Variable Flow System Design for HVAC Engineers

Variable Flow System Design for HVAC Engineers

Variable flow systems modulate airflow or water flow to match actual loads, reducing fan and pump energy by 30-60% compared to constant-volume systems. Proper design requires understanding minimum flow limits, diversity factors, and control strategies.

Fan Energy Savings

Fan affinity laws:

$$\frac{CFM_2}{CFM_1} = \frac{RPM_2}{RPM_1}$$

$$\frac{Power_2}{Power_1} = \left(\frac{RPM_2}{RPM_1}\right)^3$$

At 50% airflow:

$$Power_{50%} = 0.5^3 = 0.125 = 12.5%$$ of full power

Energy savings: 87.5% reduction at half flow

VAV System Design

Minimum Airflow

Reasons for minimum flow:

1. Meet ventilation requirements (ASHRAE 62.1)
2. Maintain air circulation
3. Prevent stratification
4. Ensure diffuser performance

Typical minimum: 30-50% of design airflow

Calculation:

$$CFM_{min} = \max\left(CFM_{ventilation}, 0.3 \times CFM_{design}\right)$$

Diversity Factor

Not all zones at peak load simultaneously

System diversity:

$$Diversity = \frac{\sum CFM_{design,zones}}{CFM_{AHU,actual}}$$

Typical values:

• Office buildings: 1.2-1.4
• Schools: 1.1-1.3
• Hospitals: 1.0-1.1 (less diversity)

Impact: Allows smaller AHU and ductwork sizing

Static Pressure Control

Setpoint location: 2/3 distance from fan to furthest terminal

Control strategy:

$$SP_{setpoint} = SP_{design} \times \left(\frac{CFM_{actual}}{CFM_{design}}\right)^2$$

Static pressure reset:

• Monitors all VAV damper positions
• If all dampers < 90% open, reduce static pressure setpoint
• Saves fan energy without sacrificing control

graph TD
    A[Measure all VAV damper positions] --> B{Any damper > 90% open?}
    B -->|Yes| C[Increase SP setpoint by 0.1 \"w.g.]
    B -->|No| D{All dampers < 50% open?}
    D -->|Yes| E[Decrease SP setpoint by 0.1 \"w.g.]
    D -->|No| F[Maintain current SP setpoint]
    C --> G[Limit: SP_min to SP_max]
    E --> G
    F --> G
    G --> H[Modulate fan speed to maintain SP]

Pressure-Independent VAV Terminals

Advantages:

• Maintains design airflow regardless of duct pressure variations
• Simplified balancing
• Better control stability

Components:

• Airflow sensor (pressure grid or hot-wire anemometer)
• Controller with airflow setpoint
• Modulating damper with actuator

Control:

$$Damper% = f\left(CFM_{setpoint} - CFM_{measured}\right)$$

Variable Primary Flow Chilled Water

Replaces constant primary / variable secondary with single variable loop

Benefits:

• Eliminates primary pumps
• Reduces piping complexity
• Lower first cost and energy

Requirements:

1. Minimum flow through chillers: Use bypass valve or 3-way valve
2. Chiller isolation: Two-way valves at each chiller
3. Differential pressure control: Maintain 15-20 psi across system

Minimum flow calculation:

$$GPM_{min,chiller} = 2-3 \text{ ft/s} \times A_{evaporator}$$

Typically 30-50% of design flow

Pump Energy Savings

Pump affinity laws:

$$\frac{GPM_2}{GPM_1} = \frac{RPM_2}{RPM_1}$$

$$\frac{Head_2}{Head_1} = \left(\frac{RPM_2}{RPM_1}\right)^2$$

$$\frac{Power_2}{Power_1} = \left(\frac{RPM_2}{RPM_1}\right)^3$$

Variable vs. constant speed:

VFD Sizing and Selection

VFD must match motor:

• Voltage (230V, 460V, etc.)
• Current (nameplate FLA)
• Horsepower rating

Typical efficiency:

• 95-97% at full load
• 92-95% at 50% load

Harmonics mitigation:

• Use line reactors (3-5% impedance)
• Or passive harmonic filters

Practical Design Considerations

1. Duct design: Size for actual diversity, not sum of peaks
2. Minimum flow: Verify ventilation requirements at all loads
3. Bypass dampers: Avoid—waste energy
4. Pressure sensors: Place at representative location (2/3 point)
5. Commissioning: Verify minimum flow, diversity factors, reset strategies

Related Technical Guides:

• HVAC Control Strategies
• Fan Selection & Performance
• Duct Static Pressure Calculations

References:

• ASHRAE Handbook of HVAC Systems and Equipment, Chapter 48: Variable-Flow Systems
• ASHRAE Guideline 36: High-Performance Sequences of Operation for HVAC Systems
• Taylor Engineering: “Optimizing Design & Control of Chilled Water Plants”