Free RETA CARO Exam Flashcards

The standard unit of cooling capacity: 12,000 Btu/hr, based on the heat required to melt one ton of ice in 24 hours.

The amount of heat required to raise the temperature of 1 pound of water by 1°F. The base heat-measurement unit used throughout refrigeration calculations.

Heat that changes a substance's temperature without changing its physical state (e.g., cooling liquid ammonia or heating vapor). Measured directly with a thermometer.

Heat that changes a substance's state (liquid to vapor or vapor to liquid) at a constant temperature. Latent heat of vaporization is what the evaporator absorbs from the load.

The temperature rise of refrigerant vapor above its saturation temperature at a given pressure, on the low (suction) side. Confirms all liquid has boiled off before vapor reaches the compressor.

The temperature drop of liquid refrigerant below its saturation temperature at a given pressure, on the high (liquid) side after the condenser. Ensures solid liquid feeds the expansion device.

Compressor (raises pressure/temp) → Condenser (rejects heat) → Expansion device (drops pressure) → Evaporator (absorbs heat) → back to the compressor.

Meters refrigerant flow into the low side and drops pressure from condensing to evaporating pressure, causing a portion of the liquid to flash into vapor and cool the remaining liquid.

The condenser must reject the total heat absorbed by the evaporator PLUS the heat of compression added by the compressor. Condenser capacity is always sized above evaporator load.

Two independent refrigeration circuits linked by a cascade condenser: the high-temperature circuit's evaporator cools the low-temperature circuit's condenser, reaching much lower temperatures than a single ammonia circuit can efficiently reach.

R-717 — the '7' prefix marks it as an inorganic refrigerant, and '17' is ammonia's molecular weight (NH3).

-28°F at atmospheric pressure (0 psig). This is why ammonia systems commonly operate under vacuum at very low evaporator temperatures.

Roughly 15%-28% ammonia by volume in air (NIOSH Pocket Guide to Chemical Hazards), with an ignition temperature of about 1,204°F. Rated a lower-flammability refrigerant.

Ammonia vapor is lighter than air (relative density about 0.6), so it rises and disperses when released. Its strong odor is detectable at roughly 5-50 ppm, well below hazardous exposure levels, giving a natural warning property.

Each pressure corresponds to exactly one saturation temperature at equilibrium. Anchor points to memorize: 0 psig = -28°F (atmospheric boiling point), 0°F ≈ 16 psig, and 68°F ≈ 110 psig. Rising head pressure with a clean condenser signals a real problem such as non-condensables, not a calibration error.

Reciprocating (pistons, positive displacement, strong at high pressure ratios), Screw/rotary (continuous, oil-flooded rotors, best for high volume), Centrifugal (high-volume, low-pressure-rise, no positive displacement), and Scroll (orbiting spiral, small capacity).

Absolute discharge pressure divided by absolute suction pressure (both in psia, not psig). Higher ratios raise discharge temperature and reduce volumetric efficiency.

The ratio of refrigerant vapor actually pumped by a compressor to its theoretical (swept) displacement. It decreases as compression ratio increases because clearance-volume gas re-expands instead of being discharged.

Devices that hold reciprocating compressor suction valves open to prevent pumping on selected cylinders, reducing starting load or providing stepped capacity control.

The primary capacity-control device on a screw compressor. It also sets the internal volume ratio (Vi), which should be matched to the system's actual compression ratio for efficient operation.

Liquid ammonia entering the compression chamber instead of vapor. Causes a sharp knocking sound and can destroy valves, pistons, or rotors; usually caused by flood-back from the evaporator or an aggressive defrost.

Rises with higher compression ratio and lower suction superheat. Excessive discharge temperature breaks down lubricating oil and risks compressor damage — a key troubleshooting indicator.

A compressor starting and stopping too frequently. Common causes include a faulty low-pressure control, an oversized compressor for the load, or a low refrigerant charge.

Ammonia and refrigeration lubricating oil are NOT miscible (unlike most CFC/HFC systems) — the oil separates from the liquid ammonia rather than dissolving in it, so it must be mechanically returned or periodically drained.

A vessel installed just after compressor discharge that removes oil entrained in the hot discharge gas and returns it to the compressor crankcase or system oil reservoir, keeping oil out of the evaporators and condensers.

