Oklahoma Refrigeration Contractor

Key Concepts

Fundamentals of electricity - One volt is the electromotive force required to send one ampere of current through a resistance of one ohm (Section 12.1.1). - Resistance usually increases with an increase in temperature, an increase in conductor length, and a decrease in conductor diameter (Section 12.1.2). - Ohm's law: E = I × R; resistance equals voltage divided by current (Section 12.1.3). - A microfarad is one-millionth (0.000001) of a farad; most HVACR capacitors are rated in microfarads (Section 12.1.4). A high-voltage capacitor may store as much as 600 V — drain it with a 20,000 Ω (20 kΩ) resistor across both terminals for a couple of seconds while wearing insulated gloves (Section 12.1.4, Safety Note).

Circuits - Series: one path, so current is the same at every point; total voltage drop equals the total applied voltage (VDT = VD1 + VD2 + VD3 + …) (Sections 12.4.2, 12.4.5). - Parallel: each load sees the full source voltage; the load with the highest resistance receives the lowest current (Section 12.4.3). - Voltage drop VD = RL × IL (Section 12.4.5).

Key Concepts

Classification & numbering - ASHRAE 000–300 series: first digit from the right = fluorine atoms; second from the right = hydrogen atoms plus one; third from the right = carbon atoms minus one (Page 171 (22nd ed.; 176 in the 20th), Section 9.3.2). - A blend containing an HFC and an HCFC but no CFC is classified as an HCFC (Page 172, Section 9.2.4). - R-22 is an HCFC, being phased out: illegal since 2010 to manufacture new equipment containing R-22; by 2020 all production and importing was to cease. Retrofits use HFCs such as R-404A, R-407C, R-410A (Page 249 (22nd ed.; 265 in the 20th)). - Zeotropic blends (400 series) fractionate — component refrigerants have different boiling points, so a leak or vapor charging leaves an improper mixture (Pages 164–165 (22nd ed.; 173–174 in the 20th), 259). - EPA SNAP allows hydrocarbon refrigerants only in new equipment, not retrofits (Page 175, Section 9.2.6). - EPA pressure classes by boiling point at atmospheric pressure: high-pressure −58°F to 50°F; very high-pressure below −58°F; low-pressure above 50°F (Page 1315 (22nd ed.; 1568 in the 20th), Appendix G).

Oklahoma Refrigeration Contractor

Key Concepts

Tubing sizing (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 size is designated by the actual outside diameter. Copper water tubing nominal size = OD minus 1/8″. - ACR copper is available in soft and hard types, in wall thicknesses Type K (heavy wall) and Type L (medium wall); Type L is more commonly used. ACR tubing is usually charged and sealed with gaseous nitrogen. - Soft ACR tubing can be hardened by oxidation or by repeated bending and hammering — this is called work hardened. Annealing (heating to a dull cherry red, then slow air cooling) softens it. - Stainless steel tubing Type 304 (Sec. 8.1.2, p. 72 (22nd ed.; 139 in the 20th)) is used in food processing, ice cream manufacturing, milk handling, and specialized cooling coils. It is easily connected to fittings by flaring or brazing.

Cutting and bending (Section 5.2 (22nd ed.; 8.3 in the 20th), 8.4) - Hacksaw cutting: use a blade with a wave set pattern and 32 teeth per inch. More teeth per inch gives a cleaner cut on thin-walled tubing. - Many lever-type benders bend more than one size; common size compatibilities are 3/16″, 1/4″, 5/16″, and 3/8″. - Minimum radius for a 90° tubing bend is five times the tubing diameter.

Key Concepts

Drive configurations - An open-drive compressor is driven by a power source separate from the compressor unit; because the shaft passes from inside to outside the case, a shaft seal is required to keep refrigerant vapor from flowing out and air from flowing in (Ch. 18, Sec. 18.1.1). Unbalanced loading from improper belt tension or drive shaft misalignment increases shaft seal wear, making it a common failure point (Sec. 18.1.1 — Shaft Seal Wear). - Hermetic compressors seal the motor inside the same dome as the compressor, so no shaft seal is needed (Sec. 18.1.2). Fully hermetic units are sealed in a welded casing and cannot be serviced without cutting open the shell; semi-hermetic units use a bolted, multipart shell that can be unbolted for repair ("serviceable" hermetics) (Sec. 18.1.2). - Fully hermetic construction: compressor and motor are usually spring-mounted inside the dome; discharge and suction lines inside the dome are flexible (Sec. 18.1.2 — Construction). Incoming suction vapor cools the motor and picks up some oil (less than 1%) before entering the suction chamber (Sec. 18.1.2 — Lubrication). - Engine-driven compressors: a pressure control connects to the engine throttle with the sensing device in the low-side suction line (Sec. 18.1.1). - Many small hermetic systems have no service valves; use clamp-on tapping (piercing) valves for gauge connections (Sec. 18.3.1).

Oklahoma Refrigeration Contractor

Key Concepts

Cycle fundamentals - The compressor and the metering device are the two dividers between the high side and low side (pp. 126–127 (22nd ed.; 91–92 in the 20th)). - Refrigerant entering the compressor during normal operation is superheated vapor (pp. 126–128 (22nd ed.; 91–93 in the 20th)). - In the condenser, refrigerant temperature does not drop below the condensing temperature until after all vapor has changed to saturated liquid; further heat removal produces subcooling (pp. 128–129 (22nd ed.; 93–95 in the 20th)). - Saturated vapor is most commonly found in the two heat exchangers: evaporator and condenser (p. 109 (22nd ed.; 84 in the 20th)). - Refrigerant that vaporizes immediately at the metering device is flash gas; it absorbs heat from the remaining liquid, cooling it (p. 1271 (22nd ed.; 1370 in the 20th)). Flash gas in the liquid line can damage an expansion valve and reduces evaporator efficiency (p. 130 (22nd ed.; 97 in the 20th)). - Ideal compression is isentropic — constant entropy on the P-h diagram (pp. 1271–1275 (22nd ed.; 1370–1374 in the 20th)).

Key Concepts

Fire protection and extinguishers - A fire extinguisher rated not less than 2A for each 3,000 sq ft of protected building area, or major fraction thereof; travel distance from any point of the protected area to the nearest extinguisher shall not exceed 100 feet (§1926.150(c)(1)(i)). - One or more 2A-rated extinguishers 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 liquid or 5 pounds of flammable gas are in use (§1926.150(c)(1)(vi)). - Substitutions: one 55-gallon open drum of water with two fire pails, or a ½-inch garden-type hose not over 100 ft discharging at least 5 gpm with a 30-foot horizontal stream, may replace a 2A extinguisher (§1926.150(c)(1)(ii)–(iii)). - 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 close of work (§1926.150(d)(1)(ii)). - Fire classes: Class A ordinary combustibles; Class B flammable liquids (grease, gasoline, oils, paints — solvents); Class C electrical equipment; Class D combustible metals. Never use water on a Class C fire (Modern Refrigeration and Air Conditioning, §2.2.2).