PE Mechanical HVAC flashcards

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

Ton of refrigeration

A rate of heat removal equal to 12,000 Btu/h, 200 Btu/min, or 3.517 kW; it is used to size cooling equipment and to convert catalog data to kW per ton.

Air-side sensible heat equation

Qs in Btu/h equals 1.08 times the airflow in cfm times the dry-bulb temperature difference in F, valid for standard sea-level air at 0.075 lbm/ft3.

Water-side sensible heat equation

Q in Btu/h equals 500 times the water flow in gpm times the temperature difference in F, based on water density and specific heat near 60 F.

Fan and pump power

Air horsepower equals cfm times total pressure in inches of water divided by 6356 and by efficiency; pump horsepower equals gpm times psi divided by 1714 and efficiency.

Steady-flow energy equation

For a control volume at steady state, heat added minus work delivered equals the change in enthalpy plus kinetic and potential energy between inlet and outlet.

Carnot COP

The maximum coefficient of performance: for a refrigerator, evaporating absolute temperature divided by the difference between condensing and evaporating absolute temperatures.

Isentropic compression relation

For an ideal gas compressed without heat transfer, T2/T1 equals the pressure ratio raised to the power (k-1)/k, with k equal to 1.4 for air.

Throttling process

Expansion through a valve or capillary is adiabatic with no work, so enthalpy stays constant while pressure falls and both temperature and entropy change.

Steam quality

The mass fraction of vapor in a saturated liquid-vapor mixture; mixture enthalpy equals hf plus quality times hfg, so quality is (h minus hf) over hfg.

Humidity ratio

Mass of water vapor per mass of dry air, equal to 0.621945 times vapor pressure divided by total pressure minus vapor pressure; it is unchanged by sensible heating.

Relative humidity

The ratio of actual water-vapor partial pressure to the saturation pressure at the same dry-bulb temperature; it falls when air is heated at constant humidity ratio.

Moist air enthalpy

Enthalpy in Btu per pound of dry air equals 0.240 times dry-bulb F plus humidity ratio times the quantity 1061 plus 0.444 times dry-bulb F.

Sensible heat ratio

Sensible load divided by total load; it fixes the slope of the room process line on a psychrometric chart and helps locate the coil apparatus dew point.

Apparatus dew point

The saturation-curve point where the line through entering and leaving coil conditions intersects saturation; leaving air lies between it and the entering air condition.

Reynolds number

Velocity times diameter divided by kinematic viscosity, representing inertial over viscous forces; pipe flow is laminar below about 2300 and turbulent above 4000.

Darcy-Weisbach equation

Friction head loss equals the friction factor times length divided by diameter times the velocity head V squared over 2g; the factor is 64/Re in laminar flow.

Velocity pressure

For standard air, velocity pressure in inches of water equals the velocity in feet per minute divided by 4005, then squared; velocity equals 4005 times its square root.

Hydraulic diameter

Four times the flow area divided by the wetted perimeter; it extends round-duct friction and Reynolds number correlations to rectangular and annular ducts.

NPSH available

Atmospheric head minus vapor pressure head, plus static suction head, minus suction line losses; it must exceed the pump's required NPSH to prevent cavitation.

Thermal resistance

Conduction resistance equals thickness divided by conductivity and area; convection resistance equals one divided by coefficient and area; resistances in series add directly.

Log mean temperature difference

The difference of the two terminal temperature differences divided by the natural log of their ratio; counterflow arrangement produces the largest possible value.

Effectiveness-NTU method

Effectiveness is actual heat transfer divided by the thermodynamic maximum; it increases with NTU, defined as overall conductance divided by the minimum capacity rate.

Critical insulation radius

The outer insulation radius at which heat loss from a cylinder stops increasing, equal to insulation conductivity divided by the outside convection coefficient.

Biot number

The ratio of internal conduction resistance to surface convection resistance, equal to h times characteristic length over k; below 0.1 lumped capacitance is valid.

Breathing-zone outdoor airflow

The ASHRAE 62.1 rate equals the people component Rp times occupants plus the area component Ra times zone floor area, before correcting for zone air distribution.

Economizer cycle

A strategy that cools with outdoor air when its temperature or enthalpy is below the return-air value, reducing or eliminating mechanical cooling energy.

VAV box minimum airflow

The lowest airflow a variable-air-volume terminal delivers, set by ventilation and minimum cooling needs; higher minimums limit turndown and support system ventilation efficiency.

Coil bypass factor

The fraction of leaving air treated as missing the coil surface: leaving-air enthalpy minus apparatus dew point enthalpy, divided by the same difference for entering air.

Vapor-compression COP

Evaporator refrigerating effect divided by compressor work, computed as (h1 minus h4) over (h2 minus h1) using evaporator outlet, discharge, and expansion inlet enthalpies.

Isenthalpic expansion

The expansion valve drops refrigerant pressure at constant enthalpy, forming a liquid-vapor mixture at the evaporator inlet; flash gas reduces the net refrigerating effect.

Condenser approach temperature

Leaving condensing refrigerant temperature minus entering cooling-medium temperature; it indicates condenser cleanliness and runs roughly 1 to 3 F on clean water-cooled shells.

Chiller kW per ton

An efficiency measure equal to 3.517 divided by COP, or 12 divided by EER; lower values are better, and typical centrifugal chillers run about 0.5 to 0.6.