Calculate Gpm From Btu

GPM Formula:

\[ GPM = \frac{BTU/hr}{\Delta T \times 500} \]

GPM (gallons per minute):

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1. What is the GPM Calculation?

The GPM (gallons per minute) calculation determines the flow rate of water based on heating load and temperature difference. This formula is essential for HVAC system design, hydronic heating systems, and industrial process calculations.

2. How Does the Calculator Work?

The calculator uses the GPM formula:

\[ GPM = \frac{BTU/hr}{\Delta T \times 500} \]

Where:

\( GPM \) — Flow rate in gallons per minute
\( BTU/hr \) — Heating load in BTU per hour
\( \Delta T \) — Temperature difference in degrees Fahrenheit
\( 500 \) — Constant factor (BTU/hr per GPM per °F)

Explanation: The constant 500 represents the heat capacity of water, where 1 GPM of water flowing with a 1°F temperature difference transfers approximately 500 BTU/hour.

3. Importance of GPM Calculation

Details: Accurate GPM calculation is crucial for proper sizing of pumps, pipes, and heat exchangers in heating and cooling systems. It ensures efficient energy transfer and prevents system inefficiencies.

4. Using the Calculator

Tips: Enter BTU/hr (heating load) and ΔT (temperature difference) in positive values. Both inputs must be greater than zero for accurate calculation.

5. Frequently Asked Questions (FAQ)

Q1: What is the significance of the constant 500?
A: The constant 500 comes from the specific heat of water (1 BTU/lb-°F) and the weight of water (8.34 lb/gal), multiplied by 60 minutes: 8.34 × 60 ≈ 500.

Q2: Can this formula be used for other fluids?
A: This specific formula is designed for water. For other fluids, you would need to adjust the constant based on the fluid's specific heat and density.

Q3: What is a typical ΔT value for HVAC systems?
A: Typical ΔT values range from 10°F to 20°F for chilled water systems and 20°F to 40°F for heating systems, depending on system design.

Q4: How does GPM affect pump sizing?
A: Higher GPM requires larger pumps with greater horsepower to overcome increased friction losses in the piping system.

Q5: What if I need to calculate for metric units?
A: For metric units (L/s and kW), use the formula: Flow rate (L/s) = Power (kW) / (4.18 × ΔT°C), where 4.18 is the specific heat of water in kJ/kg-°C.