21.2 Chilled Water, Water Towers & Secondary Coolant

21.2 Chilled Water, Water Towers & Secondary Coolant

Trade C’s large-system cluster is not only “pick a bigger rooftop.” The Class A outline (and the theory Class B still needs) asks you to install secondary coolant systems, chilled-water loops with chillers/evaporators, release vents and air separators, and water towers — evaporative cooling, forced-draft and induced-draft fans, and secondary-coolant condensers. Refrigeration & Air Conditioning Technology, 9th Edition (2021), Pipefitters Handbook, 3rd Edition (1967), and FBC Mechanical 2023 are the books. The hydronic math is the same 500-constant used in Chapter 18; the new pieces are the two water loops, the tower, and the air-management devices that keep a 200-gpm circuit from air-locking.

Quick Answer: Secondary coolant (chilled water, brine, or glycol) carries the building load to the chiller evaporator. At a typical 10°F chilled-water ΔT, flow is about 2.4 gpm per ton. Tower range is water ΔT; approach is leaving water minus entering wet-bulb. Vents and an air separator belong on the loop, not as extras.

Secondary coolant versus refrigerant

A secondary coolant is a liquid that moves heat between the occupied load and the refrigeration machine. The refrigerant boils and condenses inside the chiller. Chilled water is the usual comfort-cooling secondary coolant. Brine or glycol blends are used when the leaving temperature is near or below freezing, or when a burst coil would damage a Florida process or a rooftop coil that sees 28°F nights in the Panhandle. Glycol has a lower specific heat and a different density than water — do not blindly use the 500 constant on a 40 percent propylene-glycol loop. Water at standard density is the 500 in

$\text{GPM} = \dfrac{\text{Btu/h}}{500 \times \Delta T}$

500 = 8.33 lb/gal × 60 min/h × 1 Btu/lb·°F. One ton is 12,000 Btu/h, so at a 10°F ΔT:

$\text{gpm/ton} = 12{,}000 \div (500 \times 10) = 2.4$

That is the planning number the outline expects: about 2.4 gpm per ton at 10°F. Stretching ΔT to 12°F drops flow to 2.0 gpm/ton; a 5°F ΔT doubles it to 4.8. The coil, control valve, and chiller evaporator must actually be rated at that flow. Inventing a 16°F ΔT on the balancing valve after the pipe is already 1-1/4 inch is not a method.

Worked evaporator flow. An 80-ton chiller at 10°F ΔT:

$\text{GPM} = 80 \times 2.4 = 192\ \text{gpm}$

or $80 \times 12{,}000 \div 5{,}000 = 192$ gpm. If the schedule is 44°F leaving / 54°F return, that is a 10°F ΔT. A 100-ton plant at the same ΔT is 240 gpm. A 25-ton Class B-legal chiller is 60 gpm.

Chilled-water loops, evaporators, vents, and separators

The chiller evaporator is a heat exchanger: refrigerant boils on one side, secondary coolant is chilled on the other. Water-cooled machines also have a condenser barrel that rejects heat into condenser water. Air-cooled chillers reject that heat to outdoor air and skip the tower — still a secondary-coolant evaporator loop on the building side.

Primary-secondary piping is the usual large-building layout. A primary loop holds nearly constant (or manufacturer-required) flow through each evaporator. A secondary loop, often with a VFD, serves the air handlers. A decoupler (common pipe) connects them so primary and secondary flows can differ without starving the barrels. Variable-primary designs exist; they still need the minimum evaporator flow the listing states. Pumping the building through the evaporator with no bypass, then closing two-way valves until flow collapses, is how you freeze a barrel.

Air in a closed loop is not a character builder. It air-locks coils, rusts steel, and makes pumps cavitate.

FBC Mechanical still wants access, listed vessels, and relief where the listing requires it. Release vents on the outline are those automatic air vents — they are not refrigerant relief valves. Refrigerant relief is a different device on the chiller, piped per the listing and Chapter 11, not dumped into a return-air plenum.

