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LoadCairn / Chilled water flow

Chilled water flow (GPM) calculator.

Find the chilled-water flow a cooling load needs at its supply and return temperatures, and check the velocity in the pipe.

Inputs

Illustrative example · editable

Starts with a 100-ton cooling load, 44°F supply, 56°F return, plain-water factor, and an example pipe diameter. These are editable planning examples, not site operating data.

500 for plain water (8.33 lb/gal × 60 min/hr × 1 BTU/lb·°F). For glycol mixes, use the factor from your glycol supplier.

4.026 in is 4-inch Schedule 40 steel. Leave blank to skip velocity.

Planning estimate

Chilled-water flow requirement

Cooling load converts to flow using the entered fluid factor and water temperature difference.

Cooling load
1,200,000 BTU/hr
Equivalent load
100.00 tons
ΔT
12 °F
Flow
200.0 GPM
Metric flow
12.62 L/s
Metric flow
45.4 m³/h
Pipe velocity
5.04 ft/s
Calculation details

Estimate formula

  1. Load in BTU/hr = tons × 12,000; kW thermal × 3,412.142; or BTU/hr as entered.
  2. ΔT (°F) = return − supply.
  3. Flow (GPM) = BTU/hr ÷ (fluid factor × ΔT). Equivalent water expression: GPM = tons × 24 ÷ ΔT.
  4. Flow (L/s) = GPM × 0.0630902.
  5. Flow (m³/h) = GPM × 0.2271247.
  6. If pipe ID is entered, velocity (ft/s) = 0.4085 × GPM ÷ ID².

Current calculation

  1. 100 tons × 12,000 = 1,200,000 BTU/hr
  2. 56 − 44 = 12°F ΔT
  3. 1,200,000 ÷ (500 × 12) = 200 GPM
  4. 100 tons × 24 ÷ 12 = 200 GPM (water expression)
  5. 200 × 0.0630902 = 12.62 L/s; 200 × 0.2271247 = 45.4 m³/h
  6. 0.4085 × 200 ÷ 4.026² = 5.04 ft/s

Assumptions

  • Sensible load carried by the water loop at steady state.
  • The 500 factor assumes water near 60°F and changes slightly with temperature and more with glycol.
  • Velocity uses the pipe's inside diameter and a full pipe.

Sources

  • ASHRAE Handbook—Fundamentals (hydronic heat-transfer relationship q = 500 × GPM × ΔT)
  • ASHRAE Handbook—HVAC Systems and Equipment, hydronic heating and cooling chapter

Planning estimate only. Pump selection, pipe sizing and balancing need a mechanical engineer's design and manufacturer data.

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LoadCairn / Chilled-water guide

Chilled-water flow for data centers: from cooling load to pipe velocity.

Cooling planners use the relationship between heat load and water temperature difference to estimate how much chilled water must circulate. This guide explains the formulas behind LoadCairn's calculator and how pipe diameter changes the resulting velocity.

Open the chilled-water flow calculator

Model at a glance

Hydronic planning only

ΔT = return temperature − supply temperature

GPM = BTU/hr ÷ (fluid factor × ΔT)

Velocity = 0.4085 × GPM ÷ pipe diameter²

01 / The relationship

More heat or less ΔT means more flow

Chilled water carries heat away from a data center load. For a fixed fluid factor, a larger cooling load requires more gallons per minute. For the same load, a smaller supply-to-return temperature difference also requires more flow because each gallon carries less heat.

The calculator defines ΔT as return temperature minus supply temperature. That makes a warmer return a positive temperature difference. If the return is equal to or colder than the supply, the input does not describe a valid cooling rise and the calculator rejects it.

At the same GPM, a smaller pipe diameter makes velocity rise. Flow tells you how much water moves; diameter determines how quickly that water moves through the pipe.

02 / Formula

Normalize the load, then solve for GPM

LoadCairn keeps the unit conversion visible so a planner can inspect the same arithmetic used in the live calculator.

