🌡️ Chilled Water System Design
Comprehensive HVAC Design Guide with Equations, Standards & Interactive Calculators
Chilled Water System Fundamentals
A chilled water system is a centralized cooling system that uses water as the heat transfer medium. The chiller removes heat from water, and this chilled water is then circulated through the building to cool the air.
System Components
- Chiller: Removes heat from water (typically 6-7°C supply)
- Cooling Tower: Rejects heat to atmosphere
- Pumps: Primary, secondary, and condenser water pumps
- Air Handling Units (AHUs): Transfer cooling to air
- Fan Coil Units (FCUs): Terminal cooling units
- Piping Distribution: Supply and return piping network
- Expansion Tank: Accommodates thermal expansion
- Controls: Building automation and control systems
Design Temperatures (ASHRAE Standards)
| Parameter | Typical Value | Range |
|---|---|---|
| Chilled Water Supply (CHW Supply) | 6-7°C (42-44°F) | 4-10°C |
| Chilled Water Return (CHW Return) | 12-14°C (54-57°F) | 10-16°C |
| ΔT (Temperature Difference) | 6°C (10.8°F) | 4-8°C |
| Condenser Water Supply | 29-32°C (85-90°F) | 27-35°C |
| Condenser Water Return | 35-38°C (95-100°F) | 32-40°C |
System Configurations
1. Primary-Only System
Single pump configuration with variable flow through chillers. Suitable for smaller systems (<500 TR).
2. Primary-Secondary System
Most common configuration with constant flow through chillers (primary) and variable flow to loads (secondary). Provides chiller protection and flexible operation.
3. Variable Primary Flow (VPF)
Modern approach with variable flow through chillers. Requires minimum flow protection. Higher efficiency potential.
Note: ASHRAE Standard 90.1 mandates variable flow systems for energy efficiency in most commercial applications.
Fundamental Design Equations
1. Cooling Load Calculation
Heat Transfer (Sensible Cooling)
Q = ṁ × Cp × ΔT
Q = Cooling load (kW or BTU/hr)
ṁ = Mass flow rate (kg/s or lb/hr)
Cp = Specific heat of water = 4.186 kJ/(kg·K) or 1 BTU/(lb·°F)
ΔT = Temperature difference (°C or °F)
Simplified Form (SI Units)
Q (kW) = V̇ (L/s) × ΔT (°C) × 4.186
Or approximately: Q ≈ 4.2 × V̇ × ΔT
Simplified Form (Imperial Units)
Q (tons) = GPM × ΔT (°F) / 24
Or: Q (BTU/hr) = 500 × GPM × ΔT
2. Flow Rate Calculations
Volumetric Flow Rate (SI)
V̇ (L/s) = Q (kW) / (4.186 × ΔT)
For standard ΔT = 6°C:
V̇ (L/s) = Q (kW) / 25.1 ≈ Q / 25
Volumetric Flow Rate (Imperial)
GPM = (Tons × 24) / ΔT
For standard ΔT = 10°F:
GPM = Tons × 2.4
3. Pump Head Calculations
Total Dynamic Head (TDH)
H = h_friction + h_static + h_equipment + h_control
h_friction = Friction losses in piping
h_static = Elevation difference
h_equipment = Losses through equipment (coils, chillers)
h_control = Control valve authority
Darcy-Weisbach Equation (Friction Loss)
h_f = f × (L/D) × (v²/2g)
f = Friction factor (0.015-0.025 for turbulent flow)
L = Pipe length (m or ft)
D = Pipe diameter (m or ft)
v = Flow velocity (m/s or ft/s)
g = Gravitational constant (9.81 m/s² or 32.2 ft/s²)
4. Pump Power
Pump Power (SI Units)
P (kW) = (ρ × g × V̇ × H) / (η × 1000)
Or simplified:
P (kW) = (V̇ (L/s) × H (m)) / (102 × η)
ρ = Water density ≈ 1000 kg/m³
g = 9.81 m/s²
η = Pump efficiency (typically 0.70-0.85)
Pump Power (Imperial Units)
BHP = (GPM × H (ft)) / (3960 × η)
Where 3960 is a conversion constant
5. Pipe Sizing
Velocity Method
v = (4 × V̇) / (π × D²)
Recommended velocities:
– Chilled water mains: 1.5 – 3.0 m/s (5-10 ft/s)
– Branches: 1.0 – 2.4 m/s (3-8 ft/s)
– Risers: max 1.5 m/s (5 ft/s) to prevent noise
6. Expansion Tank Sizing
ASHRAE Method
V_tank = (V_system × e × P_a) / (P_a – P_f)
V_system = Total system water volume
e = Coefficient of thermal expansion ≈ 0.0004 per °C
P_a = Absolute fill pressure
P_f = Absolute final pressure
7. Chiller Performance
Coefficient of Performance (COP)
COP = Q_cooling / W_input
Typical values:
