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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We apologise for this. Please note that the options for this question are: Figure 6 If sending in the tear-out page from the magazine, just write in one of these options under question one. The outline of the chart is as shown in Figure 1 and it may be used for individual rooms or processes as well as describing the changes to properties that take place in a complete air handling system.

When illustrating the air condition in chsrt 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 and processes are not lost in the plethora of printed lines. A photo-enlarged chart or a specific area from the chart produced by computer software will improve the resolution.

When lines are drawn on the chart they reflect changes in sensible heat or latent heat or a combination of the two. Sensible heat is that added to, or taken frommoist air without a change in moisture content and chaet heat is that added to, or taken frommoist air without a change of dry bulb temperature.


The energy contained in the air as indicated by Enthalpy, h relating to both the sensible and latent energy increases towards the right and the top of the chart as each of dry-bulb temperature and moisture content increase.

When using the Psychrometric Chart for plotting cycles that include both latent and sensible change the individual processes are simplified to straight lines. Sensible heating is shown in Figure 2. This may be achieved, for example, with a heater battery made up from rows of hot water or steam coils, or electric resistance heaters or as the result of some heat recovery mechanism such as a flat plate heat recovery device.

The opposite process, sensible cooling, is shown in Figure 3.

cibse guide Psychrometric

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. This combined process will produce an off coil temperature somewhat higher than that of the actual temperature of the coil known as the Apparatus Dew Point, ADP and shown as ckbse X on Figure 5.

How close the dry-bulb temperature of point B approaches the ADP will depend on the effectiveness of the psychometric coil.

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 process that would typically result from a steam humidifier almost pure latent heating is indicated in Figure 4 — it can reasonably be psydhrometric as a vertical line following a constant dry-bulb temperature.

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 cibs increase the temperature of the air and there will be a very slight sensible heating effect from the steam itself. 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.


Where two air streams combine adiabatically as they normally would the resulting chartt will have a psychrometric state that is related to the proportions of each of psycrometric combining air mass flowrates. This is indicated in Figure 7 as air stream A mixing with air stream B to produce mixed air at condition M. 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.

Module 3: The properties of air

The psychrometric values were based on the on-coil and off-coil dry-bulb and wet-bulb temperatures as indicated in the diagram. These calculations may be reorganised to determine unknown variables for any of the processes illustrated in this article. A future article will combine these processes to examine system psychrometry. Figure 1 — Outline Psychrometric Chart. Figure 2 — Psychrometry of Sensible Heating. Figure 3 — Psychrometry of Sensible Cooling. Figure psychrmetric — Psychrometry of Steam Humidification.

Figure 5 — Psychrometry of Sensible and Latent Cooling. Figure 6 — Psychrometry of Adiabatic Humidification. Figure 7 — Psychrometry of Mixing Air Streams. Figure 8 — Example Psychrometric Process.

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