21.3 Centrifugal, Absorption & Ammonia Systems

21.3 Centrifugal, Absorption & Ammonia Systems

The last large-system cluster on Trade C is machine type. The Class A outline includes centrifugal compressor systems; the Class B outline omits that install bullet. Absorption systems appear on both outlines. Ammonia is a Class A Trade C item (industrial refrigeration), not a light-commercial rooftop refrigerant. Refrigeration & Air Conditioning Technology, 9th Edition (2021) and Florida Building Code — Mechanical, 2023 Chapter 11 (Refrigeration) support the cluster. IIAR 2 (International Institute of Ammonia Refrigeration) is the closed-circuit ammonia design standard the industry actually builds to; this exam wants the qualitative engine-room picture, not a memorized unofficial detector chart.

Quick Answer: A centrifugal raises pressure with an impeller. Lithium bromide absorption uses water as the refrigerant and LiBr as the absorbent, driven by heat. Ammonia plants live in machinery/engine rooms with ventilation, detection, and emergency shutdown. Class B studies all three; Class B installs none of the over-25-ton or centrifugal or ammonia plants.

Centrifugal compressors — Class A install, everyone’s theory

A centrifugal compressor accelerates refrigerant vapor in an impeller and converts that velocity to pressure in a diffuser (and often a volute). It is a dynamic compressor, not a positive-displacement piston or screw. Typical comfort-cooling centrifugals start in the over-100-ton neighborhood and run into the hundreds or thousands of tons — which is why they sit on the Class A outline next to the over-100-ton install item. Smaller machines exist; do not treat “any chiller over 25 tons” as centrifugal. Many 40- to 90-ton plants are screws or multiple scrolls.

Capacity control on a centrifugal is inlet guide vanes, a variable-speed drive (VFD) on the impeller, or both. At too low a flow the impeller surges: flow breaks down, the machine hunts, and bearings and the impeller pay for it. Hot-gas bypass is a last-resort fake load, not a substitute for staying on the map. Purge units appear on low-pressure centrifugals because noncondensable air leaks in; high-pressure machines leak out. Oil systems lubricate gears and bearings even when the refrigerant circuit is described as “oil-free” at the impeller — read the listing.

Open-drive centrifugals put the motor outside the refrigerant; hermetic and semi-hermetic put it in the housing. The electrical picture is still 489.105: a 460-volt, several-hundred-amp feeder is an electrical contractor; HVAC lands controls, flow interlocks, and the dedicated-circuit repairs the statute actually grants. Dead load of a water-filled centrifugal plus the barrel water is a structural number (Section 21.1).

Class B candidates still need to explain surge, vanes, and why a 200-ton centrifugal is not a 20-ton scroll. They do not bid, permit, or install that machine as Class B.

Absorption — lithium bromide, on both outlines

A vapor-compression chiller uses a mechanical compressor. An absorption chiller uses a thermal compressor: heat boils refrigerant out of a liquid absorbent, the refrigerant condenses, expands, and evaporates (chilling the secondary coolant), then the vapor is absorbed back into the solution and pumped to the generator again. No piston, no impeller of refrigerant vapor as the driving machine.

Lithium bromide–water is the comfort-cooling absorption pair:

• Water (R-718) is the refrigerant. It evaporates at low pressure (a vacuum) around typical chilled-water temperatures (about 40–45°F leaving). You do not use LiBr-water to make ice-cream-room suction temperatures — the refrigerant would freeze.
• Lithium bromide is the absorbent. Strong (concentrated) LiBr drinks water vapor in the absorber. Weak solution is pumped to the generator (concentrator), where steam, hot water, or a direct-fired burner boils the water vapor back out.
• The water vapor goes to a condenser, rejects heat, and returns through a metering device to the evaporator.
• A solution heat exchanger between strong and weak streams is ordinary textbook practice.

Crystallization is the absorption exam word. If the LiBr becomes too concentrated or too cold, the salt comes out of solution and plugs the machine. Causes include loss of condenser water, too much firing at low load, or a cold machine after an improper shutdown. The cure is prevention and the manufacturer’s dilution/stop sequence — not “add acid.”

