7.6 Refrigerants, Cooling Towers, and Specialized HVAC Technologies (Absorption, VRF, Chilled Beams)

Refrigerants, Cooling Towers, and Specialized HVAC Technologies

Beyond the core vapor-compression chiller and air-distribution systems, the CEM exam tests several specialized HVAC technologies that drive both energy use and regulatory compliance. This section covers refrigerant regulations and selection, cooling-tower performance, absorption chillers, variable refrigerant flow (VRF) systems, and chilled beams—technologies that appear throughout the HVAC Systems and Building Envelope domain.

Refrigerants and Global Warming Potential (GWP)

A refrigerant is the working fluid in a vapor-compression cycle, absorbing heat at the evaporator and rejecting it at the condenser. Historically, CFCs and HCFCs (e.g., R-22) were phased out under the Montreal Protocol for ozone depletion. Today's fleet relies heavily on HFCs, which are ozone-safe but have high Global Warming Potential (GWP). GWP measures how much heat a greenhouse gas traps relative to CO2 over 100 years; CO2 has a GWP of 1.

Common high-GWP HFCs include R-410A (GWP ≈ 2,088) and R-134a (GWP ≈ 1,430). Under the Kigali Amendment to the Montreal Protocol and the U.S. AIM (American Innovation and Manufacturing) Act, HFCs are being phased down roughly 85% by 2036. The industry is transitioning to low-GWP hydrofluoroolefins (HFOs) and HFC/HFO blends, including:

• R-1234ze(E) — GWP ≈ 1, used in centrifugal chillers and as a foam-blowing agent.
• R-1233zd(E) — GWP ≈ 1, a low-pressure refrigerant for centrifugal chillers.
• R-32 — GWP ≈ 675, a lower-GWP HFC used in residential and light-commercial split systems.

ASHRAE Standard 34 classifies refrigerants by toxicity (A = lower, B = higher) and flammability (1 = no flame propagation, 2L = lower flammability, 2 = flammable, 3 = higher flammability). Most modern low-GWP refrigerants carry a 2L flammability classification, requiring equipment and installation adjustments. Refrigerant leak detection is also an energy issue—leaks degrade chiller capacity and efficiency over time.

Cooling Towers

A cooling tower rejects condenser heat from a water-cooled chiller through evaporative cooling. A portion of the recirculating condenser water is evaporated, carrying away latent heat; the rest is cooled and returned to the chiller. Two performance metrics dominate:

• Range = hot water temperature in − cold water temperature out (the degrees the tower cools the water).
• Approach = cold water temperature out − ambient wet-bulb temperature (how close the tower gets to the theoretical limit).

The wet-bulb temperature sets the floor for the cold-water temperature; a tighter (smaller) approach requires more tower fan energy and/or a larger tower. Energy managers optimize the trade-off between chiller kW/ton (improved by colder condenser water) and tower fan kW (increased by a tighter approach). Cycling tower fans or using two-speed/VFD fans allows the tower to match part-load conditions efficiently.

Water management is equally important. Cycles of concentration compare the dissolved solids in the recirculating water to the makeup water; higher cycles reduce water consumption but increase scaling/fouling risk. Drift eliminators reduce water (and chemical) loss. Unmanaged towers risk Legionella growth and scale formation on condenser tubes, both of which raise condensing pressure and chiller energy use.

Absorption Chillers

An absorption chiller uses a heat source (steam, hot water, direct gas firing, or waste heat) instead of a mechanical compressor to drive the refrigeration cycle. The working pair is typically lithium bromide (LiBr) and water, where LiBr is the absorbent and water is the refrigerant; ammonia-water is used for low-temperature industrial applications.

Because the cycle is heat-driven, the COP (defined as cooling output ÷ heat input, not electricity) is far lower than a vapor-compression chiller:

• Single-effect (low-grade steam or hot water): COP ≈ 0.6–0.8.
• Double-effect (higher-pressure steam or direct-fired): COP ≈ 1.0–1.2.
• Direct gas-fired: COP ≈ 1.2.

An absorption chiller makes economic sense when free or low-cost heat is available—engine jacket/exhaust heat from a CHP system, process waste heat, solar thermal, or surplus steam. They consume very little electricity (mainly small solution pumps), shifting load from peak electrical demand to thermal input.

Variable Refrigerant Flow (VRF) Systems

VRF systems use an inverter-driven outdoor unit connected to multiple indoor fan-coil units via refrigerant piping, modulating the refrigerant flow to each zone. Key advantages include simultaneous heating and cooling (with heat-recovery VRF, one zone can be heated while another is cooled by transferring heat between them), high part-load efficiency from inverter compressors, and long refrigerant runs that reduce ductwork. They are common in hotels, multifamily, and office buildings. The CEM should weigh their high installed cost and refrigerant charge against their efficiency and zoning flexibility.

Chilled Beams

Chilled beams are radiant terminal devices that cool primarily by absorbing heat from the room rather than by blowing cold air. Passive chilled beams rely on natural convection; active chilled beams mix primary ventilation air with induced room air for higher capacity. They use warmer chilled water (typically 55–58°F) to avoid condensation, which raises chiller efficiency. Their low fan energy and quiet operation make them attractive for perimeter zones, but condensation management and fresh-air delivery (a separate dedicated outdoor air system is required) must be designed correctly.