Tennessee CMC-C Mechanical HVAC/Refrigeration Contractor

Key Concepts

Circuit diagrams - Pictorial diagrams show all connections among a circuit's devices and the approximate physical location of devices in a unit; they may also give wire colors (Ch. 16, Sec. 16.1.1). - In a ladder diagram, the vertical lines are the power supply and the horizontal lines (rungs) contain the controls and loads. Each rung is an individual circuit with a load, and may also contain switches, relays, or control devices (Ch. 16, Sec. 16.1.2). - Ladder diagrams reflect the order of component operation: devices near the top rung are energized and started before devices further down (Ch. 16, Sec. 16.1.2).

Topic: Piping — Refrigeration, Hydronic, Steam and Process

Key Concepts

Tubing and pipe materials - ACR tubing size is designated by the actual outside diameter; copper water tubing nominal size = OD minus 1/8″ (Modern Refrigeration and Air Conditioning, Ch. 5 (22nd ed.; 8 in the 20th), Section 5.1.1 (22nd ed.; 8.1.1 in the 20th)). - ACR tubing is charged and sealed with gaseous nitrogen to keep it clean and dry until used (Page 71 (22nd ed.; 138 in the 20th)). - Soft ACR tubing hardened by oxidation or repeated bending/hammering is work hardened; it may crack at stress points when flared, and is softened by annealing (Section 5.1.1 (22nd ed.; 8.1.1 in the 20th)). - Hard-drawn ACR tubing is hard and stiff, so it needs few clamps or supports, particularly in larger diameters; it should not be bent,…

Key Concepts

• The two dividers between high side and low side are the compressor and the metering device. The compressor draws low-pressure vapor from the evaporator and compresses it into high-pressure vapor, pushing it into the high side; the metering device separates the high side from the low side at the other end of the cycle by restricting flow and lowering pressure (Modern Refrigeration and Air Conditioning, Pages 128 (22nd ed.; 91 in the 20th)–93).
• Flash gas. When high-pressure liquid passes through the metering device, the pressure drop causes some refrigerant to vaporize immediately. This vapor is called flash gas, and it absorbs heat from the remaining liquid, cooling it (Page 1271 (22nd ed.; 1370 in the 20th)). In an expansion-valve system, flash gas in the liquid line can damage the expansion valve and reduces evaporator efficiency…

Tennessee CMC-C Mechanical HVAC/Refrigeration Contractor

Key Concepts

Compressors - Scroll: refrigerant pockets are pushed toward the center, reducing in volume and increasing in pressure; the vapor discharges through a port at the center of the scrolls (p. 441–442). - Screw: an unloader mechanism (often a slide valve) repositions the discharge port or opens passages that recirculate trapped vapor back to suction. A slide valve stuck in the unloaded position lets the compressor run without building adequate discharge pressure (p. 442–443). - Reciprocating unloading: an eight-cylinder compressor unloaded in pairs drops 25% per step — 8 cyl = 100%, 6 = 75%, 4 = 50%, 2 = 25% (p. 549 (22nd ed.; 879 in the 20th)). - Crankcase heater: warms the compressor so refrigerant does not condense into liquid during the Off cycle, preventing liquid slugging/flooded starts (p. 1039 (22nd…

Key Concepts

Evacuation levels and targets - Low-pressure refrigerants (e.g., R-123): evacuate to 25 mm Hg absolute (≈2500 microns or 29 in. Hg vacuum) with recovery equipment manufactured after November 15, 1993 — EPA 608; Page 1318 (22nd ed.; 1572 in the 20th) (Appendix G). - High-pressure appliances such as R-410A systems under 200 lb charge: 0 psig — EPA 608. - Most manufacturers recommend 500 microns prior to charging, to boil off all moisture; may take several hours — Page 210 (22nd ed.; 223 in the 20th). - Deep vacuum: pull to 250 microns; after shutting off the pump the reading rises for about one minute — if it levels off and holds at 500 microns or less, the system is dry and leak-free — Page 239 (22nd ed.; 255 in the 20th). - A reading that levels…

Tennessee CMC-C Mechanical HVAC/Refrigeration Contractor · 10 questions (10% of exam)

