Understanding Psychrometric Charts, Mechanical (HVAC) Courses from Mid Career College. MCC has been providing training for building services. The Psychrometric charts provide a graphical representation of the thermodynamic properties of moist air using the data contained in CIBSE Guide C. The. A practical introduction to psychrometry and the use of the psychrometric chart as a design tool. KS20 is essential reading for those involved in designing and.
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The designer whether aware of it or not will already be applying frequency-based weather data when undertaking any building load analysis, even when calculating a simple steady state building heat loss. The use of computer modelling tools enables designers to evaluate options of building design and plant selections at the press of a button.
The psychrometric values were based on the on-coil and off-coil dry-bulb and wet-bulb temperatures as indicated in the diagram. The data have been taken from Figure 2 and added to the outline chart as shown in Figure 3.
Some of the sprayed water will evaporate into the air — the heat to cause that evaporation being taken from the air itself and so the air dry bulb temperature will drop until the point where the air is saturated with water vapour. It is a graphical representation that shows properties of air at 1 atmosphere pressure This is the nominal air pressure at sea level. Similarly the data can be quickly used to determine extreme summer time dry bulb temperatures that may be used, for example, to aid in the selection of an appropriately sized air cooled condenser.
On the graph itself, the curved lines show relative humidity and the various straight lines show dry bulb and wet bulb temperatures, moisture content and specific volume. For a cooling coil to provide sensible cooling only and no latent cooling the coil surface temperature must be greater than the dew-point temperature of the air. The Psychrometric Chart The psychrometric chart is a very useful tool that can unlock the mystery surrounding the properties of air.
The chart is now a tool that not only allows examination of the properties of air but also clearly includes the dimension of time. So, to take the yellow highlighted data as an example, over a year it might be expected that the dry bulb temperature of between The historic data are available in tables that include amongst other things coincident wet-bulb and dry-bulb temperatures, and are ideally suited to act as a basis for the analysis of design conditions.
My thanks to Dr Roger Legg for his publications, which provided underlying material for this article. This vapour pressurePv measured in Pa or mbaris related to the mass of water vapour in the air — the more molecules present, the higher the vapour pressure. If molecules of water vapour were to be continually added to a volume of static air for example, from a room humidifier then at some point the air would not be able to support any more molecules — the air is said to be saturated at that point and any further water molecules added will simply condense out.
So, for example, if we were selecting an external dry-bulb bin temperature of -3 o C, the total frequency of temperature below that bin is 0.
The molecular mass of chzrt vapour is 18 and so the specific gas constant may be obtained from Figure 5 — Psychrometry of Sensible and Latent Cooling. But these data are limited to just temperature: The actual underlying data as collected by the weather station would be based on the same sets of hourly averages data; however, the longer averaging period will moderate out the high and low values. On the curved edge of the chart there are scales for wet bulb temperatures and specific volume with the specific enthalpy scale drawn in steps.
Dry bulb temperature and wet bulb depression tables or calculations can be used to determine the air moisture content. Figure 8 — Example Psychrometric Process.
Module 9: The basic psychrometric processes
A photo-enlarged chart or a specific area from the chart produced by computer software will improve the resolution. Alongside the computer model it is often useful to have a simpler method not only to aid understanding of the more complex outputs of the software, but also to provide swift feedback as a check of the overall validity of the model. A steam humidifier will produce a small amount of sensible heating since the higher temperature of the device the tube carrying the steam into the air will increase the temperature of the air and there will be a very slight sensible heating effect from the steam itself.
Figure 2 — Psychrometry of Sensible Heating. This will relate such parameters as the number and shape of coil rows, the spacing and type of fins on the coil, and the air velocity. The opposite process, sensible cooling, is shown in Figure 3.
Module 9: The basic psychrometric processes – CIBSE Journal
The graphical representation provides, at the least, a swift visual check on the value, but also can inform and communicate the effect of alternative values on the consequent plant operation. Through the incorporation of appropriate weather data, a psychrometric chart can provide the tool to undertake such checks.
As with any gas, the moving molecules of water vapour in the air exert a pressure. This will not just be, as a standard psychrometric calculation, in terms of power, but by including the frequency information, and hence time, comparative energy psychromteric may be examined. Also on the vertical axis is the continuation of the specific enthalpy scale. And, of course, is most frequently sensed using an electrical or electronic device. When illustrating the air condition in a project or job notes it might be clearer to refer to a printed chart possibly conveniently plastic coated and reusable to read off the data and then convert that into a sketch similar to that shown in the figure where the data psjchrometric processes are not lost in the plethora of printed lines.
Figure 4 — Psychrometry of Steam Humidification. So considering the yellow shaded area, the -3 o C bin — this bin is the sum of historic occurrences of 24 hourly averaged outdoor temperature between Water vapour H 2 O constitutes about 0.
A couple of the data points appear to exist in the impossible area to the left of the saturation curve; this is due to the approximated sketch of the psychrometric chart and should not distract from the bigger overall picture.
The guide also includes sets cihse equations that will allow determination of values for other gases that may be used in spreadsheets or computer programmes to the Characteristic Equation may be used in conjunction with the Gas Laws.
On the next post, I will look at how to represent air with a particular set of properties on the chart. There osychrometric also a scale showing the specific enthalpy of the air below the temperature scale. Sensible heating is shown in Figure 2. Figure 6 If sending in the tear-out page from the magazine, just write in one of these options under question one.
The atmospheric pressure will alter with both weather and location and so the relative psgchrometric of the vapour pressure will also change. This equation allows calculation of the properties of gases under different pressures and temperatures. Air is normally considered to be a mix of dry air and water vapour.
The chart was later refined by others in the US, including the famous Willis Carrier at the beginning of the psychrlmetric century, by which time the uncertainties of wet bulb temperature measurement were well understood. The saturated vapour pressurePswill have been reached — the actual value of this is related to the dry bulb temperature of the air; air with a higher dry bulb temperature will have a greater potential to hold water vapour, and so will have a higher saturated vapour pressure.
Glaisher had problems reliably determining the wet psychrometrkc temperature — these problems arose from the differences between screen and sling wet bulb temperatures.
Module Travelling into time with psychrometry – CIBSE Journal
Figure 6 — Psychrometry of Adiabatic Humidification. If any two properties are known, then the others may be read from the chart — so, for example, in the field this would typically be wet bulb and dry bulb temperature measured with a sling psychrometer.
The two values are likely to vary slightly from each other.