Single-effect machines have a textbook COP around 0.7 (they use a lot of heat per ton). Double-effect machines add a second generator and land a textbook COP around 1.2. Those are order-of-magnitude teaching numbers from the listed technology text, not a substitute for the unit’s rating plate. Because the driving heat plus the cooling load both leave through the cooling water, the tower on an absorption machine is larger than the tower on an electric chiller of the same tons. Electrical load is mostly pumps and tower fans, which is why absorption shows up where waste steam or cheap gas is available — a hospital, a campus, a cogen plant — including Florida jobs that already have boilers (boilers themselves remain Class A statutory scope).

Absorption install of a 15-ton machine can be inside Class B’s 25-ton cap. A 60-ton absorption chiller is still absorption theory on Class B and Class A install on capacity. The outline did not drop absorption from Class B the way it dropped centrifugal.

Ammonia — Class A, engine rooms, IIAR qualitative

Ammonia (R-717) is an efficient industrial refrigerant used in food plants, cold storage, and process systems, not in a 5-ton strip-center split. It is toxic, has a sharp odor, and is classified in the B2L neighborhood of ASHRAE 34 (toxic, lower flammability). It is lighter than air (molecular weight 17 versus air at about 29), so leaked vapor rises — ventilation design is not “floor sweeps only because it is a gas.”

FBC Mechanical 2023 Chapter 11 treats refrigeration machinery rooms, occupancy, and relief. Closed-circuit ammonia plants are designed to IIAR 2. Teach the rooms and the functions, not an unofficial ppm table:

• A dedicated machinery room (engine room) for the compressors, vessels, and headers that the code and IIAR require to be indoors in a machinery room.
• Mechanical ventilation capable of exhausting a leak, with emergency ventilation on detection — running a residential bath fan is not a machinery room.
• Ammonia detection that alarms and starts ventilation / emergency shutdown per the published standard used on the job (IIAR 2 / ASHRAE 15 / the listing). OSHA publishes workplace exposure limits (the federal 8-hour PEL is 50 ppm); do not invent IIAR alarm setpoints that you did not look up in the standard on the table.
• Emergency shutdown that stops compressors and isolates the plant the way the standard and the listing require.
• PPE, eyewash, and restricted access as the plant’s program and OSHA 1926/1910 require — this is not a Class B changeout in a hallway.
• Relief piping to a safe discharge, not into an air handler.

Ammonia systems are typically open-drive compressors (reciprocating or screw) with steel piping, oil management, and high latent heat. Water-cooled condensers and evaporative condensers are common; the “tower” may be an evaporative condenser rather than a separate centrifugal chiller tower. Class A HVAC or a mechanical contractor is the ticket conversation; Class B’s 25-ton cap and omitted centrifugal/ammonia install path are not a workaround. EPA Section 608 still applies to refrigerant handling as a federal technician credential; it does not replace a CILB Class A certificate, and ammonia plants have their own IIAR/OSHA overlay on top of 608.

Side-by-side — what the outline is really asking

Florida scenario

Three drawings land on the same qualifying agent’s desk in Duval County. Sheet M-1 is a Jacksonville high-rise with a 400-ton water-cooled centrifugal, inlet vanes plus VFD, 44/54 chilled water, induced-draft tower. That is Class A on tons, Class A on centrifugal install, and a surge/vane theory item even for Class B readers. Sheet M-2 is an Orlando hospital 200-ton double-effect LiBr absorber using campus steam; water is the refrigerant, LiBr is the absorbent, the tower is oversized because absorber and condenser both reject heat, and crystallization is the shutdown risk. Absorption theory is on both outlines; installing 200 tons is still Class A. Sheet M-3 is a frozen-food plant ammonia engine room: open-drive screws, IIAR 2 machinery room, mechanical ventilation, detectors, emergency stop, steel pipe, evaporative condensers. Class B does not contract it. The qualifier who treats all three as “just big AC” fails the outline. The qualifier who can name impeller versus generator versus engine room, and who knows which ticket may pull the permit, is the one Trade C is paying 23 percent to find.

Traps: (1) Calling every chiller over 25 tons a centrifugal. (2) Saying LiBr is the refrigerant. (3) Inventing ammonia detector ppm numbers that are not on the open-book table. (4) Thinking Class B may skip this section because centrifugal install was dropped. (5) Treating EPA 608 as a substitute for Class A on an ammonia plant.