Free Chilled Water Loop Sizing Calculator
Size central chilled water loops. Calculate cooling flow rate (GPM) from chiller tonnage, temperature differential (ΔT), pipe diameter, and friction loss.
❄️ Chiller Capacity & Temperature Delta
Flow Metric: 2.40 GPM per Ton of Cooling
• Frictional Head Loss: 2.1 ft H2O / 100 ft
• Pressure Drop: 0.91 psi / 100 ft
The "Low ΔT Syndrome" Danger: If cooling coils degrade or 2-way control valves bypass water, return temperature drops (e.g. from 54°F down to 48°F). This halves chiller cooling capacity while forcing pumps to run at 100%, causing massive plant inefficiency.
Recommended Tools & Equipment
Tested hardware and components for high reliability
The Universal Hydronic Flow Equation
In HVAC hydronic systems, the relationship between thermal heat transfer rate (BTU/hr), flow rate (GPM), and temperature difference ($Delta T$) is governed by the heat transfer equation:
GPM = Q / (500 × ΔT) = (Tons × 12,000) / (500 × ΔT) = (Tons × 24) / ΔT
The constant $500$ is derived from water density ($8.33, ext{lbs/gal}$) multiplied by 60 minutes/hr and specific heat ($1.0, ext{BTU/lb}cdot^circ ext{F}$). When glycol solutions are introduced, fluid density increases while specific heat decreases, lowering the constant to ~475-485.
Standard Pipe Sizing Guidelines (ASHRAE 90.1)
- Pipes 2" and Smaller: Sized based on head loss not exceeding 4.0 ft of water per 100 ft of pipe length.
- Pipes Larger than 2": Sized based on maintaining maximum water velocity below 8.5 feet per second (FPS) to prevent inner wall erosion-corrosion and noise transmission through building structures.
Frequently Asked Questions
Why is a 10°F Delta T standard for chillers?
Standard ARI 550/590 rating conditions specify entering water at 54°F and leaving chilled water at 44°F (10°F Delta T), which yields 2.4 GPM per cooling ton. Modern chilled water plants frequently design for 12°F to 16°F Delta T to reduce pump horsepower and pipe diameter by 20% to 35%.
How does propylene glycol affect pump flow rate and head loss?
A 30% glycol solution reduces specific heat capacity from 1.0 down to ~0.93 BTU/lb°F. To achieve identical cooling, pump flow rate (GPM) must increase by approximately 7% to 8%, and friction head loss rises by 15% to 25% due to higher fluid viscosity.
What is the primary difference between Primary-Only and Primary-Secondary pumping?
Primary-Secondary systems decouple the constant-flow chiller evaporator loop from the variable-flow building distribution loop using a common neutral bridge pipe. Modern plants use Primary-Only Variable Flow (VPF), varying flow directly through the chiller evaporators to eliminate secondary pump hardware.