 | | | Humidification terminology
Absolute humidity, humidity ratio, specific humidity Duct equivalent relative humidity HumidityAir, for the purpose of this discussion, is defined as a gaseous mixture of nitrogen, oxygen, carbon dioxide, water vapor (humidity), several inert gases, and traces of ozone and submicroscopic solid matter, sometimes called permanent atmospheric substances. All other airborne substances are considered contaminants. The water vapor (humidity), being a gas, occupies space along with the other gases of the air. In HVAC engineering, air is considered as being made up of only two components, dry air and water vapor. The properties of the dry air, which is composed of nitrogen, oxygen, carbon dioxide and rare gases, remain relatively unchanged as the temperature of the air rises and falls. The water vapor, on the other hand, may undergo considerable alteration as the temperature changes, including changes of state (condensing and freezing). Substantial amounts of energy are involved in these transformations. In measuring these changes and when working with the processing of air for various air conditioning problems, the engineer is concerned with two basic laws, Thermodynamics and Psychrometry. Thermodynamics is the study of heat energy transformations and substances that are affected by them. Psychrometry deals with the measurement of atmospheric conditions, particularly the moisture mixed with air. A psychrometric chart is a graphic representation of the thermodynamic tables that the HVAC engineer finds convenient to use when plotting solutions for the various air-conditioning processes involving water vapor and temperature changes. Relative humidity (RH)When we wish to describe the wetness or dryness of air at a given temperature and pressure, we use the term relative humidity. RH tells us the amount of moisture present in the air at a given temperature compared to what the air could hold at that temperature if it were saturated. It is expressed as a percentage. Absolute humidity, humidity ratio, specific humidityEach of the above terms is expressed as a number that describes a unit weight of water vapor associated with a unit weight of dry air. It is commonly expressed as fractional pounds (or kilograms) of water vapor per pound (or kilogram) of dry air, or if the use of whole numbers is preferred, it is expressed in grains of moisture per pound of dry air. There are 7000 grains in one pound. Duct equivalent relative humidityThis is the relative humidity of a duct airstream at a given temperature as compared to the relative humidity of the space served, which is usually at a different temperature. For example, a 55°F duct airstream has a duct equivalent relative humidity of 80% when compared to a room condition of 72°F and 45% relative humidity. This data is needed when absorption distance evaluations of duct steam dispersers are being made. Dry bulb temperatureThis is simply the temperature of the air indicated by any type of thermometer or thermocouple that has not been affected by evaporation or radiation. Wet bulb temperatureThis is an expression of the temperature of the air when a wick or sock, wetted with water, encases the sensing element of a dry bulb thermometer and air is passed over it at a velocity of 700 feet per minute or more. The drier the air, the greater is the cooling caused by evaporation and, therefore, the lower the wet bulb temperature. Mean radiant temperatureThis is the temperature of a uniform black enclosure in which a solid body or occupant would lose the same amount of heat by radiation only as in a non-uniform environment, such as a room that has cool walls. This factor is one of the components that make up the Standard Effective Temperature Index. Dew point temperatureThis is the saturation temperature corresponding to the humidity ratio and pressure of a given moist-air state. In other words, it is the surface temperature at which moisture begins to condense on that surface. The more humid the air, the higher the dew point temperature. Conversely, the dryer the air, the lower the dew-point temperature. Vapor migrationWater vapor, being a gas, behaves under the laws of low pressure gases. One of these laws, simply stated, says that "in a mixture of gases, the total pressure is the sum of the individual pressures exerted by each of the gases." This means that, in a mixture of water vapor and dry air, the water vapor exerts its own vapor pressure and will migrate from areas of higher vapor pressure to areas of lower vapor pressure. This migration occurs regardless of air movement. If the air movement is in the same physical direction as the force of the pressure differential, it will be accelerated. This characteristic can sometimes be taken advantage of when designing a humidification system for a large space served by more than one air distribution system. Further, the rate of movement at which this migration takes place is dependent upon the vapor pressure difference between the two areas. The greater the difference, the faster the migration. It is important to keep this phenomenon in mind when designing humidification for buildings or spaces within buildings. It may be necessary to consider the use of building materials having vapor barrier qualities in order to prevent loss of moisture, condensation and/or frost formation within the walls of the structure, resulting in damage. Latent heatLatent means hidden. In HVAC usage, latent commonly refers to change of state, which is the heat involved in fusion (freezing water or melting ice) or vaporization (creating water vapor) or condensation with no change in temperature. For water, fusion requires 144 BTU per pound and vaporization or condensation requires 970 BTU per pound. These values, which are for sea-level atmospheric pressure, vary as the pressure changes. Latent heat is not the same for all substances. Sensible heatSensible means in this case, that which can be sensed. In HVAC usage, it refers to the heat required to cause a change in temperature. The change is detected or sensed by the use of a thermometer. Specific heatThis is the heat required to cause a one-degree change in temperature in a unit mass of a substance. Common units are BTUs per pound (Fahrenheit degree) or calories per gram (Celsius degree). The specific heat of water is 1.
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