Water towers — evaporative cooling, draft, range, and approach

A cooling tower rejects condenser heat by evaporative cooling: a small fraction of the water evaporates, and that latent heat cools the rest. The leaving water cannot be colder than the entering wet-bulb of the air. That is why a Miami afternoon at 78°F wet-bulb will not produce 70°F condenser water no matter how many extra fan horsepower you buy.

Range = tower inlet water temperature minus tower outlet water temperature. It is the condenser-water ΔT. A classic design is 95°F in / 85°F out — a 10°F range.

Approach = tower outlet (leaving) water temperature minus entering wet-bulb. A common catalog point is 85°F leaving at 78°F WB — a 7°F approach. Tighter approach means more fill, more air, more money. Approach of zero is a perpetual-motion answer.

Condenser heat rejection is cooling load plus compressor heat, roughly 15,000 Btu/h per ton on a typical electric chiller (a COP near 4: 12,000 of cooling plus about 3,000 of work). At a 10°F range:

$\text{gpm/ton} \approx 15{,}000 \div (500 \times 10) = 3.0$

About 3 gpm per ton of condenser/tower water at 10°F range, versus 2.4 gpm per ton of chilled water at 10°F ΔT. Mixing those two numbers is the exam’s hydronic banana peel. An 80-ton plant: 192 gpm chilled water, about 240 gpm condenser water.

Forced-draft towers have the fan on the inlet: they push air through the fill. Induced-draft towers have the fan on the discharge: they pull air through the fill and are the usual factory-assembled HVAC tower (less recirculation of discharge into the intake). Crossflow air moves horizontally across falling water; counterflow air moves up against falling water. Secondary-coolant condensers on this outline are the water-cooled (or glycol-cooled) condenser barrels and the tower or closed-circuit fluid cooler that serves them. A closed-circuit cooler keeps the condenser water in a coil and sprays over the coil — still evaporative cooling, still range and approach against wet-bulb.

Florida wet-bulb is high. A tower selected for 78°F WB at 85°F leaving will not make 85°F leaving if the site 1% wet-bulb is 80°F; leaving water sits at wet-bulb plus approach. Undersized towers raise SDT, cut chiller tons, and trip high-pressure. Drift eliminators, basin screens, and water treatment are maintenance (Chapter 32) but they start as install accessories: equalizing lines, make-up, overflow, and a dead-leg-free condenser loop.

Florida scenario

A Tampa (Hillsborough, Climate Zone 2A) six-story office is issued with a 100-ton water-cooled chiller, 44/54 chilled water, 95/85 condenser water, induced-draft tower, primary-secondary pumps, air separator on the primary pump suction, and automatic vents at the penthouse coil high points. Chilled-water flow is $100 \times 2.4 = 240$ gpm. Condenser flow is about $100 \times 3 = 300$ gpm. Range is 10°F. If the design wet-bulb is 78°F and leaving condenser water is 85°F, approach is 7°F. A Class B qualifier who “helps” by bidding this plant as two 50-ton “halves” of the same loop is still looking at 100 tons in one system — Class A. What Class B must still know is why the 240 gpm and the 300 gpm are different, why an air vent on the fifth-floor coil does not replace the separator in the basement, and why a salesman who promises 80°F condenser water on a 78°F wet-bulb day has promised a 2°F approach the tower catalog may not own. The expansion tank on that primary loop, if it is an unfired pressure vessel in Class A’s statutory list, is another reason the ticket is Class A even before you count tons.

Traps: (1) Using 3 gpm/ton on the evaporator. (2) Using 2.4 gpm/ton on the tower. (3) Calling approach the same thing as range. (4) Selecting a tower to leave below wet-bulb. (5) Omitting the air separator because “we have automatic vents.” (6) Treating glycol like water in the 500 formula.