NVQ L4 Refrigeration & AC flashcards

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

A2L refrigerant flammability limit

A2L refrigerants such as R-32 have a lower flammability limit of about 0.3 kg/m3, so ignition sources and ventilation must be controlled within the occupied space.

Refrigerant vapour exposure limit

The UK workplace exposure limit for R-134a and R-32 is 1000 ppm over an eight-hour reference period; atmosphere monitoring is required in plant rooms where leakage could accumulate.

Confined space entry control

Oxygen must be tested between 19.5% and 23.5% by volume before entry, with a permit to work, continuous monitoring and a rescue plan in place.

Brazing nitrogen purge

Passing oxygen-free nitrogen through pipework while brazing displaces air and prevents internal oxidation, which would otherwise form scale that damages compressors and expansion valves.

Sensible heat ratio

SHR is the sensible load divided by the total load; values below about 0.75 mean the latent load dominates and reheat or desiccant control may be needed.

Refrigerant glide

The temperature difference between the bubble and dew points of a zeotropic blend at constant pressure; R-407C glides about 5-7 K while R-410A glides under 0.5 K.

Suction line pressure drop limit

Suction lines are sized so that total pressure drop equals no more than about 1 K of saturation temperature, keeping capacity loss and compressor power within acceptable limits.

Minimum oil return velocity

Horizontal suction lines need at least 3-4 m/s and vertical risers 7-8 m/s to carry oil back to the compressor; dual risers maintain velocity at part load.

Critical path float

Float equals late start minus early start; activities on the critical path have zero float, so any delay to them directly delays the whole project completion date.

F10 notification threshold

A construction project must be notified to the HSE when it lasts more than 30 working days with more than 20 workers simultaneously, or exceeds 500 person-days.

Schedule performance index

SPI is earned value divided by planned value; a value below 1.0 means the project is behind schedule and progress is slower than the baseline forecast.

Long-lead plant scheduling

Chillers and large air handling units can take 12-20 weeks to deliver, so their order dates are set backwards from the required installation date.

Strength pressure test

Pipework is strength tested with oxygen-free nitrogen at 1.43 times the maximum allowable pressure, then leak tested at 1.1 times that pressure, following BS EN 378-2.

Triple evacuation target

Evacuate to below 500 microns (0.67 mbar) three times, breaking the vacuum with nitrogen between stages, to remove moisture and non-condensables before charging.

Standing vacuum test

After evacuation the vacuum should hold around 200-500 microns for at least one hour with no significant rise, proving the system is dry and leak-free.

Air balancing tolerance

Commissioned air volumes should be within plus or minus 10% of design figures at each terminal, achieved by proportional balancing from the index circuit.

High superheat low suction

High superheat with low suction pressure and a small evaporator temperature difference usually indicates undercharge, a liquid line restriction or a blocked filter drier.

Non-condensable symptoms

Air or nitrogen trapped in the system raises discharge pressure and subcooling and causes bubbles in the liquid sight glass even when the refrigerant charge is correct.

Compressor winding resistance check

Phase-to-phase resistances should be balanced within about 2%; a large imbalance, or insulation resistance below 1 MOhm at 500 V, indicates winding damage or moisture.

Acid test on oil

An acid test kit confirms oil contamination after a compressor burnout; a positive result requires an oil change, new filter driers and often suction line filtration.

Carbon dioxide equivalent calculation

Tonnes of CO2 equivalent equal the refrigerant charge in kilograms multiplied by its GWP and divided by 1000; this figure sets the leak check frequency band.

F-gas leak check frequency

Systems containing 5 tCO2e or more need checks every 12 months, 50 tCO2e every 6 months and 500 tCO2e every 3 months; fitted detection systems halve the intervals.

Refrigerant recovery obligation

Refrigerant must be recovered before equipment is dismantled or disposed of; venting to atmosphere is prohibited and recovered gas must be recycled, reclaimed or destroyed.

GWP of R-410A

R-410A has a GWP of 2088, so a 10 kg charge equals about 20.9 tCO2e, which places the system in the six-monthly leak check band.

Toolbox talk record

A short documented briefing given before a task; the attendance record and content provide evidence of supervision and competence control under health and safety law.

SMSTS supervisor certification

The Site Management Safety Training Scheme certificate shows a supervisor's health and safety knowledge and is commonly required on UK construction sites for managing work teams.

GROW coaching model

Goal, Reality, Options, Will: a structured conversation a supervisor uses to draw the solution from the technician rather than simply issuing an instruction.

Herzberg two-factor theory

Hygiene factors such as pay and conditions only prevent dissatisfaction; motivators such as recognition, responsibility and achievement are what actually raise a team's engagement.

Inspection and test plan

An ITP lists the inspections, tests and records required at each stage of the work, marking hold points where work cannot continue without a signed release.

Five whys root cause

Repeatedly asking why a failure occurred, typically five times, moves the analysis past symptoms to the underlying process or design cause.

Chiller specific power

Chiller efficiency expressed as kW of electrical input per kW of cooling; a machine with a COP of 3.0 has a specific power of 0.333 kW/kW.

PDCA improvement cycle

Plan, Do, Check, Act: a repeating cycle used to trial a change, measure its effect and standardise what works before starting the next improvement.