer glycol back to the reservoir to be re-chilled.

Temperature control is applied by a solenoid valve or by switching the pump: a controller reading the beer temperature opens the loop when the beer is above setpoint and closes it when it is at target. The chiller itself just keeps the reservoir cold.

Why glycol

Because water freezes

The reservoir has to run below the temperature you want to achieve, and for a cold crash to 34°F that means the reservoir sits close to or below 32°F. Plain water would freeze, which stops the pump and can damage the chiller.

Propylene glycol depresses the freezing point. Mixed at the manufacturer’s stated concentration -- commonly around 25 to 35 percent -- the fluid stays liquid well below freezing while remaining pumpable.

• Propylene glycol only. Automotive antifreeze is ethylene glycol and is toxic. Propylene glycol is the food-safe alternative and is what brewing suppliers sell.
• Get the concentration right. Too dilute and it can freeze in the chiller; too concentrated and it becomes viscous, pumps poorly and actually transfers heat less effectively.
• Check it annually. Glycol degrades over time and the mix drifts as water is topped up.

Jacket or coil

Two ways to get the cold into the beer

Jacket

A channel built into the vessel wall, so glycol flows around the outside of the tank and nothing enters the beer. Cooling is even across the whole wetted surface, there is nothing extra to sanitize, and this is what makes a vessel a unitank. The downside is that jacketed vessels are expensive and start at capacities larger than most homebrewers need.

Coil

A stainless coil suspended inside the beer, with glycol flowing through it. Cheaper by a wide margin, available as a factory option on several conicals, and it achieves the same direct cooling. The trade is that it is another surface inside the fermenter to clean and sanitize, and cooling is slightly less even -- the beer nearest the coil is coldest.

For a homebrewer the coil is almost always the right answer: conical vs unitank walks through the cost difference.

Sizing

Horsepower against vessels

The load is bigger than it looks. Active fermentation releases real heat for two to three days, and a crash cool asks the chiller to remove the thermal mass of the entire batch. An undersized chiller runs continuously and never reaches setpoint, which is both expensive and ineffective. Err one size up -- the chiller is the component you keep when the vessels change.

Installation

The three mistakes that ruin performance

• No exhaust path. A chiller is a refrigeration unit: it moves heat out of the glycol and into the surrounding air. Put it in a sealed cupboard and it heats the air it is trying to reject heat into, and efficiency collapses. Clearance on all sides and a route for warm air to leave the space.
• Uninsulated lines. Every uninsulated foot of line loses cooling capacity to the room and sweats condensation onto the floor. Foam pipe insulation on every run, including the return.
• Probe in the air. The same rule as a chamber. The controller should read the beer -- ideally through a thermowell -- rather than the glycol temperature or the room. See fermentation temperature control.