– Air-cooled chillers: 2.5 – 3.5
– Water-cooled chillers: 4.5 – 6.5
Energy Efficiency Ratio (EER)
EER (BTU/W·h) = COP × 3.412
kW/Ton = 12 / EER
8. Cooling Tower Performance
Heat Rejection
Q_reject = Q_cooling × (1 + 1/COP)
Or: Q_reject ≈ Q_cooling × 1.25 (typical)
Approach Temperature
Approach = T_cw_leaving – T_wb_ambient
Typical approach: 3-5°C (5-9°F)
Applicable Codes & Standards
ASHRAE Standards
- ASHRAE 90.1 – Energy Standard for Buildings Mandatory requirements for chilled water systems including minimum efficiency (COP), pipe insulation, variable flow pumping, and waterside economizers for systems ≥300 tons.
- ASHRAE 15 – Safety Standard for Refrigeration Systems Requirements for refrigerant safety, machinery room ventilation, pressure relief devices, and system testing.
- ASHRAE 62.1 – Ventilation for Acceptable Indoor Air Quality Minimum ventilation rates and indoor air quality requirements affecting cooling load calculations.
- ASHRAE 55 – Thermal Environmental Conditions Comfort criteria for occupied spaces, establishing temperature and humidity setpoints.
- ASHRAE Guideline 14 – Measurement of Energy Demand Methods for measuring and verifying building energy performance.
International Building Codes
- IBC – International Building Code General building requirements including structural support for equipment and seismic considerations.
- IMC – International Mechanical Code Mechanical system design, installation, and maintenance requirements. Chapter 12 covers hydronic piping.
- IPC – International Plumbing Code Water supply and drainage requirements for HVAC systems.
- IECC – International Energy Conservation Code Energy efficiency requirements similar to ASHRAE 90.1, often adopted by local jurisdictions.
Industry Standards
- AHRI 550/590 – Performance Rating of Water Chilling Packages Standard rating conditions: 44°F leaving chilled water, 85°F entering condenser water.
- AHRI 210/240 – Performance Rating of Unitary Air-Conditioning Equipment Rating standards for air-cooled equipment including SEER, EER, and IEER.
- CTI Standards – Cooling Technology Institute Performance and certification standards for cooling towers (STD-201, ATC-105).
- HI Standards – Hydraulic Institute Centrifugal pump standards including ANSI/HI 9.6.3 for variable frequency drives.
Piping Standards
- ASME B31.9 – Building Services Piping Design requirements for building service piping including materials, pressure ratings, and supports.
- ASTM Standards Material specifications: A53 (steel pipe), B88 (copper tube), D2846 (CPVC), F2389 (PE-RT).
Key Design Requirements (ASHRAE 90.1)
Minimum Equipment Efficiency
| Equipment Type | Size Category | Min. Efficiency | Test Standard |
|---|---|---|---|
| Water-Cooled Centrifugal | <150 tons | 0.610 kW/ton | AHRI 550/590 |
| Water-Cooled Centrifugal | 150-300 tons | 0.596 kW/ton | AHRI 550/590 |
| Water-Cooled Centrifugal | ≥300 tons | 0.570 kW/ton | AHRI 550/590 |
| Air-Cooled Chiller | ≥150 tons | 9.562 EER | AHRI 550/590 |
| Water-Cooled Screw | All sizes | 0.640 kW/ton | AHRI 550/590 |
Mandatory Requirements
- Variable speed drives on all pumps ≥10 HP
- Two-way control valves on all cooling coils
- Differential pressure reset for variable flow systems
- Waterside economizer for systems ≥300 tons (climate dependent)
- Chilled water supply temperature reset based on outdoor air or load
- Pipe insulation: R-3 minimum for 1″ and smaller, R-5 for larger pipes
- Automatic shutoff for pumps when no demand exists
Design Guidelines
Diversity Factors
| Building Type | Diversity Factor |
|---|---|
| Office Buildings | 0.75 – 0.85 |
| Hotels | 0.60 – 0.75 |
| Hospitals | 0.85 – 0.95 |
| Retail | 0.80 – 0.90 |
| Schools | 0.70 – 0.80 |
Important: Diversity factors account for the fact that not all spaces reach peak load simultaneously. Apply to total connected load, not individual zone calculations.