Key Concepts

Combustion and flame appearance - A proper flame has a blue inner core surrounded by light blue. A yellow flame tip means insufficient primary air and incomplete combustion — unburned carbon particles. Orange outer edges come from dust drawn in with primary air and are not a sign of lack of primary air. Too much primary air makes the flame noisy and lift above the burner. (Modern Refrigeration and Air Conditioning, p. 757 (22nd ed.; 1118 in the 20th)) - Natural gas flammability limits: 4.0% lower, 14.0% upper; ideal concentration 8–10%. (MRAC, Fig. 41-3, p. 757 (22nd ed.; 1118 in the 20th) area)

Key Concepts

Appliance categories and type matching - A Category IV furnace is a positive-pressure venting, condensing furnace; flue gas temperature is less than 140°F (60°C), below its dew point, so some flue gas condenses (secondary heat exchanger, PVC vent). A Category I furnace is negative-pressure, noncondensing, with flue gas at least 140°F (60°C) — Modern Refrigeration and Air Conditioning, p. 776 (22nd ed.; 1134 in the 20th). - Design, sizing and installation of vents for Category II, III and IV appliances follow the appliance manufacturer's instructions (IFGC 501.14, 503.6.10.3). Venting type by appliance is set in Table 503.4 (e.g., listed vented wall furnaces → Type B-W gas vent). - Low-heat appliance (residential appliance): products of combustion at the flue entrance are 1,000°F (538°C) or less (IMC Chapter 2 (22nd ed.; 2 in the 20th), Appliance Type). Medium-heat: more…

Tennessee CMC-C Mechanical HVAC/Refrigeration Contractor · 14 questions (14%)

Key Concepts

Duct pressure classification and material selection - When the designer does not specify a pressure class, variable volume duct upstream of VAV boxes defaults to 2 in. wg (500 Pa); all other ducts default to 1 in. wg (250 Pa) (SMACNA HVAC-DCS S1.4, p. 1.1). - The greater duct dimension determines the gage for all sides, reinforced or unreinforced (SMACNA Section 2.1.1, Item 3, p. 2.1). - Duct gage/reinforcement tables are based on G-60 coated galvanized steel of lockforming grade, ASTM A653 and A924 (S1.11, p. 2.4). Casing in chambers with moisture present and metal drain pans must be G-90 (S6.6, S6.7, Ch. 9). - Aluminum: modulus of elasticity is one-third that of steel, yield strength about one-half, so aluminum must be about 44% thicker (Section 2.10, p.…

Tennessee CMC-C Mechanical (HVAC/Refrigeration) — 5 questions (5%)

Key Concepts

• Indoor design conditions. The Manual J "bulls eye" for heating is 70°F dry-bulb; 75°F is the cooling design dry-bulb (with 50% RH wet-coil / 45% RH dry-coil climates) — Manual J, Section 3, Page 3-6. MRAC states the indoor design temperature is generally 70°F or 72°F.
• Outdoor design temperature (ODT). ASHRAE gives three values per locality: 99.6% for residences and small, poorly insulated office buildings; 99% for well-constructed, well-insulated buildings with a standard number of windows; 97.5% for large buildings with considerable thermal capacity and small total window area — MRAC, Page 722 (22nd ed.; 991 in the 20th).
• Material properties. K-value (thermal conductivity) = heat through one square foot one inch thick, in one hour, per 1°F difference; units Btu·in/hr·ft²·°F. C-value uses the material's **actual…

Key Concepts

Fire protection (Subpart F, §1926.150) - A fire extinguisher rated not less than 2A shall be provided for each 3,000 square feet of protected building area, or major fraction thereof; travel distance to the nearest extinguisher shall not exceed 100 feet — §1926.150(c)(1)(i). - One or more extinguishers rated not less than 2A on each floor; in multistory buildings at least one adjacent to the stairway — §1926.150(c)(1)(iv). - A 10B-rated extinguisher within 50 feet wherever more than 5 gallons of flammable/combustible liquids or 5 pounds of flammable gas are used — §1926.150(c)(1)(vi). - An alarm system (telephone, siren, etc.) alerting employees and the local fire department — §1926.150(e)(1); the alarm code and reporting instructions must be conspicuously posted at phones and employee entrances — §1926.150(e)(2). - During demolition or alterations, **sprinkler control valves checked daily at…