What Size Chiller Do I Need? A Step-by-Step Sizing Guide

Picking the right chiller size is the single most important decision you'll make when buying an industrial cooling system. Undersized chillers run 100% of the time, never reach setpoint, and burn out early. Oversized chillers short-cycle, waste energy, and stress the compressor. Right-sized chillers just work — for 15 to 20 years.

This guide walks you through exactly how to size a chiller for your process, with formulas, worked examples, and the derate factors that trip people up.

The Two Ways to Size a Chiller

There are two scenarios, and they use two different formulas.

Scenario A — Continuous process: You have a steady flow of warm fluid coming back from your process, and you need to cool it continuously. This covers most CNC machining, injection molding, laser cooling, and welding applications.

Scenario B — Batch cool-down: You need to cool a fixed volume of fluid from one temperature to another within a specific time window. This is common in brewing, wine production, cannabis extraction, and lab work.

Figure out which one you are, then use the matching formula below.

Scenario A: Continuous Process Sizing

For any steady-state cooling loop, the formula is:

BTU/hr = GPM × 500 × ΔT (°F)

Where:

• GPM = gallons per minute of fluid flowing through the chiller
• 500 = a constant combining water density and specific heat (use ~460 for 50% glycol)
• ΔT = temperature rise across your process (return temp − supply temp)

Then convert to tons:

Tons = BTU/hr ÷ 12,000

And add your safety margin:

Sized tons = Calculated tons × 1.20 to 1.25

Worked Example: 3-Axis CNC Machining Center

Your spindle manufacturer specifies a 10 GPM flow rate and a 10°F temperature rise at full load.

• BTU/hr = 10 × 500 × 10 = 50,000 BTU/hr
• Tons = 50,000 ÷ 12,000 = 4.17 tons
• Sized tons (×1.2) = 5.0 tons

A 5-ton portable chiller is the right answer. The Freeze 5-ton unit would be a direct fit.

Scenario B: Batch Cool-Down Sizing

For a fixed volume of fluid that needs to come down in temperature over a set period, use:

BTU = Gallons × 8.33 × ΔT (°F)

Where:

• Gallons = total volume to cool
• 8.33 = weight of one gallon of water in pounds (specific heat = 1 BTU/lb·°F)
• ΔT = temperature drop you need

Then divide by the time available:

BTU/hr = Total BTU ÷ Hours

And convert to tons and apply your safety margin as before.

Worked Example: Post-Boil Wort Cooldown

A brewery needs to cool a 200-gallon batch of wort from 200°F to 70°F in 60 minutes (after a heat exchanger pre-cools to 100°F, leaving the chiller to pull it from 100°F to 70°F in the fermenter jacket).

• BTU = 200 × 8.33 × 30 = 49,980 BTU
• BTU/hr over 1 hour = 49,980 BTU/hr
• Tons = 49,980 ÷ 12,000 = 4.17 tons
• Sized tons (×1.25) = 5.2 tons

A 5-ton chiller paired with Fluxwrap tank jackets handles this comfortably.

Don't Forget the Derate Factors

Your calculated tonnage assumes ideal conditions. The real world is less forgiving. Apply these derates before picking a chiller model:

Glycol concentration

Glycol reduces heat transfer and increases viscosity. Typical derate factors for propylene glycol:

Size the chiller so that its derated capacity still meets your requirement. Don't compare nameplate ratings — they're measured in pure water.

Ambient temperature (air-cooled units)

Air-cooled chillers lose capacity as the air gets hotter:

• At 75°F ambient: 100% of rated capacity
• At 95°F ambient: ~90–95% of rated capacity
• At 105°F ambient: ~80–85% of rated capacity
• At 115°F ambient: ~70–75% of rated capacity

If you're in Phoenix, Houston, or anywhere with regular 100°F+ summer days, size up or consider water-cooled.

Altitude

Above 3,000 ft, air density drops and air-cooled condenser performance falls 3–5% per additional 1,000 ft. Rule of thumb: derate 5% per 2,000 ft above 2,500 ft.

Long pipe runs and elevation

Every 50 ft of pipe run and every 10 ft of vertical rise adds pump head loss. If your chiller is far from the process or upstairs from it, verify the pump head rating — not just the flow rate.

Quick Sizing Cheat Sheet

For common applications, here's where most customers land:

These are starting points, not guarantees. Your specific process, ambient conditions, and glycol mix can shift you a size up or down.

The Five Most Common Sizing Mistakes

1. Skipping the safety margin. A chiller sized exactly to the calculated load will run at 100% forever and never stabilize. Always add 20–25%.

2. Using nameplate capacity with glycol. The published BTU/hr is tested in pure water. A 5-ton rating drops to about 4.1 tons at 50% glycol. Size off the derated number.

3. Ignoring ambient. A chiller rated at 100°F ambient in a factory that routinely hits 110°F will run short and short-cycle when it can't keep up. Check the capacity curve at your worst-case ambient.

4. Forgetting pump heat. The circulation pump itself adds 1,000–3,000 BTU/hr to the loop. Include it in the cooling load for tightly-controlled applications.

5. Sizing for average instead of peak. Your peak load — the hottest summer day at maximum production — is what the chiller has to handle. Size for peak, not average.

When to Call an Engineer

You can size most single-process applications yourself with the formulas above. Call an engineer when:

• Your process has variable or intermittent loads
• You're feeding multiple pieces of equipment from one chiller
• You need precision temperature control (±0.5°F or tighter)
• You're in a cleanroom, pharma, or regulatory-controlled environment
• You're designing a new cooling system from scratch and want a sanity check

North Slope's engineering team sizes chillers every day. Call us at (866) 826-2993 or request a quote with your numbers — we'll confirm the sizing and recommend a specific model in about 15 minutes.

Still Not Sure? Use the AI Tool

If you'd rather talk it through than do the math, our AI tool asks a few questions about your process, runs the calculations, and hands the results to our team for a formal quote. It's the fastest way to get a right-sized recommendation.