COMFORT AND PSYCHROMETRICS
To understand the psychrometric processes of heating, cooling, humidification and dehumidification.
The air that surrounds us has to be maintained at the correct conditions for us to be comfortable. The air in our homes is treated by the processes of heating, cooling, dehumidifying, humidifying, and cleaning it so that our bodies will give off the correct amount of heat for comfort.
Certain combinations of air temperature, relative humidity, air movement and mean radiant temperature (MRT) will result in what most people consider thermal comfort. When these combinations are plotted in a psychrometric chart, they define an area as the comfort zone. This chart also indicates the type of discomfort one experiences outside of the comfort zone. The comfort zone should be the goal of the thermal design of a building because it defines those conditions that most people in our society find comfortable.
The study of air and its properties is called psychrometrics. When we move through a room, we are not aware of the air inside the room, but the air has weight and occupies space like water in swimming pool. Air weights 0.075 lb/ft3 at standard atmospheric conditions and the density of water is 62.4 lb/ft3 . Air like water, offers resistance to movement. For example invert an empty glass and push it down in water. The air in the glass resists the water going up into the glass. Cooling, heating, humidifying, dehumidifying and cleaning out air describes the air conditioning profession. Air consists of approximately 78% nitrogen, 21% oxygen and 1% other gasses. Water in the form of lower-pressure vapor is suspended in the air and called humidity.
The moisture content in air (humidity) is measured by weight, expressed in pounds or grains (7000gr/lb). Relative humidity is the most practical and most used for field measurements. It is based on the weight of water vapor in a pound of air compared to the weight of water vapor that a pound of air could hold if it were 100% saturated.
DRY BULB AND WET BULB TEMPERATURES
The moisture content of air can be checked by using a combination of dry-bulb (DB) and wet-bulb (WB) temperatures. DB temperature is the sensible-heat level of air and is taken with an ordinary thermometer. WB temperature is taken with a thermometer with a wick on the end that is soaked with distilled water. The reading from a WB thermometer takes into account the moisture content of the
air. It reflects the total heat content of air. Note that a 100 % relative humidity, DB and WB are equal.
DEW POINT TEMPERATURE (DP)
The dew-point temperature is the temperature at which moisture begins to condense out of the air. For example, if you were to set a glass of warm water in a room with temperature of 75oF and 50% RH, the water in the glass would evaporate slowly to the room. If you gradually cool the glass with ice, when the glass surface temperature becomes 55.5oF, water will begin to form on the surface of the glass. This temperature at which water forms is called the dew point temperature of the air. Air can be dehumidified by passing it over a surface that is below the dew point temperature of the air, moisture will collect on the cold surface for example, an air conditioning coil. The condensed moisture is drained and this is the moisture that we see running out of the condensate line of an air conditioner.
Enthalpy is the total heat contained in lb of a substance. It is measured from a reference (datum) point. This reference point is 0oF (-17.8oC) for dry air, 32oF (0oC) for water vapor and –40oF (-40oC) for refrigerants. A psychrometric chart in metric units uses 0oC as a reference base.
The following examples show how different processes of air conditioning are plotted.
- Air is heated. Movement through the heating equipment can be followed as a sensible-heat direction on the chart
- Air is cooled. There is no moisture. This shows a sensible-heat direction on the chart.
- Air is humidified. No heat is added or removed. An increase in moisture content and dew point temperature shows
- Air is dehumidified. No heat is added or removed. A decrease in moisture content and dew point temperature shows
- Air is cooled and humidified using an evaporative cooler.
- Air is heated and humidified is the most common in the winter application. This will show both a rise in temperature and an increase in moisture and dew point temperature .
- Air is cooled and dehumidified is the most common in the summer application. A decrease in temperature, moisture content and dew point temperature will occur
It is important to notice that any change in heat content or moisture content of air will cause a change in the wet bulb reading and therefore, a change in the total heat content.