18.2 Control Requirements, Fan Motors, CFM & Compressor Capacity

18.2 Control Requirements, Fan Motors, CFM & Compressor Capacity

Trade Area A does not stop at pipe diameter. The outline asks you to determine control requirements, fan motors and CFM, and compressor capacity. Those three sit on the same drawing: the coil cannot use the gpm you just calculated unless the compressor (or chiller) actually makes that capacity at the real suction and discharge temperatures, and the air side cannot use that capacity unless the fan delivers the scheduled CFM on the real system curve. Refrigeration & Air Conditioning Technology, 9th Edition (2021), the Trane Ductulator, and FBC Mechanical 2023 support this cluster. Class B still needs the theory of large-machine capacity and unloaders even though the Class B outline drops the 25–100 ton, over 100 ton, and centrifugal compressor install items.

Quick Answer: CFM = area (ft²) × velocity (fpm). Fan horsepower cubes with speed. Compressor capacity comes from the manufacturer’s table at the actual SST and SDT, not from the nominal ton nameplate. Safety controls cut the machine out; operating controls stage or unload it. Pneumatic control piping is Class A only.

Control requirements — operating versus safety

A control requirement on a design set is not “put a thermostat on the wall.” It is the combination of devices the listing, FBC Mechanical, and NFPA 90A/90B need so the system starts, runs, and stops without destroying itself or moving smoke into a corridor.

Operating controls match capacity to load: space thermostat or sensor, humidistat, economizer changeover, occupancy or time clock, DDC setpoint reset, capacity-staging or unloading, and the low-voltage heating/ventilating/air-conditioning control wiring that F.S. 489.105(3)(f) and (g) expressly allow both Class A and Class B to install, disconnect, and reconnect.

Safety controls open the circuit when a limit is crossed. Typical stack on a DX or chilled-water air handler:

• High-pressure cutout on the refrigerant discharge
• Low-pressure cutout on the suction (loss of charge, closed valve, failed evaporator fan)
• Freeze stat or low-limit on a chilled-water or DX coil that can ice
• High-limit on a furnace or electric heat
• Flow or differential-pressure switch on a water-cooled condenser or chiller evaporator (no water, no compressor)
• Overflow / auxiliary drain float or pan switch (FBC Mechanical condensate)
• Smoke detector in the supply (and often return) on systems that meet the NFPA 90A size/airflow trigger, shutting down fans so the air handler does not feed a fire

Those safeties are not the same as a fire-alarm contractor’s building smoke-control panel, and they are not a license to install fire-sprinkler flow switches. Interlocks with kitchen suppression are Section 18.4: makeup air typically stops; grease exhaust typically continues.

Pneumatic controls use compressed air in small-diameter pneumatic control piping to move damper motors and valves. F.S. 489.105(3)(f) lists pneumatic control piping in the Class A scope. Class B’s definition does not. Class B may still install electric/electronic low-voltage HVAC controls. If the drawings show a pneumatic compressor, high-pressure main, reducing station, and 1/4-inch plastic or copper control tubing, that is a Class A (or mechanical contractor) job. DDC that replaces pneumatics is still low-voltage control wiring plus sensors — in both classes’ statutory grant.

Power wiring stays inside the HVAC electrical fence: replace, disconnect, or reconnect on the line or load side of a dedicated existing disconnect on single-phase; repair or replace power wiring, disconnects, breakers, or fuses for dedicated HVAC circuits with a circuit-breaker lock. New three-phase feeders and new disconnects that are not those exceptions belong to an electrical contractor.

Fan motors and CFM

Airflow in cubic feet per minute is:

(\text{CFM} = A \times V)

where A is internal cross-section in square feet and V is velocity in feet per minute. For rectangular duct, (A = (W \times H) / 144) with width and height in inches.

Worked CFM example. A 16-inch by 12-inch supply main (use inside dimensions on the exam unless the problem gives OD and gauge) at 800 fpm:

(A = (16 \times 12) / 144 = 1.333\ \text{ft}^2)

(\text{CFM} = 1.333 \times 800 \approx 1{,}067\ \text{cfm})

A round 14-inch duct has area (\pi d^2 / 4 / 144). Diameter 14 inches → area ≈ 1.069 ft². At 800 fpm that is about 855 cfm. Treating “16×12” as 192 cfm, or treating fpm as if it were already cfm, is the usual wrong answer.

A planning number of 350 to 450 cfm per ton is a starting point, not FBC Mechanical. Florida latent loads often want the lower end so the coil can dehumidify. The scheduled CFM still has to sit on the fan curve at the external static pressure of the real duct, filter, and coil. The operating point is the intersection of the manufacturer’s CFM-versus-static plot and the system curve (static rises roughly with CFM squared).

Fan laws (same fan, same duct system):

• CFM varies directly with RPM
• Static pressure varies with RPM squared
• Brake horsepower varies with RPM cubed

Need 20 percent more air by speeding the blower? (1.2^3 = 1.728) — about 73 percent more power, more noise, and a motor that may leave the nameplate FLA. The better design change is more duct area or a different fan, not a bigger pulley on an undersized motor. Motor service factor is not a license to park at 115 percent of FLA all afternoon in a 105°F Miami penthouse.

Select the motor from brake horsepower at the operating point, plus the drive loss, at the scheduled voltage and phase. A 208-volt three-phase motor on 240-volt single-phase is not a field conversion. Belt length, pitch diameter, and RPM are handbook/drive-table items; rotation must match the scroll. Grease-exhaust fans have extra rules (motor out of the airstream, UL 762) in Section 18.4.

Compressor capacity

One refrigeration ton = 12,000 Btu/h. A “10-ton” nameplate is a nominal rating-test capacity, not a promise at every outdoor condition. Manufacturer expanded performance data list capacity against saturated suction temperature (SST) and saturated discharge (condensing) temperature (SDT) — or against evaporator and condenser entering-fluid temperatures on a water-cooled machine.

Capacity falls when:

• Condensing temperature rises (dirty condenser, recirculated discharge air, high outdoor wet-bulb on a cooling tower, undersized condenser water)
• Suction temperature drops (starved evaporator, low airflow, iced coil, low chilled-water flow)

Capacity rises when SST rises and SDT falls, which is why a 95°F AHRI point and a 115°F Florida roof are not the same tons. The exam item is: read the table at the stated SST/SDT, do not recite the nameplate.

Capacity control keeps the compressor from short-cycling when the building load is below design:

Unloaders reduce capacity to match load; they do not increase capacity when head pressure is high. If the table says 9.2 tons at 45°F SST / 125°F SDT and the nameplate says 10, the design capacity is 9.2, and the condenser or tower is what you fix.

Florida scenario. A Volusia County (Climate Zone 2) office is issued with a 20-ton packaged rooftop — inside Class B’s 25-ton cap — scheduled at 8,000 cfm. The supply main is 20×12. Area = 1.667 ft², so the duct velocity at 8,000 cfm would be 4,800 fpm, which will not sit on a packaged fan curve and is not a duct-design velocity. The correct control/fan recommendation is to enlarge the duct (or split mains) until CFM = area × a reasonable fpm, then pick the motor from the fan chart at that static — not to overspeed the stock blower until horsepower cubes. A second drawing set for the same campus shows an 80-ton air-cooled chiller with unloaders. Class B still needs to read that capacity table and explain unloaders; Class B does not install that 80-ton machine as one system, and does not run pneumatic mains to its valves.