NATE Core flashcards
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32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Sensible Heat
- Heat that changes the temperature of a substance without changing its state. In air, calculated as 1.08 × CFM × temperature difference in °F.
- Latent Heat
- Heat that causes a change of state (e.g., water vapor to liquid) without a temperature change. In air, calculated as 0.68 × CFM × humidity ratio difference in grains per pound.
- Enthalpy
- Total heat content of air per pound of dry air, combining sensible and latent energy. Used in the total heat formula: Qt = 4.5 × CFM × Δh.
- Superheat
- The number of degrees a vapor is above its saturation temperature at a given pressure. Measured as actual line temperature minus saturation temperature.
- Subcooling
- The number of degrees a liquid is below its saturation temperature at a given pressure. Measured as saturation temperature minus actual liquid line temperature.
- Saturation Temperature
- The temperature at which a refrigerant changes state at a given pressure. Also called boiling point or condensing point depending on direction of heat flow.
- Refrigerant Glide
- The temperature difference between the bubble point and dew point of a zeotropic refrigerant blend at a given pressure. Causes composition change during leakage.
- Bubble Point
- The temperature at which the first bubble of vapor forms in a liquid refrigerant mixture at a given pressure. Used for subcooling calculations on blends.
- Dew Point
- The temperature at which the first drop of liquid forms from a vapor refrigerant mixture at a given pressure. Used for superheat calculations on blends.
- PSI to Inches Water
- Conversion: 1 psi equals 2.31 feet of water column, or 27.7 inches of water column. Used for duct static pressure and gas pressure measurements.
- Standard Air Density
- The density of air at 70°F and sea level (29.92 inHg) is approximately 0.075 pounds per cubic foot. Used in heat load and airflow formulas.
- Specific Heat of Air
- The amount of heat required to raise one pound of air by 1°F, approximately 0.24 BTU/lb·°F at constant pressure. Used in sensible heat calculations.
- Ohm's Law
- The relationship between voltage (E), current (I), and resistance (R): E = I × R. Also expressed as I = E/R or R = E/I in electrical circuits.
- Electrical Power
- Power in watts equals voltage times current (P = E × I) for resistive circuits. For three-phase, P = E × I × 1.732 × power factor.
- BTU
- British Thermal Unit: the amount of heat required to raise one pound of water by 1°F. Standard unit of heat quantity in HVACR.
- Ton of Refrigeration
- A cooling capacity equal to 12,000 BTU per hour, or 200 BTU per minute. Based on the heat absorbed by melting one ton of ice in 24 hours.
- Sensible Heat Ratio
- The ratio of sensible heat to total heat (SHR = Qs / Qt). Used to determine coil performance and airflow requirements in air conditioning.
- Dry Bulb Temperature
- The temperature of air measured by a standard thermometer, indicating sensible heat only. Does not account for moisture content.
- Wet Bulb Temperature
- The lowest temperature achievable by evaporative cooling, measured with a thermometer whose bulb is covered by a wet wick. Indicates total heat content.
- Dew Point Temperature
- The temperature at which air becomes saturated (100% relative humidity) and water vapor begins to condense. Used to assess latent load and condensation risk.
- Relative Humidity
- The ratio of actual water vapor in air to the maximum amount it can hold at that temperature, expressed as a percentage. Changes with temperature even if moisture content remains constant.
- Humidity Ratio
- The weight of water vapor per pound of dry air, usually expressed in grains per pound (7000 grains = 1 lb). Used in latent heat calculations.
- Psychrometric Chart
- A graph showing the relationships among dry bulb, wet bulb, dew point, relative humidity, humidity ratio, and enthalpy of air at a given pressure.
- Pressure Conversion
- 1 atmosphere equals 14.696 psi or 29.92 inches of mercury. 1 psi equals 2.31 feet of water column or 27.7 inches of water column.
- Temperature Conversion
- To convert Celsius to Fahrenheit: °F = (°C × 1.8) + 32. To convert Fahrenheit to Celsius: °C = (°F − 32) / 1.8.
- Absolute Zero
- The theoretical lowest temperature where molecular motion stops, equal to −459.67°F or −273.15°C, or 0 Kelvin. Used in gas law calculations.
- Gas Laws
- Relationships among pressure, volume, and temperature of gases: Boyle's law (PV = constant), Charles's law (V/T = constant), and the ideal gas law (PV = nRT).
- Heat Transfer Modes
- Conduction (through solids), convection (through fluids), and radiation (electromagnetic waves). HVAC systems use all three in heating, cooling, and insulation.
- Latent Heat of Vaporization
- The heat required to change a substance from liquid to vapor at constant temperature. For water at 212°F, approximately 970 BTU per pound.
- Airflow Formula
- CFM = (sensible heat) / (1.08 × temperature difference). Used to determine required airflow for a given heating or cooling load.
- Static Pressure
- The outward force exerted by air on duct walls, measured in inches of water column. Used to determine fan performance and duct sizing.
- Velocity Pressure
- The pressure due to the velocity of air, measured in inches of water column. Total pressure equals static pressure plus velocity pressure.