Glycol Calculator for Heat Transfer: How to Use It

A glycol calculator for heat transfer works out the required flow rate, pipe size, or pressure drop for a water-glycol mixture in a heating or cooling loop. You enter the heat load, fluid temperature, and glycol concentration, and you get back the flow rate and recommended pipe diameter. This page explains the formulas behind the calculation and how to apply them to real systems.

Formula

The basic heat transfer formula is:

Q = � × c_p × ΔT

Where:

• Q = heat load (kW)
• á¹� = mass flow rate (kg/s)
• c_p = specific heat capacity of the fluid (kJ/kg·°C)
• ΔT = temperature difference between supply and return (°C)

For water, c_p is about 4.18 kJ/kg·°C. For a glycol mixture, c_p is lower. A 30% propylene glycol mixture at 20°C has a c_p of roughly 3.8 kJ/kg·°C. The calculator above uses a linear approximation: c_p = 4.18 × (1 – concentration/100). This is accurate enough for most design work, but for exact values consult the glycol manufacturer’s data sheet.

Worked Example

Using the default values: heat load = 100 kW, ΔT = 10°C, glycol concentration = 30%, average temperature = 20°C.

1. Calculate the effective specific heat: c_p = 4.18 × (1 – 0.30) = 2.926 kJ/kg·°C
2. Convert heat load to watts: 100 kW = 100,000 W
3. Mass flow rate: � = Q / (c_p × ΔT) = 100,000 / (2,926 × 10) = 3.42 kg/s
4. Convert to m³/h (assuming density ≈ 1,000 kg/m³): 3.42 kg/s × 3,600 / 1,000 = 12.3 m³/h

The calculator gives 12.3 m³/h, matching this result.

Why Glycol Changes the Numbers

Glycol (ethylene or propylene) is added to water to lower the freezing point and raise the boiling point. But it also changes the fluid’s properties:

• Specific heat capacity decreases with higher glycol concentration, so more flow is needed to carry the same heat.
• Viscosity increases, especially at low temperatures, raising pressure drop and pump energy.
• Density changes slightly, affecting mass flow calculations.

For example, a 50% ethylene glycol mixture at 0°C is about 5 times more viscous than water at the same temperature. This means a pipe that works for water may be undersized for glycol.

How to Choose the Right Glycol Concentration

The concentration you need depends on the coldest expected ambient temperature and the required freeze protection. Common guidelines:

• For freeze protection down to -10°C: 25-30% glycol
• For -20°C: 35-40%
• For -30°C: 45-50%

But concentration also affects heat transfer. Higher glycol means lower c_p and higher viscosity, so you may need a larger pump or bigger pipes. Always balance freeze protection with system efficiency.

Common Mistakes in Glycol Calculations

Here are the most frequent errors people make:

• Using water’s specific heat for a glycol mixture – this underestimates flow rate by up to 30%.
• Ignoring viscosity – pressure drop can be much higher than water, so the pump may be undersized.
• Mixing up percent by volume vs. percent by weight – glycol is often sold by volume, but some formulas use weight. Check the label.
• Forgetting that c_p changes with temperature – at low temperatures, the error grows.

Limitations of the Simple Formula

The formula Q = � × c_p × ΔT assumes a constant specific heat and no heat loss. In reality, c_p varies with temperature and concentration, and pipe heat loss can be significant for long runs. For precise design, use engineering software or manufacturer charts. Also, the calculator gives flow rate but not pressure drop – you need the pipe length, fittings, and fluid viscosity to calculate that.

Safety and Maintenance Notes

Glycol is toxic (ethylene glycol) or less toxic but still not food-grade (propylene glycol). Always handle it with gloves and avoid spills. Check the fluid’s freeze point and pH annually, as glycol degrades over time and becomes acidic. Use a corrosion inhibitor if the system contains metals other than stainless steel. Never mix ethylene and propylene glycol in the same system.

Frequently Asked Questions

What is the difference between ethylene and propylene glycol?

Ethylene glycol has better heat transfer properties and is cheaper, but it is highly toxic. Propylene glycol is safer for food processing and potable water systems, but it is more viscous and has slightly lower heat capacity. For most industrial loops, ethylene glycol is common; for residential or food-related systems, propylene is preferred.

Can I use the same pump for water and glycol?

Not without checking. Glycol’s higher viscosity increases friction losses, so the pump may not deliver the required flow. You may need a larger pump or a different impeller. Always recalculate the system curve with glycol properties.

How do I convert flow rate from m³/h to GPM?

Multiply by 4.4029. For example, 12.3 m³/h × 4.4029 = 54.2 GPM. Many designers use GPM in the US and m³/h elsewhere, so unit conversion is a common source of error.

What is the ideal velocity in a glycol pipe?

For water, 1-3 m/s is typical. For glycol, because of higher viscosity, keep velocity below 2 m/s to limit pressure drop and noise. The calculator shows velocity for a 2-inch pipe, but you should adjust pipe size to stay in the recommended range.

Does the calculator account for altitude or pressure?

No. The boiling point of glycol changes with pressure, and at high altitudes the freezing point is slightly different. For most systems at sea level, the error is small. If your system operates above 2,000 meters, consult a specialist.

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in load | Heat load | kW | 100 | 0
in dt | Temperature difference | °C | 10 | 0
in conc | Glycol concentration | % | 30 | 0
in temp | Average fluid temperature | °C | 20 | 0
out flow | Required flow rate | m³/h | (load * 3600) / (dt * 4.18 * (1 - conc / 100)) | 2
out vel | Velocity in 2" pipe | m/s | (flow / 3600) / (pi * (0.0525 / 2)^2) | 2