ASHRAE Terminology

In a mixture of water vapor and dry air, the ratio of the mass of water vapor to the volume occupied by the mixture.

(1) chemicals impregnated into small paper cards that change color with specific relative humidities. (2) instrument in which the relative humidity is determined from the absorption of water vapor by a hygroscopic material.

the temperature at which water (liquid or solid) at temperature t, by evaporating into moist air at dry-bulb temperature t and humidity ratio W, can bring air to saturation adiabatically at the same temperature t while total pressure p is constant. Also known as thermodynamic-wet-bulb temperature.

assembly of equipment for the simultaneous control of air temperature, relative humidity, purity, and motion. Compare to air-conditioning system.

the process of treating air to meet the requirements of a conditioned space by controlling its temperature, humidity, cleanliness, and distribution.

(1) distribution of the air in a space, called the treated space, in a manner to satisfy certain specified conditions such as air change rate, pressure, cleanliness, temperature, humidity, air velocity, and noise level, in a specified zone within this treated space, which is called the occupied zone. It is usually achieved by means of air terminal devices which form the common boundaries between the treated space and the air-distribution system. (2) the introduction of air into a building space for the purpose of providing acceptable velocity and temperature distribution in the occupied zone.

in enclosed spaces, combined treatment of the air to control, as specified, temperature, relative humidity, velocity of motion, and radiant heat energy level, including consideration of the need for removal of airborne particles and contaminant gases. Some partial air conditioners that may not accomplish all of these controls are selected for their capability to control specific phases of air treatment.

assembly of equipment for air treatment to simultaneously control its temperature, humidity, cleanliness, and distribution to meet the requirements of a conditioned space. See air conditioner.

characteristics of the environment. For example, temperature, relative humidity, pressure, and motion.

chilled ceiling systems have a very low profile (often flush with the ceiling) and work by means of convection heat transfer and induced air movement in the room in which they are placed. Chilled ceilings lack the ability to control the humidity or provide ventilation and must be paired with a ventilation system in order to maintain latent heat gains.

specially constructed, enclosed area environmentally controlled with respect to airborne particulates, temperature, humidity, air pressure, air-pressure flow patterns, air motion, vibration, viable organisms, and lighting.

treating air to control its temperature, relative humidity, cleanliness, and distribution to meet the comfort requirements of the occupants of the conditioned space.

(also treated air) air treated to control its temperature, relative humidity, purity, pressure, and movement.

that part of a building that is heated and/or cooled and/or humidity controlled for the comfort of occupants. Compare unconditioned space.

in either a closed-loop or open-loop control system, the measured physical phenomenon (e.g., temperature, humidity, pressure, lighting, CO₂) that causes a controller to respond in an effort to reduce or minimize the deviation from a desired value.

temperature at which water vapor has reached the saturation point (100% relative humidity). Temperature of the air at which it must be cooled at constant barometric pressure for water vapor to condense.

increase in moisture content (specific humidity) of air expressed in terms of a rise in dew-point temperature.

temperature of moist air saturated at pressure p, with the same humidity ratio W as that of the given sample of moist air. It is defined as the solution td(p, W) of the equation: Ws(p, td) = W

system of a central plant that produces conditioned air at two temperatures and humidity levels to supply air through two independent duct systems to the points of usage where mixing may be carried out.

device that indicates relative humidity from the electrical resistance of a hygroscopic salt.

systems that primarily use a combination of mechanical equipment, airflow, water flow, and electrical energy to provide heating, ventilating, air conditioning, filtration, humidification, and dehumidification for the purpose of human comfort or process control of temperature and humidity.

an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature (how hot it feels, also termed the felt air temperature). When the relative humidity is high, the evaporation rate is reduced, so heat is removed from the body at a lower rate, causing it to retain more heat than it would in dry air. Compare wind chill.

ratio of increase of enthalpy of moist air to the rise of temperature expressed per unit mass of the dry air component under conditions of constant pressure and humidity ratio.

an automatic control device used to maintain humidity at a fixed or adjustable setpoint.

automatic controls used to maintain humidity at a fixed or adjustable set point.

ratio in percent of the mass of moisture at a given temperature to the maximum possible at the same temperature. See humidity.

ratio of the mass of water vapor to the mass of dry air in a sample of moist air. Also known as mixing ratio (or humidity value from Mollier’s diagram).

ratio of the specific humidity to that at saturation at the same temperature and pressure, usually expressed as a percentage.

instrument responsive to relative humidity, usually relative humidity in the atmosphere. Compare to psychrometer.

branch of science that deals with the measurement of humidity

attributes of the respirable air inside a building (indoor climate), including gaseous composition, humidity, temperature, and contaminants. See also indoor environment quality (IEQ). See also sick building syndrome.

state of constant relative humidity.

the change in enthalpy associated with a change in humidity ratio, caused by the addition or removal of moisture.

element inserted in the ductwork that facilitates the determination of air temperature, air humidity, airflow rate, and/or pressure.

conditions such as temperature, humidity, and motion of air within an enclosure or limited outdoor area.

a type of air-distribution system in which conditioned air is delivered to the space at a velocity sufficient to promote complete mixing of supply air with room air, thereby maintaining the entire volume of air in the space at a relatively uniform temperature, humidity, and air quality condition. A conventional overhead air distribution, which supplies and returns air at ceiling level, is an example of a mixing system.

in humid air, the ratio of the number of water vapor mols to the total number of mols in the mixture. Compare to humidity.

