Proper chiller plant sequencing optimizes energy efficiency while meeting cooling loads. This guide covers staging strategies for multi-chiller plants.
Building Load
↓
Primary Pumps → Chillers → Condenser Pumps → Cooling Towers
↓
Secondary Pumps → Building Distribution
| Equipment | Sequencing Consideration |
|---|---|
| Chillers | Load-based staging |
| Primary CHW pumps | Match to operating chillers |
| Secondary CHW pumps | Variable speed, pressure-based |
| Condenser pumps | Match to operating chillers |
| Cooling towers | Match to operating chillers + fans |
Stage 1: Lead chiller only
- Chiller 1: ON
- Chiller 2: OFF
- Pumps/towers: Match chiller 1
Stage 2: Lead + Lag
- Chiller 1: ON
- Chiller 2: ON
- Pumps/towers: Full capacity
Rotate lead chiller to equalize runtime:
| Method | Logic |
|---|---|
| Time-based | Rotate weekly or monthly |
| Runtime-based | Rotate when lag catches lead |
| Calendar-based | Specific rotation schedule |
Stage up to next chiller when:
1. Operating chiller(s) > 85-90% capacity
AND
2. Condition persists > 15-30 minutes
AND
3. Load is increasing (trending up)
Stage down when:
1. Total load can be met by (N-1) chillers
AND
2. Operating chillers < 50% capacity
AND
3. Condition persists > 15-30 minutes
AND
4. Load is stable or decreasing
% Capacity = Actual_Load / Rated_Capacity × 100
For multiple chillers:
% Capacity = Total_Load / Total_Online_Capacity × 100
If CHWR_Temp > Setpoint + Deadband:
Stage up (more capacity needed)
If CHWR_Temp < Setpoint - Deadband:
Stage down (excess capacity)
If CHWS_Temp > Setpoint + Deadband:
Stage up (cannot meet setpoint)
If CHWS_Temp < Setpoint - Deadband:
May stage down OR reset setpoint up
Load (tons) = GPM × ΔT × 500 / 12000
Where:
GPM = Chilled water flow
ΔT = CHWR - CHWS (°F)
500 = Constant (BTU/hr per GPM per °F)
12000 = BTU/hr per ton
| Equipment | Minimum Run |
|---|---|
| Chiller | 15-30 minutes |
| Chiller (start to start) | 10-15 minutes |
| Cooling tower fan | 5 minutes |
Before starting chiller:
1. CHW flow proven (flow switch or ΔP)
2. Condenser flow proven
3. Tower fan(s) running
4. Chiller not in alarm
5. Minimum off-time satisfied
Stage-Up Sequence:
1. Start condenser pump
2. Verify CW flow
3. Start cooling tower fan(s)
4. Start primary CHW pump
5. Verify CHW flow
6. Start chiller
7. Wait for chiller to load
Stage-Down Sequence:
1. Unload chiller
2. Stop chiller
3. Wait 5 minutes
4. Stop primary CHW pump
5. Stop condenser pump
6. Reduce tower fans (or stop)
Chillers are most efficient at specific loading:
| Load | Typical kW/ton |
|---|---|
| 25% | 0.75-0.85 |
| 50% | 0.55-0.65 |
| 75% | 0.50-0.55 |
| 100% | 0.55-0.65 |
Sweet spot: 60-80% load typically most efficient
Equal loading: All chillers at same % capacity
Optimized loading: Load based on efficiency curves
When load is low:
Lower CWT setpoint → Improves chiller efficiency
But: Increases tower fan energy
Optimal CWT = Function of load, wet bulb, equipment curves
Eliminates primary/secondary pumping:
Variable speed primary pumps → Chillers → Building
Critical requirement: Maintain minimum flow through operating chillers
Chiller_Min_Flow = Typically 50% of design flow
Total_Flow must be ≥ (Online_Chillers × Chiller_Min_Flow)
Stage up when:
Total flow approaching (Online_Chillers × Max_Flow)
Stage down when:
Total flow can be handled by (N-1) chillers
While maintaining minimum flow through each
| Point | Purpose |
|---|---|
| Chiller kW | Efficiency tracking |
| Load (tons) | Staging reference |
| CHWS/CHWR temps | Performance monitoring |
| CWS/CWR temps | Tower performance |
| Runtime hours | Maintenance planning |
Plant kW/ton = Total_Plant_kW / Load_Tons
Includes: Chillers + Pumps + Tower fans
Symptoms: Chillers cycle frequently Causes:
Solutions:
Symptoms: One chiller loaded, other lightly loaded Causes:
Solutions: