BTEB Refrigeration & AC Exam flashcards

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

Coefficient of Performance

Ratio of refrigerating effect to compressor work input, COP = Q_evap / W. Higher COP means more cooling per unit of energy, and it falls as the temperature lift increases.

Ton of Refrigeration

Cooling rate equal to 3.517 kW, 12,000 Btu/h or 3024 kcal/h, defined as the heat needed to melt one short ton of ice in 24 hours.

Sensible Heat

Heat that changes the dry-bulb temperature of a substance without changing its moisture content, calculated as mass times specific heat times temperature difference.

Latent Heat

Heat absorbed or released during a phase change or moisture change without a dry-bulb temperature change, such as evaporation of water or condensation on a cooling coil.

Superheat

Difference between the actual suction line temperature and the saturation temperature at the same pressure, confirming that all liquid has boiled off before the compressor.

Subcooling

Difference between the condensing saturation temperature and the actual liquid line temperature, showing how far the liquid has been cooled below its condensing point.

Absolute Pressure

Pressure measured from a perfect vacuum, equal to gauge pressure plus atmospheric pressure of about 101.3 kPa; all refrigerant saturation tables use this value.

Refrigerant Glide

Temperature difference between bubble point and dew point in a zeotropic blend, which causes composition and temperature change during evaporation and condensation.

Vapour Compression Cycle

Continuous sequence of compression, condensation, expansion and evaporation in which a refrigerant absorbs heat at low pressure and rejects it at high pressure.

Thermostatic Expansion Valve

Modulating valve that maintains constant evaporator superheat by balancing bulb pressure against evaporator pressure plus an adjustable spring force.

Capillary Tube

Fixed restriction of small-bore tubing that meters refrigerant and equalises pressure at shutdown; performance depends critically on exact charge and tube length.

Filter Drier

Device in the liquid line containing a desiccant and screen that removes moisture and solid particles, protecting the expansion valve and compressor from damage.

Suction Accumulator

Vessel installed in the suction line that traps liquid refrigerant and oil, allowing controlled return to the compressor and preventing damaging liquid slugging.

Flooded Evaporator

Evaporator kept completely filled with liquid refrigerant, requiring a surge drum or low-side float and giving high heat transfer with careful liquid level control.

Scroll Compressor

Positive-displacement compressor using two interleaved spiral wraps, one orbiting, to compress gas continuously with few moving parts, low noise and no suction valves.

Condenser Heat Rejection

Total heat rejected by the condenser equals refrigerating effect plus compressor work, so the condenser load is always greater than the evaporator load.

Sensible Heat Ratio

Ratio of sensible load to total load; a low value indicates a large latent or moisture load that needs a lower apparatus dew point for dehumidification.

Bypass Factor

Fraction of air passing through a cooling coil without contacting the surface, calculated from leaving and entering dry-bulb temperatures and the apparatus dew point.

Psychrometric Chart

Graph plotting dry-bulb, wet-bulb, dew point, humidity ratio, relative humidity and enthalpy, used to find coil and room conditions and process lines.

Apparatus Dew Point

Effective coil surface temperature at which air leaving the coil would be saturated, found by extending the entering-to-leaving condition line to the saturation curve.

Ventilation Load

Heat and moisture introduced by outdoor air supplied for indoor air quality, calculated from airflow and the enthalpy or humidity difference between outdoor and room air.

Solar Heat Gain

Radiant heat entering through glazing and roof, depending on orientation, time of day, shading and glass properties, and a major component of peak cooling load.

Energy Efficiency Ratio

Ratio of cooling capacity in Btu/h to electrical power input in watts at rated conditions; a higher value indicates a more efficient unit.

Air Handling Unit

Factory or site-assembled casing containing filters, coils, fan and dampers that conditions and moves air through ductwork to the served spaces.

Fan Laws

Relations stating that airflow varies with speed, pressure with speed squared and power with speed cubed, used to predict performance at changed speed or size.

Cooling Tower Approach

Difference between the water leaving the cooling tower and the entering wet-bulb temperature; it cannot reach zero and typically ranges from three to five kelvin.

Chilled Water System

Arrangement in which a chiller cools water that is pumped to air handling or fan coil units, allowing central plant and refrigerant containment away from occupied spaces.

Triple Evacuation

Method of evacuating a system, breaking the vacuum with dry nitrogen, and repeating three times to remove moisture and non-condensable gases.

Micron Vacuum

Unit of vacuum equal to one thousandth of a millimetre of mercury; a standing vacuum of about 500 microns indicates an acceptable moisture-free, leak-free system.

Weigh-In Charging

Method of charging refrigerant by measuring the cylinder on a scale and adding the nameplate quantity, giving the most accurate charge for critically charged systems.

Nitrogen Pressure Test

Leak test using dry nitrogen at about one and a half times working pressure with soap solution or an electronic detector, never using oxygen or air.

Liquid Slugging

Condition in which liquid refrigerant reaches the compressor, causing noise, valve damage and oil dilution; prevented by correct superheat and an accumulator.

High Discharge Pressure

Abnormally high condensing pressure usually caused by a dirty condenser, non-condensable gases, overcharge, high ambient temperature or restricted cooling water.

Refrigerant Recovery

Process of removing refrigerant from a system into a recovery cylinder for reuse or disposal, required by law instead of venting to the atmosphere.

Lockout Tagout

Safety procedure of isolating electrical energy, applying a personal lock and tag, and verifying zero voltage before maintenance work begins.