Load (BTU/hr) = tons × 12,000

Load (BTU/hr) = thermal kW × 3,412.142

ΔT (°F) = return temperature − supply temperature

Flow (GPM) = BTU/hr ÷ (fluid factor × ΔT)

A BTU/hr input is already in the calculation unit and passes through unchanged. The default fluid factor is 500 BTU/(GPM × °F), representing plain water under the calculator's standard planning assumption. A different positive factor can be entered for another fluid or condition.

03 / Worked example

Follow the calculator's starting sample

The values below match the current calculator sample: 100 tons, 44°F supply, 56°F return, a 500 fluid factor, and a 4.026-inch pipe diameter.

  1. Load = 100 tons × 12,000 = 1,200,000 BTU/hr
  2. ΔT = 56°F − 44°F = 12°F
  3. GPM = 1,200,000 ÷ (500 × 12) = 200 GPM
  4. Velocity = 0.4085 × 200 ÷ 4.026² = 5.04 ft/s

The same sample is also about 12.62 L/s or 45.4 m³/h. Change the load, temperatures, fluid factor, or optional pipe diameter in the calculator to trace another scenario.

04 / Inputs

What the calculator needs

Cooling load
Enter the load as tons, thermal kW, or BTU/hr. The calculator converts tons and kW to BTU/hr before calculating flow.
Supply and return temperature
Enter the water temperature leaving the load and the temperature returning from it. Return temperature must be warmer than supply.
Fluid factor
The default is 500 BTU/(GPM × °F), the common plain-water planning factor. Enter another positive factor when the fluid or conditions call for it.
Pipe diameter
This input is optional. Leave it blank for flow only, or enter the pipe inside diameter to estimate velocity.

05 / Planning boundary

A transparent estimate, not a final design

Use the result to understand the load-and-ΔT relationship and to screen a scenario. A mechanical engineer still needs to review pipe sizing, pump head, balancing, controls, redundancy, glycol or water properties, and manufacturer data.

06 / Next step

Run the flow estimate with your scenario

Enter your own load, temperatures, fluid factor, and optional pipe diameter to see the live calculation trace.

Try the calculator

Questions / Engineering estimates

Chilled-water flow questions

How does the chilled-water flow calculator determine GPM?

It calculates the temperature rise as return temperature minus supply temperature, converts the entered load to BTU/hr when needed, and divides that load by the fluid factor multiplied by ΔT. With the default factor of 500 BTU/(GPM × °F), the result is a simplified planning flow for the values entered.

Why must the return water be warmer than the supply water?

The calculator models heat carried away as a positive return-minus-supply temperature difference. If return temperature is equal to or below supply, the entered pair does not provide the positive ΔT required by this calculation, so the form flags it instead of reporting a flow estimate.

When should I change the default fluid factor?

The default 500 factor is the calculator’s plain-water planning assumption. Use a different documented factor when the fluid or operating conditions require it. The page does not calculate fluid properties from concentration or temperature, so use project-specific engineering data rather than guessing a correction.

Does the pipe diameter input select a pipe size?

No. The optional inside diameter is used to estimate velocity for the entered flow. The result does not choose a nominal pipe size or check pressure drop, fittings, valves, pump head, noise, erosion limits, or applicable project criteria.

How is a cooling ton converted to BTU/hr?

The calculator uses 12,000 BTU/hr per refrigeration ton before solving for flow. For example, 100 tons becomes 1,200,000 BTU/hr. That is a unit conversion within the calculator’s load calculation; it does not add a reserve or determine a chiller selection.

Can I use the result to size a pump or complete a hydronic design?

No. The result is a flow estimate from load, temperature difference, and the selected factor. Pump selection and hydronic design also require system pressure losses, equipment data, fluid properties, control strategy, operating modes, and project review.

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Estimates for planning only, not a substitute for a licensed engineer’s design or manufacturer data.