![]() We are given point (A) as 27☌ db, 50%SAT, and point (B) as 21☌ db, 17☌ wb. To place the statements made above into context we will examine the process displayed in Fig 3, which shows a cooling process from a starting point (A) to finishing point (B). Therefore combinations of heating or cooling combined with changes in the moisture content can be displayed by lines sloping upwards or downwards from the (SP) point. The removal of latent heat energy has the reverse effect (dehumidification).Īny process that involves an increase in the moisture content (humidification) of the air is displayed by a line moving above the (SP) point, while any process that decreases the moisture content of the air can be shown by a line moving below the (SP) point. The addition of latent heat to air results in an increase in moisture content (humidification). This sensible cooling process results in a horizontal line to the left of (SP). Conversely, if sensible heat energy only is removed from the air, the air is reduced in temperature and is therefore cooled without any change in moisture content. ![]() ![]() If the starting point of the process is at (SP) on the chart and sensible heat only is applied or added to the air then a horizontal line moving to the right of (SP) displays the characteristics of the air. The processes that can be displayed and calculated from a psychrometric chart are shown in Fig 2. ![]() In Fig 1 this would be achieved by transposing the process curve from the main body of the chart to the protractor shown upper left. Most charts enable the plotting of the sensible heat ratio (SHR) line. Percentage saturation (more popularly known as relative humidity %RH) is shown as dashed lines in Fig 1 in order to differentiate between all the other lines shown on the chart and is termed %SAT. Constant volume lines are usually shown widely spaced and readings have to be interpolated for a given condition. These lines show the specific volume of the air in m3/kg. Superimposed on the chart are lines of constant volume better termed specific volume. Some charts use a separate set of scales external to the body of the chart and one uses a straight edge on these scales to determine the value within the area of the chart. The specific enthalpy of the air is read from the enthalpy scale in kJ/kg. Horizontal lines intersecting the Y1 axis shows the moisture content of the air, in this example in grams of moisture per kilogram (g/kg) of dry air. However wb lines slope diagonally to the right and the 20☌ wb line intersects all dry bulb temperatures greater than 20☌. For example, the 20☌ db line intersects the wb curve at 20☌. Dry bulb (db) temperatures are read vertically from the X axis, until the db temperature lines intersect the saturation curve or wet bulb (wb) curve. The psychrometric chart will help us find the heat carrying capability of the air and enable us to design the “air side” of an air conditioning system.įigure 1 shows the outlines of the major functions. These two figures, sensible and latent heat, will determine both the characteristics of the evaporator and size of the air handler. The heat gain calculation will determine the absolute maximum amounts of sensible and latent heat gains to the space together with peak time of day and month of year. ![]() This total quantity of heat energy comprises two forms, namely a given amount of sensible heat and the remaining amount of latent heat. In order to determine the quantity of air that has to pass through the evaporator we have to know the total amount of heat that has to be removed. Therefore, in order to remove heat from the space the heat must be transported in some way to the evaporator and the obvious medium used is the air contained by the room which is recirculated through the evaporator coil by the fan within the air handling unit. The only source of cooling on any air conditioning system is the evaporator coil. ![]()
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