Chilled Water Pipe Sizing & Condenser Water Pipe Sizing Calculation Sheet

Upper limit of water velocity shall be 1.2m/s for diameter 50mm and smaller pipes.

Upper limit of pressure drop shall be 400Pa/m for diameter over 50mm pipes.

The velocity should not exceed 4.6m/s in any case.

Enter the Water Flowrate (L/s) or (L/min) in yellow color highlight.

Here is ASHRAE Pipe Sizing. If you use this calculation sheet, you can select the pipe size in a second.

Chilled Water Pipe Sizing & Condenser Water Pipe Sizing according to ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) standards involves several considerations to ensure efficient operation, appropriate pressure drop, and energy conservation. Here’s a step-by-step guide to sizing these pipes.

1. Determine System Requirements

Chilled Water System:

• Cooling Load (Q): The total cooling load in BTU/hr or kW.
• Flow Rate (GPM or L/s): Based on the cooling load and the temperature difference (ΔT) of the chilled water supply and return.
  • Flow Rate (GPM)=[Cooling Load (BTU/hr)]/[500×Δ𝑇(°F)]
  • Flow Rate (GPM)=500×ΔT(°F)Cooling Load (BTU/hr)​Flow Rate (L/s)=Cooling Load (kW)4.186×Δ𝑇(°C)
  • Flow Rate (L/s)=4.186×ΔT(°C)Cooling Load (kW)​

Condenser Water System:

• Heat Rejection Load: Usually higher than the cooling load due to additional heat from the compressor.
• Flow Rate: Similarly calculated using the heat rejection load and temperature difference.Flow Rate (GPM)=Heat Rejection Load (BTU/hr)500×Δ𝑇(°F)Flow Rate (GPM)=500×ΔT(°F)Heat Rejection Load (BTU/hr)​Flow Rate (L/s)=Heat Rejection Load (kW)4.186×Δ𝑇(°C)Flow Rate (L/s)=4.186×ΔT(°C)Heat Rejection Load (kW)​

2. Calculate Pipe Size

Pipe Sizing Criteria:

• Velocity: Maintain a velocity within the recommended range to avoid erosion, noise, and high-pressure drop. For chilled water, ASHRAE recommends 2-12 ft/s (0.6-3.7 m/s). For condenser water, 3-10 ft/s (0.9-3.0 m/s) is typical.
• Pressure Drop: Keep the pressure drop within acceptable limits, usually around 2-4 ft of water per 100 ft of pipe (20-40 kPa per 100 m).

3. Using Pipe Sizing Charts or Software

Use ASHRAE charts, tables, or specialized software to find the appropriate pipe diameter. Here’s how to do it manually using a typical procedure:

1. Select Initial Diameter:
   • Estimate an initial pipe diameter based on flow rate.
2. Calculate Velocity:
   • For a given pipe diameter 𝐷D and flow rate 𝑄Q:

Velocity(𝑉)=4×𝑄𝜋×𝐷2Velocity(V)=π×D24×Q​

Ensure the velocity is within recommended limits.

3. Calculate Pressure Drop:
   • Use the Darcy-Weisbach equation or the Hazen-Williams formula to calculate the pressure drop per unit length.

Δ𝑃=𝑓(𝐿𝐷)(𝜌𝑉22)ΔP=f(DL​)(2ρV2​)

Where:

• Δ𝑃ΔP is the pressure drop.
• 𝑓f is the friction factor (can be obtained from the Moody chart).
• 𝐿L is the length of the pipe.
• 𝐷D is the diameter.
• 𝜌ρ is the density of the fluid.
• 𝑉V is the velocity.

4. Iterate as Necessary:
   • Adjust the pipe diameter to balance between acceptable velocity and pressure drop.

4. Verify with ASHRAE Standards and Guidelines

Refer to ASHRAE Handbook—Fundamentals, HVAC Systems and Equipment, and other relevant standards for detailed recommendations and verification.

Example Calculation

Given:

• Cooling load: 500,000 BTU/hr
• Temperature difference (ΔT): 10°F for chilled water, 15°F for condenser water.

Calculate Flow Rate:

Chilled Water:

Flow Rate (GPM)=500,000500×10=100 GPMFlow Rate (GPM)=500×10500,000​=100 GPM

Condenser Water: Assume heat rejection load is 600,000 BTU/hr.

Flow Rate (GPM)=600,000500×15=80 GPMFlow Rate (GPM)=500×15600,000​=80 GPM

Select Initial Pipe Diameter and Calculate Velocity:

For chilled water with 100 GPM flow:

• Assume initial diameter of 3 inches.

Velocity=4×100𝜋×(3/12)2=7.07 ft/sVelocity=π×(3/12)24×100​=7.07 ft/s

This velocity is within the acceptable range.

Calculate Pressure Drop:

Using Hazen-Williams formula (simplified for demonstration):

Δ𝑃=0.002083×𝐿×𝑄1.85𝐶1.85×𝐷4.8655ΔP=0.002083×C1.85×D4.8655L×Q1.85​

Where:

• 𝐿L = length of the pipe.
• 𝑄Q = flow rate (GPM).
• 𝐶C = Hazen-Williams coefficient (typically 140 for clean water pipes).
• 𝐷D = diameter (inches).

For detailed calculations, refer to specific pipe sizing software or ASHRAE guidelines for precise results.

Conclusion

Following ASHRAE standards ensures proper sizing of chilled water and condenser water pipes, optimizing system performance and energy efficiency. Proper pipe sizing involves balancing flow rate, velocity, and pressure drop, and can be facilitated using charts, tables, and software tools provided by ASHRAE and other engineering resources.

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