PE Mechanical flashcards

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

Ton of refrigeration

Rate of heat removal equal to 12,000 Btu per hour, 200 Btu per minute, or 3.517 kilowatts; the standard basis for sizing cooling and refrigeration equipment.

Sensible heat equation for air

Qs equals 1.08 times cfm times the dry-bulb temperature difference in F, derived from 60 minutes per hour, 0.075 lb per cubic foot, and 0.24 Btu per lb-F.

Fan affinity laws

For a fixed fan and system, airflow varies directly with speed, static pressure with the square of speed, and brake horsepower with the cube of speed.

Water flow heat equation

Q in Btu per hour equals 500 times gallons per minute times the water temperature change in F, based on 8.33 lb per gallon and specific heat of 1.0.

Standard air properties

At 70 F and sea level, air has density 0.075 lb per cubic foot, specific heat 0.24 Btu per lb-F, and specific volume 13.3 cubic feet per pound of dry air.

Humidity ratio formula

W equals 0.622 times water vapor partial pressure divided by total pressure minus vapor pressure, giving pounds of water per pound of dry air.

Moist air enthalpy

Enthalpy equals 0.240T plus W times (1061 plus 0.444T) in Btu per pound of dry air, where T is dry-bulb temperature in F.

Coil bypass factor

Fraction of entering air that leaves a cooling coil untreated; leaving enthalpy equals apparatus dew point plus bypass factor times the entering-to-ADP enthalpy difference.

Sensible heat ratio

Sensible load divided by total load for a space or coil; it fixes the slope of the process line on the psychrometric chart.

Relative humidity

Water vapor partial pressure divided by the saturation vapor pressure at the same dry-bulb temperature, expressed as a percent; it depends on both moisture content and temperature.

Conduction heat transfer equation

Q equals U times area times temperature difference; the U-factor is the reciprocal of total R-value including surface films and each assembly layer.

CLTD cooling load method

Estimates envelope load as U times area times a corrected cooling load temperature difference, adjusted for latitude, month, wall color, and indoor-outdoor temperature swing.

Ventilation latent load

QL equals about 4,840 times cfm times the humidity ratio difference, or roughly 0.68 times cfm times the difference in grains per pound.

Heating degree days

Sum over the heating season of 65 F minus the daily mean outdoor temperature for days below the balance point; used to estimate annual heating energy.

Internal heat gain from lighting

All lighting wattage inside a conditioned space becomes heat at 3.412 Btu per hour per watt; a portion is radiant and appears as load after a time lag.

Sensible effectiveness

For an energy recovery device, the temperature change of one airstream divided by the maximum possible difference between the two entering airstreams.

Enthalpy wheel

Rotary total-energy exchanger that transfers both sensible heat and moisture between airstreams, typically achieving total effectiveness between 50 and 80 percent.

Runaround coil energy recovery

Two coils joined by a pumped glycol loop that transfers sensible heat only; there is no cross-contamination and the airstreams may be located far apart.

Latent effectiveness

Change in humidity ratio of the supply airstream divided by the maximum humidity ratio difference between the entering exhaust and outside airstreams.

Cooling tower approach

Leaving condenser water temperature minus entering wet-bulb temperature; a smaller approach requires a larger tower but lowers chiller condensing temperature and power.

Chiller efficiency kW per ton

Compressor input power per ton of cooling; it equals 12 divided by EER, and COP equals 3.412 divided by the kW-per-ton value.

Duct velocity pressure

For standard air, velocity pressure in inches of water equals duct velocity in feet per minute divided by 4,005, with the quotient then squared.

VAV terminal minimum airflow

Lowest airflow setting of a variable air volume box, established by ventilation requirements, box turndown, and diffuser throw rather than by space load.

Air-side economizer

Uses cool outside air for free cooling when its temperature or enthalpy is below the return air, modulating dampers and reducing or stopping mechanical cooling.

Carnot refrigeration COP

Maximum coefficient of performance between a cold region at TL and a warm region at TH, both absolute: TL divided by (TH minus TL).

Thermostatic expansion valve

Meters refrigerant into the evaporator to hold a nearly constant superheat at the outlet, modulating in response to bulb temperature and evaporator pressure.

Subcooling

Liquid refrigerant temperature below the saturation temperature at condenser pressure; it increases refrigeration effect and prevents flash gas ahead of the expansion valve.

Refrigerant temperature glide

Difference between bubble and dew point temperatures of a zeotropic blend at a given pressure; it complicates charging, leak detection, and coil design.

Proportional band

The error span, expressed as a percent of sensor range, over which a proportional controller moves its output from minimum to maximum.

Integral control action

Controller output changes in proportion to the accumulated error over time; this eliminates steady-state offset but slows response and can cause overshoot.

RTD versus thermistor

Resistance temperature detectors use platinum and are stable and near-linear; thermistors are cheaper and more sensitive but nonlinear and limited in temperature range.

Offset in proportional control

Steady-state deviation from setpoint that persists with proportional-only control because a nonzero error is required to hold the output at the load value.