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Chilled Water Pipe GPM Calculator

Chilled Water GPM Formula

1. What is the Chilled Water Pipe GPM Calculator?

Definition: The Chilled Water Pipe GPM Calculator determines the flow rate in Gallons Per Minute (GPM) required for a Fan Coil Unit (FCU) or Air Handling Unit (AHU) based on its tonnage and the temperature difference between chilled water supply (CHWS) and return (CHWR).

Purpose: This tool helps HVAC engineers and technicians size piping and pumps for efficient chilled water systems, supporting temperature inputs in °F or °C.

2. How Does the Calculator Work?

The calculator uses the following formula:

\( GPM = \frac{TR \times 24}{\Delta T} \)

Where:

  • \( GPM \): Flow rate in Gallons Per Minute;
  • \( TR \): Tonnage of FCU or AHU;
  • \( \Delta T \): Temperature difference in °F (\( CHWR - CHWS \));
  • \( 24 \): Constant factor (based on 1 ton = 12,000 BTU/hr and water properties).

Steps:

  • Enter the tonnage of the FCU or AHU.
  • Enter the CHWR temperature and select its unit (°F or °C).
  • Enter the CHWS temperature and select its unit (°F or °C).
  • Convert temperatures to °F if entered in °C: \( T_{\text{°F}} = T_{\text{°C}} \times 1.8 + 32 \).
  • Calculate temperature difference: \( \Delta T = CHWR_{\text{°F}} - CHWS_{\text{°F}} \).
  • Calculate GPM using the formula.
  • Display the result to four decimal places, using scientific notation for values less than 0.0001.

3. Importance of GPM Calculation

Calculating GPM is essential for:

  • System Design: Ensures proper water flow for cooling capacity.
  • Efficiency: Optimizes pump and pipe sizing to reduce energy consumption.
  • Maintenance: Helps diagnose flow issues in chilled water systems.

4. Using the Calculator

Example 1 (in °F): Calculate GPM for an FCU with 5 TR, CHWR at 55°F, and CHWS at 45°F:

  • Input: Tonnage: 5; CHWR: 55°F; CHWS: 45°F.
  • \( \Delta T = 55 - 45 = 10^\circ F \).
  • \( GPM = \frac{5 \times 24}{10} = 12 \).
  • Result: GPM = 12.0000.

Example 2 (in °C): Calculate GPM for an AHU with 10 TR, CHWR at 12.8°C, and CHWS at 7.2°C:

  • Input: Tonnage: 10; CHWR: 12.8°C; CHWS: 7.2°C.
  • CHWR in °F: \( 12.8 \times 1.8 + 32 = 55^\circ F \).
  • CHWS in °F: \( 7.2 \times 1.8 + 32 = 45^\circ F \).
  • \( \Delta T = 55 - 45 = 10^\circ F \).
  • \( GPM = \frac{10 \times 24}{10} = 24 \).
  • Result: GPM = 24.0000.

5. Frequently Asked Questions (FAQ)

Q: What is GPM in a chilled water system?
A: GPM (Gallons Per Minute) is the flow rate of chilled water required to meet the cooling load of an FCU or AHU.

Q: Why use 24 in the formula?
A: The factor 24 comes from 1 ton of cooling (12,000 BTU/hr) divided by the specific heat and density of water (500 GPM per ton per °F).

Q: Can I use Celsius?
A: Yes, the calculator converts °C to °F internally using \( T_{\text{°F}} = T_{\text{°C}} \times 1.8 + 32 \).

Reference: https://www.youtube.com/watch?v=8eRqacWnY4Y&ab_channel=HVACSIMPLIFIED

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