Glycol Flow Rate Calculator
Calculate the required GPM for hydronic systems based on heat load and fluid properties.
1 Ton = 12,000 BTU/hr
Difference between Supply and Return temps
Water = 1.0, 30% Propylene Glycol ≈ 0.90
Water = 1.0, 30% Propylene Glycol ≈ 1.035
Required Flow Rate
0.00 GPM
Mass Flow: 0 lbs/hr
Understanding Glycol Flow Rate Calculations
In hydronic HVAC systems, chillers, and process cooling applications, calculating the correct flow rate is critical for ensuring efficient heat transfer. When water is mixed with glycol (either Ethylene or Propylene) for freeze protection, the physical properties of the heat transfer fluid change, requiring adjustments to the standard flow equations.
The Physics Behind the Calculation
The calculation determines how much fluid must circulate to move a specific amount of thermal energy. The standard hydronic formula is derived from the thermodynamics equation:
Q = 500 × GPM × SG × Cp × ΔT
Where:
- Q (Heat Load): The amount of heat energy to be added or removed, measured in BTU/hr.
- GPM (Flow Rate): Gallons Per Minute.
- 500: A conversion constant (derived from 8.33 lbs/gal × 60 min/hr).
- SG (Specific Gravity): The density of the fluid relative to water. Glycol is denser than water (SG > 1.0).
- Cp (Specific Heat): The amount of heat required to raise the fluid temperature by 1°F. Glycol holds less heat than pure water (Cp < 1.0).
- ΔT (Delta T): The design temperature drop or rise across the heat exchanger (in °F).
Why Glycol Affects Flow Rate
Pure water has a Specific Heat ($C_p$) of 1.0 and a Specific Gravity ($SG$) of 1.0. This simplifies the equation to $Q = 500 \times GPM \times \Delta T$.
However, when you add glycol:
- Specific Heat Drops: The fluid carries less heat per pound. This requires a higher flow rate to move the same amount of energy.
- Specific Gravity Increases: The fluid becomes heavier per gallon. This slightly offsets the drop in specific heat.
- Viscosity Increases: While not part of the thermal equation, higher viscosity increases pump head pressure (pump work), which is a crucial consideration for pump sizing after determining the GPM.
Typical Values for Estimation
If you do not have the exact manufacturer data sheet for your glycol mixture, the following approximations are commonly used for 30% Propylene Glycol at standard operating temperatures:
- Specific Heat ($C_p$): ~0.90 BTU/lb·°F
- Specific Gravity ($SG$): ~1.035
Note: As temperature decreases, viscosity increases significantly, but specific heat and gravity remain relatively stable for flow rate sizing purposes.