Static Pressure Regain at Fan Discharge Calculator

1. What is a Static Pressure Regain at Fan Discharge Calculator?

Definition: This calculator computes the static pressure regain (\( \text{Regain} \)) at a fan discharge, representing the increase in static pressure when a fan discharges into a duct with reduced velocity, assuming 75% efficiency.

Purpose: It is used in HVAC design to quantify the static pressure recovered due to velocity reduction, aiding in fan selection and duct system efficiency.

2. How Does the Calculator Work?

The calculator uses the following formula for static pressure regain:

Static Pressure Regain: \[ \text{Regain} = 0.75 (P_v \text{ fan} - P_v \text{ duct}) \]

Where:

\( \text{Regain} \): Static pressure regain (in. of water, Pa)
\( P_v \text{ fan} \): Velocity pressure at fan outlet (in. of water, Pa)
\( P_v \text{ duct} \): Velocity pressure in duct (in. of water, Pa)

Unit Conversions:

Velocity Pressures (\( P_v \text{ fan} \), \( P_v \text{ duct} \)): in. of water, Pa (1 Pa = 0.00401463 in. of water)
Static Pressure Regain (\( \text{Regain} \)): in. of water, Pa (1 in. of water = 248.84 Pa)

Steps:

Enter the velocity pressure at fan outlet (\( P_v \text{ fan} \)) and velocity pressure in duct (\( P_v \text{ duct} \)), and select their units.
Convert \( P_v \text{ fan} \) and \( P_v \text{ duct} \) to in. of water.
Calculate the static pressure regain using the formula.
Convert the result to the selected unit (in. of water or Pa).
Display the result with 5 decimal places, or in scientific notation if the value is greater than 10,000 or less than 0.00001.

3. Importance of Static Pressure Regain Calculation

Calculating static pressure regain is crucial for:

HVAC System Design: Accounts for pressure recovery at fan discharge, optimizing fan sizing and duct design.
Energy Efficiency: Maximizes system efficiency by leveraging velocity reduction to regain static pressure.
System Performance: Ensures proper air distribution and pressure balance in duct systems.

4. Using the Calculator

Examples:

Example 1: For \( P_v \text{ fan} = 0.36 \, \text{in. of water} \), \( P_v \text{ duct} = 0.16 \, \text{in. of water} \), static pressure regain in in. of water:
- \( \text{Regain} = 0.75 \times (0.36 - 0.16) = 0.15 \)
- Since 0.15 < 10000 and > 0.00001, display with 5 decimal places: \( 0.15000 \)
Example 2: For \( P_v \text{ fan} = 100 \, \text{Pa} \), \( P_v \text{ duct} = 50 \, \text{Pa} \), static pressure regain in Pa:
- Convert: \( P_v \text{ fan} = 100 \times 0.00401463 \approx 0.401463 \, \text{in. of water} \)
- \( P_v \text{ duct} = 50 \times 0.00401463 \approx 0.200732 \, \text{in. of water} \)
- \( \text{Regain} = 0.75 \times (0.401463 - 0.200732) \approx 0.150546 \, \text{in. of water} \)
- Convert to Pa: \( 0.150546 \times 248.84 \approx 37.462 \)
- Since 37.462 < 10000 and > 0.00001, display with 5 decimal places: \( 37.46200 \)
Example 3: For \( P_v \text{ fan} = 0.5 \, \text{in. of water} \), \( P_v \text{ duct} = 0.2 \, \text{in. of water} \), static pressure regain in in. of water:
- \( \text{Regain} = 0.75 \times (0.5 - 0.2) = 0.225 \)
- Since 0.225 < 10000 and > 0.00001, display with 5 decimal places: \( 0.22500 \)

5. Frequently Asked Questions (FAQ)

Q: What does static pressure regain represent?
A: Static pressure regain (\( \text{Regain} \)) quantifies the increase in static pressure when a fan discharges into a duct with reduced velocity, improving system efficiency.

Q: How can I determine the velocity pressures?
A: Velocity pressure at fan outlet (\( P_v \text{ fan} \)) and in duct (\( P_v \text{ duct} \)) are calculated from air velocity and density using the formula \( P_v = \rho \left( \frac{V}{4005} \right)^2 \), or obtained from measurements or HVAC design software.

Q: Why is static pressure regain important in HVAC design?
A: It helps engineers account for pressure recovery, ensuring accurate fan sizing and efficient duct system performance.