12.2 Water-Cooled Condensers, Cooling Towers, and Chilled-Water Systems
Key Takeaways
A water-cooled condenser rejects heat to water rather than air, and a water regulating valve modulates flow to maintain the target condensing pressure.
A cooling tower rejects heat by evaporating roughly 1% of the circulating water for every 10 degrees Fahrenheit of range, and its performance is limited by the outdoor wet-bulb temperature.
Tower approach is condenser water leaving temperature minus outdoor wet bulb, and range is entering minus leaving water temperature.
Blowdown or bleed removes concentrated dissolved solids; cycles of concentration equal makeup water divided by blowdown, and inadequate blowdown causes scale.
A flooded chiller evaporator holds liquid refrigerant around water tubes and uses a float or orifice metering device rather than a thermostatic expansion valve.
12.2 Water-Cooled Condensers, Cooling Towers, and Chilled-Water Systems
The Commercial Air Conditioning sheet names "condensers water cooled," "installing a water cooled system and adjusting a water regulating valve," "describing a chilled water system and its operation," "describing cooling towers and their operating limitations," and "describing the operation and function of a flooded evaporator and its metering device." Water is a far better heat-transfer medium than air, and once a building is large enough, rejecting heat to water becomes the efficient choice.
1. Water-Cooled Condensers
| Type | Construction | Application |
|---|---|---|
| Tube-in-tube (double pipe) | One tube inside another; refrigerant in the annulus, water in the inner tube, counterflow | Small systems, ice machines, self-contained units |
| Shell-and-coil | A coiled tube inside a shell | Small to medium |
| Shell-and-tube | A bundle of straight tubes inside a cylindrical shell, with removable heads for mechanical cleaning | Medium to large; the standard for chillers |
Why water wins. Water's specific heat is about 1.0 BTU/lb·°F versus air's 0.24, and its density is roughly 800 times greater. A water-cooled condenser can hold condensing temperature within about 10°F of the entering water, while an air-cooled condenser typically runs 20–30°F above ambient. Lower condensing temperature means lower compression ratio, higher capacity, and lower power draw.
The heat-rejection equation for water:
where 500 = 8.33 lb/gal × 60 min/hr × 1.0 BTU/lb·°F. A condenser circulating 30 GPM with a 10°F rise rejects .
Rule of thumb: about 3 GPM per ton of cooling for a cooling-tower condenser water loop, producing a 10°F range.
The water regulating valve
A water regulating valve (sometimes called a head pressure control valve) is a pressure-actuated valve in the condenser water line. It senses discharge pressure through a capillary and modulates water flow to hold condensing pressure at setpoint.
- Rising head pressure opens the valve, admitting more water.
- Falling head pressure closes it, conserving water and preventing the head from dropping too low to feed the metering device.
- On city-water (once-through) systems it also conserves water, which is the reason many jurisdictions now prohibit once-through cooling entirely.
- Adjustment: run the system at steady load, then turn the adjusting stem until discharge pressure stabilizes at the manufacturer's target condensing temperature — commonly about 105°F saturated for a tower system.
- Failure signs: a valve stuck closed produces high head and eventual high-pressure trips; stuck open produces low head, low capacity, and possible TXV starving in cool weather.
2. Cooling Towers
A cooling tower rejects building heat to the atmosphere by evaporating a small fraction of the circulating water. Evaporating one pound of water absorbs roughly 1,000 BTU, which is why a tower can reject enormous heat with modest water flow.
Types
- Induced draft, counterflow — fan at the top pulls air up through falling water; the most common package tower.
- Induced draft, crossflow — air moves horizontally across falling water; lower fan power, easier access.
- Forced draft — fan at the base pushes air through; more prone to recirculation of saturated discharge air.
- Closed-circuit (fluid cooler) — the building loop stays inside a coil while spray water evaporates over it; keeps the building loop clean at the cost of a higher approach.
Performance metrics
| Term | Definition | Typical value |
|---|---|---|
| Range | Entering water temperature − leaving water temperature | 10°F |
| Approach | Leaving water temperature − outdoor wet-bulb temperature | 7–10°F |
| Design condition | e.g. 95°F in, 85°F out, 78°F wet bulb | Range 10, approach 7 |
Wet bulb, not dry bulb, is the limit. Because the process is evaporative, the theoretical minimum water temperature is the outdoor wet-bulb temperature (the same physics as Process 7 in Section 11.2). A tower can never produce water colder than the ambient wet bulb, and a real tower approaches it by 7–10°F. This is the single most important operating limitation on the exam: on a humid day with a 78°F wet bulb, an 85°F leaving-water design is achievable; a 70°F leaving-water expectation is physically impossible.
A rising approach is the classic tower fault. If leaving water temperature climbs while wet bulb stays the same, the tower is losing capacity: scaled or clogged fill, plugged spray nozzles, low water flow, a slipping or reversed fan, restricted air inlet louvers, or discharge air recirculating back into the inlet.
