Why “How Many Tons?” Is the Wrong First Question
A ton of cooling is a rate of heat removal — 12,000 BTU per hour. It’s a useful number, but on its own it’s incomplete, because a chiller’s ability to move heat depends entirely on how cold you’re asking it to run.
Refrigeration capacity falls as the target temperature drops. The colder the evaporator has to get, the harder the compressor works and the less heat it can move per hour. That means rated capacity is always tied to a specific fluid temperature. A spec sheet that says “5 tons” without a temperature is telling you half the story.
The practical consequence: if you pick a chiller on tonnage alone and then discover it can’t reach your temperature, no amount of extra tonnage fixes it. You bought the wrong class of machine.
Temperature Sets the Class. Tonnage Sizes It.
Think of it as two decisions in order:
- What’s the coldest fluid temperature my process actually needs? This determines which series of chiller is even capable of the job — the refrigeration architecture behind it.
- How much heat do I need to remove at that temperature? This determines the size of the unit within that series.
Get step one right and step two is straightforward math. Get step one wrong and step two is irrelevant. Here’s how the required temperature maps to AG Chill’s lineup:
| Coldest fluid temp you need | Series that reaches it | Typical applications |
|---|---|---|
| Down to 40°F (4°C) | Standard | Hydroponics, glycol loops, general process |
| Down to 10°F (-12°C) | LT | Deeper process cooling, food & beverage |
| Down to -22°F (-30°C) | ELT | Sub-zero process, air-cooled extraction |
| Down to -58°F (-50°C) | SLT | Hydrocarbon condensing, deep ethanol pre-cool |
| Down to -112°F (-80°C) | ULT (cascade) | Freeze-drying, supercritical CO₂, semiconductor test |
You can always run a series warmer than its minimum — a ULT will happily hold 20°F. What you can’t do is push a Standard unit below its floor. That’s why the coldest point your process ever needs to hit is the number that anchors everything. See the full breakdown on the series comparison page.
Now Size the Tonnage: The Three Inputs
Once temperature has selected the series, the cooling load determines the size. Three things drive it:
- Heat load — how much heat your process dumps into the fluid, in BTU/hr or tons. This can come from equipment heat, an exothermic reaction, or the energy needed to pull a batch down to temperature in a set time.
- Flow rate — how fast fluid moves through the loop (GPM). It sets how the load and temperature relate through the system.
- Delta-T (ΔT) — the temperature difference between the fluid entering and leaving your process. Load, flow, and ΔT are locked together: for water, roughly BTU/hr = GPM × ΔT × 500.
That relationship is why a single number can’t size a chiller. A 10°F pull-down at high flow and a 40°F pull-down at low flow can demand very different machines even at the same “tonnage.”
Don’t forget the glycol penalty
Any time you run below about 40°F you’ll be using a glycol/water mix to prevent freezing — and glycol carries less heat than water and pumps harder. That reduces effective capacity, so a system sized on pure-water numbers will come up short in the real world. Our glycol guide covers the concentration charts and the capacity derate.
A Quick Example of the Trap
Say an extraction operator needs to condense solvent and figures “we run a lot of volume, let’s get a big 10-ton unit.” They buy on tonnage. But the process needs -50°F fluid. A Standard or LT chiller — no matter how many tons — simply can’t get there. The right move was to identify -50°F first, which points to the SLT Series, and then size the tonnage for the actual condensing load. Temperature first would have avoided a five-figure mistake.
Rule of thumb: if a vendor quotes you a chiller by tonnage without asking your target fluid temperature, flow, and ΔT, they’re guessing. Those numbers aren’t optional — they’re the whole job.
How to Get It Right in Practice
- Write down your coldest required fluid temperature. Not typical — the coldest the process ever needs. That picks the series.
- Gather your load, flow, and ΔT. Even approximate figures get you a real answer.
- Account for glycol if you’re running sub-40°F.
- Let the calculator do the arithmetic. The AG Chill sizing calculator takes temperature, flow, and load and returns the series and tonnage together — in the right order.
Temperature decides what machine. Tonnage decides how big. Do them in that order and sizing stops being guesswork.
Related: How Chillers Reach -80°C: Single-Stage, Two-Stage, and Cascade Refrigeration Explained ›
Get the Right Series and Size the First Time
Give us your temperature, flow, and load — we’ll return the machine that fits, and tell you honestly if you need less than you think.