PE Mechanical key terms

Relationships for a fan or pump at constant impeller or wheel diameter: flow varies with speed, head varies with speed squared, and power varies with speed cubed.

Ratio of the sum of individual peak loads to the coincident peak load of the combined system. It lets designers size central equipment smaller than the arithmetic sum of all connected loads.

Method that sums first cost, energy, maintenance, and replacement costs over an owning period, discounted to present value, so alternatives with different capital and operating costs can be compared fairly.

Overall heat transfer coefficient for an assembly such as a wall or window, expressed in Btu/h·ft²·°F. It includes conductive, convective, and radiative effects and is the reciprocal of total R-value.

Thermal resistance of a material or assembly per unit area, equal to thickness divided by thermal conductivity. Higher R-value means better resistance to heat flow; it is the reciprocal of U-factor.

Temperature of moist air measured by an ordinary thermometer, unaffected by moisture content or air motion. It is the horizontal axis variable on a standard psychrometric chart.

Lowest temperature attainable by evaporating water into an air stream, measured with a wetted thermometer in moving air. It reflects both sensible heat and moisture content and sets cooling tower performance.

Temperature at which moist air becomes saturated when cooled at constant pressure and constant moisture content. Cooling below it causes condensation on coils or duct surfaces.

Mass of water vapor per unit mass of dry air, often expressed in grains or pounds of moisture per pound of dry air. It is unchanged by heating or cooling without condensation.

Ratio of the actual water vapor partial pressure to the saturation vapor pressure at the same dry-bulb temperature, expressed as a percentage. It changes when air is heated or cooled.

Total heat content of moist air per unit mass of dry air, combining sensible heat and the latent heat of water vapor. Coil load equals mass flow times enthalpy difference.

Sensible heat gain divided by total heat gain for a space or coil process. On a psychrometric chart it sets the slope of the room process line from supply to return air.

Selected outdoor weather conditions, typically a percentile dry-bulb and coincident wet-bulb, used to size heating and cooling equipment. Heating design often uses 99% and cooling 1% frequency.

Fictitious outdoor air temperature that would produce the same heat gain through a surface as actual air temperature plus solar radiation and sky radiation effects. Used in cooling load calculations.

Tabulated temperature difference for walls and roofs that accounts for thermal storage, time lag, and solar effects, multiplied by U-factor and area to estimate conduction cooling load.

Uncontrolled leakage of outdoor air into a building through cracks, joints, and openings driven by wind and stack pressure. It adds sensible and latent load and is distinct from designed ventilation.

Cooling load that removes moisture without changing dry-bulb temperature, from people, infiltration, and process moisture. It equals mass flow times latent heat of vaporization and drives dehumidification requirements.

Cooling or heating load that changes air dry-bulb temperature without changing moisture content, from conduction, solar gain, people, lights, and equipment. It is removed by raising air temperature at the coil.

Energy needed to heat, cool, humidify, or dehumidify outdoor air supplied for occupancy and pressurization, calculated from outdoor-to-indoor enthalpy or temperature difference and airflow.

Device that transfers both sensible heat and moisture between exhaust and incoming outdoor air streams, reducing ventilation heating and cooling loads. It uses a permeable core or desiccant wheel.

Device that transfers only sensible heat between exhaust and supply air streams through a metal or plate core. It saves heating energy but does not control humidity.

Ratio of actual heat or moisture transfer in an energy recovery device to the maximum possible transfer between the two air streams. It is reported separately for sensible and latent performance.

Factory or field assembled box containing a fan, coils, filters, dampers, and controls that conditions and moves air to served spaces. It may be draw-through or blow-through.

System that maintains a zone set point by varying supply airflow at a roughly constant supply temperature, using VAV boxes with dampers and often reheat. It saves fan energy at part load.

Refrigeration machine that removes heat from chilled water or another secondary fluid for cooling coils. It may be vapor-compression, absorption, air-cooled, or water-cooled, and is rated in tons and COP.

Heat rejection device that evaporates a portion of condenser water to cool the remainder toward the ambient wet-bulb temperature. Performance is described by range and approach.

Graph of the pressure a fan can develop versus airflow at a given speed and air density. The operating point is where the fan curve intersects the system curve.

Graph of the pressure loss through a duct or piping network versus flow. It rises roughly with flow squared and shifts when dampers, valves, or filters change.

Control strategy that uses cool outdoor air instead of mechanical cooling when outdoor air enthalpy or temperature is low enough to satisfy the load. It reduces compressor energy.

Ratio of useful refrigeration effect to the work input for a refrigeration cycle, dimensionless. Higher COP means more cooling per unit of compressor energy.

Ratio of cooling output in Btu/h to electrical power input in watts at a specified rating condition. It is numerically equal to COP times 3.412.

Amount by which refrigerant vapor temperature exceeds its saturation temperature at the same pressure, usually measured at the evaporator outlet to protect the compressor and set expansion valve operation.

Amount by which liquid refrigerant temperature falls below its saturation temperature at the same pressure, typically measured at the condenser outlet. It indicates how much the condenser has fully condensed the refrigerant.

Refrigerant metering device that modulates flow to maintain a set superheat at the evaporator outlet, using a bulb sensing suction line temperature and pressure.

Control algorithm combining proportional, integral, and derivative actions to reduce error. Proportional action responds to current error, integral removes offset, and derivative damps rate of change.

Range of controlled variable over which the controller output moves from minimum to maximum in proportional-only control. A narrow band gives faster response but risks instability.

Range of controlled variable values around the set point over which no control action occurs, preventing rapid cycling of equipment. It is common in thermostats and staged systems.

Networked digital controls that monitor and command HVAC equipment, schedules, alarms, and energy use from a central interface. It coordinates set points, sequences, and data trending.

Ratio of valve pressure drop at full flow to the total pressure drop of the circuit branch it controls. Low authority causes nonlinear, unstable valve response and poor temperature control.

Target value that a control system tries to maintain, such as space temperature or supply air static pressure. It may be fixed or reset by schedules or outdoor conditions.