(1) a plan for gathering of relevant measurement data over time to evaluate equipment or system performance. The plan defines specific M&V methods to be used, including baseline determination, performance period measurements, savings verification calculations, and acceptance criteria. The M&V methods chosen are consistent with the current facility requirements (CFR). During the implementation phase, a list is developed of specific instrumentation and data-gathering equipment that must be maintained at the site. During the hand-off phase, the type, frequency, and distribution of M&V reports to be submitted for approval is confirmed. (2) equipment to measure and record the parameters of the HVAC&R systems (i.e., temperature, humidity, pressure, electric current, kW, and volts). (3) gathering of relevant measurement data over time to evaluate equipment or system performance (e.g., chiller electric demand, inlet evaporator temperature and flow, outlet evaporator temperature, condenser inlet temperature, and ambient dry-bulb temperature and relative humidity or wet-bulb temperature, for use in developing a chiller performance map (e.g., kW/ton versus cooling load and versus condenser inlet temperature).

(1) air circulation caused by air movement or pressure differences. (2) air circulation caused by differences of density induced by differences of temperature and/or humidity.

uses a pipe surrounded by a coil in order to form a radiator system. Passive chilled beams have no method for maintaining the humidity of a room and must be paired with a ventilation system in order to maintain latent heat gains.

time rate of water vapor transmission through unit area of flat material of unit thickness induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions. When permeability varies with psychrometric conditions, the spot or specific permeability defines the property at a specific condition. Note: permeability is a property of a material, but the permeability of a body that performs like a material may be used. Permeability is the arithmetic product of permeance and thickness.

(1) ratio of the mole fraction of water vapor to the mole fraction of water vapor saturated at the same temperature and barometric pressure. (2) ratio of the partial pressure or density of water vapor to the saturation pressure or density, respectively, at the same dry-bulb temperature and barometric pressure of the ambient air.

relative humidity (rh, RH)

system that reduces heat gain through a roof by cooling the outside surface with a water spray, suited for only temporary treatment because high humidity may be introduced by air intakes on the roof.

(1) condition for coexistence in stable equilibrium of a vapor and liquid or a vapor and solid phase of the same substance. As an example, steam over the water from which it is being generated. See vapor pressure. (2) physical adsorbent that contains all the contaminant it can hold at the challenge concentration, temperature, and humidity. This point is its activity, expressed as a percentage of the carbon mass or fraction (i.e., g contaminant/g adsorbent). (3) condition for coexistence in stable equilibrium of a vapor and liquid, or a vapor and solid phase of the same substance. As an example, steam over the water from which it is being generated. See vapor pressure.

dimensionless difference between the humidity ratio of an air sample and the saturation humidity ratio of air at the same temperature and pressure.

hygrometer of two matched thermometers, one with its bulb wetted and the other dry, capable of being whirled rapidly on a sling to indicate the temperature differences related to relative humidity.

(1) ratio of the mass of water to the total mass of a moist air sample. (2) ratio of the mass of water vapor to total mass of a moist air sample (including water vapor and dry air) in a particular volume. Specific humidity is expressed as a ratio in units of lb of moisture/lb of dry air or grains of moisture/lb of dry air (kilograms of water vapor per kilogram of mixture). See also absolute humidity.

the air that flows through the device being tested. During the test, test air should be at the temperature, humidity, pressure, and atmospheric dust concentration prevailing at the time of the test. Test air for arrestance and dust-holding capacity measurement may be indoor ambient air.

the surrounding atmosphere characterized by parameters such as air temperature, wet-bulb temperature, dew-point temperature, water vapor pressure, total atmospheric pressure, relative humidity, and specific humidity.

(also known as absolute humidity or water vapor density number), in a mixture of water vapor and dry air, the mass of water vapor in a specific volume of the mixture. Compare to relative humidity.

(1) the process of supplying air to or removing air from a space for the purpose of controlling air contaminant levels, humidity, or temperature within the space. (2) the process of supplying or removing air by natural or mechanical means to or from any space. Such air may or may not have been conditioned.

steady vapor pressure difference that induces unit time rate of vapor flow through unit area and unit thickness of a flat material (or construction that acts like a homogeneous body) for specific conditions of temperature and relative humidity at each surface. Vapor resistivity is the reciprocal of vapor permeability.

steady-state vapor flow in unit time through unit area of a body, normal to specified parallel surfaces, under specific conditions of temperature and humidity at each surface.

those combinations of air temperature, mean radiant temperature, and humidity that are predicted to be an acceptable thermal environment at particular values of air speed, metabolic rate, and clothing insulation (Icl).