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 $500 \times 30 \times 10 = 150{,}000\text{ BTU/hr}$.
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 $1{,}200{,}000 \div (500 \times 10) = 240\text{ GPM}$, 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?
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?
Why does a flooded chiller evaporator use a float valve or orifice rather than a thermostatic expansion valve?