PE Mechanical HVAC key terms

A method of comparing design options by totaling initial cost plus present value of operating, maintenance, energy, and replacement costs over the equipment's expected service life.

Ratio of the maximum coincident load on a system to the sum of individual peak loads; less than one when peaks do not occur simultaneously.

Extra capacity or safety factor added above calculated load to cover uncertainty, future growth, or unusual conditions; excessive margin harms efficiency and increases first cost.

Graph plotting dry-bulb temperature, humidity ratio, relative humidity, enthalpy, and wet-bulb temperature so moist-air properties and processes can be read graphically.

Temperature of air measured by an ordinary thermometer, unaffected by moisture content or air motion; used with humidity ratio to fix a psychrometric state.

Temperature indicated by a thermometer with a wetted bulb exposed to moving air; approximates the adiabatic saturation temperature and indicates evaporative cooling potential.

Temperature at which air becomes saturated when cooled at constant pressure and humidity ratio; further cooling produces condensation.

Ratio of actual water vapor pressure in air to the saturation vapor pressure at the same dry-bulb temperature, expressed as a percentage.

Mass of water vapor per unit mass of dry air in a moist-air mixture, often expressed in kg/kg or lb/lb; also called specific humidity.

Fraction of total coil or space heat load that is sensible; the remainder is latent. It determines the slope of the process line on a psychrometric chart.

Effective coil surface temperature found by extending the room-to-supply-air process line to saturation on a psychrometric chart; indicates coil dehumidifying capability.

Fraction of air passing through a coil or filter that contacts no effective surface and leaves unchanged; used to calculate leaving-air conditions.

Cooling capacity equal to 12,000 Btu per hour, or about 3.517 kW; historically the heat absorbed melting one short ton of ice per day.

Ratio of useful cooling or heating delivered to the work or energy input required; higher values mean more efficient operation.

Temperature change of a zeotropic refrigerant blend during evaporation or condensation at constant pressure; affects coil design, charging, and leak detection.

Amount by which refrigerant vapor temperature exceeds its saturation temperature at the same pressure; measured at the evaporator outlet to check expansion-valve control.

Amount by which liquid refrigerant temperature falls below its saturation temperature at the same pressure; ensures solid liquid reaches the expansion device.

Heat exchanger in which low-pressure refrigerant absorbs heat from air or water and boils, producing the refrigerating effect.

Heat exchanger that rejects heat from hot refrigerant vapor to ambient air or water, causing the refrigerant to condense to liquid.

Refrigerant metering device that maintains a set superheat at the evaporator outlet by modulating flow in response to bulb temperature and pressure.

Packaged refrigeration machine that cools water or another secondary fluid for distribution to air-handling units or terminal devices.

Difference between the water leaving a cooling tower and the entering air wet-bulb temperature; a smaller approach means a larger tower and better performance.

Factory-assembled or field-built enclosure containing fans, coils, filters, dampers, and controls that conditions and moves air to served spaces.

HVAC system that varies supply airflow to zones while holding supply-air temperature roughly constant, using terminal boxes and dampers to match loads.

Control strategy that uses cool outdoor air instead of mechanical cooling when outdoor conditions are favorable, reducing compressor energy.

Pressure exerted by air against duct walls, measured relative to atmosphere; used to select fans and control variable-air-volume systems.

Graph of fan static pressure versus airflow at a fixed speed, showing the operating point where it intersects the system resistance curve.

Relationships showing that for geometrically similar pumps, flow varies with speed, head with speed squared, and power with speed cubed.

Energy conservation for steady, incompressible, frictionless flow along a streamline, relating pressure, velocity, and elevation heads.

Dimensionless ratio of inertial to viscous forces in fluid flow in a pipe or duct; indicates whether flow is laminar or turbulent.

Absolute pressure at a pump suction, above the liquid vapor pressure, available to prevent cavitation; must exceed the pump manufacturer's required value.

Proportionality constant relating heat flux to temperature difference between a surface and adjacent fluid; depends on fluid properties and flow conditions.

Overall heat transfer coefficient for an assembly such as a wall or window, describing heat flow per unit area per unit temperature difference including all resistances.

Opposition to heat flow through a material or assembly, equal to thickness divided by thermal conductivity for conduction; resistances in series add.

Average temperature difference between hot and cold fluids in a heat exchanger, calculated from terminal differences; used with U-factor and area to find heat transfer.

Ideal reversible thermodynamic cycle operating between two constant-temperature reservoirs; sets the maximum possible efficiency for any heat engine or refrigeration cycle.

Ratio of actual work output to ideal work for a compressor or turbine undergoing the same pressure change with no entropy change.

Energy conservation principle stating that energy entering a system minus energy leaving equals the change in stored energy; heat and work are energy transfers.

Thermodynamic property measuring energy dispersal or irreversibility; it increases in real adiabatic processes and remains constant in an ideal isentropic process.