The actual pressure driving lubrication to bearings: oil pump discharge pressure minus crankcase (suction) pressure — not the gauge reading alone. Low net oil pressure triggers a compressor safety trip.

Oil accumulating in low-side vessels (evaporators, accumulators, surge drums) because it does not mix with liquid ammonia. Left unchecked, it insulates heat-transfer surfaces and starves the compressor of oil; must be periodically drained.

Isolate the vessel, relieve pressure to a safe low level, wear proper PPE (chemical goggles, gloves, protective clothing), and drain slowly through a dedicated oil pot to avoid a sudden ammonia release.

Synthetic oils (e.g., PAG/PAO-based) flow better at low evaporator temperatures and are common in modern ammonia systems. Mineral oils are more traditional but thicken and lose lubricity at very low temperatures.

Low net oil pressure, a compressor tripping on its low-oil-pressure safety switch, unusual bearing or piston noise, and rising bearing or discharge temperatures.

Oil is added to the compressor crankcase or system oil reservoir under positive pressure using an oil pump or pressurized charging drum — the system is never opened to atmosphere while pressurized.

Refrigerant flow is metered by a TXV that controls the superheat leaving the coil. Uses a smaller ammonia charge and simpler controls than a flooded system; common on smaller units.

The coil stays liquid-full, with level controlled by a float or surge drum rather than superheat. Higher heat-transfer efficiency than DX; common on large industrial systems.

Senses suction line superheat through a sensing bulb and (usually) an external equalizer, then modulates refrigerant flow into the evaporator to hold a set superheat.

A vessel installed ahead of the compressor suction line that traps any liquid ammonia carryover from the evaporator, protecting the compressor from slugging.

Frost on an evaporator coil insulates the surface and reduces heat transfer and cooling capacity. Defrost (hot gas, water, electric, or air) removes the frost and restores performance; hot gas defrost routes discharge gas back through the coil.

Rejects heat using both a water spray and forced airflow across the coil. The most common and most efficient condenser type in industrial ammonia plants.

Rejects heat using fans and airflow only, with no water spray. Simpler and lower-maintenance than evaporative condensers, but less efficient in hot climates.

The difference between condensing temperature and outdoor wet-bulb (evaporative) or dry-bulb (air-cooled) temperature. A rising approach with clean coils and good airflow points to non-condensables or overcharge.

A vessel that stores liquid ammonia after the condenser, buffering the system's refrigerant charge to handle varying loads and maintain a steady liquid feed to the low side.

The main liquid outlet (shutoff) valve on the high-pressure receiver, used to isolate the receiver's liquid ammonia from the rest of the system.

A safety device that automatically vents excess pressure from a vessel to atmosphere or a flare/relief header once the set pressure is exceeded. Sized, installed, and tested per ASME and IIAR requirements.

Air or other gases trapped in an ammonia system that will not condense at normal condenser conditions. They raise head pressure and reduce condenser efficiency even when charge and airflow are otherwise normal.

A device, usually near the condenser or receiver high point, that periodically collects and vents accumulated non-condensable gas while minimizing ammonia loss. Purge logs should be checked regularly to catch chronic air in-leakage.

OSHA's 8-hour time-weighted average exposure limit for ammonia: 50 ppm.

35 ppm as a 15-minute short-term exposure limit — the maximum average concentration workers may be exposed to for a brief period.

300 ppm — NIOSH's Immediately Dangerous to Life or Health concentration. Only self-contained breathing apparatus (SCBA) may be used at or above this level, never a filtering respirator.

10,000 pounds of anhydrous ammonia on-site triggers coverage under OSHA's Process Safety Management standard (29 CFR 1910.119) and EPA's Risk Management Program rule (40 CFR Part 68).

Self-contained breathing apparatus is required for entry into any atmosphere at or above the IDLH (300 ppm for ammonia); air-purifying (cartridge/filter) respirators are not adequate at these concentrations.

IIAR 2 covers safe design of closed-circuit systems, IIAR 6 covers inspection/testing/maintenance, IIAR 7 covers developing operating procedures, and IIAR 9 covers minimum safety criteria for existing systems.