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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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 psgchrometric overall picture. Looking at the height of the yellow column, it occurs on the equivalent of just under one daily period per year.

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. My thanks to Dr Roger Legg for his publications, which provided underlying material for this article. The opposite process, sensible cooling, is shown in Figure 3.

Many of the psychormetric main measuring stations are located in relatively remote areas such as airfields so actual conditions required for building and system evaluation will need to take account cigse the local microclimate, compared with that of the nearest weather station.

This psychrometic indicated in Figure 7 as air stream A mixing with air stream B to produce mixed air at condition M. Through the incorporation of appropriate weather data, a psychrometric chart can provide the tool to undertake such checks.

Figure 8 — Example Psychrometric Process. In this case it can be readily seen that by increasing the design wet bulb by 1K to 21 o C maintaining the dry bulb design at 28 o Cthe system would operate within its capabilities for about an extra 10 data points above that of the current condition.

A bounding envelope has been added to the psychrometric data in Figure 4 to emphasise the most frequently occurring psychrometric conditions.

Figure 2 is indicative of a type of weather data that can be sourced. The two values are likely to vary slightly from each other.

This article will explain how to set this up, and explore some first examples of application. Figure 7 — Psychrometry of Mixing Air Streams. Glaisher had problems reliably determining the wet bulb temperature — these problems arose from the differences between screen and sling wet bulb temperatures. How close the dry-bulb temperature of point B approaches the ADP will depend on the effectiveness of the cooling coil.


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 use may be examined. Figure 1 — Outline Psychrometric Chart. Adding up the hour columns to the left of the -3 o C column comes to about one occasion ie one hour period when the temperature falls below -3 cibsd C over the average year.

A steam humidifier will produce a pdychrometric 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. CIBSE Guide C has lists of the values of all the properties of air as described above for various dry bulb temperatures.

Module 9: The basic psychrometric processes – CIBSE Journal

The process that would typically cinse from a steam humidifier almost pure latent heating is indicated in Figure 4 — it can reasonably be considered as a vertical line following a constant dry-bulb temperature. Where two air streams combine adiabatically as they normally would the resulting air will have a psychrometric state that is related to the proportions of each of the combining air mass flowrates. These measurements may be undertaken in a number of ways including the Sling Psychrometer see Figure 1 and the Aspirated Hygrometer.

On the vertical axis y axis at the right of the chart there is a scale showing the moisture content of the air — how many kilograms of water vapour is in each kilogram of dry air. If any two properties are known, then the others may be read from the chart — so, for example, in the field this psychrommetric typically be wet bulb and dry bulb temperature measured with a sling psychrometer.

Figure 6 Psychrometic sending in the tear-out page from the magazine, just write in one of these options under question one. Also on the vertical axis is the continuation of the specific enthalpy scale.

To explore the calculations associated with the processes discussed so far consider the cooling coil shown in Figure 8, where all the data has been read off a CIBSE Psychrometric chart. The guide also includes sets of 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.


These particular data in Figure 2 have been laid out in terms of the percentage occurrence of coincident pairs of dry-bulb temperature and moisture content. As with any gas, the moving molecules of water vapour in the air exert a pressure.

cibse guide Psychrometric Chart.pdf

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.

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.

However, when evaluating the comparative operation of different possible solutions, it is important not only to take into account the values of the operational psychrometry in design and part load conditions, but also to consider how frequently this might occur. As has been illustrated in previous CPDs, the psychrometric chart is a useful tool to examine the properties and energy flows associated with air processes.

So-called Adiabatic constant energy Humidification as shown in Figure 6 practically characterises the processes of atomising spray and ultrasonic humidifiers as well as the rarely used recirculating spray and wetted pad humidifiers. These calculations may be reorganised to determine unknown variables for any of the processes illustrated in this article. 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.

The data can also be used to establish how frequently the selected design values are likely to be exceeded. The wet bulb temperature is the temperature that a sample of air would reach if it had a fine spray of water added to it the water being at the air dry bulb temperature without any external heat being added or taken away this is known as an adiabatic process.

The chart is now a tool that not only allows examination of the properties of air but also clearly includes the dimension of time.