Water losses and treatment
| Loss | Mechanism | Magnitude |
|---|---|---|
| Evaporation | The cooling process itself | ≈ 1% of flow per 10°F of range |
| Drift | Water droplets carried out in the airstream | 0.005–0.2% (drift eliminators minimize it) |
| Blowdown (bleed) | Deliberate discharge of concentrated water | Set by water chemistry |
Evaporation removes pure water and leaves dissolved minerals behind, so the remaining water concentrates. Cycles of concentration = makeup ÷ blowdown. Without adequate blowdown, calcium carbonate scale forms on the condenser tubes and the fill, and heat transfer collapses.
Treatment addresses four problems: scale (hardness), corrosion (dissolved oxygen and low pH), biological growth, and suspended solids. Legionella is the safety-critical one: warm, nutrient-rich, aerosolized tower water is a proven transmission route, which is why ASHRAE Standard 188 requires a written water management plan for building water systems including cooling towers, and why drift eliminators, biocide programs, and regular cleaning are not optional.
Freeze protection in cold climates: a basin heater, an indoor sump or remote sump, bypass piping around the fill, and variable-speed or cycling fan control to prevent overcooling.
3. Chilled-Water Systems
Instead of moving refrigerant to the load, a chilled-water system moves water and keeps the refrigerant in a machine room.
Components: the chiller (evaporator, compressor, condenser), chilled-water pumps, an expansion tank, an air separator, air-handling units or fan coils with chilled-water coils and control valves, and a cooling tower with condenser-water pumps on water-cooled machines.
Typical design: 44°F supply, 54°F return — a 10°F ΔT. Flow follows the same equation:
A 100-ton load (1,200,000 BTU/hr) at 10°F ΔT requires , which is the origin of the 2.4 GPM per ton rule of thumb for chilled water.
Low delta-T syndrome is the classic chilled-water plant complaint: return water comes back too cold, so the plant must circulate excess flow to meet the load, wasting pump energy. Causes include coil control valves stuck open, three-way valves bypassing, fouled coils, improper coil selection, and uncontrolled bypasses.
Flooded evaporators
Large chillers use a flooded evaporator: refrigerant fills the shell and water flows through the tubes, with the tube bundle submerged in boiling liquid refrigerant. Advantages are excellent heat transfer (the tubes are wetted continuously) and a very small temperature difference between refrigerant and water.
A flooded evaporator does not use a TXV. Because the shell is deliberately full of liquid, there is no superheat at the outlet for a TXV to sense. Metering is by:
- Low-side float valve — maintains a liquid level in the evaporator; opens as level falls.
- High-side float valve — maintains a level in the condenser and passes everything else downstream.
- Fixed orifice plates — common on modern centrifugal machines.
Because there is no superheat, a flooded evaporator requires a suction line accumulator or an eliminator section in the shell to prevent liquid carryover to the compressor.
Freeze protection is critical. Water in the tubes at 44°F is only 12°F from freezing. Chillers carry a low chilled-water temperature cutout, a flow switch proving water flow before the compressor can start, and interlocks with the pump starter. The Type III EPA rules in Section 5.2 exist for exactly this reason: pulling a low-pressure chiller into a deep vacuum with no water flowing will freeze and split the tubes.
A cooling tower is designed for 95 degrees Fahrenheit entering water, 85 degrees Fahrenheit leaving water, at a 78 degree Fahrenheit outdoor wet bulb. What are the range and approach, and what limits the tower's performance?
Range 17 degrees and approach 10 degrees; performance is limited by outdoor dry-bulb temperature
Range 10 degrees and approach 7 degrees; performance is limited by the outdoor wet-bulb temperature, which the tower can approach but never reach
Range 7 degrees and approach 10 degrees; performance is limited by condenser water flow rate only
Range 10 degrees and approach 17 degrees; performance is limited by the fan motor horsepower
A chiller plant serves a 60-ton load with 44 degree Fahrenheit supply and 54 degree Fahrenheit return chilled water. What flow rate is required?
144 GPM, calculated as load in BTU per hour divided by 500 times the temperature difference
720 GPM, calculated as tons times 12 GPM per ton
60 GPM, one gallon per minute per ton
1,440 GPM, calculated as load in BTU per hour divided by the temperature difference
Why does a flooded chiller evaporator use a float valve or orifice rather than a thermostatic expansion valve?
Because a TXV cannot handle the low pressures found in a low-pressure chiller
Because the shell is deliberately kept full of liquid refrigerant so there is no superheat at the outlet for a TXV to sense; a float valve maintains liquid level instead
Because float valves are less expensive than TXVs at chiller capacities
Because a flooded evaporator operates with a fixed load, so no modulation of refrigerant flow is ever required
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