ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment
ashrae.org/ashraeterms

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z
external resistance
external static pressure loss
friction loss

pressure loss due to friction between a flowing fluid and its contact surface.

initial resistance

the pressure loss of the device operating at a specified airflow rate with no dust load, expressed in Pa (in. of water).

internal static pressure loss
loss coefficient

a dimensionless fluid resistance coefficient having the same value in dynamically similar streams (i.e., streams with geometrically similar stretches, equal Reynolds numbers, and equal values of other criteria necessary for dynamic similarity). The loss coefficient represents the ratio of total pressure loss to velocity pressure at the referenced cross section - C = ?Pt / pvt where C = total pressure loss coefficient, dimensionless, ?pt = total pressure loss, in. of water (Pa), pvi = velocity pressure at referenced cross section i, in. of water (Pa).

pressure drop

(1) difference in pressure between two points in a flow system, usually caused by frictional resistance to fluid flow in a conduit, filter, or other flow system. Compare to pressure loss. (2) loss in pressure, as from one end of a refrigerant line to the other, from friction, static, heat, etc.

pressure loss

decrease in total pressure due to friction and turbulence. It is a measure of the mechanical energy lost by the flow per unit volume of the fluid.

pressure loss coefficient

factor for mechanical energy loss as a result of flow.

rated final resistance

the operating pressure loss at the airflow rate at which a disposable device (e.g., filter) should be replaced or renewed, as recommended by the manufacturer, expressed in Pa (in. of water).

static pressure loss

(1) external static pressure loss for forced-air systems using packaged air-handling equipment, the static pressure loss resulting from airflow through the supply and return ductwork and other elements external to the packaged air-handling unit. Compare to internal static pressure loss and total static pressure loss. (2) internal static pressure loss for forced-air systems, the static pressure loss resulting from airflow through the internal elements of the packaged air-handling unit. These internal elements may include such items as filters, coils, dampers, mixing sections, etc. Compare to internal static pressure loss and total static pressure loss. (3) total static pressure loss for forced-air systems, the sum of the external static pressure loss and the internal static pressure loss. See internal static pressure loss and external static pressure loss. The term is not applicable to an individual fan or a fan located within a builtup air-handling unit. Fans are rated by the term fan total static pressure, which is not the same as fan total pressure. Compare to fan total pressure.

test pressure loss

differential in total pressure between the inlet and the outlet sections of a test duct or across a test fitting. For test fittings, the fitting is assumed to have zero length. For multiflow fittings, the total pressure loss shall be determined for each stream separately.

total static pressure loss