Interactive Design Calculators
1. Cooling Load & Flow Rate Calculator
2. Pump Head Calculator
3. Pump Power Calculator
4. Pipe Sizing Calculator
5. Chiller Efficiency Calculator
System Design Considerations
Primary-Secondary System Design
Most reliable configuration for medium to large systems (500-5000 tons).
Primary Loop (Chiller Loop)
- Constant flow through chillers for stable operation
- Flow rate: Q / 25 L/s per 100 kW (2.4 GPM per ton)
- Primary pump head: Chiller evaporator + piping losses (typically 15-25m)
- Decoupler pipe connects primary and secondary loops
Secondary Loop (Building Loop)
- Variable flow to match building load
- Two-way control valves at all cooling coils
- Differential pressure sensor in most remote circuit
- DP reset based on valve position (min 50% open)
- VFD controlled pumps for energy savings
Variable Primary Flow (VPF) Design
Modern approach for high-efficiency systems.
Key Requirements
- Chiller minimum flow protection (typically 30-40% of design flow)
- Automatic bypass valve or dedicated bypass loop
- Headered chillers with individual isolation
- Lead-lag staging based on load optimization
- Lower first cost (eliminates primary pumps)
- Higher energy efficiency potential
Critical: VPF requires careful control design to prevent low flow through operating chillers. Always verify chiller manufacturer approval for variable flow operation.
Chiller Sequencing Strategies
| Load Range | Number of Operating Chillers | Notes |
|---|---|---|
| 0-40% | 1 Chiller | Base load operation |
| 40-80% | 2 Chillers | Stage on at 95% capacity |
| 80-120% | 3 Chillers | Stage on at 90% capacity |
| >120% | All + Reserve | Peak demand |
Pipe Insulation Requirements
| Pipe Size | Min. Insulation (ASHRAE 90.1) | Recommended |
|---|---|---|
| <1″ (25mm) | R-3 (0.5m²K/W) | 1″ (25mm) thickness |
| 1″-2″ (25-50mm) | R-4 (0.7m²K/W) | 1″ (25mm) thickness |
| 2″-4″ (50-100mm) | R-5 (0.9m²K/W) | 1.5″ (38mm) thickness |
| 4″-8″ (100-200mm) | R-6 (1.1m²K/W) | 2″ (50mm) thickness |
| >8″ (200mm) | R-8 (1.4m²K/W) | 2.5″ (65mm) thickness |
Energy Conservation Measures
- Chilled Water Reset: Increase supply temperature during low loads (saves 1-2% per °F increase)
- Condenser Water Reset: Lower condenser water temp when possible (1% energy savings per °F)
- Waterside Economizer: Free cooling when outdoor conditions permit (required ≥300 tons)
- Variable Speed Cooling Towers: Match fan speed to load and wet-bulb
- Heat Recovery: Use rejected heat for reheat or domestic hot water
- Thermal Storage: Shift cooling production to off-peak hours
- VFD Pumps: Reduce pumping energy at part load (energy ∝ flow³)
Balancing and Commissioning
Per ASHRAE Guideline 1.1 and Standard 202:
- Verify design flow rates at all coils (±10% tolerance)
- Test and balance all pumps to design conditions
- Verify control valve authority (25-50% of system ΔP)
- Commission all control sequences and interlocks
- Measure chiller efficiency at multiple loads (25%, 50%, 75%, 100%)
- Verify waterside economizer operation and switchover
- Document system performance and create O&M manuals