Coils, Chillers and Cooling Towers
Chilled-water flow, chiller COP, condenser water, cooling tower range and approach, and water use.
The cooling chain
A central cooling plant moves heat in three steps: air coils give their heat to chilled water at about 6 °C; a chiller (see Refrigeration and Heat Pumps) lifts that heat to a higher temperature with a compressor; and a condenser water loop carries the heat plus the compressor work to a cooling tower, which rejects it by evaporating some of the water. Each step is a heat balance of the form Q = ṁ c ΔT for the water.
Chilled water and the coil
Water carries heat very effectively: 4.186 kJ per kilogram per kelvin, so each liter per second with a 6 K rise carries 25.1 kW (the plant figure of HVAC Basics and Thermal Comfort puts air at 1.2 W per L/s per K). Hence the pipes are far smaller than the ducts. Typical chilled water runs 6 °C supply and 12 °C return (a 6 K range; 7 °C and 13 °C in some countries); the coil entering the air side at 27 °C and leaving at 13 °C needs a cold surface (Psychrometric Processes).
Worked example: water flow to the office coil
- The 100 m² office needs 7.27 kW (Heating and Cooling Load Calculations). With a 6 K water rise: ṁ = 7.27 ÷ (4.186 × 6) = 0.290 kg/s (1 043 L/h).
- A pipe of DN20 (inside diameter 21.6 mm) at that flow has a velocity of 0.79 m/s: small enough for a branch.
The chiller and its COP
The chiller is a vapor-compression machine with an evaporator (cooling the water) and a condenser (heating the condenser water). Its performance is the coefficient of performance COP = cooling ÷ electrical input, or the older kW per ton (0.70 kW/ton here, equivalent to a COP of 5.0). Types: screw and scroll (small and medium), centrifugal (large, most efficient, COP 6 to 7 at full load), absorption (heat driven). A real machine reaches roughly half to 60 percent of the Carnot COP; between 6 °C evaporating and 35 °C condensing the Carnot value is 9.6, so about 5.3 is realistic. Because buildings run at part load most of the time, the integrated part-load value (IPLV) and the seasonal figure matter more than the full-load COP; variable-speed compressors raise them a great deal.
Condensing temperature matters. Each degree the condenser water is colder reduces the compressor lift and saves about 2.5 percent of the power: 5 K lower saves 12 percent. This is why the tower is run as cold as the wet bulb allows (floating the condenser water setpoint).
Cooling towers
The tower cools the condenser water by letting a small part evaporate. The two numbers to know are the range (hot water in minus cold water out) and the approach (cold water out minus the ambient wet-bulb). The approach can never be zero because the water cannot get colder than the wet-bulb, and a small approach needs a large tower: 3 to 5 K is typical. Water is lost three ways: evaporation (about 1 percent of the flow for every 5.5 K of range, because each kg evaporated takes 2.43 MJ), blowdown (bleed to stop minerals concentrating) and drift (droplets, 0.005 percent with good eliminators). The cycles of concentration C relate them: blowdown = evaporation ÷ (C − 1).
A tower must be protected against Legionella: keep the water treated, drain dead legs, clean basins, and follow the local code (ASHRAE 188 or the national guidance). Put it away from air intakes.
Worked example: a 100 kW plant
Worked example: chiller and tower for a 100 kW load
Cooling load 100 kW, chilled water 6 °C and 12 °C, chiller COP 5.0, ambient wet-bulb 25.5 °C, tower approach 4 K, condenser water range 5.5 K.
- Chilled water flow = 100 ÷ (4.186 × 6) = 3.98 kg/s (14.3 m³/h, 63 gpm).
- Chiller power = 100 ÷ 5.0 = 20 kW (0.70 kW/ton).
- Heat to the tower = cooling + compressor work = 100 + 20 = 120 kW.
- Tower water: cold 29.5 °C (wet-bulb + approach), hot 35.0 °C; flow = 120 ÷ (4.186 × 5.5) = 5.21 kg/s.
- Evaporation = 120 ÷ 2 430 kJ/kg = 49 g/s = 178 L/h (0.95 percent of the circulating flow). With 4 cycles of concentration the blowdown is 59 L/h, so the make-up is 237 L/h, about 711 m³ over 3 000 hours of full-load running.
- Chilled water pump: 3.98 L/s at a 25 m head and 70 percent efficiency: P = ρ g Q H ÷ η = 1.39 kW (see Pump Power and Sizing Calculator).
The low delta-T problem
Plants are designed for a 6 K water range, but many run at 2 or 3 K because coils are oversized, valves are leaking or control valves are open too far. Then the same load needs more flow: at 3 K the example needs 7.96 kg/s, 2.0 times the design flow, and the pump power (rising as the cube of the flow for the same pipework, see Ducts, Fans and Air Distribution for the fan laws that also apply to pumps) climbs from 1.39 kW to about 11.2 kW, while the chiller sees colder return water and loses efficiency. Fix with the right coil and valve selection, pressure-independent control valves and a variable-flow primary system.
Water-cooled or air-cooled
| Type | COP (typical) | For | Against |
|---|---|---|---|
| Water-cooled with a tower | 5 to 7 (full load) | large buildings; efficient; compact chiller | tower water use and treatment; plant room and pumps; Legionella control |
| Air-cooled chiller or heat pump | 2.5 to 3.5 at 35 °C ambient | simple, no water, no tower; roof or ground mount | larger, noisier, loses efficiency in hot weather; 100 kW would need about 33 kW at COP 3.0 against 20 kW |
| Free cooling (a dry cooler in winter) | very high when it works | data centers, process cooling | only below about 5 to 10 °C ambient |
Related
- Previous: Ducts, Fans and Air Distribution. Next: Hydronic Heating and Cooling.
- Refrigeration and Heat Pumps.
- Pump Power and Sizing Calculator and Pumps, Curves and NPSH.
- HVAC and Psychrometrics: the whole guide.
FAQ
What does kW per ton mean?
Electrical input per ton of cooling: 3.517 ÷ COP. A COP of 5 is 0.70 kW/ton; good chillers are below 0.6.
Why does a cooling tower lose water?
Mainly evaporation: that is how it cools. The rest is blowdown to keep the dissolved solids from building up, and drift.
Why is the approach never zero?
The water cannot be cooled below the wet-bulb temperature of the air: you would need an infinite tower.
Why 6 °C chilled water?
It is cold enough to dehumidify the air at the coil, but warm enough to avoid freezing and to keep the chiller efficient. Raising it to 8 or 10 °C saves energy where humidity is controlled otherwise.
References
- ASHRAE, Handbook – HVAC Systems and Equipment (water chillers, cooling towers) and Refrigeration; ASHRAE Guideline 12 and Standard 188 (Legionellosis risk management).
- AHRI Standard 550/590 (chiller performance, IPLV); CTI (Cooling Technology Institute) tower standards.
- Taylor, S. T., Optimizing Design and Control of Chilled Water Plants, ASHRAE Journal series.
Last reviewed: October 2026. Efficiencies and water-use figures are typical.