HVAC Excellence Master LC Refrigeration flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Sensible heat

Heat that changes a substance's temperature without changing its state. In air it is estimated with Q = 1.08 x CFM x temperature difference, using dry-bulb readings.

Latent heat

Heat absorbed or released during a phase change at constant temperature, such as boiling or condensing refrigerant. It does not show on a thermometer but moves most of the system's capacity.

Superheat definition

The number of degrees a vapor is above its saturation temperature at the measured pressure. Suction superheat is measured at the evaporator outlet and indicates metering device performance.

Subcooling definition

The number of degrees a liquid is below its saturation temperature at the measured pressure. Liquid-line subcooling confirms the condenser has fully condensed refrigerant and is used for TXV charging.

TXV operation

A thermostatic expansion valve modulates refrigerant flow to hold a set evaporator superheat. Its bulb senses outlet temperature, and rising superheat opens the valve to admit more refrigerant.

Accumulator purpose

A vessel on the suction line that traps liquid refrigerant and oil and meters them back as vapor, protecting the compressor from liquid floodback in systems without a receiver or with long line sets.

Zeotropic blend glide

The temperature spread between bubble point and dew point for a blend such as R-407C. Charge as liquid, and use dew point for superheat and bubble point for subcooling calculations.

Compressor locked rotor amps

The current drawn when the rotor cannot turn, typically five to seven times running load amps. Compare measured inrush against the nameplate LRA to judge winding and start components.

Nitrogen purge while brazing

A low-pressure nitrogen flow through the tubing during brazing displaces oxygen and prevents copper oxide scale. Scale flakes later clog driers, screens, and capillary tubes and cause restrictions.

Suction line oil return

Horizontal suction runs slope about a quarter inch per ten feet toward the compressor, and traps at riser bases plus periodic riser traps keep oil moving back to the crankcase.

Evacuation micron target

Most manufacturers specify 500 microns or lower, verified by a standing vacuum decay test. A rising vacuum after isolation indicates moisture or a leak in the circuit.

Triple evacuation

Evacuate, break the vacuum with dry nitrogen, and repeat. The nitrogen sweeps absorb and carry out residual moisture, which is useful on large or contaminated systems.

Phosphorus brazing filler

BCuP filler is used for copper-to-copper joints without flux. Copper to brass or steel requires silver-bearing filler and flux, and flux residue must be cleaned after brazing.

Liquid line pressure drop

Excessive liquid-line drop causes flash gas ahead of the metering device and reduces capacity. Keep drop low, typically under two to three psi, by sizing lines per manufacturer tables.

Equipment clearance and pad

Rooftop and condensing units need level mounting, nameplate service clearances, unobstructed airflow, and vibration isolators torqued to specification to prevent noise and line fatigue.

Line set support spacing

Horizontal refrigerant tubing is typically supported about every six feet, with vertical risers supported at each floor. Hangers must not crush insulation or restrict thermal movement.

Ohm's law and power

E = I x R and P = I x E. For three-phase loads, P = 1.732 x V x A x power factor. These formulas size circuits and verify measured current against nameplate ratings.

Hermetic winding check

Measure resistance from each terminal to common and to the shell. Infinite resistance means an open winding, near zero means a short, and continuity to the shell means a grounded compressor.

Run capacitor test

Use the capacitance function or microfarad scale and compare with the nameplate rating, usually within about six percent. A weak or bulged capacitor causes high current and poor starting.

Contactor failure signs

Pitted, burned, or welded contacts cause intermittent operation or a compressor that will not shut off. Verify coil voltage and pull-in before replacing the contactor.

Low pressure cutout

A safety switch that stops the compressor when suction pressure falls below a setpoint, protecting against low charge and coil freeze-up. It should never be bypassed as a permanent repair.

Oil pressure safety switch

A control that trips the compressor if oil pressure does not rise within a timed interval, commonly 120 seconds, after startup. It protects against bearing damage from oil failure.

Heat pump reversing valve

The valve is energized or de-energized through the O or B terminal to swap evaporator and condenser roles. A valve stuck mid-travel causes poor heating and cooling performance.

Nameplate MCA and MOP

Minimum circuit ampacity sizes the branch-circuit conductors, and maximum overcurrent protection sizes the breaker or fuse. Both come from the equipment nameplate, not from rules of thumb.

Weigh-in charging

The most accurate charging method: nameplate factory charge plus additional charge per foot of installed liquid line, adjusted for line size. Use it whenever the total charge is known.

TXV subcooling target

For TXV systems, charge to the nameplate subcooling, commonly eight to twelve degrees Fahrenheit, measured at the condenser outlet with steady-state conditions and correct airflow.

Fixed orifice superheat

Superheat for a piston system is set from the manufacturer chart using indoor wet-bulb and outdoor dry-bulb. Target values commonly range from about five to fifteen degrees.

Condenser approach method

Condensing temperature minus outdoor ambient. A high approach with clean coils and correct airflow indicates overcharge or non-condensables rather than a load problem.

Steady-state readings

Pressures and temperatures are valid only after fifteen to twenty minutes of operation with panels in place, correct airflow, and stable readings. Charging before stabilization misleads diagnosis.

Evaporator air temperature split

For comfort cooling, the difference between return and supply air is commonly about eighteen to twenty-two degrees Fahrenheit. A low split suggests low load or excess airflow.

Non-condensable symptoms

High discharge pressure and high subcooling with a normal or high condenser approach, often after poor evacuation. Correct by recovering refrigerant, evacuating, and recharging by weight.

Frozen evaporator diagnosis

Frost indicates low airflow, low charge, a restricted metering device, or a stuck-open valve. Find and correct the cause before adding refrigerant or the coil will refreeze.