A large building cools itself with three loops of fluid, and two machines sit between them. The chiller makes cold water. The cooling tower throws the building's heat, plus the chiller's own work, out into the air. They are usually bought, serviced and optimized separately, and managing them together is where extra energy and water savings are found.
The three loops
Chilled-water loop. Water leaves the chiller at roughly 42–46 °F, runs to air handlers on every floor, picks up the building's heat and comes back about 10 °F warmer. The standard rating condition for water-cooled chillers (AHRI Standard 550/590) is 44 °F leaving chilled water.
Refrigerant circuit, inside the chiller. The chiller's evaporator pulls heat out of the chilled water by boiling refrigerant. An electric compressor raises that refrigerant to a higher pressure and temperature so it can condense and give the heat up, in the condenser, to a second water loop.
Condenser-water loop. Water leaves the chiller's condenser warm and goes to the cooling tower, which returns it cooler. The same rating standard uses 85 °F entering condenser water and 3.0 gpm per ton, which puts the water leaving the condenser in the mid-90s °F: a range of roughly 9–10 °F.
Where the points sit in the sample plant
- CDWL_RW_TEMP: Condenser water to tower
- CDWL_SW_TEMP: Condenser water from tower
- CHL_POW_1: Chiller power
- CHL_POW_2: Chiller power
- CHL_POW_3: Chiller power
- CT_FAN_SPD_1: Cooling-tower fan speed
- CT_POW_1: Cooling-tower fan power
- CT_POW_2: Cooling-tower fan power
- CT_POW_3: Cooling-tower fan power
- CWL_PRI_CW_FLOW: Chilled-water flow
- CWL_PRI_RW_TEMP: Chilled-water return temperature
- CWL_PRI_SW_TEMP: Chilled-water supply temperature
- CWL_SEC_LOAD: Measured cooling load
- OA_TEMP: Outdoor wet-bulb temperature
- OA_TEMP_WB: Outdoor dry-bulb temperature
- 8 pump power points: CDWL_PM_POW_1, CDWL_PM_POW_2, CDWL_PM_POW_3, CWL_PRI_PM_POW_1, CWL_PRI_PM_POW_2, CWL_PRI_PM_POW_3, CWL_SEC_PM_POW_1, CWL_SEC_PM_POW_2
- Make-up, blowdown and chemical feed: no point in dataset.
What each machine costs to run
| Chiller | Cooling tower | |
|---|---|---|
| Job | Moves heat out of the building's water | Rejects that heat to outdoor air |
| Main input | Electricity (the compressor) | Water, fan electricity, treatment chemicals |
| Key metric | kW per ton of cooling | Approach to wet-bulb, cycles of concentration |
| Usually serviced by | The chiller maker's service arm or a mechanical contractor | A water-treatment company, plus mechanical for fans and fill |
A ton is 12,000 BTU per hour of cooling. kW/ton is electricity per ton; lower is better. The tower has to reject more heat than the building produces, because the compressor's work ends up in the condenser water too.
How a cooling tower uses water
The tower cools water by evaporating a small share of it. A widely used approximation is that evaporation is about 0.00085 × circulating flow (gpm) × range (°F): roughly 1% of the flow for every 10 °F of range, less the share of heat that leaves as sensible heat. A 500-ton plant at 3.0 gpm per ton and a 10 °F range evaporates about 12.75 gpm at full load (try the calculator).
Evaporation leaves minerals behind. To keep them from scaling, the tower drains some concentrated water (blowdown) and replaces it with fresh makeup. The ratio of makeup to blowdown is the cycles of concentration, and it sets how much water the tower uses beyond evaporation. FEMP notes that many towers run at two to four cycles, while six or more may be possible (more on cycles).
Why the two have to be run together
The condenser-water loop connects the machines, and decisions on one side change the other:
- Colder condenser water helps the chiller. The compressor's lift, the pressure difference it has to overcome, falls with the temperature of the water it rejects heat into. Getting colder water costs tower fan energy and depends on the outdoor wet-bulb.
- Water chemistry reaches the chiller's tubes. Pushing cycles up saves water. Pushing them too high, or letting treatment slip, lets scale and biofilm build on condenser tubes. Fouled tubes raise the condenser approach and the chiller's kW/ton.
- The tower's condition shows up as fan energy first. In LBNL's public chiller-plant data (the sample plant above), reducing one tower's heat-transfer capacity to 65% barely moved chiller kW/ton in July. That tower's fan ran at full speed in most running hours, against about one hour in ten for the healthy plant (details).
Chiller setpoints and water chemistry are usually managed by different teams, each focused on its own side. Measuring both from the same data, on the same timeline, shows how they affect each other. Plant Check covers the energy side from a BMS export: kW/ton, tower approach and fan energy. Putting the water chemistry on the same timeline is what a pilot will add.